nan-tagging.js

// assumes little endian

// constants taken from http://nikic.github.com/2012/02/02/Pointer-magic-for-efficient-dynamic-value-representations.html
var MAXDOUBLE = 0xfff80000,
    INT32TAG  = 0xfff90000,
    BOOLTAG   = 0xfffa0000;

var heap = new ArrayBuffer(24),
    f64  = new Float64Array(heap),
    ui32 = new Uint32Array(heap),
    i32  = new Int32Array(heap),
    ui8  = new Uint8Array(heap);


function setInt32(index, value){
  var index32 = index << 1; // index is 8 bytes, we need to look at 4 bytes
  i32[index32] = value >> 0; // store int32
  ui32[index32 + 1] = INT32TAG; // tag as int32
}

function getInt32(index){
  var index32 = index << 1;
  if (ui32[index32 + 1] === INT32TAG) { // check tag
    return i32[index32]; // return int32 payload
  }
  throw new TypeError('Value is not an int32');
}

function isInt32(index){
  return ui32[(index << 1) + 1] === INT32TAG;
}



function setBool(index, value){
  var index32 = index << 1;
  ui32[index32] = value ? 1 : 0;
  ui32[index32 + 1] = BOOLTAG;
}

function getBool(index){
  var index32 = index << 1;
  if (ui32[index32 + 1] === BOOLTAG) {
    return ui32[index32] ? true : false;
  }
  throw new TypeError('Value is not a boolean');
}

function isBool(index){
  return ui32[(index << 1) + 1] === BOOLTAG;
}


function getType(index){
  var tag = ui32[(index << 1) + 1];

  if (tag <= MAXDOUBLE) {
    return 'double';
  }

  switch (tag) {
    case INT32TAG: return 'int32';
    case BOOLTAG: return 'bool';
  }

  throw new Error('not reached');
}

function toHex(){
  return [].map.call(ui8, function(byte, i){
    return ('00'+byte.toString(16)).slice(-2) + ((i + 1) % 8 ? ' ' : '|');
  }).join('').slice(0, -1);
}

function types(){
  return [].map.call(f64, function(_, index){
    return getType(index);
  });
}


console.log(isInt32(1));  // false
setInt32(1, 10000);
console.log(isInt32(1));  // true
console.log(getInt32(1)); // 10000
setBool(2, true);
console.log(types());     // [ 'double', 'int32', 'bool' ]
console.log(f64);         // [ 0, NaN, NaN ]
console.log(toHex());     // 00 00 00 00 00 00 00 00|10 27 00 00 00 00 f9 ff|01 00 00 00 00 00 fa ff