dsdecmp/CSharp/DSDecmp/Formats/LZOvl.cs

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using System;
using System.Collections.Generic;
using System.Text;
namespace DSDecmp.Formats
{
/// <summary>
/// The LZ-Overlay compression format. Compresses part of the file from end to start.
/// Is used for the 'overlay' files in NDS games, as well as arm9.bin
/// </summary>
public class LZOvl : CompressionFormat
{
#region Method: Supports(Stream, long)
public override bool Supports(System.IO.Stream stream, long inLength)
{
// assume the 'inLength' does not include the 12 bytes at the end of arm9.bin
// only allow integer-sized files
if (inLength > 0xFFFFFFFFL)
return false;
// the header is 4 bytes minimum
if (inLength < 4)
return false;
long streamStart = stream.Position;
byte[] header = new byte[Math.Min(inLength, 0x20)];
stream.Position += inLength - header.Length;
stream.Read(header, 0, header.Length);
// reset the stream
stream.Position = streamStart;
uint extraSize = IOUtils.ToNDSu32(header, header.Length - 4);
if (extraSize == 0)
return true;
// if the extrasize is nonzero, the minimum header length is 8 bytes
if (header.Length < 8)
return false;
byte headerLen = header[header.Length - 5];
if (inLength < headerLen)
return false;
// the compressed length is assumed to be valid.
// verify that the rest of the header is filled with 0xFF
for (int i = header.Length - 9; i >= header.Length - headerLen; i--)
if (header[i] != 0xFF)
return false;
return true;
}
#endregion
public override void Decompress(System.IO.Stream instream, long inLength, System.IO.Stream outstream)
{
// Overlay LZ compression is basically just LZ-0x10 compression.
// however the order if reading is reversed: the compression starts at the end of the file.
// Assuming we start reading at the end towards the beginning, the format is:
/*
* u32 extraSize; // decompressed data size = file length (including header) + this value
* u8 headerSize;
* u24 compressedLength; // can be less than file size (w/o header). If so, the rest of the file is uncompressed.
* u8[headerSize-8] padding; // 0xFF-s
*
* 0x10-like-compressed data follows (without the usual 4-byte header).
* The only difference is that 2 should be added to the DISP value in compressed blocks
* to get the proper value.
* the u32 and u24 are read most significant byte first.
* if extraSize is 0, there is no headerSize, decompressedLength or padding.
* the data starts immediately, and is uncompressed.
*
* arm9.bin has 3 extra u32 values at the 'start' (ie: end of the file),
* which may be ignored. (and are ignored here) These 12 bytes also should not
* be included in the computation of the output size.
*/
throw new NotImplementedException();
}
public override int Compress(System.IO.Stream instream, long inLength, System.IO.Stream outstream)
{
throw new NotImplementedException();
}
}
}