/////////////////////////////////////////////////////////////////////////////////
//
// Photoshop PSD FileType Plugin for Paint.NET
// http://psdplugin.codeplex.com/
//
// This software is provided under the MIT License:
//   Copyright (c) 2006-2007 Frank Blumenberg
//   Copyright (c) 2010-2019 Tao Yue
//
// Portions of this file are provided under the BSD 3-clause License:
//   Copyright (c) 2006, Jonas Beckeman
//
// See LICENSE.txt for complete licensing and attribution information.
//
/////////////////////////////////////////////////////////////////////////////////

using System;
using System.Diagnostics;
using System.IO;

namespace PhotoshopFile
{
  public class RleReader
  {
    private Stream stream;

    public RleReader(Stream stream)
    {
      this.stream = stream;
    }

    /// <summary>
    /// Decodes a PackBits RLE stream.
    /// </summary>
    /// <param name="buffer">Output buffer for decoded data.</param>
    /// <param name="offset">Offset at which to begin writing.</param>
    /// <param name="count">Number of bytes to decode from the stream.</param>
    unsafe public int Read(byte[] buffer, int offset, int count)
    {
      if (!Util.CheckBufferBounds(buffer, offset, count))
      {
        throw new ArgumentOutOfRangeException();
      }
      if (count == 0)
      {
        return 0;
      }

      // Pin the entire buffer now, so that we don't keep pinning and unpinning
      // for each RLE packet.
      fixed (byte* ptrBuffer = &buffer[0])
      {
        int bytesLeft = count;
        int bufferIdx = offset;
        while (bytesLeft > 0)
        {
          // ReadByte returns an unsigned byte, but we want a signed byte.
          var flagCounter = unchecked((sbyte)stream.ReadByte());

          // Raw packet
          if (flagCounter > 0)
          {
            var readLength = flagCounter + 1;
            if (bytesLeft < readLength)
              throw new RleException("Raw packet overruns the decode window.");

            stream.Read(buffer, bufferIdx, readLength);

            bufferIdx += readLength;
            bytesLeft -= readLength;
          }
          // RLE packet
          else if (flagCounter > -128)
          {
            var runLength = 1 - flagCounter;
            var byteValue = (byte)stream.ReadByte();
            if (runLength > bytesLeft)
              throw new RleException("RLE packet overruns the decode window.");

            byte* ptr = ptrBuffer + bufferIdx;
            byte* ptrEnd = ptr + runLength;
            while (ptr < ptrEnd)
            {
              *ptr = byteValue;
              ptr++;
            }

            bufferIdx += runLength;
            bytesLeft -= runLength;
          }
          else
          {
            // The canonical PackBits algorithm will never emit 0x80 (-128), but
            // some programs do.  Simply skip over the byte.
          }
        }

        Debug.Assert(bytesLeft == 0);
        return count - bytesLeft;
      }
    }
  }
}