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✏️ Editing: bitreader.js

/*
node-bzip - a pure-javascript Node.JS module for decoding bzip2 data

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Based on micro-bunzip by Rob Landley (rob@landley.net).

Based on bzip2 decompression code by Julian R Seward (jseward@acm.org),
which also acknowledges contributions by Mike Burrows, David Wheeler,
Peter Fenwick, Alistair Moffat, Radford Neal, Ian H. Witten,
Robert Sedgewick, and Jon L. Bentley.
*/

var BITMASK = [0x00, 0x01, 0x03, 0x07, 0x0F, 0x1F, 0x3F, 0x7F, 0xFF];

// offset in bytes
var BitReader = function(stream) {
  this.stream = stream;
  this.bitOffset = 0;
  this.curByte = 0;
  this.hasByte = false;
};

BitReader.prototype._ensureByte = function() {
  if (!this.hasByte) {
    this.curByte = this.stream.readByte();
    this.hasByte = true;
  }
};

// reads bits from the buffer
BitReader.prototype.read = function(bits) {
  var result = 0;
  while (bits > 0) {
    this._ensureByte();
    var remaining = 8 - this.bitOffset;
    // if we're in a byte
    if (bits >= remaining) {
      result <<= remaining;
      result |= BITMASK[remaining] & this.curByte;
      this.hasByte = false;
      this.bitOffset = 0;
      bits -= remaining;
    } else {
      result <<= bits;
      var shift = remaining - bits;
      result |= (this.curByte & (BITMASK[bits] << shift)) >> shift;
      this.bitOffset += bits;
      bits = 0;
    }
  }
  return result;
};

// seek to an arbitrary point in the buffer (expressed in bits)
BitReader.prototype.seek = function(pos) {
  var n_bit = pos % 8;
  var n_byte = (pos - n_bit) / 8;
  this.bitOffset = n_bit;
  this.stream.seek(n_byte);
  this.hasByte = false;
};

// reads 6 bytes worth of data using the read method
BitReader.prototype.pi = function() {
  var buf = new Buffer(6), i;
  for (i = 0; i < buf.length; i++) {
    buf[i] = this.read(8);
  }
  return buf.toString('hex');
};

module.exports = BitReader;

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