Esempio n. 1
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        // Format of the dynamic block header:
        //      5 Bits: HLIT, # of Literal/Length codes - 257 (257 - 286)
        //      5 Bits: HDIST, # of Distance codes - 1        (1 - 32)
        //      4 Bits: HCLEN, # of Code Length codes - 4     (4 - 19)
        //
        //      (HCLEN + 4) x 3 bits: code lengths for the code length
        //          alphabet given just above, in the order: 16, 17, 18,
        //          0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15
        //
        //          These code lengths are interpreted as 3-bit integers
        //          (0-7); as above, a code length of 0 means the
        //          corresponding symbol (literal/length or distance code
        //          length) is not used.
        //
        //      HLIT + 257 code lengths for the literal/length alphabet,
        //          encoded using the code length Huffman code
        //
        //       HDIST + 1 code lengths for the distance alphabet,
        //          encoded using the code length Huffman code
        //
        // The code length repeat codes can cross from HLIT + 257 to the
        // HDIST + 1 code lengths.  In other words, all code lengths form
        // a single sequence of HLIT + HDIST + 258 values.
        bool DecodeDynamicBlockHeader() {
            switch (state) {
            case InflaterState.ReadingNumLitCodes:               
                literalLengthCodeCount = input.GetBits(5);
                if( literalLengthCodeCount < 0) {
                    return false;
                }
                literalLengthCodeCount += 257;
                state = InflaterState.ReadingNumDistCodes;
                goto case InflaterState.ReadingNumDistCodes;

            case InflaterState.ReadingNumDistCodes:
                distanceCodeCount = input.GetBits(5);
                if( distanceCodeCount < 0) {
                    return false;
                }
                distanceCodeCount += 1;
                state = InflaterState.ReadingNumCodeLengthCodes;
                goto case InflaterState.ReadingNumCodeLengthCodes;

            case InflaterState.ReadingNumCodeLengthCodes:
                codeLengthCodeCount = input.GetBits(4);
                if( codeLengthCodeCount < 0) {
                    return false;
                }
                codeLengthCodeCount += 4;
                loopCounter = 0;
                state = InflaterState.ReadingCodeLengthCodes;
                goto case InflaterState.ReadingCodeLengthCodes;

            case InflaterState.ReadingCodeLengthCodes:
                while(loopCounter < codeLengthCodeCount) {
                    int bits = input.GetBits(3);
                    if( bits < 0) {
                        return false;
                    }
                    codeLengthTreeCodeLength[codeOrder[loopCounter]] = (byte)bits;
                    ++loopCounter;
                }
                
                for (int i = codeLengthCodeCount; i < codeOrder.Length; i++) {
                    codeLengthTreeCodeLength[ codeOrder[i] ] = 0;
                }

                // create huffman tree for code length
                codeLengthTree = new HuffmanTree(codeLengthTreeCodeLength);
                codeArraySize = literalLengthCodeCount + distanceCodeCount;
                loopCounter = 0;     // reset loop count

                state = InflaterState.ReadingTreeCodesBefore;
                goto case InflaterState.ReadingTreeCodesBefore;

            case InflaterState.ReadingTreeCodesBefore:
            case InflaterState.ReadingTreeCodesAfter:                
                while (loopCounter < codeArraySize) {
                    if( state == InflaterState.ReadingTreeCodesBefore) {
                        if( (lengthCode = codeLengthTree.GetNextSymbol(input)) < 0) {
                            return false;
                        }
                    }

                    // The alphabet for code lengths is as follows:
                    //  0 - 15: Represent code lengths of 0 - 15
                    //  16: Copy the previous code length 3 - 6 times.
                    //  The next 2 bits indicate repeat length
                    //         (0 = 3, ... , 3 = 6)
                    //      Example:  Codes 8, 16 (+2 bits 11),
                    //                16 (+2 bits 10) will expand to
                    //                12 code lengths of 8 (1 + 6 + 5)
                    //  17: Repeat a code length of 0 for 3 - 10 times.
                    //    (3 bits of length)
                    //  18: Repeat a code length of 0 for 11 - 138 times
                    //    (7 bits of length)
                    if (lengthCode <= 15) {  
                        codeList[loopCounter++] = (byte)lengthCode;
                    }
                    else {
                        if( !input.EnsureBitsAvailable(7)) { // it doesn't matter if we require more bits here
                            state = InflaterState.ReadingTreeCodesAfter;
                            return false;
                        }

                        int repeatCount;
                        if (lengthCode == 16) {
                            if (loopCounter == 0) {          // can't have "prev code" on first code
                                throw new InvalidDataException();
                            }

                            byte previousCode = codeList[loopCounter-1];
                            repeatCount = input.GetBits(2) + 3;

                            if (loopCounter + repeatCount > codeArraySize) {
                                throw new InvalidDataException();
                            }

                            for (int j = 0; j < repeatCount; j++) {
                                codeList[loopCounter++] = previousCode;
                            }
                        } 
                        else if (lengthCode == 17) {
                            repeatCount = input.GetBits(3) + 3;

                            if (loopCounter + repeatCount > codeArraySize) {
                                throw new InvalidDataException();
                            }

                            for (int j = 0; j < repeatCount; j++) {
                                codeList[loopCounter++] = 0;
                            }
                        } 
                        else { // code == 18
                            repeatCount = input.GetBits(7) + 11;

                            if (loopCounter + repeatCount > codeArraySize) {
                                throw new InvalidDataException();
                            }

                            for (int j = 0; j < repeatCount; j++) {
                                codeList[loopCounter++] = 0;
                            }
                        }
                    }
                    state = InflaterState.ReadingTreeCodesBefore; // we want to read the next code.
                }
                break;

            default:
                Debug.Assert(false, "check why we are here!");
                throw new InvalidDataException(SR.GetString(SR.UnknownState));
            }

            byte[]  literalTreeCodeLength  = new byte[HuffmanTree.MaxLiteralTreeElements];
            byte[]  distanceTreeCodeLength = new byte[HuffmanTree.MaxDistTreeElements];

            // Create literal and distance tables
            Array.Copy(codeList, literalTreeCodeLength, literalLengthCodeCount);
            Array.Copy(codeList, literalLengthCodeCount, distanceTreeCodeLength, 0, distanceCodeCount);

            // Make sure there is an end-of-block code, otherwise how could we ever end?
            if (literalTreeCodeLength[HuffmanTree.EndOfBlockCode] == 0) {
                throw new InvalidDataException();
            }                

            literalLengthTree = new HuffmanTree(literalTreeCodeLength);
            distanceTree = new HuffmanTree(distanceTreeCodeLength);
            state = InflaterState.DecodeTop;
            return true;
        }