xref.js

/* Copyright 2021 Mozilla Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import {
  assert,
  bytesToString,
  FormatError,
  info,
  InvalidPDFException,
  warn,
} from "../shared/util.js";
import { CIRCULAR_REF, Cmd, Dict, isCmd, Ref, RefSet } from "./primitives.js";
import {
  DocStats,
  MissingDataException,
  ParserEOFException,
  XRefEntryException,
  XRefParseException,
} from "./core_utils.js";
import { Lexer, Parser } from "./parser.js";
import { BaseStream } from "./base_stream.js";
import { CipherTransformFactory } from "./crypto.js";

class XRef {
  constructor(stream, pdfManager) {
    this.stream = stream;
    this.pdfManager = pdfManager;
    this.entries = [];
    this.xrefstms = Object.create(null);
    this._cacheMap = new Map(); // Prepare the XRef cache.
    this._pendingRefs = new RefSet();
    this.stats = new DocStats(pdfManager.msgHandler);
    this._newRefNum = null;
  }

  getNewRef() {
    if (this._newRefNum === null) {
      this._newRefNum = this.entries.length || 1;
    }
    return Ref.get(this._newRefNum++, 0);
  }

  resetNewRef() {
    this._newRefNum = null;
  }

  setStartXRef(startXRef) {
    // Store the starting positions of xref tables as we process them
    // so we can recover from missing data errors
    this.startXRefQueue = [startXRef];
  }

  parse(recoveryMode = false) {
    let trailerDict;
    if (!recoveryMode) {
      trailerDict = this.readXRef();
    } else {
      warn("Indexing all PDF objects");
      trailerDict = this.indexObjects();
    }
    trailerDict.assignXref(this);
    this.trailer = trailerDict;

    let encrypt;
    try {
      encrypt = trailerDict.get("Encrypt");
    } catch (ex) {
      if (ex instanceof MissingDataException) {
        throw ex;
      }
      warn(`XRef.parse - Invalid "Encrypt" reference: "${ex}".`);
    }
    if (encrypt instanceof Dict) {
      const ids = trailerDict.get("ID");
      const fileId = ids && ids.length ? ids[0] : "";
      // The 'Encrypt' dictionary itself should not be encrypted, and by
      // setting `suppressEncryption` we can prevent an infinite loop inside
      // of `XRef_fetchUncompressed` if the dictionary contains indirect
      // objects (fixes issue7665.pdf).
      encrypt.suppressEncryption = true;
      this.encrypt = new CipherTransformFactory(
        encrypt,
        fileId,
        this.pdfManager.password
      );
    }

    // Get the root dictionary (catalog) object, and do some basic validation.
    let root;
    try {
      root = trailerDict.get("Root");
    } catch (ex) {
      if (ex instanceof MissingDataException) {
        throw ex;
      }
      warn(`XRef.parse - Invalid "Root" reference: "${ex}".`);
    }
    if (root instanceof Dict) {
      try {
        const pages = root.get("Pages");
        if (pages instanceof Dict) {
          this.root = root;
          return;
        }
      } catch (ex) {
        if (ex instanceof MissingDataException) {
          throw ex;
        }
        warn(`XRef.parse - Invalid "Pages" reference: "${ex}".`);
      }
    }

    if (!recoveryMode) {
      throw new XRefParseException();
    }
    // Even recovery failed, there's nothing more we can do here.
    throw new InvalidPDFException("Invalid Root reference.");
  }

  processXRefTable(parser) {
    if (!("tableState" in this)) {
      // Stores state of the table as we process it so we can resume
      // from middle of table in case of missing data error
      this.tableState = {
        entryNum: 0,
        streamPos: parser.lexer.stream.pos,
        parserBuf1: parser.buf1,
        parserBuf2: parser.buf2,
      };
    }

    const obj = this.readXRefTable(parser);

    // Sanity check
    if (!isCmd(obj, "trailer")) {
      throw new FormatError(
        "Invalid XRef table: could not find trailer dictionary"
      );
    }
    // Read trailer dictionary, e.g.
    // trailer
    //    << /Size 22
    //      /Root 20R
    //      /Info 10R
    //      /ID [ <81b14aafa313db63dbd6f981e49f94f4> ]
    //    >>
    // The parser goes through the entire stream << ... >> and provides
    // a getter interface for the key-value table
    let dict = parser.getObj();

    // The pdflib PDF generator can generate a nested trailer dictionary
    if (!(dict instanceof Dict) && dict.dict) {
      dict = dict.dict;
    }
    if (!(dict instanceof Dict)) {
      throw new FormatError(
        "Invalid XRef table: could not parse trailer dictionary"
      );
    }
    delete this.tableState;

    return dict;
  }

  readXRefTable(parser) {
    // Example of cross-reference table:
    // xref
    // 0 1                    <-- subsection header (first obj #, obj count)
    // 0000000000 65535 f     <-- actual object (offset, generation #, f/n)
    // 23 2                   <-- subsection header ... and so on ...
    // 0000025518 00002 n
    // 0000025635 00000 n
    // trailer
    // ...

    const stream = parser.lexer.stream;
    const tableState = this.tableState;
    stream.pos = tableState.streamPos;
    parser.buf1 = tableState.parserBuf1;
    parser.buf2 = tableState.parserBuf2;

    // Outer loop is over subsection headers
    let obj;

    while (true) {
      if (!("firstEntryNum" in tableState) || !("entryCount" in tableState)) {
        if (isCmd((obj = parser.getObj()), "trailer")) {
          break;
        }
        tableState.firstEntryNum = obj;
        tableState.entryCount = parser.getObj();
      }

      let first = tableState.firstEntryNum;
      const count = tableState.entryCount;
      if (!Number.isInteger(first) || !Number.isInteger(count)) {
        throw new FormatError(
          "Invalid XRef table: wrong types in subsection header"
        );
      }
      // Inner loop is over objects themselves
      for (let i = tableState.entryNum; i < count; i++) {
        tableState.streamPos = stream.pos;
        tableState.entryNum = i;
        tableState.parserBuf1 = parser.buf1;
        tableState.parserBuf2 = parser.buf2;

        const entry = {};
        entry.offset = parser.getObj();
        entry.gen = parser.getObj();
        const type = parser.getObj();

        if (type instanceof Cmd) {
          switch (type.cmd) {
            case "f":
              entry.free = true;
              break;
            case "n":
              entry.uncompressed = true;
              break;
          }
        }

        // Validate entry obj
        if (
          !Number.isInteger(entry.offset) ||
          !Number.isInteger(entry.gen) ||
          !(entry.free || entry.uncompressed)
        ) {
          throw new FormatError(
            `Invalid entry in XRef subsection: ${first}, ${count}`
          );
        }

        // The first xref table entry, i.e. obj 0, should be free. Attempting
        // to adjust an incorrect first obj # (fixes issue 3248 and 7229).
        if (i === 0 && entry.free && first === 1) {
          first = 0;
        }

        if (!this.entries[i + first]) {
          this.entries[i + first] = entry;
        }
      }

      tableState.entryNum = 0;
      tableState.streamPos = stream.pos;
      tableState.parserBuf1 = parser.buf1;
      tableState.parserBuf2 = parser.buf2;
      delete tableState.firstEntryNum;
      delete tableState.entryCount;
    }

    // Sanity check: as per spec, first object must be free
    if (this.entries[0] && !this.entries[0].free) {
      throw new FormatError("Invalid XRef table: unexpected first object");
    }
    return obj;
  }

  processXRefStream(stream) {
    if (!("streamState" in this)) {
      // Stores state of the stream as we process it so we can resume
      // from middle of stream in case of missing data error
      const streamParameters = stream.dict;
      const byteWidths = streamParameters.get("W");
      let range = streamParameters.get("Index");
      if (!range) {
        range = [0, streamParameters.get("Size")];
      }

      this.streamState = {
        entryRanges: range,
        byteWidths,
        entryNum: 0,
        streamPos: stream.pos,
      };
    }
    this.readXRefStream(stream);
    delete this.streamState;

    return stream.dict;
  }

  readXRefStream(stream) {
    const streamState = this.streamState;
    stream.pos = streamState.streamPos;

    const [typeFieldWidth, offsetFieldWidth, generationFieldWidth] =
      streamState.byteWidths;

    const entryRanges = streamState.entryRanges;
    while (entryRanges.length > 0) {
      const [first, n] = entryRanges;

      if (!Number.isInteger(first) || !Number.isInteger(n)) {
        throw new FormatError(`Invalid XRef range fields: ${first}, ${n}`);
      }
      if (
        !Number.isInteger(typeFieldWidth) ||
        !Number.isInteger(offsetFieldWidth) ||
        !Number.isInteger(generationFieldWidth)
      ) {
        throw new FormatError(
          `Invalid XRef entry fields length: ${first}, ${n}`
        );
      }
      for (let i = streamState.entryNum; i < n; ++i) {
        streamState.entryNum = i;
        streamState.streamPos = stream.pos;

        let type = 0,
          offset = 0,
          generation = 0;
        for (let j = 0; j < typeFieldWidth; ++j) {
          const typeByte = stream.getByte();
          if (typeByte === -1) {
            throw new FormatError("Invalid XRef byteWidths 'type'.");
          }
          type = (type << 8) | typeByte;
        }
        // if type field is absent, its default value is 1
        if (typeFieldWidth === 0) {
          type = 1;
        }
        for (let j = 0; j < offsetFieldWidth; ++j) {
          const offsetByte = stream.getByte();
          if (offsetByte === -1) {
            throw new FormatError("Invalid XRef byteWidths 'offset'.");
          }
          offset = (offset << 8) | offsetByte;
        }
        for (let j = 0; j < generationFieldWidth; ++j) {
          const generationByte = stream.getByte();
          if (generationByte === -1) {
            throw new FormatError("Invalid XRef byteWidths 'generation'.");
          }
          generation = (generation << 8) | generationByte;
        }
        const entry = {};
        entry.offset = offset;
        entry.gen = generation;
        switch (type) {
          case 0:
            entry.free = true;
            break;
          case 1:
            entry.uncompressed = true;
            break;
          case 2:
            break;
          default:
            throw new FormatError(`Invalid XRef entry type: ${type}`);
        }
        if (!this.entries[first + i]) {
          this.entries[first + i] = entry;
        }
      }

      streamState.entryNum = 0;
      streamState.streamPos = stream.pos;
      entryRanges.splice(0, 2);
    }
  }

  indexObjects() {
    // Simple scan through the PDF content to find objects,
    // trailers and XRef streams.
    const TAB = 0x9,
      LF = 0xa,
      CR = 0xd,
      SPACE = 0x20;
    const PERCENT = 0x25,
      LT = 0x3c;

    function readToken(data, offset) {
      let token = "",
        ch = data[offset];
      while (ch !== LF && ch !== CR && ch !== LT) {
        if (++offset >= data.length) {
          break;
        }
        token += String.fromCharCode(ch);
        ch = data[offset];
      }
      return token;
    }
    function skipUntil(data, offset, what) {
      const length = what.length,
        dataLength = data.length;
      let skipped = 0;
      // finding byte sequence
      while (offset < dataLength) {
        let i = 0;
        while (i < length && data[offset + i] === what[i]) {
          ++i;
        }
        if (i >= length) {
          break; // sequence found
        }
        offset++;
        skipped++;
      }
      return skipped;
    }
    const objRegExp = /^(\d+)\s+(\d+)\s+obj\b/;
    const endobjRegExp = /\bendobj[\b\s]$/;
    const nestedObjRegExp = /\s+(\d+\s+\d+\s+obj[\b\s<])$/;
    const CHECK_CONTENT_LENGTH = 25;

    const trailerBytes = new Uint8Array([116, 114, 97, 105, 108, 101, 114]);
    const startxrefBytes = new Uint8Array([
      115, 116, 97, 114, 116, 120, 114, 101, 102,
    ]);
    const objBytes = new Uint8Array([111, 98, 106]);
    const xrefBytes = new Uint8Array([47, 88, 82, 101, 102]);

    // Clear out any existing entries, since they may be bogus.
    this.entries.length = 0;
    this._cacheMap.clear();

    const stream = this.stream;
    stream.pos = 0;
    const buffer = stream.getBytes(),
      length = buffer.length;
    let position = stream.start;
    const trailers = [],
      xrefStms = [];
    while (position < length) {
      let ch = buffer[position];
      if (ch === TAB || ch === LF || ch === CR || ch === SPACE) {
        ++position;
        continue;
      }
      if (ch === PERCENT) {
        // %-comment
        do {
          ++position;
          if (position >= length) {
            break;
          }
          ch = buffer[position];
        } while (ch !== LF && ch !== CR);
        continue;
      }
      const token = readToken(buffer, position);
      let m;
      if (
        token.startsWith("xref") &&
        (token.length === 4 || /\s/.test(token[4]))
      ) {
        position += skipUntil(buffer, position, trailerBytes);
        trailers.push(position);
        position += skipUntil(buffer, position, startxrefBytes);
      } else if ((m = objRegExp.exec(token))) {
        const num = m[1] | 0,
          gen = m[2] | 0;

        let contentLength,
          startPos = position + token.length,
          updateEntries = false;
        if (!this.entries[num]) {
          updateEntries = true;
        } else if (this.entries[num].gen === gen) {
          // Before overwriting an existing entry, ensure that the new one won't
          // cause *immediate* errors when it's accessed (fixes issue13783.pdf).
          try {
            const parser = new Parser({
              lexer: new Lexer(stream.makeSubStream(startPos)),
            });
            parser.getObj();
            updateEntries = true;
          } catch (ex) {
            if (ex instanceof ParserEOFException) {
              warn(`indexObjects -- checking object (${token}): "${ex}".`);
            } else {
              // The error may come from the `Parser`-instance being initialized
              // without an `XRef`-instance (we don't have a usable one yet).
              updateEntries = true;
            }
          }
        }
        if (updateEntries) {
          this.entries[num] = {
            offset: position - stream.start,
            gen,
            uncompressed: true,
          };
        }

        // Find the next "obj" string, rather than "endobj", to ensure that
        // we won't skip over a new 'obj' operator in corrupt files where
        // 'endobj' operators are missing (fixes issue9105_reduced.pdf).
        while (startPos < buffer.length) {
          const endPos = startPos + skipUntil(buffer, startPos, objBytes) + 4;
          contentLength = endPos - position;

          const checkPos = Math.max(endPos - CHECK_CONTENT_LENGTH, startPos);
          const tokenStr = bytesToString(buffer.subarray(checkPos, endPos));

          // Check if the current object ends with an 'endobj' operator.
          if (endobjRegExp.test(tokenStr)) {
            break;
          } else {
            // Check if an "obj" occurrence is actually a new object,
            // i.e. the current object is missing the 'endobj' operator.
            const objToken = nestedObjRegExp.exec(tokenStr);

            if (objToken && objToken[1]) {
              warn(
                'indexObjects: Found new "obj" inside of another "obj", ' +
                  'caused by missing "endobj" -- trying to recover.'
              );
              contentLength -= objToken[1].length;
              break;
            }
          }
          startPos = endPos;
        }
        const content = buffer.subarray(position, position + contentLength);

        // checking XRef stream suspect
        // (it shall have '/XRef' and next char is not a letter)
        const xrefTagOffset = skipUntil(content, 0, xrefBytes);
        if (xrefTagOffset < contentLength && content[xrefTagOffset + 5] < 64) {
          xrefStms.push(position - stream.start);
          this.xrefstms[position - stream.start] = 1; // Avoid recursion
        }

        position += contentLength;
      } else if (
        token.startsWith("trailer") &&
        (token.length === 7 || /\s/.test(token[7]))
      ) {
        trailers.push(position);
        position += skipUntil(buffer, position, startxrefBytes);
      } else {
        position += token.length + 1;
      }
    }
    // reading XRef streams
    for (let i = 0, ii = xrefStms.length; i < ii; ++i) {
      this.startXRefQueue.push(xrefStms[i]);
      this.readXRef(/* recoveryMode */ true);
    }
    // finding main trailer
    let trailerDict;
    for (let i = 0, ii = trailers.length; i < ii; ++i) {
      stream.pos = trailers[i];
      const parser = new Parser({
        lexer: new Lexer(stream),
        xref: this,
        allowStreams: true,
        recoveryMode: true,
      });
      const obj = parser.getObj();
      if (!isCmd(obj, "trailer")) {
        continue;
      }
      // read the trailer dictionary
      const dict = parser.getObj();
      if (!(dict instanceof Dict)) {
        continue;
      }
      // Do some basic validation of the trailer/root dictionary candidate.
      try {
        const rootDict = dict.get("Root");
        if (!(rootDict instanceof Dict)) {
          continue;
        }
        const pagesDict = rootDict.get("Pages");
        if (!(pagesDict instanceof Dict)) {
          continue;
        }
        const pagesCount = pagesDict.get("Count");
        if (!Number.isInteger(pagesCount)) {
          continue;
        }
        // The top-level /Pages dictionary isn't obviously corrupt.
      } catch (ex) {
        continue;
      }
      // taking the first one with 'ID'
      if (dict.has("ID")) {
        return dict;
      }
      // The current dictionary is a candidate, but continue searching.
      trailerDict = dict;
    }
    // No trailer with 'ID', taking last one (if exists).
    if (trailerDict) {
      return trailerDict;
    }
    // No trailer dictionary found, taking the "top"-dictionary (if exists).
    if (this.topDict) {
      return this.topDict;
    }
    // nothing helps
    throw new InvalidPDFException("Invalid PDF structure.");
  }

  readXRef(recoveryMode = false) {
    const stream = this.stream;
    // Keep track of already parsed XRef tables, to prevent an infinite loop
    // when parsing corrupt PDF files where e.g. the /Prev entries create a
    // circular dependency between tables (fixes bug1393476.pdf).
    const startXRefParsedCache = new Set();

    try {
      while (this.startXRefQueue.length) {
        const startXRef = this.startXRefQueue[0];

        if (startXRefParsedCache.has(startXRef)) {
          warn("readXRef - skipping XRef table since it was already parsed.");
          this.startXRefQueue.shift();
          continue;
        }
        startXRefParsedCache.add(startXRef);

        stream.pos = startXRef + stream.start;

        const parser = new Parser({
          lexer: new Lexer(stream),
          xref: this,
          allowStreams: true,
        });
        let obj = parser.getObj();
        let dict;

        // Get dictionary
        if (isCmd(obj, "xref")) {
          // Parse end-of-file XRef
          dict = this.processXRefTable(parser);
          if (!this.topDict) {
            this.topDict = dict;
          }

          // Recursively get other XRefs 'XRefStm', if any
          obj = dict.get("XRefStm");
          if (Number.isInteger(obj)) {
            const pos = obj;
            // ignore previously loaded xref streams
            // (possible infinite recursion)
            if (!(pos in this.xrefstms)) {
              this.xrefstms[pos] = 1;
              this.startXRefQueue.push(pos);
            }
          }
        } else if (Number.isInteger(obj)) {
          // Parse in-stream XRef
          if (
            !Number.isInteger(parser.getObj()) ||
            !isCmd(parser.getObj(), "obj") ||
            !((obj = parser.getObj()) instanceof BaseStream)
          ) {
            throw new FormatError("Invalid XRef stream");
          }
          dict = this.processXRefStream(obj);
          if (!this.topDict) {
            this.topDict = dict;
          }
          if (!dict) {
            throw new FormatError("Failed to read XRef stream");
          }
        } else {
          throw new FormatError("Invalid XRef stream header");
        }

        // Recursively get previous dictionary, if any
        obj = dict.get("Prev");
        if (Number.isInteger(obj)) {
          this.startXRefQueue.push(obj);
        } else if (obj instanceof Ref) {
          // The spec says Prev must not be a reference, i.e. "/Prev NNN"
          // This is a fallback for non-compliant PDFs, i.e. "/Prev NNN 0 R"
          this.startXRefQueue.push(obj.num);
        }

        this.startXRefQueue.shift();
      }

      return this.topDict;
    } catch (e) {
      if (e instanceof MissingDataException) {
        throw e;
      }
      info("(while reading XRef): " + e);

      this.startXRefQueue.shift();
    }

    if (recoveryMode) {
      return undefined;
    }
    throw new XRefParseException();
  }

  getEntry(i) {
    const xrefEntry = this.entries[i];
    if (xrefEntry && !xrefEntry.free && xrefEntry.offset) {
      return xrefEntry;
    }
    return null;
  }

  fetchIfRef(obj, suppressEncryption = false) {
    if (obj instanceof Ref) {
      return this.fetch(obj, suppressEncryption);
    }
    return obj;
  }

  fetch(ref, suppressEncryption = false) {
    if (!(ref instanceof Ref)) {
      throw new Error("ref object is not a reference");
    }
    const num = ref.num;

    // The XRef cache is populated with objects which are obtained through
    // `Parser.getObj`, and indirectly via `Lexer.getObj`. Neither of these
    // methods should ever return `undefined` (note the `assert` calls below).
    const cacheEntry = this._cacheMap.get(num);
    if (cacheEntry !== undefined) {
      // In documents with Object Streams, it's possible that cached `Dict`s
      // have not been assigned an `objId` yet (see e.g. issue3115r.pdf).
      if (cacheEntry instanceof Dict && !cacheEntry.objId) {
        cacheEntry.objId = ref.toString();
      }
      return cacheEntry;
    }
    let xrefEntry = this.getEntry(num);

    if (xrefEntry === null) {
      // The referenced entry can be free.
      this._cacheMap.set(num, xrefEntry);
      return xrefEntry;
    }
    // Prevent circular references, in corrupt PDF documents, from hanging the
    // worker-thread. This relies, implicitly, on the parsing being synchronous.
    if (this._pendingRefs.has(ref)) {
      this._pendingRefs.remove(ref);

      warn(`Ignoring circular reference: ${ref}.`);
      return CIRCULAR_REF;
    }
    this._pendingRefs.put(ref);

    try {
      if (xrefEntry.uncompressed) {
        xrefEntry = this.fetchUncompressed(ref, xrefEntry, suppressEncryption);
      } else {
        xrefEntry = this.fetchCompressed(ref, xrefEntry, suppressEncryption);
      }
      this._pendingRefs.remove(ref);
    } catch (ex) {
      this._pendingRefs.remove(ref);
      throw ex;
    }
    if (xrefEntry instanceof Dict) {
      xrefEntry.objId = ref.toString();
    } else if (xrefEntry instanceof BaseStream) {
      xrefEntry.dict.objId = ref.toString();
    }
    return xrefEntry;
  }

  fetchUncompressed(ref, xrefEntry, suppressEncryption = false) {
    const gen = ref.gen;
    let num = ref.num;
    if (xrefEntry.gen !== gen) {
      throw new XRefEntryException(`Inconsistent generation in XRef: ${ref}`);
    }
    const stream = this.stream.makeSubStream(
      xrefEntry.offset + this.stream.start
    );
    const parser = new Parser({
      lexer: new Lexer(stream),
      xref: this,
      allowStreams: true,
    });
    const obj1 = parser.getObj();
    const obj2 = parser.getObj();
    const obj3 = parser.getObj();

    if (obj1 !== num || obj2 !== gen || !(obj3 instanceof Cmd)) {
      throw new XRefEntryException(`Bad (uncompressed) XRef entry: ${ref}`);
    }
    if (obj3.cmd !== "obj") {
      // some bad PDFs use "obj1234" and really mean 1234
      if (obj3.cmd.startsWith("obj")) {
        num = parseInt(obj3.cmd.substring(3), 10);
        if (!Number.isNaN(num)) {
          return num;
        }
      }
      throw new XRefEntryException(`Bad (uncompressed) XRef entry: ${ref}`);
    }
    if (this.encrypt && !suppressEncryption) {
      xrefEntry = parser.getObj(this.encrypt.createCipherTransform(num, gen));
    } else {
      xrefEntry = parser.getObj();
    }
    if (!(xrefEntry instanceof BaseStream)) {
      if (
        typeof PDFJSDev === "undefined" ||
        PDFJSDev.test("!PRODUCTION || TESTING")
      ) {
        assert(
          xrefEntry !== undefined,
          'fetchUncompressed: The "xrefEntry" cannot be undefined.'
        );
      }
      this._cacheMap.set(num, xrefEntry);
    }
    return xrefEntry;
  }

  fetchCompressed(ref, xrefEntry, suppressEncryption = false) {
    const tableOffset = xrefEntry.offset;
    const stream = this.fetch(Ref.get(tableOffset, 0));
    if (!(stream instanceof BaseStream)) {
      throw new FormatError("bad ObjStm stream");
    }
    const first = stream.dict.get("First");
    const n = stream.dict.get("N");
    if (!Number.isInteger(first) || !Number.isInteger(n)) {
      throw new FormatError("invalid first and n parameters for ObjStm stream");
    }
    let parser = new Parser({
      lexer: new Lexer(stream),
      xref: this,
      allowStreams: true,
    });
    const nums = new Array(n);
    const offsets = new Array(n);
    // read the object numbers to populate cache
    for (let i = 0; i < n; ++i) {
      const num = parser.getObj();
      if (!Number.isInteger(num)) {
        throw new FormatError(
          `invalid object number in the ObjStm stream: ${num}`
        );
      }
      const offset = parser.getObj();
      if (!Number.isInteger(offset)) {
        throw new FormatError(
          `invalid object offset in the ObjStm stream: ${offset}`
        );
      }
      nums[i] = num;
      offsets[i] = offset;
    }

    const start = (stream.start || 0) + first;
    const entries = new Array(n);
    // read stream objects for cache
    for (let i = 0; i < n; ++i) {
      const length = i < n - 1 ? offsets[i + 1] - offsets[i] : undefined;
      if (length < 0) {
        throw new FormatError("Invalid offset in the ObjStm stream.");
      }
      parser = new Parser({
        lexer: new Lexer(
          stream.makeSubStream(start + offsets[i], length, stream.dict)
        ),
        xref: this,
        allowStreams: true,
      });

      const obj = parser.getObj();
      entries[i] = obj;
      if (obj instanceof BaseStream) {
        continue;
      }
      const num = nums[i],
        entry = this.entries[num];
      if (entry && entry.offset === tableOffset && entry.gen === i) {
        if (
          typeof PDFJSDev === "undefined" ||
          PDFJSDev.test("!PRODUCTION || TESTING")
        ) {
          assert(
            obj !== undefined,
            'fetchCompressed: The "obj" cannot be undefined.'
          );
        }
        this._cacheMap.set(num, obj);
      }
    }
    xrefEntry = entries[xrefEntry.gen];
    if (xrefEntry === undefined) {
      throw new XRefEntryException(`Bad (compressed) XRef entry: ${ref}`);
    }
    return xrefEntry;
  }

  async fetchIfRefAsync(obj, suppressEncryption) {
    if (obj instanceof Ref) {
      return this.fetchAsync(obj, suppressEncryption);
    }
    return obj;
  }

  async fetchAsync(ref, suppressEncryption) {
    try {
      return this.fetch(ref, suppressEncryption);
    } catch (ex) {
      if (!(ex instanceof MissingDataException)) {
        throw ex;
      }
      await this.pdfManager.requestRange(ex.begin, ex.end);
      return this.fetchAsync(ref, suppressEncryption);
    }
  }

  getCatalogObj() {
    return this.root;
  }
}

export { XRef };