/////////////////////////////////////////////////////////////////////////////////
//
// 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) 2010-2017 Tao Yue
//
// See LICENSE.txt for complete licensing and attribution information.
//
/////////////////////////////////////////////////////////////////////////////////

using System;
using System.Drawing;
using System.IO.Compression;

namespace PhotoshopFile.Compression
{
  public class ZipPredict16Image : ImageData
  {
    private ImageData zipImage;

    protected override bool AltersWrittenData => true;

    public ZipPredict16Image(byte[] zipData, Size size)
      : base(size, 16)
    {
      // 16-bitdepth images are delta-encoded word-by-word.  The deltas
      // are thus big-endian and must be reversed for further processing.
      var zipRawImage = new ZipImage(zipData, size, 16);
      zipImage = new EndianReverser(zipRawImage);
    }

    internal override void Read(byte[] buffer)
    {
      if (buffer.Length == 0)
      {
        return;
      }

      zipImage.Read(buffer);
      unsafe
      {
        fixed (byte* ptrData = &buffer[0])
        {
          Unpredict((UInt16*)ptrData);
        }
      }
    }

    public override byte[] ReadCompressed()
    {
      return zipImage.ReadCompressed();
    }

    internal override void WriteInternal(byte[] array)
    {
      if (array.Length == 0)
      {
        return;
      }

      unsafe
      {
        fixed (byte* ptrData = &array[0])
        {
          Predict((UInt16*)ptrData);
        }
      }

      zipImage.WriteInternal(array);
    }

    unsafe private void Predict(UInt16* ptrData)
    {
      // Delta-encode each row
      for (int i = 0; i < Size.Height; i++)
      {
        UInt16* ptrDataRow = ptrData;
        UInt16* ptrDataRowEnd = ptrDataRow + Size.Width;

        // Start with the last column in the row
        ptrData = ptrDataRowEnd - 1;
        while (ptrData > ptrDataRow)
        {
          *ptrData -= *(ptrData - 1);
          ptrData--;
        }
        ptrData = ptrDataRowEnd;
      }
    }

    /// <summary>
    /// Unpredicts the decompressed, native-endian image data.
    /// </summary>
    unsafe private void Unpredict(UInt16* ptrData)
    {
      // Delta-decode each row
      for (int i = 0; i < Size.Height; i++)
      {
        UInt16* ptrDataRowEnd = ptrData + Size.Width;

        // Start with column index 1 on each row
        ptrData++;
        while (ptrData < ptrDataRowEnd)
        {
          *ptrData += *(ptrData - 1);
          ptrData++;
        }

        // Advance pointer to the next row
        ptrData = ptrDataRowEnd;
      }
    }
  }
}