90 lines
2.8 KiB
C#
90 lines
2.8 KiB
C#
// Copyright (c) Microsoft Corporation. All rights reserved.
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namespace System.IO.Compression {
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using System.Diagnostics;
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internal static class GZipConstants {
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internal const int CompressionLevel_3 = 3;
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internal const int CompressionLevel_10 = 10;
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internal const long FileLengthModulo = 4294967296;
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internal const byte ID1 = 0x1F;
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internal const byte ID2 = 0x8B;
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internal const byte Deflate = 0x8;
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internal const int Xfl_HeaderPos = 8;
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internal const byte Xfl_FastestAlgorithm = 4;
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internal const byte Xfl_MaxCompressionSlowestAlgorithm = 2;
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}
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internal class GZipFormatter : IFileFormatWriter {
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private byte[] headerBytes = new byte[] {
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GZipConstants.ID1, // ID1
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GZipConstants.ID2, // ID2
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GZipConstants.Deflate, // CM = deflate
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0, // FLG, no text, no crc, no extra, no name, no comment
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// MTIME (Modification Time) - no time available
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0,
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0,
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0,
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0,
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// XFL
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// 2 = compressor used max compression, slowest algorithm
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// 4 = compressor used fastest algorithm
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GZipConstants.Xfl_FastestAlgorithm,
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// OS: 0 = FAT filesystem (MS-DOS, OS/2, NT/Win32)
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0
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};
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private uint _crc32;
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private long _inputStreamSizeModulo;
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internal GZipFormatter() : this(GZipConstants.CompressionLevel_3) { }
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internal GZipFormatter(int compressionLevel) {
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if (compressionLevel == GZipConstants.CompressionLevel_10) {
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headerBytes[GZipConstants.Xfl_HeaderPos] = GZipConstants.Xfl_MaxCompressionSlowestAlgorithm;
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}
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}
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public byte[] GetHeader() {
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return headerBytes;
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}
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public void UpdateWithBytesRead(byte[] buffer, int offset, int bytesToCopy) {
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_crc32 = Crc32Helper.UpdateCrc32(_crc32, buffer, offset, bytesToCopy);
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long n = _inputStreamSizeModulo + (uint) bytesToCopy;
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if (n >= GZipConstants.FileLengthModulo) {
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n %= GZipConstants.FileLengthModulo;
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}
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_inputStreamSizeModulo = n;
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}
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public byte[] GetFooter() {
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byte[] b = new byte[8];
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WriteUInt32(b, _crc32, 0);
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WriteUInt32(b, (uint)_inputStreamSizeModulo, 4);
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return b;
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}
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internal void WriteUInt32(byte[] b, uint value, int startIndex) {
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b[startIndex] = (byte)value;
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b[startIndex + 1] = (byte)(value >> 8);
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b[startIndex + 2] = (byte)(value >> 16);
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b[startIndex + 3] = (byte)(value >> 24);
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}
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}
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}
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