nan-tagging-2.js

var assert = console.assert;

var DOUBLE = 0,
    INT32  = 1,
    BOOL   = 2,
    PTR    = 3;

var MAXDOUBLE = 0xfff80000,
    INT32TAG  = 0xfff90000,
    BOOLTAG   = 0xfffa0000,
    PTRTAG    = 0xfffb0000;

var tags = [MAXDOUBLE, INT32TAG, BOOLTAG, PTRTAG];

var heap = new ArrayBuffer(128),
    f64  = new Float64Array(heap),
    ui32 = new Uint32Array(heap),
    i32  = new Int32Array(heap),
    ui8  = new Uint8Array(heap);

var hp = 0;

// super basic alloc just as an example
function alloc(){
  return hp++;
}

var NULL = alloc();


function Value(type, value){
  var addr = alloc();

  if (type === DOUBLE) {
    f64[addr] = value;
  } else {
    var addr32 = addr << 1;
    i32[addr32] = value;
    ui32[addr32 + 1] = tags[type];
  }

  return addr;
}



Value.isDouble = function isDouble(addr){
  return ui32[(addr << 1) + 1] < MAXDOUBLE;
};

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

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

Value.isPointer = function isPointer(addr){
  return ui32[(addr << 1) + 1] === PTRTAG;
};


Value.getDouble = function getDouble(addr){
  assert(Value.isDouble(addr));
  return f64[addr];
};

Value.getInt32 = function getInt32(addr){
  assert(Value.isInt32(addr));
  return i32[addr << 1];
};

Value.getBool = function getBool(addr){
  assert(Value.isBool(addr));
  return i32[addr << 1] ? true : false;
};

Value.getPointer = function getPointer(addr){
  assert(Value.isPointer(addr));
  return ui32[addr << 1];
};

Value.get = function get(addr){
  var addr32 = addr << 1;
  var tag = ui32[addr32 + 1];

  if (tag < MAXDOUBLE) {
    return f64[addr];
  }

  switch (tag) {
    case INT32TAG:
        return i32[addr32];
    case BOOLTAG:
        return i32[addr32] ? true : false;
    case PTRTAG:
        return ui32[addr32];
  }

};


var bool = Value(BOOL, true);
console.log(Value.get(bool));

var int32 = Value(INT32, 200000);
console.log(Value.get(int32));

var double = Value(DOUBLE, 2000000.45);
console.log(Value.get(double));