// copy-on-set

function Cos(state, parent) {
  'use strict';
  
  if (typeof this === 'undefined') {
  	return new Cos(state, parent);
  }

  state = state || {};

  let copied = false;
  
  this.get = name => 
  	state[name];

  this.set = (name, value) => {
    if (!copied) {
      state = copy(state);
    }
    state[name] = value;
  };

  this.spawn = () => new Cos(state, this);
  
  this.state = () => state;
  
  this.parent = () => parent;
}

// deap copy
function copy(obj) {

  if (obj === null || typeof obj !== 'object') {
    // primitive types are passed by value
    return obj;
  }
  	
  if (Array.isArray(obj)) {
    // copy array
    return obj.map(item => copy(item));
  }


  if (obj instanceof Date) {
    // copy date
    return new Date(obj.getTime());
  }
  // copy object
  return Object.keys(obj).reduce((obj, item) => obj[item] = copy(obj[item]) && obj,
    Object.assign({}, obj));
}