var d3_timer_queue = null,

    d3_timer_interval, // is an interval (or frame) active?

    d3_timer_timeout; // is a timeout active?

// The timer will continue to fire until callback returns true.

d3.timer = function(callback, delay, then) {

  var found = false,

      t0,

      t1 = d3_timer_queue;

  if (arguments.length < 3) {

    if (arguments.length < 2) delay = 0;

    else if (!isFinite(delay)) return;

    then = Date.now();

  }

  // See if the callback's already in the queue.

  while (t1) {

    if (t1.callback === callback) {

      t1.then = then;

      t1.delay = delay;

      found = true;

      break;

    }

    t0 = t1;

    t1 = t1.next;

  }

  // Otherwise, add the callback to the queue.

  if (!found) d3_timer_queue = {

    callback: callback,

    then: then,

    delay: delay,

    next: d3_timer_queue

  };

  // Start animatin'!

  if (!d3_timer_interval) {

    d3_timer_timeout = clearTimeout(d3_timer_timeout);

    d3_timer_interval = 1;

    d3_timer_frame(d3_timer_step);

  }

}

function d3_timer_step() {

  var elapsed,

      now = Date.now(),

      t1 = d3_timer_queue;

  while (t1) {

    elapsed = now - t1.then;

    if (elapsed >= t1.delay) t1.flush = t1.callback(elapsed);

    t1 = t1.next;

  }

  var delay = d3_timer_flush() - now;

  if (delay > 24) {

    if (isFinite(delay)) {

      clearTimeout(d3_timer_timeout);

      d3_timer_timeout = setTimeout(d3_timer_step, delay);

    }

    d3_timer_interval = 0;

  } else {

    d3_timer_interval = 1;

    d3_timer_frame(d3_timer_step);

  }

}

d3.timer.flush = function() {

  var elapsed,

      now = Date.now(),

      t1 = d3_timer_queue;

  while (t1) {

    elapsed = now - t1.then;

    if (!t1.delay) t1.flush = t1.callback(elapsed);

    t1 = t1.next;

  }

  d3_timer_flush();

};

// Flush after callbacks, to avoid concurrent queue modification.

function d3_timer_flush() {

  var t0 = null,

      t1 = d3_timer_queue,

      then = Infinity;

  while (t1) {

    if (t1.flush) {

      t1 = t0 ? t0.next = t1.next : d3_timer_queue = t1.next;

    } else {

      then = Math.min(then, t1.then + t1.delay);

      t1 = (t0 = t1).next;

    }

  }

  return then;

}

var d3_timer_frame = window.requestAnimationFrame

    || window.webkitRequestAnimationFrame

    || window.mozRequestAnimationFrame

    || window.oRequestAnimationFrame

    || window.msRequestAnimationFrame

    || function(callback) { setTimeout(callback, 17); };