bitview.js

BitView = function(buf) {
  this.buffer = buf;
  this.u8 = new Uint8Array(buf);
};

BitView.prototype.getBit = function(idx) {
  var v = this.u8[idx >> 3];
  var off = idx & 0x7;
  return (v & (0x80 >> off)) >> (7-off);
};

BitView.prototype.setBit = function(idx, val) {
  var bidx = idx >> 3;
  var v = this.u8[bidx];
  var off = idx & 0x7;
  if (val) {
    this.u8[bidx] = v | (0x80 >> off);
  } else {
    this.u8[bidx] = v & ~(0x80 >> off);
  }
};

BitView.prototype.getInt12 = function(idx) {
  var bidx = idx/8 | 0;
  var a = this.u8[bidx];
  var b = this.u8[bidx + 1];
  var c = this.u8[bidx + 2];
  var off = idx % 8;
  var abits = 8-off;
  var bbits = Math.min(12-abits, 8);
  var cbits = Math.max(12-abits-bbits, 0);
  var am = ~(0xff << (abits));
  var bm = (0xff << (8-bbits));
  var cm = (0xff << (8-cbits));
  a &= am;
  b &= bm;
  c &= cm;
  return (((a << 16) + (b << 8) + c) >> (12-off)) - 2048;
};

BitView.prototype.setInt12 = function(idx, val) {
  val += 2048;
  var bidx = idx/8 | 0;
  var off = idx % 8;
  var v = val << (12-off);
  var a = (v & 0xff0000) >> 16;
  var b = (v & 0x00ff00) >> 8;
  var c = v & 0x0000ff;
  var abits = 8-off;
  var bbits = Math.min(12-abits, 8);
  var cbits = Math.max(12-abits-bbits, 0);
  var am = (0xff << (abits));
  this.u8[bidx] = (this.u8[bidx] & am) + a;
  var bm = ~(0xff << (8-bbits));
  this.u8[bidx+1] = (this.u8[bidx+1] & bm) + b;
  var cm = ~(0xff << (8-cbits));
  this.u8[bidx+2] = (this.u8[bidx+2] & cm) + c;
};

BitView.prototype.getInt6 = function(idx) {
  var bidx = idx/8 | 0;
  var a = this.u8[bidx];
  var b = this.u8[bidx + 1];
  var off = idx % 8;
  var abits = 8-off;
  var bbits = Math.max(6-abits, 0);
  var am = ~((0xff << (abits)) + (0xff >> (8-(2-off))));
  var bm = (0xff << (8-bbits));
  a &= am;
  b &= bm;
  return (((a << 8) + b) >> (10-off)) - 32;
};

BitView.prototype.setInt6 = function(idx, val) {
  val += 32;
  var bidx = idx/8 | 0;
  var off = idx % 8;
  var v = val << (10-off);
  var a = (v & 0xff00) >> 8;
  var b = (v & 0x00ff);
  var abits = 8-off;
  var bbits = Math.max(6-abits, 0);
  var am = ((0xff << (abits)) + (0xff >> (8-(2-off))));
  this.u8[bidx] = (this.u8[bidx] & am) + a;
  var bm = ~(0xff << (8-bbits));
  this.u8[bidx+1] = (this.u8[bidx+1] & bm) + b;
};

BitView.test = function() {
  var buf = new ArrayBuffer(3);
  var bv = new BitView(buf);
  var i,j;
  for (j=0; j<12; j++) {
    for (i=-2048; i<2048; i++) { 
      bv.setInt12(j,i); 
      if (bv.getInt12(j) != i) {
        console.log('12-bit prob at', j, i);
        console.log('expected', i, 'got', bv.getInt12(j));
        break;
      }
    }
  }
  for (j=0; j<18; j++) {
    for (i=-32; i<32; i++) { 
      bv.setInt6(j,i); 
      if (bv.getInt6(j) != i) {
        console.log('6-bit prob at', j, i);
        console.log('expected', i, 'got', bv.getInt6(j));
        break;
      }
    }
  }
  return bv;
};