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var d3_timer_id = 0,
    d3_timer_byId = {},
    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) {
  if (arguments.length < 3) {
    if (arguments.length < 2) delay = 0;
    else if (!isFinite(delay)) return;
    then = Date.now();
  }

  // If the callback's already in the queue, update it.
  var timer = d3_timer_byId[callback.id];
  if (timer && timer.callback === callback) {
    timer.then = then;
    timer.delay = delay;
  }

  // Otherwise, add the callback to the queue.
  else d3_timer_byId[callback.id = ++d3_timer_id] = 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) {
      delete d3_timer_byId[t1.callback.id];
      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); };
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