Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System/sys/system/IO/compression/GZipUtils.cs

@@ -0,0 +1,89 @@
+//   Copyright (c) Microsoft Corporation.  All rights reserved.
+
+namespace System.IO.Compression {
+    using System.Diagnostics;
+
+    internal static class GZipConstants {
+        internal const int CompressionLevel_3 = 3;
+        internal const int CompressionLevel_10 = 10;
+
+        internal const long FileLengthModulo = 4294967296;
+
+        internal const byte ID1 = 0x1F;
+        internal const byte ID2 = 0x8B;
+        internal const byte Deflate = 0x8;
+
+        internal const int Xfl_HeaderPos = 8;
+        internal const byte Xfl_FastestAlgorithm = 4;
+        internal const byte Xfl_MaxCompressionSlowestAlgorithm = 2;
+    }
+
+    internal class GZipFormatter : IFileFormatWriter {
+
+        private byte[] headerBytes = new byte[] {
+                GZipConstants.ID1,      // ID1
+                GZipConstants.ID2,      // ID2
+                GZipConstants.Deflate,  // CM = deflate
+                0,                      // FLG, no text, no crc, no extra, no name, no comment
+
+                // MTIME (Modification Time) - no time available
+                0,    
+                0, 
+                0, 
+                0, 
+
+                // XFL
+                // 2 = compressor used max compression, slowest algorithm
+                // 4 = compressor used fastest algorithm
+                GZipConstants.Xfl_FastestAlgorithm,
+
+                // OS: 0 = FAT filesystem (MS-DOS, OS/2, NT/Win32)
+                0     
+            };
+
+        private uint _crc32;
+        private long _inputStreamSizeModulo;
+
+        internal GZipFormatter() : this(GZipConstants.CompressionLevel_3) { }
+
+        internal GZipFormatter(int compressionLevel) {
+            if (compressionLevel == GZipConstants.CompressionLevel_10) {
+                headerBytes[GZipConstants.Xfl_HeaderPos] = GZipConstants.Xfl_MaxCompressionSlowestAlgorithm;
+            }
+        }
+
+        public byte[] GetHeader() {
+            return headerBytes;
+        }
+
+        public void UpdateWithBytesRead(byte[] buffer, int offset, int bytesToCopy) {
+            _crc32 = Crc32Helper.UpdateCrc32(_crc32, buffer, offset, bytesToCopy);
+
+            long n = _inputStreamSizeModulo + (uint) bytesToCopy;
+            if (n >= GZipConstants.FileLengthModulo) {
+                n %= GZipConstants.FileLengthModulo;
+            }
+            _inputStreamSizeModulo = n;
+        }
+
+        public byte[] GetFooter() {
+            byte[] b = new byte[8];
+
+            WriteUInt32(b, _crc32, 0);
+            WriteUInt32(b, (uint)_inputStreamSizeModulo, 4);
+
+            return b;
+
+        }
+
+        internal void WriteUInt32(byte[] b, uint value, int startIndex) {
+            b[startIndex] = (byte)value;
+            b[startIndex + 1] = (byte)(value >> 8);
+            b[startIndex + 2] = (byte)(value >> 16);
+            b[startIndex + 3] = (byte)(value >> 24);
+        }
+    }
+
+
+}
+