/*! echo v1.5.0 | (c) 2014 @toddmotto | mit license | github.com/toddmotto/echo */
(function (root, factory) {
  if (typeof define === 'function' && define.amd) {
    define(factory);
  } else if (typeof exports === 'object') {
    module.exports = factory;
  } else {
    root.echo = factory(root);
  }
})(this, function (root) {

  'use strict';

  var exports = {};

  var callback = function () {};

  var offset, poll, throttle, unload;

  var inview = function (element, view) {
    var box = element.getboundingclientrect();
    return (box.right >= view.l && box.bottom >= view.t && box.left <= view.r && box.top <= view.b);
  };

  var debounce = function () {
    cleartimeout(poll);
    poll = settimeout(exports.render, throttle);
  };

  exports.init = function (opts) {
    opts = opts || {};
    var offsetall = opts.offset || 0;
    var offsetvertical = opts.offsetvertical || offsetall;
    var offsethorizontal = opts.offsethorizontal || offsetall;
    var optiontoint = function (opt, fallback) {
      return parseint(opt || fallback, 10);
    };
    offset = {
      t: optiontoint(opts.offsettop, offsetvertical),
      b: optiontoint(opts.offsetbottom, offsetvertical),
      l: optiontoint(opts.offsetleft, offsethorizontal),
      r: optiontoint(opts.offsetright, offsethorizontal)
    };
    throttle = optiontoint(opts.throttle, 250);
    unload = !!opts.unload;
    callback = opts.callback || callback;
    exports.render();
    if (document.addeventlistener) {
      root.addeventlistener('scroll', debounce, false);
      root.addeventlistener('load', debounce, false);
    } else {
      root.attachevent('onscroll', debounce);
      root.attachevent('onload', debounce);
    }
  };

  exports.render = function () {
    var nodes = document.queryselectorall('img[data-echo]');
    var length = nodes.length;
    var src, elem;
    var view = {
      l: 0 - offset.l,
      t: 0 - offset.t,
      b: (root.innerheight || document.documentelement.clientheight) + offset.b,
      r: (root.innerwidth || document.documentelement.clientwidth) + offset.r
    };
    for (var i = 0; i < length; i++) {
      elem = nodes[i];
      if (inview(elem, view)) {
        if (unload) {
          elem.setattribute('data-echo-placeholder', elem.src);
        }
        elem.src = elem.getattribute('data-echo');
        if (!unload) {
          elem.removeattribute('data-echo');
        }
        callback(elem, 'load');
      } else if (unload && !!(src = elem.getattribute('data-echo-placeholder'))) {
        elem.src = src;
        elem.removeattribute('data-echo-placeholder');
        callback(elem, 'unload');
      }
    }
    if (!length) {
      exports.detach();
    }
  };

  exports.detach = function () {
    if (document.removeeventlistener) {
      root.removeeventlistener('scroll', debounce);
    } else {
      root.detachevent('onscroll', debounce);
    }
    cleartimeout(poll);
  };

  return exports;

});