Method parseStream

src/parser/xref-parser.ts:195–340

* Parse xref stream format (PDF 1.5+). * Scanner must be positioned at stream object start ("N M obj"). * * XRef streams encode cross-reference data as binary in a stream object. * The stream dictionary contains: * - /Type /XRef * - /Size - total number of objects * - /W [w1 w2

()

   * - /Index [first count ...] - object number ranges (optional, defaults to [0 Size])
   */
  parseStream(): XRefData {
    // Parse the indirect object containing the xref stream
    const parser = new IndirectObjectParser(this.scanner);
    const indirectObj = parser.parseObject();

    if (!(indirectObj.value instanceof PdfStream)) {
      throw new XRefParseError("Expected XRef stream object");
    }

    const stream = indirectObj.value;

    // Validate /Type is /XRef (optional per spec, but good to check)
    const type = stream.getName("Type");
    if (type !== undefined && type.value !== "XRef") {
      throw new XRefParseError(`Expected /Type /XRef, got /Type /${type.value}`);
    }

    // Get required /W array (field widths)
    const wArray = stream.getArray("W");
    if (wArray === undefined || wArray.length < 3) {
      throw new XRefParseError("XRef stream missing or invalid /W array");
    }

    const w0 = wArray.at(0);
    const w1Val = wArray.at(1);
    const w2Val = wArray.at(2);

    const w1 = w0 instanceof PdfNumber ? w0.value : 0; // Type field width
    const w2 = w1Val instanceof PdfNumber ? w1Val.value : 0; // Offset field width
    const w3 = w2Val instanceof PdfNumber ? w2Val.value : 0; // Generation field width

    // Validate field widths to prevent integer overflow during parsing.
    // JavaScript safely handles integers up to 2^53-1, but bitwise operations
    // are limited to 32 bits. Fields wider than 6 bytes (48 bits) could cause
    // issues with shift operations. Most PDFs use 1-4 byte widths.
    const MAX_FIELD_WIDTH = 6;

    if (w1 < 0 || w1 > MAX_FIELD_WIDTH) {
      throw new XRefParseError(`Invalid XRef stream field width w1=${w1} (max ${MAX_FIELD_WIDTH})`);
    }

    if (w2 < 0 || w2 > MAX_FIELD_WIDTH) {
      throw new XRefParseError(`Invalid XRef stream field width w2=${w2} (max ${MAX_FIELD_WIDTH})`);
    }

    if (w3 < 0 || w3 > MAX_FIELD_WIDTH) {
      throw new XRefParseError(`Invalid XRef stream field width w3=${w3} (max ${MAX_FIELD_WIDTH})`);
    }

    const entrySize = w1 + w2 + w3;

    // Get /Size (total object count)
    const size = stream.getNumber("Size")?.value;
    if (size === undefined) {
      throw new XRefParseError("XRef stream missing /Size");
    }

    // Get /Index array or default to [0 Size]