/**
 Docs: http://learn.javascript.ru/tutorial/lib
*/

function animate(opts) {

  var start = new Date;
  var delta = opts.delta || linear;

  var timer = setInterval(function() {
    var progress = (new Date - start) / opts.duration;

    if (progress > 1) progress = 1;

    opts.step( delta(progress) );

    if (progress == 1) {
      clearInterval(timer);
      opts.complete && opts.complete();
    }
  }, opts.delay || 13);

  return timer;
}

/**
 Анимация стиля opts.prop у элемента opts.elem
 от opts.from до opts.to продолжительностью opts.duration
 в конце opts.complete

 Например: animateProp({ elem: ..., prop: 'height', start:0, end: 100, duration: 1000 })
*/
function animateProp(opts) {
  var start = opts.start, end = opts.end, prop = opts.prop;

  opts.step = function(delta) {
    var value = Math.round(start + (end - start)*delta);
    opts.elem.style[prop] = value + 'px';
  }
  return animate(opts);
}

// ------------------ Delta ------------------

function elastic(progress) {
  return Math.pow(2, 10 * (progress-1)) * Math.cos(20*Math.PI*1.5/3*progress);
}

function linear(progress) {
  return progress;
}

function quad(progress) {
  return Math.pow(progress, 2);
}

function quint(progress) {
  return Math.pow(progress, 5);
}

function circ(progress) {
  return 1 - Math.sin(Math.acos(progress));
}

function back(progress) {
  return Math.pow(progress, 2) * ((1.5 + 1) * progress - 1.5);
}


function bounce(progress) {
  for(var a = 0, b = 1, result; 1; a += b, b /= 2) {
    if (progress >= (7 - 4 * a) / 11) {
      return -Math.pow((11 - 6 * a - 11 * progress) / 4, 2) + Math.pow(b, 2);
    }
  }
}

function makeEaseInOut(delta) {
  return function(progress) {
    if (progress < .5)
      return delta(2*progress) / 2;
    else
      return (2 - delta(2*(1-progress))) / 2;
  }
}


function makeEaseOut(delta) {
  return function(progress) {
    return 1 - delta(1 - progress);
  }
}