mono/netcore/System.Private.CoreLib/shared/System/Text/StringBuilder.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System.Text;
using System.Runtime;
using System.Runtime.Serialization;
using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Runtime.Versioning;
using System.Security;
using System.Threading;
using System.Globalization;
using System.Diagnostics;
using System.Collections.Generic;

namespace System.Text
{
    // This class represents a mutable string.  It is convenient for situations in
    // which it is desirable to modify a string, perhaps by removing, replacing, or
    // inserting characters, without creating a new String subsequent to
    // each modification.
    //
    // The methods contained within this class do not return a new StringBuilder
    // object unless specified otherwise.  This class may be used in conjunction with the String
    // class to carry out modifications upon strings.
    [Serializable]
    [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
    public sealed partial class StringBuilder : ISerializable
    {
        // A StringBuilder is internally represented as a linked list of blocks each of which holds
        // a chunk of the string.  It turns out string as a whole can also be represented as just a chunk,
        // so that is what we do.

        /// <summary>
        /// The character buffer for this chunk.
        /// </summary>
        internal char[] m_ChunkChars;

        /// <summary>
        /// The chunk that logically precedes this chunk.
        /// </summary>
        internal StringBuilder? m_ChunkPrevious;

        /// <summary>
        /// The number of characters in this chunk.
        /// This is the number of elements in <see cref="m_ChunkChars"/> that are in use, from the start of the buffer.
        /// </summary>
        internal int m_ChunkLength;

        /// <summary>
        /// The logical offset of this chunk's characters in the string it is a part of.
        /// This is the sum of the number of characters in preceding blocks.
        /// </summary>
        internal int m_ChunkOffset;

        /// <summary>
        /// The maximum capacity this builder is allowed to have.
        /// </summary>
        internal int m_MaxCapacity;

        /// <summary>
        /// The default capacity of a <see cref="StringBuilder"/>.
        /// </summary>
        internal const int DefaultCapacity = 16;

        private const string CapacityField = "Capacity"; // Do not rename (binary serialization)
        private const string MaxCapacityField = "m_MaxCapacity"; // Do not rename (binary serialization)
        private const string StringValueField = "m_StringValue"; // Do not rename (binary serialization)
        private const string ThreadIDField = "m_currentThread"; // Do not rename (binary serialization)

        // We want to keep chunk arrays out of large object heap (< 85K bytes ~ 40K chars) to be sure.
        // Making the maximum chunk size big means less allocation code called, but also more waste
        // in unused characters and slower inserts / replaces (since you do need to slide characters over
        // within a buffer).
        internal const int MaxChunkSize = 8000;

        /// <summary>
        /// Initializes a new instance of the <see cref="StringBuilder"/> class.
        /// </summary>
        public StringBuilder()
        {
            m_MaxCapacity = int.MaxValue;
            m_ChunkChars = new char[DefaultCapacity];
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="StringBuilder"/> class.
        /// </summary>
        /// <param name="capacity">The initial capacity of this builder.</param>
        public StringBuilder(int capacity)
            : this(capacity, int.MaxValue)
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="StringBuilder"/> class.
        /// </summary>
        /// <param name="value">The initial contents of this builder.</param>
        public StringBuilder(string? value)
            : this(value, DefaultCapacity)
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="StringBuilder"/> class.
        /// </summary>
        /// <param name="value">The initial contents of this builder.</param>
        /// <param name="capacity">The initial capacity of this builder.</param>
        public StringBuilder(string? value, int capacity)
            : this(value, 0, value?.Length ?? 0, capacity)
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="StringBuilder"/> class.
        /// </summary>
        /// <param name="value">The initial contents of this builder.</param>
        /// <param name="startIndex">The index to start in <paramref name="value"/>.</param>
        /// <param name="length">The number of characters to read in <paramref name="value"/>.</param>
        /// <param name="capacity">The initial capacity of this builder.</param>
        public StringBuilder(string? value, int startIndex, int length, int capacity)
        {
            if (capacity < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(capacity), SR.Format(SR.ArgumentOutOfRange_MustBePositive, nameof(capacity)));
            }
            if (length < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(length), SR.Format(SR.ArgumentOutOfRange_MustBeNonNegNum, nameof(length)));
            }
            if (startIndex < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_StartIndex);
            }

            if (value == null)
            {
                value = string.Empty;
            }
            if (startIndex > value.Length - length)
            {
                throw new ArgumentOutOfRangeException(nameof(length), SR.ArgumentOutOfRange_IndexLength);
            }

            m_MaxCapacity = int.MaxValue;
            if (capacity == 0)
            {
                capacity = DefaultCapacity;
            }
            capacity = Math.Max(capacity, length);

            m_ChunkChars = new char[capacity];
            m_ChunkLength = length;

            unsafe
            {
                fixed (char* sourcePtr = value)
                {
                    ThreadSafeCopy(sourcePtr + startIndex, m_ChunkChars, 0, length);
                }
            }
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="StringBuilder"/> class.
        /// </summary>
        /// <param name="capacity">The initial capacity of this builder.</param>
        /// <param name="maxCapacity">The maximum capacity of this builder.</param>
        public StringBuilder(int capacity, int maxCapacity)
        {
            if (capacity > maxCapacity)
            {
                throw new ArgumentOutOfRangeException(nameof(capacity), SR.ArgumentOutOfRange_Capacity);
            }
            if (maxCapacity < 1)
            {
                throw new ArgumentOutOfRangeException(nameof(maxCapacity), SR.ArgumentOutOfRange_SmallMaxCapacity);
            }
            if (capacity < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(capacity), SR.Format(SR.ArgumentOutOfRange_MustBePositive, nameof(capacity)));
            }

            if (capacity == 0)
            {
                capacity = Math.Min(DefaultCapacity, maxCapacity);
            }

            m_MaxCapacity = maxCapacity;
            m_ChunkChars = new char[capacity];
        }

        private StringBuilder(SerializationInfo info, StreamingContext context)
        {
            if (info == null)
            {
                throw new ArgumentNullException(nameof(info));
            }

            int persistedCapacity = 0;
            string? persistedString = null;
            int persistedMaxCapacity = int.MaxValue;
            bool capacityPresent = false;

            // Get the data
            SerializationInfoEnumerator enumerator = info.GetEnumerator();
            while (enumerator.MoveNext())
            {
                switch (enumerator.Name)
                {
                    case MaxCapacityField:
                        persistedMaxCapacity = info.GetInt32(MaxCapacityField);
                        break;
                    case StringValueField:
                        persistedString = info.GetString(StringValueField);
                        break;
                    case CapacityField:
                        persistedCapacity = info.GetInt32(CapacityField);
                        capacityPresent = true;
                        break;
                    default:
                        // Ignore other fields for forwards-compatibility.
                        break;
                }
            }

            // Check values and set defaults
            if (persistedString == null)
            {
                persistedString = string.Empty;
            }
            if (persistedMaxCapacity < 1 || persistedString.Length > persistedMaxCapacity)
            {
                throw new SerializationException(SR.Serialization_StringBuilderMaxCapacity);
            }

            if (!capacityPresent)
            {
                // StringBuilder in V1.X did not persist the Capacity, so this is a valid legacy code path.
                persistedCapacity = Math.Min(Math.Max(DefaultCapacity, persistedString.Length), persistedMaxCapacity);
            }

            if (persistedCapacity < 0 || persistedCapacity < persistedString.Length || persistedCapacity > persistedMaxCapacity)
            {
                throw new SerializationException(SR.Serialization_StringBuilderCapacity);
            }

            // Assign
            m_MaxCapacity = persistedMaxCapacity;
            m_ChunkChars = new char[persistedCapacity];
            persistedString.CopyTo(0, m_ChunkChars, 0, persistedString.Length);
            m_ChunkLength = persistedString.Length;
            m_ChunkPrevious = null;
            AssertInvariants();
        }

        void ISerializable.GetObjectData(SerializationInfo info, StreamingContext context)
        {
            if (info == null)
            {
                throw new ArgumentNullException(nameof(info));
            }

            AssertInvariants();
            info.AddValue(MaxCapacityField, m_MaxCapacity);
            info.AddValue(CapacityField, Capacity);
            info.AddValue(StringValueField, ToString());
            // Note: persist "m_currentThread" to be compatible with old versions
            info.AddValue(ThreadIDField, 0);
        }

        [System.Diagnostics.Conditional("DEBUG")]
        private void AssertInvariants()
        {
            Debug.Assert(m_ChunkOffset + m_ChunkChars.Length >= m_ChunkOffset, "The length of the string is greater than int.MaxValue.");

            StringBuilder currentBlock = this;
            int maxCapacity = this.m_MaxCapacity;
            for (;;)
            {
                // All blocks have the same max capacity.
                Debug.Assert(currentBlock.m_MaxCapacity == maxCapacity);
                Debug.Assert(currentBlock.m_ChunkChars != null);

                Debug.Assert(currentBlock.m_ChunkLength <= currentBlock.m_ChunkChars.Length);
                Debug.Assert(currentBlock.m_ChunkLength >= 0);
                Debug.Assert(currentBlock.m_ChunkOffset >= 0);

                StringBuilder? prevBlock = currentBlock.m_ChunkPrevious;
                if (prevBlock == null)
                {
                    Debug.Assert(currentBlock.m_ChunkOffset == 0);
                    break;
                }
                // There are no gaps in the blocks.
                Debug.Assert(currentBlock.m_ChunkOffset == prevBlock.m_ChunkOffset + prevBlock.m_ChunkLength);
                currentBlock = prevBlock;
            }
        }

        public int Capacity
        {
            get { return m_ChunkChars.Length + m_ChunkOffset; }
            set
            {
                if (value < 0)
                {
                    throw new ArgumentOutOfRangeException(nameof(value), SR.ArgumentOutOfRange_NegativeCapacity);
                }
                if (value > MaxCapacity)
                {
                    throw new ArgumentOutOfRangeException(nameof(value), SR.ArgumentOutOfRange_Capacity);
                }
                if (value < Length)
                {
                    throw new ArgumentOutOfRangeException(nameof(value), SR.ArgumentOutOfRange_SmallCapacity);
                }

                if (Capacity != value)
                {
                    int newLen = value - m_ChunkOffset;
                    char[] newArray = new char[newLen];
                    Array.Copy(m_ChunkChars, 0, newArray, 0, m_ChunkLength);
                    m_ChunkChars = newArray;
                }
            }
        }

        /// <summary>
        /// Gets the maximum capacity this builder is allowed to have.
        /// </summary>
        public int MaxCapacity => m_MaxCapacity;

        /// <summary>
        /// Ensures that the capacity of this builder is at least the specified value.
        /// </summary>
        /// <param name="capacity">The new capacity for this builder.</param>
        /// <remarks>
        /// If <paramref name="capacity"/> is less than or equal to the current capacity of
        /// this builder, the capacity remains unchanged.
        /// </remarks>
        public int EnsureCapacity(int capacity)
        {
            if (capacity < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(capacity), SR.ArgumentOutOfRange_NegativeCapacity);
            }

            if (Capacity < capacity)
            {
                Capacity = capacity;
            }
            return Capacity;
        }

        public override string ToString()
        {
            AssertInvariants();

            if (Length == 0)
            {
                return string.Empty;
            }

            string result = string.FastAllocateString(Length);
            StringBuilder? chunk = this;
            unsafe
            {
                fixed (char* destinationPtr = result)
                {
                    do
                    {
                        if (chunk.m_ChunkLength > 0)
                        {
                            // Copy these into local variables so that they are stable even in the presence of race conditions
                            char[] sourceArray = chunk.m_ChunkChars;
                            int chunkOffset = chunk.m_ChunkOffset;
                            int chunkLength = chunk.m_ChunkLength;

                            // Check that we will not overrun our boundaries.
                            if ((uint)(chunkLength + chunkOffset) <= (uint)result.Length && (uint)chunkLength <= (uint)sourceArray.Length)
                            {
                                fixed (char* sourcePtr = &sourceArray[0])
                                    string.wstrcpy(destinationPtr + chunkOffset, sourcePtr, chunkLength);
                            }
                            else
                            {
                                throw new ArgumentOutOfRangeException(nameof(chunkLength), SR.ArgumentOutOfRange_Index);
                            }
                        }
                        chunk = chunk.m_ChunkPrevious;
                    }
                    while (chunk != null);

                    return result;
                }
            }
        }

        /// <summary>
        /// Creates a string from a substring of this builder.
        /// </summary>
        /// <param name="startIndex">The index to start in this builder.</param>
        /// <param name="length">The number of characters to read in this builder.</param>
        public string ToString(int startIndex, int length)
        {
            int currentLength = this.Length;
            if (startIndex < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_StartIndex);
            }
            if (startIndex > currentLength)
            {
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_StartIndexLargerThanLength);
            }
            if (length < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(length), SR.ArgumentOutOfRange_NegativeLength);
            }
            if (startIndex > currentLength - length)
            {
                throw new ArgumentOutOfRangeException(nameof(length), SR.ArgumentOutOfRange_IndexLength);
            }

            AssertInvariants();
            string result = string.FastAllocateString(length);
            unsafe
            {
                fixed (char* destinationPtr = result)
                {
                    this.CopyTo(startIndex, new Span<char>(destinationPtr, length), length);
                    return result;
                }
            }
        }

        public StringBuilder Clear()
        {
            this.Length = 0;
            return this;
        }

        /// <summary>
        /// Gets or sets the length of this builder.
        /// </summary>
        public int Length
        {
            get
            {
                return m_ChunkOffset + m_ChunkLength;
            }
            set
            {
                //If the new length is less than 0 or greater than our Maximum capacity, bail.
                if (value < 0)
                {
                    throw new ArgumentOutOfRangeException(nameof(value), SR.ArgumentOutOfRange_NegativeLength);
                }

                if (value > MaxCapacity)
                {
                    throw new ArgumentOutOfRangeException(nameof(value), SR.ArgumentOutOfRange_SmallCapacity);
                }

                if (value == 0 && m_ChunkPrevious == null)
                {
                    m_ChunkLength = 0;
                    m_ChunkOffset = 0;
                    return;
                }

                int delta = value - Length;
                if (delta > 0)
                {
                    // Pad ourselves with null characters.
                    Append('\0', delta);
                }
                else
                {
                    StringBuilder chunk = FindChunkForIndex(value);
                    if (chunk != this)
                    {
                        // Avoid possible infinite capacity growth.  See https://github.com/dotnet/coreclr/pull/16926
                        int capacityToPreserve = Math.Min(Capacity, Math.Max(Length * 6 / 5, m_ChunkChars.Length));
                        int newLen = capacityToPreserve - chunk.m_ChunkOffset;
                        if (newLen > chunk.m_ChunkChars.Length)
                        {
                            // We crossed a chunk boundary when reducing the Length. We must replace this middle-chunk with a new larger chunk,
                            // to ensure the capacity we want is preserved.
                            char[] newArray = new char[newLen];
                            Array.Copy(chunk.m_ChunkChars, 0, newArray, 0, chunk.m_ChunkLength);
                            m_ChunkChars = newArray;
                        }
                        else
                        {
                            // Special case where the capacity we want to keep corresponds exactly to the size of the content.
                            // Just take ownership of the array.
                            Debug.Assert(newLen == chunk.m_ChunkChars.Length, "The new chunk should be larger or equal to the one it is replacing.");
                            m_ChunkChars = chunk.m_ChunkChars;
                        }

                        m_ChunkPrevious = chunk.m_ChunkPrevious;
                        m_ChunkOffset = chunk.m_ChunkOffset;
                    }
                    m_ChunkLength = value - chunk.m_ChunkOffset;
                    AssertInvariants();
                }
                Debug.Assert(Length == value, "Something went wrong setting Length.");
            }
        }

        [IndexerName("Chars")]
        public char this[int index]
        {
            get
            {
                StringBuilder? chunk = this;
                for (;;)
                {
                    int indexInBlock = index - chunk.m_ChunkOffset;
                    if (indexInBlock >= 0)
                    {
                        if (indexInBlock >= chunk.m_ChunkLength)
                        {
                            throw new IndexOutOfRangeException();
                        }
                        return chunk.m_ChunkChars[indexInBlock];
                    }
                    chunk = chunk.m_ChunkPrevious;
                    if (chunk == null)
                    {
                        throw new IndexOutOfRangeException();
                    }
                }
            }
            set
            {
                StringBuilder? chunk = this;
                for (;;)
                {
                    int indexInBlock = index - chunk.m_ChunkOffset;
                    if (indexInBlock >= 0)
                    {
                        if (indexInBlock >= chunk.m_ChunkLength)
                        {
                            throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
                        }
                        chunk.m_ChunkChars[indexInBlock] = value;
                        return;
                    }
                    chunk = chunk.m_ChunkPrevious;
                    if (chunk == null)
                    {
                        throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
                    }
                }
            }
        }

        /// <summary>
        /// GetChunks returns ChunkEnumerator that follows the IEnumerable pattern and
        /// thus can be used in a C# 'foreach' statements to retreive the data in the StringBuilder
        /// as chunks (ReadOnlyMemory) of characters.  An example use is:
        /// 
        ///      foreach (ReadOnlyMemory&lt;char&gt; chunk in sb.GetChunks())
        ///         foreach(char c in chunk.Span)
        ///             { /* operation on c }
        ///
        /// It is undefined what happens if the StringBuilder is modified while the chunk
        /// enumeration is incomplete.  StringBuilder is also not thread-safe, so operating
        /// on it with concurrent threads is illegal.  Finally the ReadOnlyMemory chunks returned 
        /// are NOT guarenteed to remain unchanged if the StringBuilder is modified, so do 
        /// not cache them for later use either.  This API's purpose is efficiently extracting
        /// the data of a CONSTANT StringBuilder.  
        /// 
        /// Creating a ReadOnlySpan from a ReadOnlyMemory  (the .Span property) is expensive 
        /// compared to the fetching of the character, so create a local variable for the SPAN 
        /// if you need to use it in a nested for statement.  For example 
        /// 
        ///    foreach (ReadOnlyMemory&lt;char&gt; chunk in sb.GetChunks())
        ///    {
        ///         var span = chunk.Span;
        ///         for(int i = 0; i &lt; span.Length; i++)
        ///             { /* operation on span[i] */ }
        ///    }
        /// </summary>
        public ChunkEnumerator GetChunks() => new ChunkEnumerator(this);

        /// <summary>
        /// ChunkEnumerator supports both the IEnumerable and IEnumerator pattern so foreach 
        /// works (see GetChunks).  It needs to be public (so the compiler can use it 
        /// when building a foreach statement) but users typically don't use it explicitly.
        /// (which is why it is a nested type). 
        /// </summary>
        public struct ChunkEnumerator
        {
            private readonly StringBuilder _firstChunk; // The first Stringbuilder chunk (which is the end of the logical string)
            private StringBuilder? _currentChunk;        // The chunk that this enumerator is currently returning (Current).  
            private readonly ManyChunkInfo? _manyChunks; // Only used for long string builders with many chunks (see constructor)

            /// <summary>
            /// Implement IEnumerable.GetEnumerator() to return  'this' as the IEnumerator  
            /// </summary>
            [ComponentModel.EditorBrowsable(ComponentModel.EditorBrowsableState.Never)] // Only here to make foreach work
            public ChunkEnumerator GetEnumerator() { return this;  }

            /// <summary>
            /// Implements the IEnumerator pattern.
            /// </summary>
            public bool MoveNext()
            {
                if (_currentChunk == _firstChunk)
                {
                    return false;
                }
                    

                if (_manyChunks != null)
                {
                    return _manyChunks.MoveNext(ref _currentChunk);
                }

                StringBuilder next = _firstChunk;
                while (next.m_ChunkPrevious != _currentChunk)
                {
                    Debug.Assert(next.m_ChunkPrevious != null);
                    next = next.m_ChunkPrevious;
                }
                _currentChunk = next;
                return true;
            }

            /// <summary>
            /// Implements the IEnumerator pattern.
            /// </summary>
            public ReadOnlyMemory<char> Current
            {
                get
                {
                    if (_currentChunk == null)
                    {
                        ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumOpCantHappen();
                    }

                    return new ReadOnlyMemory<char>(_currentChunk.m_ChunkChars, 0, _currentChunk.m_ChunkLength);
                }
            }

            #region private
            internal ChunkEnumerator(StringBuilder stringBuilder)
            {
                Debug.Assert(stringBuilder != null);
                _firstChunk = stringBuilder;
                _currentChunk = null;   // MoveNext will find the last chunk if we do this.  
                _manyChunks = null;

                // There is a performance-vs-allocation tradeoff.   Because the chunks
                // are a linked list with each chunk pointing to its PREDECESSOR, walking
                // the list FORWARD is not efficient.   If there are few chunks (< 8) we
                // simply scan from the start each time, and tolerate the N*N behavior. 
                // However above this size, we allocate an array to hold pointers to all
                // the chunks and we can be efficient for large N.    
                int chunkCount = ChunkCount(stringBuilder);
                if (8 < chunkCount)
                {
                    _manyChunks = new ManyChunkInfo(stringBuilder, chunkCount);
                }
                    
            }

            private static int ChunkCount(StringBuilder? stringBuilder)
            {
                int ret = 0;
                while (stringBuilder != null)
                {
                    ret++;
                    stringBuilder = stringBuilder.m_ChunkPrevious;
                }
                return ret;
            }

            /// <summary>
            /// Used to hold all the chunks indexes when you have many chunks. 
            /// </summary>
            private class ManyChunkInfo
            {
                private readonly StringBuilder[] _chunks;    // These are in normal order (first chunk first) 
                private int _chunkPos;

                public bool MoveNext(ref StringBuilder? current)
                {
                    int pos = ++_chunkPos;
                    if (_chunks.Length <= pos)
                    {
                        return false;
                    }
                    current = _chunks[pos];
                    return true;
                }

                public ManyChunkInfo(StringBuilder? stringBuilder, int chunkCount)
                {
                    _chunks = new StringBuilder[chunkCount];
                    while (0 <= --chunkCount)
                    {
                        Debug.Assert(stringBuilder != null);
                        _chunks[chunkCount] = stringBuilder;
                        stringBuilder = stringBuilder.m_ChunkPrevious;
                    }
                    _chunkPos = -1;
                }
            }
#endregion
        }

        /// <summary>
        /// Appends a character 0 or more times to the end of this builder.
        /// </summary>
        /// <param name="value">The character to append.</param>
        /// <param name="repeatCount">The number of times to append <paramref name="value"/>.</param>
        public StringBuilder Append(char value, int repeatCount)
        {
            if (repeatCount < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(repeatCount), SR.ArgumentOutOfRange_NegativeCount);
            }

            if (repeatCount == 0)
            {
                return this;
            }

            // this is where we can check if the repeatCount will put us over m_MaxCapacity
            // We are doing the check here to prevent the corruption of the StringBuilder.
            int newLength = Length + repeatCount;
            if (newLength > m_MaxCapacity || newLength < repeatCount)
            {
                throw new ArgumentOutOfRangeException(nameof(repeatCount), SR.ArgumentOutOfRange_LengthGreaterThanCapacity);
            }

            int index = m_ChunkLength;
            while (repeatCount > 0)
            {
                if (index < m_ChunkChars.Length)
                {
                    m_ChunkChars[index++] = value;
                    --repeatCount;
                }
                else
                {
                    m_ChunkLength = index;
                    ExpandByABlock(repeatCount);
                    Debug.Assert(m_ChunkLength == 0);
                    index = 0;
                }
            }

            m_ChunkLength = index;
            AssertInvariants();
            return this;
        }

        /// <summary>
        /// Appends a range of characters to the end of this builder.
        /// </summary>
        /// <param name="value">The characters to append.</param>
        /// <param name="startIndex">The index to start in <paramref name="value"/>.</param>
        /// <param name="charCount">The number of characters to read in <paramref name="value"/>.</param>
        public StringBuilder Append(char[]? value, int startIndex, int charCount)
        {
            if (startIndex < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_GenericPositive);
            }
            if (charCount < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(charCount), SR.ArgumentOutOfRange_GenericPositive);
            }

            if (value == null)
            {
                if (startIndex == 0 && charCount == 0)
                {
                    return this;
                }

                throw new ArgumentNullException(nameof(value));
            }
            if (charCount > value.Length - startIndex)
            {
                throw new ArgumentOutOfRangeException(nameof(charCount), SR.ArgumentOutOfRange_Index);
            }

            if (charCount == 0)
            {
                return this;
            }

            unsafe
            {
                fixed (char* valueChars = &value[startIndex])
                {
                    Append(valueChars, charCount);
                    return this;
                }
            }
        }


        /// <summary>
        /// Appends a string to the end of this builder.
        /// </summary>
        /// <param name="value">The string to append.</param>
        public StringBuilder Append(string? value)
        {
            if (value != null)
            {
                // We could have just called AppendHelper here; this is a hand-specialization of that code.
                char[] chunkChars = m_ChunkChars;
                int chunkLength = m_ChunkLength;
                int valueLen = value.Length;
                int newCurrentIndex = chunkLength + valueLen;

                if (newCurrentIndex < chunkChars.Length) // Use strictly < to avoid issues if count == 0, newIndex == length
                {
                    if (valueLen <= 2)
                    {
                        if (valueLen > 0)
                        {
                            chunkChars[chunkLength] = value[0];
                        }
                        if (valueLen > 1)
                        {
                            chunkChars[chunkLength + 1] = value[1];
                        }
                    }
                    else
                    {
                        unsafe
                        {
                            fixed (char* valuePtr = value)
                            fixed (char* destPtr = &chunkChars[chunkLength])
                            {
                                string.wstrcpy(destPtr, valuePtr, valueLen);
                            }
                        }
                    }

                    m_ChunkLength = newCurrentIndex;
                }
                else
                {
                    AppendHelper(value);
                }
            }

            return this;
        }

        // We put this fixed in its own helper to avoid the cost of zero-initing `valueChars` in the
        // case we don't actually use it.
        private void AppendHelper(string value)
        {
            unsafe
            {
                fixed (char* valueChars = value)
                {
                    Append(valueChars, value.Length);
                }
            }
        }

        /// <summary>
        /// Appends part of a string to the end of this builder.
        /// </summary>
        /// <param name="value">The string to append.</param>
        /// <param name="startIndex">The index to start in <paramref name="value"/>.</param>
        /// <param name="count">The number of characters to read in <paramref name="value"/>.</param>
        public StringBuilder Append(string? value, int startIndex, int count)
        {
            if (startIndex < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);
            }
            if (count < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_GenericPositive);
            }

            if (value == null)
            {
                if (startIndex == 0 && count == 0)
                {
                    return this;
                }
                throw new ArgumentNullException(nameof(value));
            }

            if (count == 0)
            {
                return this;
            }

            if (startIndex > value.Length - count)
            {
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);
            }

            unsafe
            {
                fixed (char* valueChars = value)
                {
                    Append(valueChars + startIndex, count);
                    return this;
                }
            }
        }

        public StringBuilder Append(StringBuilder? value)
        {
            if (value != null && value.Length != 0)
            {
                return AppendCore(value, 0, value.Length);
            }
            return this;
        }

        public StringBuilder Append(StringBuilder? value, int startIndex, int count)
        {
            if (startIndex < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);
            }

            if (count < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_GenericPositive);
            }

            if (value == null)
            {
                if (startIndex == 0 && count == 0)
                {
                    return this;
                }
                throw new ArgumentNullException(nameof(value));
            }

            if (count == 0)
            {
                return this;
            }

            if (count > value.Length - startIndex)
            {
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);
            }

            return AppendCore(value, startIndex, count);
        }

        private StringBuilder AppendCore(StringBuilder value, int startIndex, int count)
        {
            if (value == this)
            {
                return Append(value.ToString(startIndex, count));
            }

            int newLength = Length + count;

            if ((uint)newLength > (uint)m_MaxCapacity)
            {
                throw new ArgumentOutOfRangeException(nameof(Capacity), SR.ArgumentOutOfRange_Capacity);
            }

            while (count > 0)
            {
                int length = Math.Min(m_ChunkChars.Length - m_ChunkLength, count);
                if (length == 0)
                {
                    ExpandByABlock(count);
                    length = Math.Min(m_ChunkChars.Length - m_ChunkLength, count);
                }
                value.CopyTo(startIndex, new Span<char>(m_ChunkChars, m_ChunkLength, length), length);

                m_ChunkLength += length;
                startIndex += length;
                count -= length;
            }

            return this;
        }

        public StringBuilder AppendLine() => Append(Environment.NewLine);

        public StringBuilder AppendLine(string? value)
        {
            Append(value);
            return Append(Environment.NewLine);
        }

        public void CopyTo(int sourceIndex, char[] destination, int destinationIndex, int count)
        {
            if (destination == null)
            {
                throw new ArgumentNullException(nameof(destination));
            }

            if (destinationIndex < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(destinationIndex), SR.Format(SR.ArgumentOutOfRange_MustBeNonNegNum, nameof(destinationIndex)));
            }

            if (destinationIndex > destination.Length - count)
            {
                throw new ArgumentException(SR.ArgumentOutOfRange_OffsetOut);
            }

            CopyTo(sourceIndex, new Span<char>(destination).Slice(destinationIndex), count);
        }

        public void CopyTo(int sourceIndex, Span<char> destination, int count)
        {
            if (count < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(count), SR.Arg_NegativeArgCount);
            }

            if ((uint)sourceIndex > (uint)Length)
            {
                throw new ArgumentOutOfRangeException(nameof(sourceIndex), SR.ArgumentOutOfRange_Index);
            }

            if (sourceIndex > Length - count)
            {
                throw new ArgumentException(SR.Arg_LongerThanSrcString);
            }

            AssertInvariants();

            StringBuilder? chunk = this;
            int sourceEndIndex = sourceIndex + count;
            int curDestIndex = count;
            while (count > 0)
            {
                Debug.Assert(chunk != null);
                int chunkEndIndex = sourceEndIndex - chunk.m_ChunkOffset;
                if (chunkEndIndex >= 0)
                {
                    chunkEndIndex = Math.Min(chunkEndIndex, chunk.m_ChunkLength);

                    int chunkCount = count;
                    int chunkStartIndex = chunkEndIndex - count;
                    if (chunkStartIndex < 0)
                    {
                        chunkCount += chunkStartIndex;
                        chunkStartIndex = 0;
                    }
                    curDestIndex -= chunkCount;
                    count -= chunkCount;

                    // We ensure that chunkStartIndex + chunkCount are within range of m_chunkChars as well as
                    // ensuring that curDestIndex + chunkCount are within range of destination
                    ThreadSafeCopy(chunk.m_ChunkChars, chunkStartIndex, destination, curDestIndex, chunkCount);
                }
                chunk = chunk.m_ChunkPrevious;
            }
        }

        /// <summary>
        /// Inserts a string 0 or more times into this builder at the specified position.
        /// </summary>
        /// <param name="index">The index to insert in this builder.</param>
        /// <param name="value">The string to insert.</param>
        /// <param name="count">The number of times to insert the string.</param>
        public StringBuilder Insert(int index, string? value, int count)
        {
            if (count < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
            }

            int currentLength = Length;
            if ((uint)index > (uint)currentLength)
            {
                throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
            }

            if (string.IsNullOrEmpty(value) || count == 0)
            {
                return this;
            }

            // Ensure we don't insert more chars than we can hold, and we don't
            // have any integer overflow in our new length.
            long insertingChars = (long)value.Length * count;
            if (insertingChars > MaxCapacity - this.Length)
            {
                throw new OutOfMemoryException();
            }
            Debug.Assert(insertingChars + this.Length < int.MaxValue);

            StringBuilder chunk;
            int indexInChunk;
            MakeRoom(index, (int)insertingChars, out chunk, out indexInChunk, false);
            unsafe
            {
                fixed (char* valuePtr = value)
                {
                    while (count > 0)
                    {
                        ReplaceInPlaceAtChunk(ref chunk!, ref indexInChunk, valuePtr, value.Length);
                        --count;
                    }

                    return this;
                }
            }
        }

        /// <summary>
        /// Removes a range of characters from this builder.
        /// </summary>
        /// <remarks>
        /// This method does not reduce the capacity of this builder.
        /// </remarks>
        public StringBuilder Remove(int startIndex, int length)
        {
            if (length < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(length), SR.ArgumentOutOfRange_NegativeLength);
            }

            if (startIndex < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_StartIndex);
            }

            if (length > Length - startIndex)
            {
                throw new ArgumentOutOfRangeException(nameof(length), SR.ArgumentOutOfRange_Index);
            }

            if (Length == length && startIndex == 0)
            {
                Length = 0;
                return this;
            }

            if (length > 0)
            {
                StringBuilder chunk;
                int indexInChunk;
                Remove(startIndex, length, out chunk, out indexInChunk);
            }

            return this;
        }

        public StringBuilder Append(bool value) => Append(value.ToString());

        public StringBuilder Append(char value)
        {
            if (m_ChunkLength < m_ChunkChars.Length)
            {
                m_ChunkChars[m_ChunkLength++] = value;
            }
            else
            {
                Append(value, 1);
            }

            return this;
        }

        [CLSCompliant(false)]
        public StringBuilder Append(sbyte value) => AppendSpanFormattable(value);

        public StringBuilder Append(byte value) => AppendSpanFormattable(value);

        public StringBuilder Append(short value) => AppendSpanFormattable(value);

        public StringBuilder Append(int value) => AppendSpanFormattable(value);

        public StringBuilder Append(long value) => AppendSpanFormattable(value);

        public StringBuilder Append(float value) => AppendSpanFormattable(value);

        public StringBuilder Append(double value) => AppendSpanFormattable(value);

        public StringBuilder Append(decimal value) => AppendSpanFormattable(value);

        [CLSCompliant(false)]
        public StringBuilder Append(ushort value) => AppendSpanFormattable(value);

        [CLSCompliant(false)]
        public StringBuilder Append(uint value) => AppendSpanFormattable(value);

        [CLSCompliant(false)]
        public StringBuilder Append(ulong value) => AppendSpanFormattable(value);

        private StringBuilder AppendSpanFormattable<T>(T value) where T : ISpanFormattable
        {
            if (value.TryFormat(RemainingCurrentChunk, out int charsWritten, format: default, provider: null))
            {
                m_ChunkLength += charsWritten;
                return this;
            }

            return Append(value.ToString());
        }

        internal StringBuilder AppendSpanFormattable<T>(T value, string? format, IFormatProvider? provider) where T : ISpanFormattable, IFormattable
        {
            if (value.TryFormat(RemainingCurrentChunk, out int charsWritten, format, provider))
            {
                m_ChunkLength += charsWritten;
                return this;
            }

            return Append(value.ToString(format, provider));
        }

        public StringBuilder Append(object? value) => (value == null) ? this : Append(value.ToString());

        public StringBuilder Append(char[]? value)
        {
            if (value?.Length > 0)
            {
                unsafe
                {
                    fixed (char* valueChars = &value[0])
                    {
                        Append(valueChars, value.Length);
                    }
                }
            }
            return this;
        }

        public StringBuilder Append(ReadOnlySpan<char> value)
        {
            if (value.Length > 0)
            {
                unsafe
                {
                    fixed (char* valueChars = &MemoryMarshal.GetReference(value))
                    {
                        Append(valueChars, value.Length);
                    }
                }
            }
            return this;
        }

        public StringBuilder Append(ReadOnlyMemory<char> value) => Append(value.Span);

        #region AppendJoin

        public unsafe StringBuilder AppendJoin(string? separator, params object?[] values)
        {
            separator = separator ?? string.Empty;
            fixed (char* pSeparator = separator)
            {
                return AppendJoinCore(pSeparator, separator.Length, values);
            }
        }

        public unsafe StringBuilder AppendJoin<T>(string? separator, IEnumerable<T> values)
        {
            separator = separator ?? string.Empty;
            fixed (char* pSeparator = separator)
            {
                return AppendJoinCore(pSeparator, separator.Length, values);
            }
        }

        public unsafe StringBuilder AppendJoin(string? separator, params string?[] values)
        {
            separator = separator ?? string.Empty;
            fixed (char* pSeparator = separator)
            {
                return AppendJoinCore(pSeparator, separator.Length, values);
            }
        }

        public unsafe StringBuilder AppendJoin(char separator, params object?[] values)
        {
            return AppendJoinCore(&separator, 1, values);
        }

        public unsafe StringBuilder AppendJoin<T>(char separator, IEnumerable<T> values)
        {
            return AppendJoinCore(&separator, 1, values);
        }

        public unsafe StringBuilder AppendJoin(char separator, params string?[] values)
        {
            return AppendJoinCore(&separator, 1, values);
        }

        private unsafe StringBuilder AppendJoinCore<T>(char* separator, int separatorLength, IEnumerable<T> values)
        {
            Debug.Assert(separator != null);
            Debug.Assert(separatorLength >= 0);

            if (values == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.values);
            }

            Debug.Assert(values != null);
            using (IEnumerator<T> en = values.GetEnumerator())
            {
                if (!en.MoveNext())
                {
                    return this;
                }

                var value = en.Current;
                if (value != null)
                {
                    Append(value.ToString());
                }

                while (en.MoveNext())
                {
                    Append(separator, separatorLength);
                    value = en.Current;
                    if (value != null)
                    {
                        Append(value.ToString());
                    }
                }
            }
            return this;
        }

        private unsafe StringBuilder AppendJoinCore<T>(char* separator, int separatorLength, T[] values)
        {
            if (values == null)
            {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.values);
            }

            Debug.Assert(values != null);
            if (values.Length == 0)
            {
                return this;
            }

            if (values[0] != null)
            {
                Append(values[0]!.ToString()); // TODO-NULLABLE: Indexer nullability tracked (https://github.com/dotnet/roslyn/issues/34644)
            }

            for (int i = 1; i < values.Length; i++)
            {
                Append(separator, separatorLength);
                if (values[i] != null)
                {
                    Append(values[i]!.ToString()); // TODO-NULLABLE: Indexer nullability tracked (https://github.com/dotnet/roslyn/issues/34644)
                }
            }
            return this;
        }

        #endregion

        public StringBuilder Insert(int index, string? value)
        {
            if ((uint)index > (uint)Length)
            {
                throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
            }

            if (value != null)
            {
                unsafe
                {
                    fixed (char* sourcePtr = value)
                        Insert(index, sourcePtr, value.Length);
                }
            }
            return this;
        }

        public StringBuilder Insert(int index, bool value) => Insert(index, value.ToString(), 1);

        [CLSCompliant(false)]
        public StringBuilder Insert(int index, sbyte value) => Insert(index, value.ToString(), 1);

        public StringBuilder Insert(int index, byte value) => Insert(index, value.ToString(), 1);

        public StringBuilder Insert(int index, short value) => Insert(index, value.ToString(), 1);

        public StringBuilder Insert(int index, char value)
        {
            unsafe
            {
                Insert(index, &value, 1);
            }
            return this;
        }

        public StringBuilder Insert(int index, char[]? value)
        {
            if ((uint)index > (uint)Length)
            {
                throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
            }

            if (value != null)
            {
                Insert(index, value, 0, value.Length);
            }
            return this;
        }

        public StringBuilder Insert(int index, char[]? value, int startIndex, int charCount)
        {
            int currentLength = Length;
            if ((uint)index > (uint)currentLength)
            {
                throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
            }

            if (value == null)
            {
                if (startIndex == 0 && charCount == 0)
                {
                    return this;
                }
                throw new ArgumentNullException(nameof(value), SR.ArgumentNull_String);
            }

            if (startIndex < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_StartIndex);
            }

            if (charCount < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(charCount), SR.ArgumentOutOfRange_GenericPositive);
            }

            if (startIndex > value.Length - charCount)
            {
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);
            }

            if (charCount > 0)
            {
                unsafe
                {
                    fixed (char* sourcePtr = &value[startIndex])
                        Insert(index, sourcePtr, charCount);
                }
            }
            return this;
        }

        public StringBuilder Insert(int index, int value) => Insert(index, value.ToString(), 1);

        public StringBuilder Insert(int index, long value) => Insert(index, value.ToString(), 1);

        public StringBuilder Insert(int index, float value) => Insert(index, value.ToString(), 1);

        public StringBuilder Insert(int index, double value) => Insert(index, value.ToString(), 1);

        public StringBuilder Insert(int index, decimal value) => Insert(index, value.ToString(), 1);

        [CLSCompliant(false)]
        public StringBuilder Insert(int index, ushort value) => Insert(index, value.ToString(), 1);

        [CLSCompliant(false)]
        public StringBuilder Insert(int index, uint value) => Insert(index, value.ToString(), 1);

        [CLSCompliant(false)]
        public StringBuilder Insert(int index, ulong value) => Insert(index, value.ToString(), 1);

        public StringBuilder Insert(int index, object? value) => (value == null) ? this : Insert(index, value.ToString(), 1);

        public StringBuilder Insert(int index, ReadOnlySpan<char> value)
        {
            if ((uint)index > (uint)Length)
            {
                throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
            }

            if (value.Length > 0)
            {
                unsafe
                {
                    fixed (char* sourcePtr = &MemoryMarshal.GetReference(value))
                        Insert(index, sourcePtr, value.Length);
                }
            }
            return this;
        }

        public StringBuilder AppendFormat(string format, object? arg0) => AppendFormatHelper(null, format, new ParamsArray(arg0));

        public StringBuilder AppendFormat(string format, object? arg0, object? arg1) => AppendFormatHelper(null, format, new ParamsArray(arg0, arg1));

        public StringBuilder AppendFormat(string format, object? arg0, object? arg1, object? arg2) => AppendFormatHelper(null, format, new ParamsArray(arg0, arg1, arg2));

        public StringBuilder AppendFormat(string format, params object?[] args)
        {
            if (args == null)
            {
                // To preserve the original exception behavior, throw an exception about format if both
                // args and format are null. The actual null check for format is in AppendFormatHelper.
                string paramName = (format == null) ? nameof(format) : nameof(args);
                throw new ArgumentNullException(paramName);
            }

            return AppendFormatHelper(null, format, new ParamsArray(args));
        }

        public StringBuilder AppendFormat(IFormatProvider? provider, string format, object? arg0) => AppendFormatHelper(provider, format, new ParamsArray(arg0));

        public StringBuilder AppendFormat(IFormatProvider? provider, string format, object? arg0, object? arg1) => AppendFormatHelper(provider, format, new ParamsArray(arg0, arg1));

        public StringBuilder AppendFormat(IFormatProvider? provider, string format, object? arg0, object? arg1, object? arg2) => AppendFormatHelper(provider, format, new ParamsArray(arg0, arg1, arg2));

        public StringBuilder AppendFormat(IFormatProvider? provider, string format, params object?[] args)
        {
            if (args == null)
            {
                // To preserve the original exception behavior, throw an exception about format if both
                // args and format are null. The actual null check for format is in AppendFormatHelper.
                string paramName = (format == null) ? nameof(format) : nameof(args);
                throw new ArgumentNullException(paramName);
            }

            return AppendFormatHelper(provider, format, new ParamsArray(args));
        }

        private static void FormatError()
        {
            throw new FormatException(SR.Format_InvalidString);
        }

        // Undocumented exclusive limits on the range for Argument Hole Index and Argument Hole Alignment.
        private const int IndexLimit = 1000000; // Note:            0 <= ArgIndex < IndexLimit
        private const int WidthLimit = 1000000; // Note:  -WidthLimit <  ArgAlign < WidthLimit

        internal StringBuilder AppendFormatHelper(IFormatProvider? provider, string format, ParamsArray args)
        {
            if (format == null)
            {
                throw new ArgumentNullException(nameof(format));
            }

            int pos = 0;
            int len = format.Length;
            char ch = '\x0';

            ICustomFormatter? cf = null;
            if (provider != null)
            {
                cf = (ICustomFormatter?)provider.GetFormat(typeof(ICustomFormatter));
            }

            while (true)
            {
                while (pos < len)
                {
                    ch = format[pos];

                    pos++;
                    // Is it a closing brace?
                    if (ch == '}')
                    {
                        // Check next character (if there is one) to see if it is escaped. eg }}
                        if (pos < len && format[pos] == '}')
                        {
                            pos++;
                        }
                        else
                        {
                            // Otherwise treat it as an error (Mismatched closing brace)
                            FormatError();
                        }
                    }
                    // Is it a opening brace?
                    if (ch == '{')
                    {
                        // Check next character (if there is one) to see if it is escaped. eg {{
                        if (pos < len && format[pos] == '{')
                        {
                            pos++;
                        }
                        else
                        {
                            // Otherwise treat it as the opening brace of an Argument Hole.
                            pos--;
                            break;
                        }
                    }
                    // If it neither then treat the character as just text.
                    Append(ch);
                }

                //
                // Start of parsing of Argument Hole.
                // Argument Hole ::= { Index (, WS* Alignment WS*)? (: Formatting)? }
                //
                if (pos == len)
                {
                    break;
                }

                //
                //  Start of parsing required Index parameter.
                //  Index ::= ('0'-'9')+ WS*
                //
                pos++;
                // If reached end of text then error (Unexpected end of text)
                // or character is not a digit then error (Unexpected Character)
                if (pos == len || (ch = format[pos]) < '0' || ch > '9') FormatError();
                int index = 0;
                do
                {
                    index = index * 10 + ch - '0';
                    pos++;
                    // If reached end of text then error (Unexpected end of text)
                    if (pos == len)
                    {
                        FormatError();
                    }
                    ch = format[pos];
                    // so long as character is digit and value of the index is less than 1000000 ( index limit )
                }
                while (ch >= '0' && ch <= '9' && index < IndexLimit);

                // If value of index is not within the range of the arguments passed in then error (Index out of range)
                if (index >= args.Length)
                {
                    throw new FormatException(SR.Format_IndexOutOfRange);
                }

                // Consume optional whitespace.
                while (pos < len && (ch = format[pos]) == ' ') pos++;
                // End of parsing index parameter.

                //
                //  Start of parsing of optional Alignment
                //  Alignment ::= comma WS* minus? ('0'-'9')+ WS*
                //
                bool leftJustify = false;
                int width = 0;
                // Is the character a comma, which indicates the start of alignment parameter.
                if (ch == ',')
                {
                    pos++;

                    // Consume Optional whitespace
                    while (pos < len && format[pos] == ' ') pos++;

                    // If reached the end of the text then error (Unexpected end of text)
                    if (pos == len)
                    {
                        FormatError();
                    }

                    // Is there a minus sign?
                    ch = format[pos];
                    if (ch == '-')
                    {
                        // Yes, then alignment is left justified.
                        leftJustify = true;
                        pos++;
                        // If reached end of text then error (Unexpected end of text)
                        if (pos == len)
                        {
                            FormatError();
                        }
                        ch = format[pos];
                    }

                    // If current character is not a digit then error (Unexpected character)
                    if (ch < '0' || ch > '9')
                    {
                        FormatError();
                    }
                    // Parse alignment digits.
                    do
                    {
                        width = width * 10 + ch - '0';
                        pos++;
                        // If reached end of text then error. (Unexpected end of text)
                        if (pos == len)
                        {
                            FormatError();
                        }
                        ch = format[pos];
                        // So long a current character is a digit and the value of width is less than 100000 ( width limit )
                    }
                    while (ch >= '0' && ch <= '9' && width < WidthLimit);
                    // end of parsing Argument Alignment
                }

                // Consume optional whitespace
                while (pos < len && (ch = format[pos]) == ' ') pos++;

                //
                // Start of parsing of optional formatting parameter.
                //
                object? arg = args[index];

                ReadOnlySpan<char> itemFormatSpan = default; // used if itemFormat is null
                // Is current character a colon? which indicates start of formatting parameter.
                if (ch == ':')
                {
                    pos++;
                    int startPos = pos;

                    while (true)
                    {
                        // If reached end of text then error. (Unexpected end of text)
                        if (pos == len)
                        {
                            FormatError();
                        }
                        ch = format[pos];

                        if (ch == '}')
                        {
                            // Argument hole closed
                            break;
                        }
                        else if (ch == '{')
                        {
                            // Braces inside the argument hole are not supported
                            FormatError();
                        }

                        pos++;
                    }

                    if (pos > startPos)
                    {
                        itemFormatSpan = format.AsSpan(startPos, pos - startPos);
                    }
                }
                else if (ch != '}')
                {
                    // Unexpected character
                    FormatError();
                }

                // Construct the output for this arg hole.
                pos++;
                string? s = null;
                string? itemFormat = null;
                
                if (cf != null)
                {
                    if (itemFormatSpan.Length != 0)
                    {
                        itemFormat = new string(itemFormatSpan);
                    }
                    s = cf.Format(itemFormat, arg, provider);
                }

                if (s == null)
                {
                    // If arg is ISpanFormattable and the beginning doesn't need padding,
                    // try formatting it into the remaining current chunk.
                    if (arg is ISpanFormattable spanFormattableArg &&
                        (leftJustify || width == 0) &&
                        spanFormattableArg.TryFormat(RemainingCurrentChunk, out int charsWritten, itemFormatSpan, provider))
                    {
                        m_ChunkLength += charsWritten;

                        // Pad the end, if needed.
                        int padding = width - charsWritten;
                        if (leftJustify && padding > 0)
                        {
                            Append(' ', padding);
                        }

                        // Continue to parse other characters.
                        continue;
                    }

                    // Otherwise, fallback to trying IFormattable or calling ToString.
                    if (arg is IFormattable formattableArg)
                    {
                        if (itemFormatSpan.Length != 0 && itemFormat == null)
                        {
                            itemFormat = new string(itemFormatSpan);
                        }
                        s = formattableArg.ToString(itemFormat, provider);
                    }
                    else if (arg != null)
                    {
                        s = arg.ToString();
                    }
                }
                // Append it to the final output of the Format String.
                if (s == null)
                {
                    s = string.Empty;
                }
                int pad = width - s.Length;
                if (!leftJustify && pad > 0)
                {
                    Append(' ', pad);
                }
                
                Append(s);
                if (leftJustify && pad > 0)
                {
                    Append(' ', pad);
                }
                // Continue to parse other characters.
            }
            return this;
        }

        /// <summary>
        /// Replaces all instances of one string with another in this builder.
        /// </summary>
        /// <param name="oldValue">The string to replace.</param>
        /// <param name="newValue">The string to replace <paramref name="oldValue"/> with.</param>
        /// <remarks>
        /// If <paramref name="newValue"/> is <c>null</c>, instances of <paramref name="oldValue"/>
        /// are removed from this builder.
        /// </remarks>
        public StringBuilder Replace(string oldValue, string? newValue) => Replace(oldValue, newValue, 0, Length);

        /// <summary>
        /// Determines if the contents of this builder are equal to the contents of another builder.
        /// </summary>
        /// <param name="sb">The other builder.</param>
        public bool Equals(StringBuilder? sb)
        {
            if (sb == null)
            {
                return false;
            }
            if (Length != sb.Length)
            {
                return false;
            }
            if (sb == this)
            {
                return true;
            }
            StringBuilder? thisChunk = this;
            int thisChunkIndex = thisChunk.m_ChunkLength;
            StringBuilder? sbChunk = sb;
            int sbChunkIndex = sbChunk.m_ChunkLength;
            for (;;)
            {
                --thisChunkIndex;
                --sbChunkIndex;

                while (thisChunkIndex < 0)
                {
                    thisChunk = thisChunk.m_ChunkPrevious;
                    if (thisChunk == null)
                    {
                        break;
                    }
                    thisChunkIndex = thisChunk.m_ChunkLength + thisChunkIndex;
                }

                while (sbChunkIndex < 0)
                {
                    sbChunk = sbChunk.m_ChunkPrevious;
                    if (sbChunk == null)
                    {
                        break;
                    }
                    sbChunkIndex = sbChunk.m_ChunkLength + sbChunkIndex;
                }

                if (thisChunkIndex < 0)
                {
                    return sbChunkIndex < 0;
                }
                if (sbChunkIndex < 0)
                {
                    return false;
                }
                
                Debug.Assert(thisChunk != null && sbChunk != null);
                if (thisChunk.m_ChunkChars[thisChunkIndex] != sbChunk.m_ChunkChars[sbChunkIndex])
                {
                    return false;
                }
            }
        }

        /// <summary>
        /// Determines if the contents of this builder are equal to the contents of <see cref="ReadOnlySpan{Char}"/>.
        /// </summary>
        /// <param name="span">The <see cref="ReadOnlySpan{Char}"/>.</param>
        public bool Equals(ReadOnlySpan<char> span)
        {
            if (span.Length != Length)
            {
                return false;
            }

            StringBuilder? sbChunk = this;
            int offset = 0;

            do
            {
                int chunk_length = sbChunk.m_ChunkLength;
                offset += chunk_length;

                ReadOnlySpan<char> chunk = new ReadOnlySpan<char>(sbChunk.m_ChunkChars, 0, chunk_length);

                if (!chunk.EqualsOrdinal(span.Slice(span.Length - offset, chunk_length)))
                {
                    return false;
                }

                sbChunk = sbChunk.m_ChunkPrevious;
            } while (sbChunk != null);

            Debug.Assert(offset == Length);
            return true;
        }

        /// <summary>
        /// Replaces all instances of one string with another in part of this builder.
        /// </summary>
        /// <param name="oldValue">The string to replace.</param>
        /// <param name="newValue">The string to replace <paramref name="oldValue"/> with.</param>
        /// <param name="startIndex">The index to start in this builder.</param>
        /// <param name="count">The number of characters to read in this builder.</param>
        /// <remarks>
        /// If <paramref name="newValue"/> is <c>null</c>, instances of <paramref name="oldValue"/>
        /// are removed from this builder.
        /// </remarks>
        public StringBuilder Replace(string oldValue, string? newValue, int startIndex, int count)
        {
            int currentLength = Length;
            if ((uint)startIndex > (uint)currentLength)
            {
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);
            }
            if (count < 0 || startIndex > currentLength - count)
            {
                throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_Index);
            }
            if (oldValue == null)
            {
                throw new ArgumentNullException(nameof(oldValue));
            }
            if (oldValue.Length == 0)
            {
                throw new ArgumentException(SR.Argument_EmptyName, nameof(oldValue));
            }

            newValue = newValue ?? string.Empty;

            int deltaLength = newValue.Length - oldValue.Length;

            int[]? replacements = null; // A list of replacement positions in a chunk to apply
            int replacementsCount = 0;

            // Find the chunk, indexInChunk for the starting point
            StringBuilder chunk = FindChunkForIndex(startIndex);
            int indexInChunk = startIndex - chunk.m_ChunkOffset;
            while (count > 0)
            {
                Debug.Assert(chunk != null, "chunk was null in replace");
                // Look for a match in the chunk,indexInChunk pointer
                if (StartsWith(chunk, indexInChunk, count, oldValue))
                {
                    // Push it on the replacements array (with growth), we will do all replacements in a
                    // given chunk in one operation below (see ReplaceAllInChunk) so we don't have to slide
                    // many times.
                    if (replacements == null)
                    {
                        replacements = new int[5];
                    }
                    else if (replacementsCount >= replacements.Length)
                    {
                        Array.Resize(ref replacements, replacements.Length * 3 / 2 + 4); // Grow by ~1.5x, but more in the begining
                    }
                    replacements[replacementsCount++] = indexInChunk;
                    indexInChunk += oldValue.Length;
                    count -= oldValue.Length;
                }
                else
                {
                    indexInChunk++;
                    --count;
                }

                if (indexInChunk >= chunk.m_ChunkLength || count == 0) // Have we moved out of the current chunk?
                {
                    // Replacing mutates the blocks, so we need to convert to a logical index and back afterwards.
                    int index = indexInChunk + chunk.m_ChunkOffset;
                    int indexBeforeAdjustment = index;

                    // See if we accumulated any replacements, if so apply them.
                    Debug.Assert(replacements != null || replacementsCount == 0, "replacements was null and replacementsCount != 0");
                    ReplaceAllInChunk(replacements, replacementsCount, chunk, oldValue.Length, newValue);
                    // The replacement has affected the logical index.  Adjust it.
                    index += ((newValue.Length - oldValue.Length) * replacementsCount);
                    replacementsCount = 0;

                    chunk = FindChunkForIndex(index);
                    indexInChunk = index - chunk.m_ChunkOffset;
                    Debug.Assert(chunk != null || count == 0, "Chunks ended prematurely!");
                }
            }

            AssertInvariants();
            return this;
        }

        /// <summary>
        /// Replaces all instances of one character with another in this builder.
        /// </summary>
        /// <param name="oldChar">The character to replace.</param>
        /// <param name="newChar">The character to replace <paramref name="oldChar"/> with.</param>
        public StringBuilder Replace(char oldChar, char newChar)
        {
            return Replace(oldChar, newChar, 0, Length);
        }

        /// <summary>
        /// Replaces all instances of one character with another in this builder.
        /// </summary>
        /// <param name="oldChar">The character to replace.</param>
        /// <param name="newChar">The character to replace <paramref name="oldChar"/> with.</param>
        /// <param name="startIndex">The index to start in this builder.</param>
        /// <param name="count">The number of characters to read in this builder.</param>
        public StringBuilder Replace(char oldChar, char newChar, int startIndex, int count)
        {
            int currentLength = Length;
            if ((uint)startIndex > (uint)currentLength)
            {
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);
            }

            if (count < 0 || startIndex > currentLength - count)
            {
                throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_Index);
            }

            int endIndex = startIndex + count;
            StringBuilder chunk = this;

            for (;;)
            {
                int endIndexInChunk = endIndex - chunk.m_ChunkOffset;
                int startIndexInChunk = startIndex - chunk.m_ChunkOffset;
                if (endIndexInChunk >= 0)
                {
                    int curInChunk = Math.Max(startIndexInChunk, 0);
                    int endInChunk = Math.Min(chunk.m_ChunkLength, endIndexInChunk);
                    while (curInChunk < endInChunk)
                    {
                        if (chunk.m_ChunkChars[curInChunk] == oldChar)
                            chunk.m_ChunkChars[curInChunk] = newChar;
                        curInChunk++;
                    }
                }
                if (startIndexInChunk >= 0)
                {
                    break;
                }

                Debug.Assert(chunk.m_ChunkPrevious != null);
                chunk = chunk.m_ChunkPrevious;
            }

            AssertInvariants();
            return this;
        }

        /// <summary>
        /// Appends a character buffer to this builder.
        /// </summary>
        /// <param name="value">The pointer to the start of the buffer.</param>
        /// <param name="valueCount">The number of characters in the buffer.</param>
        [CLSCompliant(false)]
        public unsafe StringBuilder Append(char* value, int valueCount)
        {
            // We don't check null value as this case will throw null reference exception anyway
            if (valueCount < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(valueCount), SR.ArgumentOutOfRange_NegativeCount);
            }

            // this is where we can check if the valueCount will put us over m_MaxCapacity
            // We are doing the check here to prevent the corruption of the StringBuilder.
            int newLength = Length + valueCount;
            if (newLength > m_MaxCapacity || newLength < valueCount)
            {
                throw new ArgumentOutOfRangeException(nameof(valueCount), SR.ArgumentOutOfRange_LengthGreaterThanCapacity);
            }

            // This case is so common we want to optimize for it heavily.
            int newIndex = valueCount + m_ChunkLength;
            if (newIndex <= m_ChunkChars.Length)
            {
                ThreadSafeCopy(value, m_ChunkChars, m_ChunkLength, valueCount);
                m_ChunkLength = newIndex;
            }
            else
            {
                // Copy the first chunk
                int firstLength = m_ChunkChars.Length - m_ChunkLength;
                if (firstLength > 0)
                {
                    ThreadSafeCopy(value, m_ChunkChars, m_ChunkLength, firstLength);
                    m_ChunkLength = m_ChunkChars.Length;
                }

                // Expand the builder to add another chunk.
                int restLength = valueCount - firstLength;
                ExpandByABlock(restLength);
                Debug.Assert(m_ChunkLength == 0, "A new block was not created.");

                // Copy the second chunk
                ThreadSafeCopy(value + firstLength, m_ChunkChars, 0, restLength);
                m_ChunkLength = restLength;
            }
            AssertInvariants();
            return this;
        }

        /// <summary>
        /// Inserts a character buffer into this builder at the specified position.
        /// </summary>
        /// <param name="index">The index to insert in this builder.</param>
        /// <param name="value">The pointer to the start of the buffer.</param>
        /// <param name="valueCount">The number of characters in the buffer.</param>
        private unsafe void Insert(int index, char* value, int valueCount)
        {
            if ((uint)index > (uint)Length)
            {
                throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
            }

            if (valueCount > 0)
            {
                StringBuilder chunk;
                int indexInChunk;
                MakeRoom(index, valueCount, out chunk, out indexInChunk, false);
                ReplaceInPlaceAtChunk(ref chunk!, ref indexInChunk, value, valueCount);
            }
        }

        /// <summary>
        /// Replaces strings at specified indices with a new string in a chunk.
        /// </summary>
        /// <param name="replacements">The list of indices, relative to the beginning of the chunk, to remove at.</param>
        /// <param name="replacementsCount">The number of replacements to make.</param>
        /// <param name="sourceChunk">The source chunk.</param>
        /// <param name="removeCount">The number of characters to remove at each replacement.</param>
        /// <param name="value">The string to insert at each replacement.</param>
        /// <remarks>
        /// This routine is very efficient because it does replacements in bulk.
        /// </remarks>
        private void ReplaceAllInChunk(int[]? replacements, int replacementsCount, StringBuilder sourceChunk, int removeCount, string value)
        {
            if (replacementsCount <= 0)
            {
                return;
            }

            unsafe
            {
                fixed (char* valuePtr = value)
                {
                    Debug.Assert(replacements != null, "replacements was null when replacementsCount > 0");
                    // calculate the total amount of extra space or space needed for all the replacements.
                    long longDelta = (value.Length - removeCount) * (long)replacementsCount;
                    int delta = (int)longDelta;
                    if (delta != longDelta)
                    {
                        throw new OutOfMemoryException();
                    }

                    StringBuilder targetChunk = sourceChunk;        // the target as we copy chars down
                    int targetIndexInChunk = replacements[0];

                    // Make the room needed for all the new characters if needed.
                    if (delta > 0)
                    {
                        MakeRoom(targetChunk.m_ChunkOffset + targetIndexInChunk, delta, out targetChunk, out targetIndexInChunk, true);
                    }
                    
                    // We made certain that characters after the insertion point are not moved,
                    int i = 0;
                    for (;;)
                    {
                        // Copy in the new string for the ith replacement
                        ReplaceInPlaceAtChunk(ref targetChunk!, ref targetIndexInChunk, valuePtr, value.Length);
                        int gapStart = replacements[i] + removeCount;
                        i++;
                        if (i >= replacementsCount)
                        {
                            break;
                        }

                        int gapEnd = replacements[i];
                        Debug.Assert(gapStart < sourceChunk.m_ChunkChars.Length, "gap starts at end of buffer.  Should not happen");
                        Debug.Assert(gapStart <= gapEnd, "negative gap size");
                        Debug.Assert(gapEnd <= sourceChunk.m_ChunkLength, "gap too big");
                        if (delta != 0)     // can skip the sliding of gaps if source an target string are the same size.
                        {
                            // Copy the gap data between the current replacement and the next replacement
                            fixed (char* sourcePtr = &sourceChunk.m_ChunkChars[gapStart])
                                ReplaceInPlaceAtChunk(ref targetChunk!, ref targetIndexInChunk, sourcePtr, gapEnd - gapStart);
                        }
                        else
                        {
                            targetIndexInChunk += gapEnd - gapStart;
                            Debug.Assert(targetIndexInChunk <= targetChunk.m_ChunkLength, "gap not in chunk");
                        }
                    }

                    // Remove extra space if necessary.
                    if (delta < 0)
                    {
                        Remove(targetChunk.m_ChunkOffset + targetIndexInChunk, -delta, out targetChunk, out targetIndexInChunk);
                    }
                }
            }
        }

        /// <summary>
        /// Returns a value indicating whether a substring of a builder starts with a specified prefix.
        /// </summary>
        /// <param name="chunk">The chunk in which the substring starts.</param>
        /// <param name="indexInChunk">The index in <paramref name="chunk"/> at which the substring starts.</param>
        /// <param name="count">The logical count of the substring.</param>
        /// <param name="value">The prefix.</param>
        private bool StartsWith(StringBuilder chunk, int indexInChunk, int count, string value)
        {
            for (int i = 0; i < value.Length; i++)
            {
                if (count == 0)
                {
                    return false;
                }

                if (indexInChunk >= chunk.m_ChunkLength)
                {
                    chunk = Next(chunk)!;
                    if (chunk == null)
                    {
                        return false;
                    }
                    indexInChunk = 0;
                }

                if (value[i] != chunk.m_ChunkChars[indexInChunk])
                {
                    return false;
                }

                indexInChunk++;
                --count;
            }

            return true;
        }

        /// <summary>
        /// Replaces characters at a specified location with the contents of a character buffer.
        /// This function is the logical equivalent of memcpy.
        /// </summary>
        /// <param name="chunk">
        /// The chunk in which to start replacing characters.
        /// Receives the chunk in which character replacement ends.
        /// </param>
        /// <param name="indexInChunk">
        /// The index in <paramref name="chunk"/> to start replacing characters at.
        /// Receives the index at which character replacement ends.
        /// </param>
        /// <param name="value">The pointer to the start of the character buffer.</param>
        /// <param name="count">The number of characters in the buffer.</param>
        private unsafe void ReplaceInPlaceAtChunk(ref StringBuilder? chunk, ref int indexInChunk, char* value, int count)
        {
            if (count != 0)
            {
                for (;;)
                {
                    Debug.Assert(chunk != null, "chunk should not be null at this point");
                    int lengthInChunk = chunk.m_ChunkLength - indexInChunk;
                    Debug.Assert(lengthInChunk >= 0, "Index isn't in the chunk.");

                    int lengthToCopy = Math.Min(lengthInChunk, count);
                    ThreadSafeCopy(value, chunk.m_ChunkChars, indexInChunk, lengthToCopy);

                    // Advance the index.
                    indexInChunk += lengthToCopy;
                    if (indexInChunk >= chunk.m_ChunkLength)
                    {
                        chunk = Next(chunk);
                        indexInChunk = 0;
                    }
                    count -= lengthToCopy;
                    if (count == 0)
                    {
                        break;
                    }
                    value += lengthToCopy;
                }
            }
        }

        /// <remarks>
        /// This method prevents out-of-bounds writes in the case a different thread updates a field in the builder just before a copy begins.
        /// All interesting variables are copied out of the heap into the parameters of this method, and then bounds checks are run.
        /// </remarks>
        private static unsafe void ThreadSafeCopy(char* sourcePtr, char[] destination, int destinationIndex, int count)
        {
            if (count > 0)
            {
                if ((uint)destinationIndex <= (uint)destination.Length && (destinationIndex + count) <= destination.Length)
                {
                    fixed (char* destinationPtr = &destination[destinationIndex])
                        string.wstrcpy(destinationPtr, sourcePtr, count);
                }
                else
                {
                    throw new ArgumentOutOfRangeException(nameof(destinationIndex), SR.ArgumentOutOfRange_Index);
                }
            }
        }

        private static unsafe void ThreadSafeCopy(char[] source, int sourceIndex, Span<char> destination, int destinationIndex, int count)
        {
            if (count > 0)
            {
                if ((uint)sourceIndex > (uint)source.Length || count > source.Length - sourceIndex)
                {
                    throw new ArgumentOutOfRangeException(nameof(sourceIndex), SR.ArgumentOutOfRange_Index);
                }

                if ((uint)destinationIndex > (uint)destination.Length || count > destination.Length - destinationIndex)
                {
                    throw new ArgumentOutOfRangeException(nameof(destinationIndex), SR.ArgumentOutOfRange_Index);
                }

                fixed (char* sourcePtr = &source[sourceIndex])
                    fixed (char* destinationPtr = &MemoryMarshal.GetReference(destination))
                        string.wstrcpy(destinationPtr + destinationIndex, sourcePtr, count);
            }
        }

        /// <summary>
        /// Gets the chunk corresponding to the logical index in this builder.
        /// </summary>
        /// <param name="index">The logical index in this builder.</param>
        /// <remarks>
        /// After calling this method, you can obtain the actual index within the chunk by
        /// subtracting <see cref="m_ChunkOffset"/> from <paramref name="index"/>.
        /// </remarks>
        private StringBuilder FindChunkForIndex(int index)
        {
            Debug.Assert(0 <= index && index <= Length);

            StringBuilder result = this;
            while (result.m_ChunkOffset > index)
            {
                Debug.Assert(result.m_ChunkPrevious != null);
                result = result.m_ChunkPrevious;
            }

            Debug.Assert(result != null);
            return result;
        }

        /// <summary>
        /// Gets the chunk corresponding to the logical byte index in this builder.
        /// </summary>
        /// <param name="byteIndex">The logical byte index in this builder.</param>
        private StringBuilder FindChunkForByte(int byteIndex)
        {
            Debug.Assert(0 <= byteIndex && byteIndex <= Length * sizeof(char));

            StringBuilder result = this;
            while (result.m_ChunkOffset * sizeof(char) > byteIndex)
            {
                Debug.Assert(result.m_ChunkPrevious != null);
                result = result.m_ChunkPrevious;
            }

            Debug.Assert(result != null);
            return result;
        }

        /// <summary>Gets a span representing the remaining space available in the current chunk.</summary>
        private Span<char> RemainingCurrentChunk
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get => new Span<char>(m_ChunkChars, m_ChunkLength, m_ChunkChars.Length - m_ChunkLength);
        }

        /// <summary>
        /// Finds the chunk that logically succeeds the specified chunk.
        /// </summary>
        /// <param name="chunk">The chunk whose successor should be found.</param>
        /// <remarks>
        /// Each chunk only stores the pointer to its logical predecessor, so this routine has to start
        /// from the 'this' pointer (which is assumed to represent the whole StringBuilder) and work its
        /// way down until it finds the specified chunk (which is O(n)). Thus, it is more expensive than
        /// a field fetch.
        /// </remarks>
        private StringBuilder? Next(StringBuilder chunk) => chunk == this ? null : FindChunkForIndex(chunk.m_ChunkOffset + chunk.m_ChunkLength);

        /// <summary>
        /// Transfers the character buffer from this chunk to a new chunk, and allocates a new buffer with a minimum size for this chunk.
        /// </summary>
        /// <param name="minBlockCharCount">The minimum size of the new buffer to be allocated for this chunk.</param>
        /// <remarks>
        /// This method requires that the current chunk is full. Otherwise, there's no point in shifting the characters over.
        /// It also assumes that 'this' is the last chunk in the linked list.
        /// </remarks>
        private void ExpandByABlock(int minBlockCharCount)
        {
            Debug.Assert(Capacity == Length, nameof(ExpandByABlock) + " should only be called when there is no space left.");
            Debug.Assert(minBlockCharCount > 0);

            AssertInvariants();

            if ((minBlockCharCount + Length) > m_MaxCapacity || minBlockCharCount + Length < minBlockCharCount)
            {
                throw new ArgumentOutOfRangeException("requiredLength", SR.ArgumentOutOfRange_SmallCapacity);
            }

            // - We always need to make the new chunk at least as big as was requested (`minBlockCharCount`).
            // - We'd also prefer to make it at least at big as the current length (thus doubling capacity).
            //   - But this is only up to a maximum, so we stay in the small object heap, and never allocate
            //     really big chunks even if the string gets really big.
            int newBlockLength = Math.Max(minBlockCharCount, Math.Min(Length, MaxChunkSize));

            // Check for integer overflow (logical buffer size > int.MaxValue)
            if (m_ChunkOffset + m_ChunkLength + newBlockLength < newBlockLength)
            {
                throw new OutOfMemoryException();
            }
            
            // Allocate the array before updating any state to avoid leaving inconsistent state behind in case of out of memory exception
            char[] chunkChars = new char[newBlockLength];

            // Move all of the data from this chunk to a new one, via a few O(1) pointer adjustments.
            // Then, have this chunk point to the new one as its predecessor.
            m_ChunkPrevious = new StringBuilder(this);
            m_ChunkOffset += m_ChunkLength;
            m_ChunkLength = 0;

            m_ChunkChars = chunkChars;

            AssertInvariants();
        }

        /// <summary>
        /// Creates a new chunk with fields copied from an existing chunk.
        /// </summary>
        /// <param name="from">The chunk from which to copy fields.</param>
        /// <remarks>
        /// <para>
        /// This method runs in O(1) time. It does not copy data within the character buffer
        /// <paramref name="from"/> holds, but copies the reference to the character buffer itself
        /// (plus a few other fields).
        /// </para>
        /// <para>
        /// Callers are expected to update <paramref name="from"/> subsequently to point to this
        /// chunk as its predecessor.
        /// </para>
        /// </remarks>
        private StringBuilder(StringBuilder from)
        {
            m_ChunkLength = from.m_ChunkLength;
            m_ChunkOffset = from.m_ChunkOffset;
            m_ChunkChars = from.m_ChunkChars;
            m_ChunkPrevious = from.m_ChunkPrevious;
            m_MaxCapacity = from.m_MaxCapacity;

            AssertInvariants();
        }

        /// <summary>
        /// Creates a gap at a logical index with the specified count.
        /// </summary>
        /// <param name="index">The logical index in this builder.</param>
        /// <param name="count">The number of characters in the gap.</param>
        /// <param name="chunk">Receives the chunk containing the gap.</param>
        /// <param name="indexInChunk">The index in <paramref name="chunk"/> that points to the gap.</param>
        /// <param name="doNotMoveFollowingChars">
        /// - If <c>true</c>, then room must be made by inserting a chunk before the current chunk.
        /// - If <c>false</c>, then room can be made by shifting characters ahead of <paramref name="index"/>
        ///   in this block forward by <paramref name="count"/> provided the characters will still fit in
        ///   the current chunk after being shifted.
        ///   - Providing <c>false</c> does not make a difference most of the time, but it can matter when someone
        ///     inserts lots of small strings at a position in the buffer.
        /// </param>
        /// <remarks>
        /// <para>
        /// Since chunks do not contain references to their successors, it is not always possible for us to make room
        /// by inserting space after <paramref name="index"/> in case this chunk runs out of space. Thus, we make room
        /// by inserting space before the specified index, and having logical indices refer to new locations by the end
        /// of this method.
        /// </para>
        /// <para>
        /// <see cref="ReplaceInPlaceAtChunk"/> can be used in conjunction with this method to fill in the newly created gap.
        /// </para>
        /// </remarks>
        private void MakeRoom(int index, int count, out StringBuilder chunk, out int indexInChunk, bool doNotMoveFollowingChars)
        {
            AssertInvariants();
            Debug.Assert(count > 0);
            Debug.Assert(index >= 0);

            if (count + Length > m_MaxCapacity || count + Length < count)
            {
                throw new ArgumentOutOfRangeException("requiredLength", SR.ArgumentOutOfRange_SmallCapacity);
            }

            chunk = this;
            while (chunk.m_ChunkOffset > index)
            {
                chunk.m_ChunkOffset += count;
                Debug.Assert(chunk.m_ChunkPrevious != null);
                chunk = chunk.m_ChunkPrevious;
            }
            indexInChunk = index - chunk.m_ChunkOffset;

            // Cool, we have some space in this block, and we don't have to copy much to get at it, so go ahead and use it.
            // This typically happens when someone repeatedly inserts small strings at a spot (usually the absolute front) of the buffer.
            if (!doNotMoveFollowingChars && chunk.m_ChunkLength <= DefaultCapacity * 2 && chunk.m_ChunkChars.Length - chunk.m_ChunkLength >= count)
            {
                for (int i = chunk.m_ChunkLength; i > indexInChunk; )
                {
                    --i;
                    chunk.m_ChunkChars[i + count] = chunk.m_ChunkChars[i];
                }
                chunk.m_ChunkLength += count;
                return;
            }

            // Allocate space for the new chunk, which will go before the current one.
            StringBuilder newChunk = new StringBuilder(Math.Max(count, DefaultCapacity), chunk.m_MaxCapacity, chunk.m_ChunkPrevious);
            newChunk.m_ChunkLength = count;

            // Copy the head of the current buffer to the new buffer.
            int copyCount1 = Math.Min(count, indexInChunk);
            if (copyCount1 > 0)
            {
                unsafe
                {
                    fixed (char* chunkCharsPtr = &chunk.m_ChunkChars[0])
                    {
                        ThreadSafeCopy(chunkCharsPtr, newChunk.m_ChunkChars, 0, copyCount1);

                        // Slide characters over in the current buffer to make room.
                        int copyCount2 = indexInChunk - copyCount1;
                        if (copyCount2 >= 0)
                        {
                            ThreadSafeCopy(chunkCharsPtr + copyCount1, chunk.m_ChunkChars, 0, copyCount2);
                            indexInChunk = copyCount2;
                        }
                    }
                }
            }

            // Wire in the new chunk.
            chunk.m_ChunkPrevious = newChunk;
            chunk.m_ChunkOffset += count;
            if (copyCount1 < count)
            {
                chunk = newChunk;
                indexInChunk = copyCount1;
            }

            AssertInvariants();
        }

        /// <summary>
        /// Used by <see cref="MakeRoom"/> to allocate another chunk.
        /// </summary>
        /// <param name="size">The size of the character buffer for this chunk.</param>
        /// <param name="maxCapacity">The maximum capacity, to be stored in this chunk.</param>
        /// <param name="previousBlock">The predecessor of this chunk.</param>
        private StringBuilder(int size, int maxCapacity, StringBuilder? previousBlock)
        {
            Debug.Assert(size > 0);
            Debug.Assert(maxCapacity > 0);

            m_ChunkChars = new char[size];
            m_MaxCapacity = maxCapacity;
            m_ChunkPrevious = previousBlock;
            if (previousBlock != null)
            {
                m_ChunkOffset = previousBlock.m_ChunkOffset + previousBlock.m_ChunkLength;
            }

            AssertInvariants();
        }

        /// <summary>
        /// Removes a specified number of characters beginning at a logical index in this builder.
        /// </summary>
        /// <param name="startIndex">The logical index in this builder to start removing characters.</param>
        /// <param name="count">The number of characters to remove.</param>
        /// <param name="chunk">Receives the new chunk containing the logical index.</param>
        /// <param name="indexInChunk">
        /// Receives the new index in <paramref name="chunk"/> that is associated with the logical index.
        /// </param>
        private void Remove(int startIndex, int count, out StringBuilder chunk, out int indexInChunk)
        {
            AssertInvariants();
            Debug.Assert(startIndex >= 0 && startIndex < Length);

            int endIndex = startIndex + count;

            // Find the chunks for the start and end of the block to delete.
            chunk = this;
            StringBuilder? endChunk = null;
            int endIndexInChunk = 0;
            for (;;)
            {
                if (endIndex - chunk.m_ChunkOffset >= 0)
                {
                    if (endChunk == null)
                    {
                        endChunk = chunk;
                        endIndexInChunk = endIndex - endChunk.m_ChunkOffset;
                    }
                    if (startIndex - chunk.m_ChunkOffset >= 0)
                    {
                        indexInChunk = startIndex - chunk.m_ChunkOffset;
                        break;
                    }
                }
                else
                {
                    chunk.m_ChunkOffset -= count;
                }

                Debug.Assert(chunk.m_ChunkPrevious != null);
                chunk = chunk.m_ChunkPrevious;
            }
            Debug.Assert(chunk != null, "We fell off the beginning of the string!");

            int copyTargetIndexInChunk = indexInChunk;
            int copyCount = endChunk.m_ChunkLength - endIndexInChunk;
            if (endChunk != chunk)
            {
                copyTargetIndexInChunk = 0;
                // Remove the characters after `startIndex` to the end of the chunk.
                chunk.m_ChunkLength = indexInChunk;

                // Remove the characters in chunks between the start and the end chunk.
                endChunk.m_ChunkPrevious = chunk;
                endChunk.m_ChunkOffset = chunk.m_ChunkOffset + chunk.m_ChunkLength;

                // If the start is 0, then we can throw away the whole start chunk.
                if (indexInChunk == 0)
                {
                    endChunk.m_ChunkPrevious = chunk.m_ChunkPrevious;
                    chunk = endChunk;
                }
            }
            endChunk.m_ChunkLength -= (endIndexInChunk - copyTargetIndexInChunk);

            // SafeCritical: We ensure that `endIndexInChunk + copyCount` is within range of `m_ChunkChars`, and
            // also ensure that `copyTargetIndexInChunk + copyCount` is within the chunk.

            // Remove any characters in the end chunk, by sliding the characters down.
            if (copyTargetIndexInChunk != endIndexInChunk) // Sometimes no move is necessary
            {
                ThreadSafeCopy(endChunk.m_ChunkChars, endIndexInChunk, endChunk.m_ChunkChars, copyTargetIndexInChunk, copyCount);
            }

            Debug.Assert(chunk != null, "We fell off the beginning of the string!");
            AssertInvariants();
        }
    }
}