var fs = require("fs");
var zlib = require("zlib");
var fd_slicer = require("fd-slicer");
var util = require("util");
var EventEmitter = require("events").EventEmitter;
var Transform = require("stream").Transform;
var PassThrough = require("stream").PassThrough;
var Writable = require("stream").Writable;

exports.open = open;
exports.fromFd = fromFd;
exports.fromBuffer = fromBuffer;
exports.fromRandomAccessReader = fromRandomAccessReader;
exports.dosDateTimeToDate = dosDateTimeToDate;
exports.ZipFile = ZipFile;
exports.Entry = Entry;
exports.RandomAccessReader = RandomAccessReader;

function open(path, options, callback) {
  if (typeof options === "function") {
    callback = options;
    options = null;
  }
  if (options == null) options = {};
  if (options.autoClose == null) options.autoClose = true;
  if (options.lazyEntries == null) options.lazyEntries = false;
  if (callback == null) callback = defaultCallback;
  fs.open(path, "r", function(err, fd) {
    if (err) return callback(err);
    fromFd(fd, options, function(err, zipfile) {
      if (err) fs.close(fd, defaultCallback);
      callback(err, zipfile);
    });
  });
}

function fromFd(fd, options, callback) {
  if (typeof options === "function") {
    callback = options;
    options = null;
  }
  if (options == null) options = {};
  if (options.autoClose == null) options.autoClose = false;
  if (options.lazyEntries == null) options.lazyEntries = false;
  if (callback == null) callback = defaultCallback;
  fs.fstat(fd, function(err, stats) {
    if (err) return callback(err);
    var reader = fd_slicer.createFromFd(fd, {autoClose: true});
    fromRandomAccessReader(reader, stats.size, options, callback);
  });
}

function fromBuffer(buffer, options, callback) {
  if (typeof options === "function") {
    callback = options;
    options = null;
  }
  if (options == null) options = {};
  options.autoClose = false;
  if (options.lazyEntries == null) options.lazyEntries = false;
  // i got your open file right here.
  var reader = fd_slicer.createFromBuffer(buffer);
  fromRandomAccessReader(reader, buffer.length, options, callback);
}

function fromRandomAccessReader(reader, totalSize, options, callback) {
  if (typeof options === "function") {
    callback = options;
    options = null;
  }
  if (options == null) options = {};
  if (options.autoClose == null) options.autoClose = true;
  if (options.lazyEntries == null) options.lazyEntries = false;
  if (callback == null) callback = defaultCallback;
  if (typeof totalSize !== "number") throw new Error("expected totalSize parameter to be a number");
  if (totalSize > Number.MAX_SAFE_INTEGER) {
    throw new Error("zip file too large. only file sizes up to 2^52 are supported due to JavaScript's Number type being an IEEE 754 double.");
  }

  // the matching unref() call is in zipfile.close()
  reader.ref();

  // eocdr means End of Central Directory Record.
  // search backwards for the eocdr signature.
  // the last field of the eocdr is a variable-length comment.
  // the comment size is encoded in a 2-byte field in the eocdr, which we can't find without trudging backwards through the comment to find it.
  // as a consequence of this design decision, it's possible to have ambiguous zip file metadata if a coherent eocdr was in the comment.
  // we search backwards for a eocdr signature, and hope that whoever made the zip file was smart enough to forbid the eocdr signature in the comment.
  var eocdrWithoutCommentSize = 22;
  var maxCommentSize = 0x10000; // 2-byte size
  var bufferSize = Math.min(eocdrWithoutCommentSize + maxCommentSize, totalSize);
  var buffer = new Buffer(bufferSize);
  var bufferReadStart = totalSize - buffer.length;
  readAndAssertNoEof(reader, buffer, 0, bufferSize, bufferReadStart, function(err) {
    if (err) return callback(err);
    for (var i = bufferSize - eocdrWithoutCommentSize; i >= 0; i -= 1) {
      if (buffer.readUInt32LE(i) !== 0x06054b50) continue;
      // found eocdr
      var eocdrBuffer = buffer.slice(i);

      // 0 - End of central directory signature = 0x06054b50
      // 4 - Number of this disk
      var diskNumber = eocdrBuffer.readUInt16LE(4);
      if (diskNumber !== 0) return callback(new Error("multi-disk zip files are not supported: found disk number: " + diskNumber));
      // 6 - Disk where central directory starts
      // 8 - Number of central directory records on this disk
      // 10 - Total number of central directory records
      var entryCount = eocdrBuffer.readUInt16LE(10);
      // 12 - Size of central directory (bytes)
      // 16 - Offset of start of central directory, relative to start of archive
      var centralDirectoryOffset = eocdrBuffer.readUInt32LE(16);
      // 20 - Comment length
      var commentLength = eocdrBuffer.readUInt16LE(20);
      var expectedCommentLength = eocdrBuffer.length - eocdrWithoutCommentSize;
      if (commentLength !== expectedCommentLength) {
        return callback(new Error("invalid comment length. expected: " + expectedCommentLength + ". found: " + commentLength));
      }
      // 22 - Comment
      // the encoding is always cp437.
      var comment = bufferToString(eocdrBuffer, 22, eocdrBuffer.length, false);

      if (!(entryCount === 0xffff || centralDirectoryOffset === 0xffffffff)) {
        return callback(null, new ZipFile(reader, centralDirectoryOffset, totalSize, entryCount, comment, options.autoClose, options.lazyEntries));
      }

      // ZIP64 format

      // ZIP64 Zip64 end of central directory locator
      var zip64EocdlBuffer = new Buffer(20);
      var zip64EocdlOffset = bufferReadStart + i - zip64EocdlBuffer.length;
      readAndAssertNoEof(reader, zip64EocdlBuffer, 0, zip64EocdlBuffer.length, zip64EocdlOffset, function(err) {
        if (err) return callback(err);

        // 0 - zip64 end of central dir locator signature = 0x07064b50
        if (zip64EocdlBuffer.readUInt32LE(0) !== 0x07064b50) {
          return callback(new Error("invalid ZIP64 End of Central Directory Locator signature"));
        }
        // 4 - number of the disk with the start of the zip64 end of central directory
        // 8 - relative offset of the zip64 end of central directory record
        var zip64EocdrOffset = readUInt64LE(zip64EocdlBuffer, 8);
        // 16 - total number of disks

        // ZIP64 end of central directory record
        var zip64EocdrBuffer = new Buffer(56);
        readAndAssertNoEof(reader, zip64EocdrBuffer, 0, zip64EocdrBuffer.length, zip64EocdrOffset, function(err) {
          if (err) return callback(err);

          // 0 - zip64 end of central dir signature                           4 bytes  (0x06064b50)
          if (zip64EocdrBuffer.readUInt32LE(0) !== 0x06064b50) return callback(new Error("invalid ZIP64 end of central directory record signature"));
          // 4 - size of zip64 end of central directory record                8 bytes
          // 12 - version made by                                             2 bytes
          // 14 - version needed to extract                                   2 bytes
          // 16 - number of this disk                                         4 bytes
          // 20 - number of the disk with the start of the central directory  4 bytes
          // 24 - total number of entries in the central directory on this disk         8 bytes
          // 32 - total number of entries in the central directory            8 bytes
          entryCount = readUInt64LE(zip64EocdrBuffer, 32);
          // 40 - size of the central directory                               8 bytes
          // 48 - offset of start of central directory with respect to the starting disk number     8 bytes
          centralDirectoryOffset = readUInt64LE(zip64EocdrBuffer, 48);
          // 56 - zip64 extensible data sector                                (variable size)
          return callback(null, new ZipFile(reader, centralDirectoryOffset, totalSize, entryCount, comment, options.autoClose, options.lazyEntries));
        });
      });
      return;
    }
    callback(new Error("end of central directory record signature not found"));
  });
}

util.inherits(ZipFile, EventEmitter);
function ZipFile(reader, centralDirectoryOffset, fileSize, entryCount, comment, autoClose, lazyEntries) {
  var self = this;
  EventEmitter.call(self);
  self.reader = reader;
  // forward close events
  self.reader.on("error", function(err) {
    // error closing the fd
    emitError(self, err);
  });
  self.reader.once("close", function() {
    self.emit("close");
  });
  self.readEntryCursor = centralDirectoryOffset;
  self.fileSize = fileSize;
  self.entryCount = entryCount;
  self.comment = comment;
  self.entriesRead = 0;
  self.autoClose = !!autoClose;
  self.lazyEntries = !!lazyEntries;
  self.isOpen = true;
  self.emittedError = false;

  if (!self.lazyEntries) self.readEntry();
}
ZipFile.prototype.close = function() {
  if (!this.isOpen) return;
  this.isOpen = false;
  this.reader.unref();
};

function emitErrorAndAutoClose(self, err) {
  if (self.autoClose) self.close();
  emitError(self, err);
}
function emitError(self, err) {
  if (self.emittedError) return;
  self.emittedError = true;
  self.emit("error", err);
}

ZipFile.prototype.readEntry = function() {
  var self = this;
  if (self.entryCount === self.entriesRead) {
    // done with metadata
    setImmediate(function() {
      if (self.autoClose) self.close();
      if (self.emittedError) return;
      self.emit("end");
    });
    return;
  }
  if (self.emittedError) return;
  var buffer = new Buffer(46);
  readAndAssertNoEof(self.reader, buffer, 0, buffer.length, self.readEntryCursor, function(err) {
    if (err) return emitErrorAndAutoClose(self, err);
    if (self.emittedError) return;
    var entry = new Entry();
    // 0 - Central directory file header signature
    var signature = buffer.readUInt32LE(0);
    if (signature !== 0x02014b50) return emitErrorAndAutoClose(self, new Error("invalid central directory file header signature: 0x" + signature.toString(16)));
    // 4 - Version made by
    entry.versionMadeBy = buffer.readUInt16LE(4);
    // 6 - Version needed to extract (minimum)
    entry.versionNeededToExtract = buffer.readUInt16LE(6);
    // 8 - General purpose bit flag
    entry.generalPurposeBitFlag = buffer.readUInt16LE(8);
    // 10 - Compression method
    entry.compressionMethod = buffer.readUInt16LE(10);
    // 12 - File last modification time
    entry.lastModFileTime = buffer.readUInt16LE(12);
    // 14 - File last modification date
    entry.lastModFileDate = buffer.readUInt16LE(14);
    // 16 - CRC-32
    entry.crc32 = buffer.readUInt32LE(16);
    // 20 - Compressed size
    entry.compressedSize = buffer.readUInt32LE(20);
    // 24 - Uncompressed size
    entry.uncompressedSize = buffer.readUInt32LE(24);
    // 28 - File name length (n)
    entry.fileNameLength = buffer.readUInt16LE(28);
    // 30 - Extra field length (m)
    entry.extraFieldLength = buffer.readUInt16LE(30);
    // 32 - File comment length (k)
    entry.fileCommentLength = buffer.readUInt16LE(32);
    // 34 - Disk number where file starts
    // 36 - Internal file attributes
    entry.internalFileAttributes = buffer.readUInt16LE(36);
    // 38 - External file attributes
    entry.externalFileAttributes = buffer.readUInt32LE(38);
    // 42 - Relative offset of local file header
    entry.relativeOffsetOfLocalHeader = buffer.readUInt32LE(42);

    self.readEntryCursor += 46;

    buffer = new Buffer(entry.fileNameLength + entry.extraFieldLength + entry.fileCommentLength);
    readAndAssertNoEof(self.reader, buffer, 0, buffer.length, self.readEntryCursor, function(err) {
      if (err) return emitErrorAndAutoClose(self, err);
      if (self.emittedError) return;
      // 46 - File name
      var isUtf8 = entry.generalPurposeBitFlag & 0x800
      try {
        entry.fileName = bufferToString(buffer, 0, entry.fileNameLength, isUtf8);
      } catch (e) {
        return emitErrorAndAutoClose(self, e);
      }

      // 46+n - Extra field
      var fileCommentStart = entry.fileNameLength + entry.extraFieldLength;
      var extraFieldBuffer = buffer.slice(entry.fileNameLength, fileCommentStart);
      entry.extraFields = [];
      var i = 0;
      while (i < extraFieldBuffer.length) {
        var headerId = extraFieldBuffer.readUInt16LE(i + 0);
        var dataSize = extraFieldBuffer.readUInt16LE(i + 2);
        var dataStart = i + 4;
        var dataEnd = dataStart + dataSize;
        var dataBuffer = new Buffer(dataSize);
        extraFieldBuffer.copy(dataBuffer, 0, dataStart, dataEnd);
        entry.extraFields.push({
          id: headerId,
          data: dataBuffer,
        });
        i = dataEnd;
      }

      // 46+n+m - File comment
      try {
        entry.fileComment = bufferToString(buffer, fileCommentStart, fileCommentStart + entry.fileCommentLength, isUtf8);
      } catch (e) {
        return emitErrorAndAutoClose(self, e);
      }

      self.readEntryCursor += buffer.length;
      self.entriesRead += 1;

      if (entry.uncompressedSize            === 0xffffffff ||
          entry.compressedSize              === 0xffffffff ||
          entry.relativeOffsetOfLocalHeader === 0xffffffff) {
        // ZIP64 format
        // find the Zip64 Extended Information Extra Field
        var zip64EiefBuffer = null;
        for (var i = 0; i < entry.extraFields.length; i++) {
          var extraField = entry.extraFields[i];
          if (extraField.id === 0x0001) {
            zip64EiefBuffer = extraField.data;
            break;
          }
        }
        if (zip64EiefBuffer == null) return emitErrorAndAutoClose(self, new Error("expected Zip64 Extended Information Extra Field"));
        var index = 0;
        // 0 - Original Size          8 bytes
        if (entry.uncompressedSize === 0xffffffff) {
          if (index + 8 > zip64EiefBuffer.length) return emitErrorAndAutoClose(self, new Error("Zip64 Extended Information Extra Field does not include Original Size"));
          entry.uncompressedSize = readUInt64LE(zip64EiefBuffer, index);
          index += 8;
        }
        // 8 - Compressed Size        8 bytes
        if (entry.compressedSize === 0xffffffff) {
          if (index + 8 > zip64EiefBuffer.length) return emitErrorAndAutoClose(self, new Error("Zip64 Extended Information Extra Field does not include Compressed Size"));
          entry.compressedSize = readUInt64LE(zip64EiefBuffer, index);
          index += 8;
        }
        // 16 - Relative Header Offset 8 bytes
        if (entry.relativeOffsetOfLocalHeader === 0xffffffff) {
          if (index + 8 > zip64EiefBuffer.length) return emitErrorAndAutoClose(self, new Error("Zip64 Extended Information Extra Field does not include Relative Header Offset"));
          entry.relativeOffsetOfLocalHeader = readUInt64LE(zip64EiefBuffer, index);
          index += 8;
        }
        // 24 - Disk Start Number      4 bytes
      }

      // validate file size
      if (entry.compressionMethod === 0) {
        if (entry.compressedSize !== entry.uncompressedSize) {
          var msg = "compressed/uncompressed size mismatch for stored file: " + entry.compressedSize + " != " + entry.uncompressedSize;
          return emitErrorAndAutoClose(self, new Error(msg));
        }
      }

      // validate file name
      if (entry.fileName.indexOf("\\") !== -1) return emitErrorAndAutoClose(self, new Error("invalid characters in fileName: " + entry.fileName));
      if (/^[a-zA-Z]:/.test(entry.fileName) || /^\//.test(entry.fileName)) return emitErrorAndAutoClose(self, new Error("absolute path: " + entry.fileName));
      if (entry.fileName.split("/").indexOf("..") !== -1) return emitErrorAndAutoClose(self, new Error("invalid relative path: " + entry.fileName));
      self.emit("entry", entry);

      if (!self.lazyEntries) self.readEntry();
    });
  });
};

ZipFile.prototype.openReadStream = function(entry, callback) {
  var self = this;
  if (!self.isOpen) return callback(new Error("closed"));
  // make sure we don't lose the fd before we open the actual read stream
  self.reader.ref();
  var buffer = new Buffer(30);
  readAndAssertNoEof(self.reader, buffer, 0, buffer.length, entry.relativeOffsetOfLocalHeader, function(err) {
    try {
      if (err) return callback(err);
      // 0 - Local file header signature = 0x04034b50
      var signature = buffer.readUInt32LE(0);
      if (signature !== 0x04034b50) return callback(new Error("invalid local file header signature: 0x" + signature.toString(16)));
      // all this should be redundant
      // 4 - Version needed to extract (minimum)
      // 6 - General purpose bit flag
      // 8 - Compression method
      // 10 - File last modification time
      // 12 - File last modification date
      // 14 - CRC-32
      // 18 - Compressed size
      // 22 - Uncompressed size
      // 26 - File name length (n)
      var fileNameLength = buffer.readUInt16LE(26);
      // 28 - Extra field length (m)
      var extraFieldLength = buffer.readUInt16LE(28);
      // 30 - File name
      // 30+n - Extra field
      var localFileHeaderEnd = entry.relativeOffsetOfLocalHeader + buffer.length + fileNameLength + extraFieldLength;
      var compressed;
      if (entry.compressionMethod === 0) {
        // 0 - The file is stored (no compression)
        compressed = false;
      } else if (entry.compressionMethod === 8) {
        // 8 - The file is Deflated
        compressed = true;
      } else {
        return callback(new Error("unsupported compression method: " + entry.compressionMethod));
      }
      var fileDataStart = localFileHeaderEnd;
      var fileDataEnd = fileDataStart + entry.compressedSize;
      if (entry.compressedSize !== 0) {
        // bounds check now, because the read streams will probably not complain loud enough.
        // since we're dealing with an unsigned offset plus an unsigned size,
        // we only have 1 thing to check for.
        if (fileDataEnd > self.fileSize) {
          return callback(new Error("file data overflows file bounds: " +
              fileDataStart + " + " + entry.compressedSize + " > " + self.fileSize));
        }
      }
      var readStream = self.reader.createReadStream({start: fileDataStart, end: fileDataEnd});
      var endpointStream = readStream;
      if (compressed) {
        var destroyed = false;
        var inflateFilter = zlib.createInflateRaw();
        readStream.on("error", function(err) {
          // setImmediate here because errors can be emitted during the first call to pipe()
          setImmediate(function() {
            if (!destroyed) inflateFilter.emit("error", err);
          });
        });

        var checkerStream = new AssertByteCountStream(entry.uncompressedSize);
        inflateFilter.on("error", function(err) {
          // forward zlib errors to the client-visible stream
          setImmediate(function() {
            if (!destroyed) checkerStream.emit("error", err);
          });
        });
        checkerStream.destroy = function() {
          destroyed = true;
          inflateFilter.unpipe(checkerStream);
          readStream.unpipe(inflateFilter);
          // TODO: the inflateFilter now causes a memory leak. see Issue #27.
          readStream.destroy();
        };
        endpointStream = readStream.pipe(inflateFilter).pipe(checkerStream);
      }
      callback(null, endpointStream);
    } finally {
      self.reader.unref();
    }
  });
};

function Entry() {
}
Entry.prototype.getLastModDate = function() {
  return dosDateTimeToDate(this.lastModFileDate, this.lastModFileTime);
};

function dosDateTimeToDate(date, time) {
  var day = date & 0x1f; // 1-31
  var month = (date >> 5 & 0xf) - 1; // 1-12, 0-11
  var year = (date >> 9 & 0x7f) + 1980; // 0-128, 1980-2108

  var millisecond = 0;
  var second = (time & 0x1f) * 2; // 0-29, 0-58 (even numbers)
  var minute = time >> 5 & 0x3f; // 0-59
  var hour = time >> 11 & 0x1f; // 0-23

  return new Date(year, month, day, hour, minute, second, millisecond);
}

function readAndAssertNoEof(reader, buffer, offset, length, position, callback) {
  if (length === 0) {
    // fs.read will throw an out-of-bounds error if you try to read 0 bytes from a 0 byte file
    return setImmediate(function() { callback(null, new Buffer(0)); });
  }
  reader.read(buffer, offset, length, position, function(err, bytesRead) {
    if (err) return callback(err);
    if (bytesRead < length) return callback(new Error("unexpected EOF"));
    callback();
  });
}

util.inherits(AssertByteCountStream, Transform);
function AssertByteCountStream(byteCount) {
  Transform.call(this);
  this.actualByteCount = 0;
  this.expectedByteCount = byteCount;
}
AssertByteCountStream.prototype._transform = function(chunk, encoding, cb) {
  this.actualByteCount += chunk.length;
  if (this.actualByteCount > this.expectedByteCount) {
    var msg = "too many bytes in the stream. expected " + this.expectedByteCount + ". got at least " + this.actualByteCount;
    return cb(new Error(msg));
  }
  cb(null, chunk);
};
AssertByteCountStream.prototype._flush = function(cb) {
  if (this.actualByteCount < this.expectedByteCount) {
    var msg = "not enough bytes in the stream. expected " + this.expectedByteCount + ". got only " + this.actualByteCount;
    return cb(new Error(msg));
  }
  cb();
};

util.inherits(RandomAccessReader, EventEmitter);
function RandomAccessReader() {
  EventEmitter.call(this);
  this.refCount = 0;
}
RandomAccessReader.prototype.ref = function() {
  this.refCount += 1;
};
RandomAccessReader.prototype.unref = function() {
  var self = this;
  self.refCount -= 1;

  if (self.refCount > 0) return;
  if (self.refCount < 0) throw new Error("invalid unref");

  self.close(onCloseDone);

  function onCloseDone(err) {
    if (err) return self.emit('error', err);
    self.emit('close');
  }
};
RandomAccessReader.prototype.createReadStream = function(options) {
  var start = options.start;
  var end = options.end;
  if (start === end) {
    var emptyStream = new PassThrough();
    setImmediate(function() {
      emptyStream.end();
    });
    return emptyStream;
  }
  var stream = this._readStreamForRange(start, end);

  var destroyed = false;
  var refUnrefFilter = new RefUnrefFilter(this);
  stream.on("error", function(err) {
    setImmediate(function() {
      if (!destroyed) refUnrefFilter.emit("error", err);
    });
  });
  refUnrefFilter.destroy = function() {
    stream.unpipe(refUnrefFilter);
    refUnrefFilter.unref();
    stream.destroy();
  };

  var byteCounter = new AssertByteCountStream(end - start);
  refUnrefFilter.on("error", function(err) {
    setImmediate(function() {
      if (!destroyed) byteCounter.emit("error", err);
    });
  });
  byteCounter.destroy = function() {
    destroyed = true;
    refUnrefFilter.unpipe(byteCounter);
    refUnrefFilter.destroy();
  };

  return stream.pipe(refUnrefFilter).pipe(byteCounter);
};
RandomAccessReader.prototype._readStreamForRange = function(start, end) {
  throw new Error("not implemented");
};
RandomAccessReader.prototype.read = function(buffer, offset, length, position, callback) {
  var readStream = this.createReadStream({start: position, end: position + length});
  var writeStream = new Writable();
  var written = 0;
  writeStream._write = function(chunk, encoding, cb) {
    chunk.copy(buffer, offset + written, 0, chunk.length);
    written += chunk.length;
    cb();
  };
  writeStream.on("finish", callback);
  readStream.on("error", function(error) {
    callback(error);
  });
  readStream.pipe(writeStream);
};
RandomAccessReader.prototype.close = function(callback) {
  setImmediate(callback);
};

util.inherits(RefUnrefFilter, PassThrough);
function RefUnrefFilter(context) {
  PassThrough.call(this);
  this.context = context;
  this.context.ref();
  this.unreffedYet = false;
}
RefUnrefFilter.prototype._flush = function(cb) {
  this.unref();
  cb();
};
RefUnrefFilter.prototype.unref = function(cb) {
  if (this.unreffedYet) return;
  this.unreffedYet = true;
  this.context.unref();
};

var cp437 = '\u0000☺☻♥♦♣♠•◘○◙♂♀♪♫☼►◄↕‼¶§▬↨↑↓→←∟↔▲▼ !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~⌂ÇüéâäàåçêëèïîìÄÅÉæÆôöòûùÿÖÜ¢£¥₧ƒáíóúñѪº¿⌐¬½¼¡«»░▒▓│┤╡╢╖╕╣║╗╝╜╛┐└┴┬├─┼╞╟╚╔╩╦╠═╬╧╨╤╥╙╘╒╓╫╪┘┌█▄▌▐▀αßΓπΣσµτΦΘΩδ∞φε∩≡±≥≤⌠⌡÷≈°∙·√ⁿ²■ ';
function bufferToString(buffer, start, end, isUtf8) {
  if (isUtf8) {
    return buffer.toString("utf8", start, end);
  } else {
    var result = "";
    for (var i = start; i < end; i++) {
      result += cp437[buffer[i]];
    }
    return result;
  }
}

function readUInt64LE(buffer, offset) {
  // there is no native function for this, because we can't actually store 64-bit integers precisely.
  // after 53 bits, JavaScript's Number type (IEEE 754 double) can't store individual integers anymore.
  // but since 53 bits is a whole lot more than 32 bits, we do our best anyway.
  var lower32 = buffer.readUInt32LE(offset);
  var upper32 = buffer.readUInt32LE(offset + 4);
  // we can't use bitshifting here, because JavaScript bitshifting only works on 32-bit integers.
  return upper32 * 0x100000000 + lower32;
  // as long as we're bounds checking the result of this function against the total file size,
  // we'll catch any overflow errors, because we already made sure the total file size was within reason.
}

function defaultCallback(err) {
  if (err) throw err;
}