function Mapper()
{
  var sources = {};


  this.forEach = function(callback)
  {
    for(var key in sources)
    {
      var source = sources[key];

      for(var key2 in source)
        callback(source[key2]);
    };
  };

  this.get = function(id, source)
  {
    var ids = sources[source];
    if(ids == undefined)
      return undefined;

    return ids[id];
  };

  this.remove = function(id, source)
  {
    var ids = sources[source];
    if(ids == undefined)
      return;

    delete ids[id];

    // Check it's empty
    for(var i in ids){return false}

    delete sources[source];
  };

  this.set = function(value, id, source)
  {
    if(value == undefined)
      return this.remove(id, source);

    var ids = sources[source];
    if(ids == undefined)
      sources[source] = ids = {};

    ids[id] = value;
  };
};


Mapper.prototype.pop = function(id, source)
{
  var value = this.get(id, source);
  if(value == undefined)
    return undefined;

  this.remove(id, source);

  return value;
};


module.exports = Mapper;