readBlock()

in src/core/flate_stream.js [307:474]

• 149 lines of code
• 31 McCabe index (conditional complexity)

  readBlock() {
    let buffer, hdr, len;
    const str = this.str;
    // read block header
    try {
      hdr = this.getBits(3);
    } catch (ex) {
      this.#endsStreamOnError(ex.message);
      return;
    }
    if (hdr & 1) {
      this.eof = true;
    }
    hdr >>= 1;

    if (hdr === 0) {
      // uncompressed block
      let b;

      if ((b = str.getByte()) === -1) {
        this.#endsStreamOnError("Bad block header in flate stream");
        return;
      }
      let blockLen = b;
      if ((b = str.getByte()) === -1) {
        this.#endsStreamOnError("Bad block header in flate stream");
        return;
      }
      blockLen |= b << 8;
      if ((b = str.getByte()) === -1) {
        this.#endsStreamOnError("Bad block header in flate stream");
        return;
      }
      let check = b;
      if ((b = str.getByte()) === -1) {
        this.#endsStreamOnError("Bad block header in flate stream");
        return;
      }
      check |= b << 8;
      if (check !== (~blockLen & 0xffff) && (blockLen !== 0 || check !== 0)) {
        // Ignoring error for bad "empty" block (see issue 1277)
        throw new FormatError("Bad uncompressed block length in flate stream");
      }

      this.codeBuf = 0;
      this.codeSize = 0;

      const bufferLength = this.bufferLength,
        end = bufferLength + blockLen;
      buffer = this.ensureBuffer(end);
      this.bufferLength = end;

      if (blockLen === 0) {
        if (str.peekByte() === -1) {
          this.eof = true;
        }
      } else {
        const block = str.getBytes(blockLen);
        buffer.set(block, bufferLength);
        if (block.length < blockLen) {
          this.eof = true;
        }
      }
      return;
    }

    let litCodeTable;
    let distCodeTable;
    if (hdr === 1) {
      // compressed block, fixed codes
      litCodeTable = fixedLitCodeTab;
      distCodeTable = fixedDistCodeTab;
    } else if (hdr === 2) {
      // compressed block, dynamic codes
      const numLitCodes = this.getBits(5) + 257;
      const numDistCodes = this.getBits(5) + 1;
      const numCodeLenCodes = this.getBits(4) + 4;

      // build the code lengths code table
      const codeLenCodeLengths = new Uint8Array(codeLenCodeMap.length);

      let i;
      for (i = 0; i < numCodeLenCodes; ++i) {
        codeLenCodeLengths[codeLenCodeMap[i]] = this.getBits(3);
      }
      const codeLenCodeTab = this.generateHuffmanTable(codeLenCodeLengths);

      // build the literal and distance code tables
      len = 0;
      i = 0;
      const codes = numLitCodes + numDistCodes;
      const codeLengths = new Uint8Array(codes);
      let bitsLength, bitsOffset, what;
      while (i < codes) {
        const code = this.getCode(codeLenCodeTab);
        if (code === 16) {
          bitsLength = 2;
          bitsOffset = 3;
          what = len;
        } else if (code === 17) {
          bitsLength = 3;
          bitsOffset = 3;
          what = len = 0;
        } else if (code === 18) {
          bitsLength = 7;
          bitsOffset = 11;
          what = len = 0;
        } else {
          codeLengths[i++] = len = code;
          continue;
        }

        let repeatLength = this.getBits(bitsLength) + bitsOffset;
        while (repeatLength-- > 0) {
          codeLengths[i++] = what;
        }
      }

      litCodeTable = this.generateHuffmanTable(
        codeLengths.subarray(0, numLitCodes)
      );
      distCodeTable = this.generateHuffmanTable(
        codeLengths.subarray(numLitCodes, codes)
      );
    } else {
      throw new FormatError("Unknown block type in flate stream");
    }

    buffer = this.buffer;
    let limit = buffer ? buffer.length : 0;
    let pos = this.bufferLength;
    while (true) {
      let code1 = this.getCode(litCodeTable);
      if (code1 < 256) {
        if (pos + 1 >= limit) {
          buffer = this.ensureBuffer(pos + 1);
          limit = buffer.length;
        }
        buffer[pos++] = code1;
        continue;
      }
      if (code1 === 256) {
        this.bufferLength = pos;
        return;
      }
      code1 -= 257;
      code1 = lengthDecode[code1];
      let code2 = code1 >> 16;
      if (code2 > 0) {
        code2 = this.getBits(code2);
      }
      len = (code1 & 0xffff) + code2;
      code1 = this.getCode(distCodeTable);
      code1 = distDecode[code1];
      code2 = code1 >> 16;
      if (code2 > 0) {
        code2 = this.getBits(code2);
      }
      const dist = (code1 & 0xffff) + code2;
      if (pos + len >= limit) {
        buffer = this.ensureBuffer(pos + len);
        limit = buffer.length;
      }
      for (let k = 0; k < len; ++k, ++pos) {
        buffer[pos] = buffer[pos - dist];
      }
    }
  }