nyks/buffer/writeBits.js

nodejs exupery style

"use strict";

var pow2  = [1, 2, 4, 8, 16, 32, 64, 128, 256];
var neg  = [127, 191, 223, 239, 247, 251, 253, 254]; //xor


function writeBits(target, source, bitoffset, sourcebitLength) {
  if(sourcebitLength === undefined)
    sourcebitLength = source.length << 3;

  if(!sourcebitLength)
    return 0;

  var soucebitStart = sourcebitLength < 0 ? (source.length << 3) + sourcebitLength : 0;
  sourcebitLength  = Math.min(Math.abs(sourcebitLength), (target.length << 3) - bitoffset);
  var sourceBitEnd  = soucebitStart +  sourcebitLength - 1;

  for(var i = soucebitStart, j = bitoffset; i <= sourceBitEnd; i++, j++) {
    if(source[i >> 3] & pow2[7 - i % 8])
      target[j >> 3] |= pow2[7 - j % 8];
    else
      target[j >> 3] &= neg[j % 8];
  }

  return sourcebitLength;
}


module.exports = writeBits;