// 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.Collections;
using System.Diagnostics;
using System.Reflection;
using System.Threading;

namespace System
{
    public static partial class Environment
    {
        public static int ProcessorCount { get; } = GetProcessorCount();

        /// <summary>
        /// Gets whether the current machine has only a single processor.
        /// </summary>
        internal static bool IsSingleProcessor => ProcessorCount == 1;

        // Unconditionally return false since .NET Core does not support object finalization during shutdown.
        public static bool HasShutdownStarted => false;

        public static string? GetEnvironmentVariable(string variable)
        {
            if (variable == null)
                throw new ArgumentNullException(nameof(variable));

            return GetEnvironmentVariableCore(variable);
        }

        public static string? GetEnvironmentVariable(string variable, EnvironmentVariableTarget target)
        {
            if (target == EnvironmentVariableTarget.Process)
                return GetEnvironmentVariable(variable);

            if (variable == null)
                throw new ArgumentNullException(nameof(variable));

            bool fromMachine = ValidateAndConvertRegistryTarget(target);
            return GetEnvironmentVariableFromRegistry(variable, fromMachine);
        }

        public static IDictionary GetEnvironmentVariables(EnvironmentVariableTarget target)
        {
            if (target == EnvironmentVariableTarget.Process)
                return GetEnvironmentVariables();

            bool fromMachine = ValidateAndConvertRegistryTarget(target);
            return GetEnvironmentVariablesFromRegistry(fromMachine);
        }

        public static void SetEnvironmentVariable(string variable, string? value)
        {
            ValidateVariableAndValue(variable, ref value);
            SetEnvironmentVariableCore(variable, value);
        }

        public static void SetEnvironmentVariable(string variable, string? value, EnvironmentVariableTarget target)
        {
            if (target == EnvironmentVariableTarget.Process)
            {
                SetEnvironmentVariable(variable, value);
                return;
            }

            ValidateVariableAndValue(variable, ref value);

            bool fromMachine = ValidateAndConvertRegistryTarget(target);
            SetEnvironmentVariableFromRegistry(variable, value, fromMachine: fromMachine);
        }

        public static string CommandLine => PasteArguments.Paste(GetCommandLineArgs(), pasteFirstArgumentUsingArgV0Rules: true);

        public static string CurrentDirectory
        {
            get => CurrentDirectoryCore;
            set
            {
                if (value == null)
                    throw new ArgumentNullException(nameof(value));

                if (value.Length == 0)
                    throw new ArgumentException(SR.Argument_PathEmpty, nameof(value));

                CurrentDirectoryCore = value;
            }
        }

        public static string ExpandEnvironmentVariables(string name)
        {
            if (name == null)
                throw new ArgumentNullException(nameof(name));

            if (name.Length == 0)
                return name;

            return ExpandEnvironmentVariablesCore(name);
        }

        private static string[]? s_commandLineArgs;

        internal static void SetCommandLineArgs(string[] cmdLineArgs) // invoked from VM
        {
            s_commandLineArgs = cmdLineArgs;
        }

        public static string GetFolderPath(SpecialFolder folder) => GetFolderPath(folder, SpecialFolderOption.None);

        public static string GetFolderPath(SpecialFolder folder, SpecialFolderOption option)
        {
            if (!Enum.IsDefined(typeof(SpecialFolder), folder))
                throw new ArgumentOutOfRangeException(nameof(folder), folder, SR.Format(SR.Arg_EnumIllegalVal, folder));

            if (option != SpecialFolderOption.None && !Enum.IsDefined(typeof(SpecialFolderOption), option))
                throw new ArgumentOutOfRangeException(nameof(option), option, SR.Format(SR.Arg_EnumIllegalVal, option));

            return GetFolderPathCore(folder, option);
        }

        public static bool Is64BitProcess => IntPtr.Size == 8;

        public static bool Is64BitOperatingSystem => Is64BitProcess || Is64BitOperatingSystemWhen32BitProcess;

        public static string NewLine => NewLineConst;

        private static OperatingSystem? s_osVersion;

        public static OperatingSystem OSVersion
        {
            get
            {
                if (s_osVersion == null)
                {
                    Interlocked.CompareExchange(ref s_osVersion, GetOSVersion(), null);
                }
                return s_osVersion;
            }
        }

        public static bool UserInteractive => true;

        public static Version Version
        {
            get
            {
                // FX_PRODUCT_VERSION is expected to be set by the host
                // Use AssemblyInformationalVersionAttribute as fallback if the exact product version is not specified by the host
                string? versionString = (string?)AppContext.GetData("FX_PRODUCT_VERSION") ??
                    typeof(object).Assembly.GetCustomAttribute<AssemblyInformationalVersionAttribute>()?.InformationalVersion;

                ReadOnlySpan<char> versionSpan = versionString.AsSpan();

                // Strip optional suffixes
                int separatorIndex = versionSpan.IndexOfAny("-+ ");
                if (separatorIndex != -1)
                    versionSpan = versionSpan.Slice(0, separatorIndex);

                // Return zeros rather then failing if the version string fails to parse
                return Version.TryParse(versionSpan, out Version? version) ? version : new Version();
            }
        }

        private static bool ValidateAndConvertRegistryTarget(EnvironmentVariableTarget target)
        {
            Debug.Assert(target != EnvironmentVariableTarget.Process);

            if (target == EnvironmentVariableTarget.Machine)
                return true;

            if (target == EnvironmentVariableTarget.User)
                return false;

            throw new ArgumentOutOfRangeException(nameof(target), target, SR.Format(SR.Arg_EnumIllegalVal, target));
        }

        private static void ValidateVariableAndValue(string variable, ref string? value)
        {
            if (variable == null)
                throw new ArgumentNullException(nameof(variable));

            if (variable.Length == 0)
                throw new ArgumentException(SR.Argument_StringZeroLength, nameof(variable));

            if (variable[0] == '\0')
                throw new ArgumentException(SR.Argument_StringFirstCharIsZero, nameof(variable));

            if (variable.Contains('='))
                throw new ArgumentException(SR.Argument_IllegalEnvVarName, nameof(variable));

            if (string.IsNullOrEmpty(value) || value[0] == '\0')
            {
                // Explicitly null out value if it's empty
                value = null;
            }
        }
    }
}