GZipDecoder.cs source code in C# .NET

                        Code:
                         / DotNET / DotNET / 8.0 / untmp / whidbey / REDBITS / ndp / fx / src / Sys / System / IO / compression / GZipDecoder.cs / 1 / GZipDecoder.cs
                        
                        
                            namespace System.IO.Compression 
{
    using System;
    using System.Diagnostics;
 
    internal enum GZIPHeaderState{
        // GZIP header 
        ReadingID1, 
        ReadingID2,
        ReadingCM, 
        ReadingFLG,
        ReadingMMTime, // iterates 4 times
        ReadingXFL,
        ReadingOS, 
        ReadingXLen1,
        ReadingXLen2, 
        ReadingXLenData, 
        ReadingFileName,
        ReadingComment, 
        ReadingCRC16Part1,
        ReadingCRC16Part2,
        Done, // done reading GZIP header
 
        // GZIP footer
        ReadingCRC, // iterates 4 times 
        ReadingFileSize // iterates 4 times 
    }
 

    // This class decodes GZip header and footer information.
    // See RFC 1952 for details about the format.
    internal class GZipDecoder { 
        const int FileText   = 1;
        const int CRCFlag     = 2; 
        const int ExtraFieldsFlag  = 4; 
        const int FileNameFlag   = 8;
        const int CommentFlag= 16; 

        private InputBuffer input;
        GZIPHeaderState gzipHeaderSubstate;
        GZIPHeaderState gzipFooterSubstate; 

        int gzip_header_flag; 
        int gzip_header_xlen; 
        uint gzipCrc32;
        uint gzipOutputStreamSize; 
        int loopCounter;

        public GZipDecoder(InputBuffer input) {
            this.input = input; 
            Reset();
        } 
 
        public void Reset() {
            gzipHeaderSubstate = GZIPHeaderState.ReadingID1; 
            gzipFooterSubstate = GZIPHeaderState.ReadingCRC;
            gzipCrc32 = 0;
            gzipOutputStreamSize = 0;
        } 

        public uint Crc32 { 
            get { 
                return gzipCrc32;
            } 
        }

        public uint StreamSize {
            get { 
                return gzipOutputStreamSize;
            } 
        } 

 
        public bool ReadGzipHeader() {

            while(true) {
                int bits; 
                switch (gzipHeaderSubstate) {
                case GZIPHeaderState.ReadingID1: 
                    bits = input.GetBits(8); 
                    if( bits < 0) {
                        return false; 
                    }

                    if( bits != 0x1F) {
                        throw new InvalidDataException(SR.GetString(SR.CorruptedGZipHeader)); 
                    }
                    gzipHeaderSubstate = GZIPHeaderState.ReadingID2; 
                    goto case GZIPHeaderState.ReadingID2; 

                case GZIPHeaderState.ReadingID2: 
                    bits = input.GetBits(8);
                    if( bits < 0) {
                        return false;
                    } 

                    if( bits != 0x8b) { 
                        throw new InvalidDataException(SR.GetString(SR.CorruptedGZipHeader)); 
                    }
 
                    gzipHeaderSubstate = GZIPHeaderState.ReadingCM;
                    goto case GZIPHeaderState.ReadingCM;

                case GZIPHeaderState.ReadingCM: 
                    bits = input.GetBits(8);
                    if( bits < 0) { 
                        return false; 
                    }
 
                    if( bits != 0x8) {         // compression mode must be 8 (deflate)
                        throw new InvalidDataException(SR.GetString(SR.UnknownCompressionMode));
                    }
 
                    gzipHeaderSubstate = GZIPHeaderState.ReadingFLG;;
                    goto case GZIPHeaderState.ReadingFLG; 
 
                case GZIPHeaderState.ReadingFLG:
                    bits = input.GetBits(8); 
                    if( bits < 0) {
                        return false;
                    }
 
                    gzip_header_flag = bits;
                    gzipHeaderSubstate = GZIPHeaderState.ReadingMMTime; 
                    loopCounter = 0; // 4 MMTIME bytes 
                    goto case GZIPHeaderState.ReadingMMTime;
 
                case GZIPHeaderState.ReadingMMTime:
                    bits = 0;
                    while(loopCounter < 4) {
                        bits = input.GetBits(8); 
                        if( bits < 0) {
                            return false; 
                        } 

                        loopCounter++; 
                    }

                    gzipHeaderSubstate = GZIPHeaderState.ReadingXFL;
                    loopCounter = 0; 
                    goto case GZIPHeaderState.ReadingXFL;
 
                case GZIPHeaderState.ReadingXFL:      // ignore XFL 
                    bits = input.GetBits(8);
                    if( bits < 0) { 
                            return false;
                    }

                    gzipHeaderSubstate = GZIPHeaderState.ReadingOS; 
                    goto case GZIPHeaderState.ReadingOS;
 
                case GZIPHeaderState.ReadingOS:      // ignore OS 
                    bits = input.GetBits(8);
                    if( bits < 0) { 
                            return false;
                    }

                    gzipHeaderSubstate = GZIPHeaderState.ReadingXLen1; 
                    goto case GZIPHeaderState.ReadingXLen1;
 
                case GZIPHeaderState.ReadingXLen1: 
                    if ((gzip_header_flag & ExtraFieldsFlag) == 0) {
                        goto case GZIPHeaderState.ReadingFileName; 
                    }

                    bits = input.GetBits(8);
                    if( bits < 0) { 
                        return false;
                    } 
 
                    gzip_header_xlen = bits;
                    gzipHeaderSubstate = GZIPHeaderState.ReadingXLen2; 
                    goto case GZIPHeaderState.ReadingXLen2;

                case GZIPHeaderState.ReadingXLen2:
                    bits = input.GetBits(8); 
                    if( bits < 0) {
                        return false; 
                    } 

                    gzip_header_xlen |= (bits << 8); 
                    gzipHeaderSubstate = GZIPHeaderState.ReadingXLenData;
                    loopCounter = 0; // 0 bytes of XLEN data read so far
                    goto case GZIPHeaderState.ReadingXLenData;
 
                case GZIPHeaderState.ReadingXLenData:
                    bits = 0; 
                    while(loopCounter < gzip_header_xlen) { 
                        bits = input.GetBits(8);
                        if( bits < 0) { 
                            return false;
                        }

                        loopCounter++; 
                    }
                    gzipHeaderSubstate = GZIPHeaderState.ReadingFileName; 
                    loopCounter = 0; 
                    goto case GZIPHeaderState.ReadingFileName;
 
                case GZIPHeaderState.ReadingFileName:
                    if ((gzip_header_flag & FileNameFlag) == 0) {
                        gzipHeaderSubstate = GZIPHeaderState.ReadingComment;
                        goto case GZIPHeaderState.ReadingComment; 
                    }
 
                    do { 
                        bits = input.GetBits(8);
                        if( bits < 0) { 
                            return false;
                        }

                        if( bits == 0) {   // see '\0' in the file name string 
                            break;
                        } 
                    } while (true); 

                    gzipHeaderSubstate = GZIPHeaderState.ReadingComment; 
                    goto case GZIPHeaderState.ReadingComment;

                case GZIPHeaderState.ReadingComment:
                    if ((gzip_header_flag & CommentFlag) == 0) { 
                        gzipHeaderSubstate = GZIPHeaderState.ReadingCRC16Part1;
                        goto case GZIPHeaderState.ReadingCRC16Part1; 
                    } 

                    do { 
                        bits = input.GetBits(8);
                        if( bits < 0) {
                            return false;
                        } 

                        if( bits == 0) {   // see '\0' in the file name string 
                            break; 
                        }
                    } while (true); 

                    gzipHeaderSubstate = GZIPHeaderState.ReadingCRC16Part1;
                    goto case GZIPHeaderState.ReadingCRC16Part1;
 
                case GZIPHeaderState.ReadingCRC16Part1:
                    if ((gzip_header_flag & CRCFlag) == 0) { 
                        gzipHeaderSubstate = GZIPHeaderState.Done; 
                        goto case GZIPHeaderState.Done;
                    } 

                    bits = input.GetBits(8);     // ignore crc
                    if( bits < 0) {
                        return false; 
                    }
 
                    gzipHeaderSubstate = GZIPHeaderState.ReadingCRC16Part2; 
                    goto case GZIPHeaderState.ReadingCRC16Part2;
 
                case GZIPHeaderState.ReadingCRC16Part2:
                    bits = input.GetBits(8);     // ignore crc
                    if( bits < 0) {
                        return false; 
                    }
 
                    gzipHeaderSubstate = GZIPHeaderState.Done; 
                    goto case GZIPHeaderState.Done;
 
                case GZIPHeaderState.Done:
                    return true;
                default:
                    Debug.Assert(false, "We should not reach unknown state!"); 
                    throw new InvalidDataException(SR.GetString(SR.UnknownState));
                } 
            } 
        }
 
        public bool ReadGzipFooter() {
            input.SkipToByteBoundary();
            if( gzipFooterSubstate == GZIPHeaderState.ReadingCRC) {
                while (loopCounter < 4 ) { 
                    int bits = input.GetBits(8);
                    if( bits < 0) { 
                        return false; 
                    }
 
                    gzipCrc32 |= ((uint)bits << (8*loopCounter));
                    loopCounter++;
                }
                gzipFooterSubstate = GZIPHeaderState.ReadingFileSize; 
                loopCounter = 0;
 
            } 

            if (gzipFooterSubstate == GZIPHeaderState.ReadingFileSize) { 
                if (loopCounter == 0)
                    gzipOutputStreamSize = 0;

                while (loopCounter < 4) { 
                    int bits = input.GetBits(8);
                    if( bits < 0) { 
                        return false; 
                    }
 
                    gzipOutputStreamSize |= ((uint)bits << (8*loopCounter));
                    loopCounter++;
                }
            } 

            return true; 
        } 
    }
}
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