function Queue() {
  this.queue = [];
}

Queue.prototype.push_tail = function(msg) {
  this.queue.reverse();
  this.queue.push(msg);
  this.queue.reverse();
}
 
Queue.prototype.push_head = function(msg) {
  this.queue.push(msg);
}

Queue.prototype.pop = function() {
  return this.queue.pop();
} 

Queue.prototype.is_empty = function() {
  return this.queue.length == 0;
}

var current_process;
var process_queue = new Queue();
var pid_count = 0;

function Process(thunk) {
  var p = this;
  p.pid = ++pid_count;
  p.cont = function() { current_process = p; thunk(); current_process = undefined };
  p.mailbox = new Queue();
  p.blocked = false;
}

/* Send a message from the current process to another process.
   Reschedules itself if it was blocked waiting for a message. */
function send(process, msg) {
  if(process.blocked) {
    process.blocked = false;
    schedule(process);
  }

  process.mailbox.push_tail(msg);  
  concede->();
}

function block_if_no_messages(k) {
  if(current_process.mailbox.is_empty()) {
    var p = current_process;
    p.blocked = true;
    p.cont = function() { current_process = p; k(); current_process = undefined; }
    next_process();
  }
}

function receive() {
  block_if_no_messages->();
  var msg = current_process.mailbox.pop();
  return msg;  
}

function schedule(process) {
  process_queue.push_tail(process);
  start_scheduler();
}

function unschedule() {
  return process_queue.pop();
}

function concede(k) {
  var p = current_process;
  p.cont = function() { current_process = p; k(); current_process = undefined; }
  schedule(p);
  next_process();
}


function sleep(ms,cont) {
  var s = 10;
  while(s < ms) {
    concede->();
    s += 10;
  }
}

function start(processes) {
  for(var i = 0; i < processes.length; ++i) {
    process_queue.push_head(processes[i]);
  }
  start_scheduler();
}

function spawn(func) {
  var p = new Process(func);
  start([p]);
  return p;
}

var scheduler_started = false;

function start_scheduler() {
  if(!scheduler_started) {
    scheduler_started = true;
    while(!process_queue.is_empty()) {
      next_process();
    }
  }
  scheduler_started = false;
}

function next_process() {
  var p = unschedule();
  if(p) {
    var k = p.cont;
    if(k) {
      setTimeout(k, 10);
    }
  }
}