/**
 * @ngdoc module
 * @name material.components.virtualRepeat
 */
angular.module('material.components.virtualRepeat', [
  'material.core'
])
.directive('mdVirtualRepeatContainer', VirtualRepeatContainerDirective)
.directive('mdVirtualRepeat', VirtualRepeatDirective);


/**
 * @ngdoc directive
 * @name mdVirtualRepeatContainer
 * @module material.components.virtualRepeat
 * @restrict E
 * @description
 * `md-virtual-repeat-container` provides the scroll container for md-virtual-repeat.
 *
 * Virtual repeat is a limited substitute for ng-repeat that renders only
 * enough dom nodes to fill the container and recycling them as the user scrolls.
 *
 * @usage
 * <hljs lang="html">
 *
 * <md-virtual-repeat-container>
 *   <div md-virtual-repeat="i in items" md-item-size="20">Hello {{i}}!</div>
 * </md-virtual-repeat-container>
 * </hljs>
 *
 * @param {boolean=} md-orient-horizontal Whether the container should scroll horizontally
 *     (defaults to orientation and scrolling vertically).
 * @param {boolean=} md-auto-shrink When present, the container will shrink to fit
 *     the number of items when that number is less than its original size.
 * @param {number=} md-auto-shrink-min Minimum number of items that md-auto-shrink
 *     will shrink to (default: 0).
 */
function VirtualRepeatContainerDirective() {
  return {
    controller: VirtualRepeatContainerController,
    template: virtualRepeatContainerTemplate,
    compile: function virtualRepeatContainerCompile($element, $attrs) {
      $element
          .addClass('md-virtual-repeat-container')
          .addClass($attrs.hasOwnProperty('mdOrientHorizontal')
              ? 'md-orient-horizontal'
              : 'md-orient-vertical');
    }
  };
}


function virtualRepeatContainerTemplate($element) {
  return '<div class="md-virtual-repeat-scroller">' +
    '<div class="md-virtual-repeat-sizer"></div>' +
    '<div class="md-virtual-repeat-offsetter">' +
      $element[0].innerHTML +
    '</div></div>';
}

/**
 * Maximum size, in pixels, that can be explicitly set to an element. The actual value varies
 * between browsers, but IE11 has the very lowest size at a mere 1,533,917px. Ideally we could
 * *compute* this value, but Firefox always reports an element to have a size of zero if it
 * goes over the max, meaning that we'd have to binary search for the value.
 * @const {number}
 */
var MAX_ELEMENT_SIZE = 1533917;

/**
 * Number of additional elements to render above and below the visible area inside
 * of the virtual repeat container. A higher number results in less flicker when scrolling
 * very quickly in Safari, but comes with a higher rendering and dirty-checking cost.
 * @const {number}
 */
var NUM_EXTRA = 3;

/** @ngInject */
function VirtualRepeatContainerController($$rAF, $scope, $element, $attrs) {
  this.$scope = $scope;
  this.$element = $element;
  this.$attrs = $attrs;

  /** @type {number} The width or height of the container */
  this.size = 0;
  /** @type {number} The scroll width or height of the scroller */
  this.scrollSize = 0;
  /** @type {number} The scrollLeft or scrollTop of the scroller */
  this.scrollOffset = 0;
  /** @type {boolean} Whether the scroller is oriented horizontally */
  this.horizontal = this.$attrs.hasOwnProperty('mdOrientHorizontal');
  /** @type {!VirtualRepeatController} The repeater inside of this container */
  this.repeater = null;
  /** @type {boolean} Whether auto-shrink is enabled */
  this.autoShrink = this.$attrs.hasOwnProperty('mdAutoShrink');
  /** @type {number} Minimum number of items to auto-shrink to */
  this.autoShrinkMin = parseInt(this.$attrs.mdAutoShrinkMin, 10) || 0;
  /** @type {?number} Original container size when shrank */
  this.originalSize = null;
  /** @type {number} Amount to offset the total scroll size by. */
  this.offsetSize = parseInt(this.$attrs.mdOffsetSize, 10) || 0;


  this.scroller = $element[0].getElementsByClassName('md-virtual-repeat-scroller')[0];
  this.sizer = this.scroller.getElementsByClassName('md-virtual-repeat-sizer')[0];
  this.offsetter = this.scroller.getElementsByClassName('md-virtual-repeat-offsetter')[0];

  $$rAF(angular.bind(this, this.updateSize));

  // TODO: Come up with a more robust (But hopefully also quick!) way of
  // detecting that we're not visible.
  if ($attrs.ngHide) {
    $scope.$watch($attrs.ngHide, angular.bind(this, function(hidden) {
      if (!hidden) {
        $$rAF(angular.bind(this, this.updateSize));
      }
    }));
  }
}


/** Called by the md-virtual-repeat inside of the container at startup. */
VirtualRepeatContainerController.prototype.register = function(repeaterCtrl) {
  this.repeater = repeaterCtrl;

  angular.element(this.scroller)
      .on('scroll wheel touchmove touchend', angular.bind(this, this.handleScroll_));
};


/** @return {boolean} Whether the container is configured for horizontal scrolling. */
VirtualRepeatContainerController.prototype.isHorizontal = function() {
  return this.horizontal;
};


/** @return {number} The size (width or height) of the container. */
VirtualRepeatContainerController.prototype.getSize = function() {
  return this.size;
};


/**
 * Resizes the container.
 * @private
 * @param {number} The new size to set.
 */
VirtualRepeatContainerController.prototype.setSize_ = function(size) {
  this.size = size;
  this.$element[0].style[this.isHorizontal() ? 'width' : 'height'] = size + 'px';
};


/** Instructs the container to re-measure its size. */
VirtualRepeatContainerController.prototype.updateSize = function() {
  if (this.originalSize) return;

  this.size = this.isHorizontal()
      ? this.$element[0].clientWidth
      : this.$element[0].clientHeight;
  this.repeater && this.repeater.containerUpdated();
};


/** @return {number} The container's scrollHeight or scrollWidth. */
VirtualRepeatContainerController.prototype.getScrollSize = function() {
  return this.scrollSize;
};


/**
 * Sets the scroller element to the specified size.
 * @private
 * @param {number} size The new size.
 */
VirtualRepeatContainerController.prototype.sizeScroller_ = function(size) {
  var dimension =  this.isHorizontal() ? 'width' : 'height';
  var crossDimension = this.isHorizontal() ? 'height' : 'width';

  // If the size falls within the browser's maximum explicit size for a single element, we can
  // set the size and be done. Otherwise, we have to create children that add up the the desired
  // size.
  if (size < MAX_ELEMENT_SIZE) {
    this.sizer.style[dimension] = size + 'px';
  } else {
    // Clear any existing dimensions.
    this.sizer.innerHTML = '';
    this.sizer.style[dimension] = 'auto';
    this.sizer.style[crossDimension] = 'auto';

    // Divide the total size we have to render into N max-size pieces.
    var numChildren = Math.floor(size / MAX_ELEMENT_SIZE);

    // Element template to clone for each max-size piece.
    var sizerChild = document.createElement('div');
    sizerChild.style[dimension] = MAX_ELEMENT_SIZE + 'px';
    sizerChild.style[crossDimension] = '1px';

    for (var i = 0; i < numChildren; i++) {
      this.sizer.appendChild(sizerChild.cloneNode(false));
    }

    // Re-use the element template for the remainder.
    sizerChild.style[dimension] = (size - (numChildren * MAX_ELEMENT_SIZE)) + 'px';
    this.sizer.appendChild(sizerChild);
  }
};


/**
 * If auto-shrinking is enabled, shrinks or unshrinks as appropriate.
 * @private
 * @param {number} size The new size.
 */
VirtualRepeatContainerController.prototype.autoShrink_ = function(size) {
  var shrinkSize = Math.max(size, this.autoShrinkMin * this.repeater.getItemSize());
  if (this.autoShrink && shrinkSize !== this.size) {
    if (shrinkSize < (this.originalSize || this.size)) {
      if (!this.originalSize) {
        this.originalSize = this.size;
      }

      this.setSize_(shrinkSize);
    } else if (this.originalSize) {
      this.setSize_(this.originalSize);
      this.originalSize = null;
    }
  }
};


/**
 * Sets the scrollHeight or scrollWidth. Called by the repeater based on
 * its item count and item size.
 * @param {number} itemsSize The total size of the items.
 */
VirtualRepeatContainerController.prototype.setScrollSize = function(itemsSize) {
  var size = itemsSize + this.offsetSize;
  if (this.scrollSize === size) return;

  this.sizeScroller_(size);
  this.autoShrink_(size);
  this.scrollSize = size;
};


/** @return {number} The container's current scroll offset. */
VirtualRepeatContainerController.prototype.getScrollOffset = function() {
  return this.scrollOffset;
};

/**
 * Scrolls to a given scrollTop position.
 * @param {number} position
 */
VirtualRepeatContainerController.prototype.scrollTo = function(position) {
  this.scroller[this.isHorizontal() ? 'scrollLeft' : 'scrollTop'] = position;
  this.handleScroll_();
};

VirtualRepeatContainerController.prototype.resetScroll = function() {
  this.scrollTo(0);
};


VirtualRepeatContainerController.prototype.handleScroll_ = function() {
  var offset = this.isHorizontal() ? this.scroller.scrollLeft : this.scroller.scrollTop;
  if (offset === this.scrollOffset) return;

  var itemSize = this.repeater.getItemSize();
  if (!itemSize) return;

  var numItems = Math.max(0, Math.floor(offset / itemSize) - NUM_EXTRA);

  var transform = this.isHorizontal() ? 'translateX(' : 'translateY(';
      transform +=  (numItems * itemSize) + 'px)';

  this.scrollOffset = offset;
  this.offsetter.style.webkitTransform = transform;
  this.offsetter.style.transform = transform;

  this.repeater.containerUpdated();
};


/**
 * @ngdoc directive
 * @name mdVirtualRepeat
 * @module material.components.virtualRepeat
 * @restrict A
 * @priority 1000
 * @description
 * `md-virtual-repeat` specifies an element to repeat using virtual scrolling.
 *
 * Virtual repeat is a limited substitute for ng-repeat that renders only
 * enough dom nodes to fill the container and recycling them as the user scrolls.
 * Arrays, but not objects are supported for iteration.
 * Track by, as alias, and (key, value) syntax are not supported.
 *
 * @usage
 * <hljs lang="html">
 * <md-virtual-repeat-container>
 *   <div md-virtual-repeat="i in items">Hello {{i}}!</div>
 * </md-virtual-repeat-container>
 *
 * <md-virtual-repeat-container md-orient-horizontal>
 *   <div md-virtual-repeat="i in items" md-item-size="20">Hello {{i}}!</div>
 * </md-virtual-repeat-container>
 * </hljs>
 *
 * @param {number=} md-item-size The height or width of the repeated elements (which
 *     must be identical for each element). Optional. Will attempt to read the size
 *     from the dom if missing, but still assumes that all repeated nodes have same
 *     height or width.
 * @param {string=} md-extra-name Evaluates to an additional name to which
 *     the current iterated item can be assigned on the repeated scope. (Needed
 *     for use in md-autocomplete).
 * @param {boolean=} md-on-demand When present, treats the md-virtual-repeat argument
 *     as an object that can fetch rows rather than an array.
 *     NOTE: This object must implement the following interface with two (2) methods:
 *     getItemAtIndex: function(index) -> item at that index or null if it is not yet
 *         loaded (It should start downloading the item in that case).
 *     getLength: function() -> number The data legnth to which the repeater container
 *         should be sized. Ideally, when the count is known, this method should return it.
 *         Otherwise, return a higher number than the currently loaded items to produce an
 *         infinite-scroll behavior.
 */
function VirtualRepeatDirective($parse) {
  return {
    controller: VirtualRepeatController,
    priority: 1000,
    require: ['mdVirtualRepeat', '^^mdVirtualRepeatContainer'],
    restrict: 'A',
    terminal: true,
    transclude: 'element',
    compile: function VirtualRepeatCompile($element, $attrs) {
      var expression = $attrs.mdVirtualRepeat;
      var match = expression.match(/^\s*([\s\S]+?)\s+in\s+([\s\S]+?)\s*$/);
      var repeatName = match[1];
      var repeatListExpression = $parse(match[2]);
      var extraName = $attrs.mdExtraName && $parse($attrs.mdExtraName);

      return function VirtualRepeatLink($scope, $element, $attrs, ctrl, $transclude) {
        ctrl[0].link_(ctrl[1], $transclude, repeatName, repeatListExpression, extraName);
      };
    }
  };
}


/** @ngInject */
function VirtualRepeatController($scope, $element, $attrs, $browser, $document, $$rAF) {
  this.$scope = $scope;
  this.$element = $element;
  this.$attrs = $attrs;
  this.$browser = $browser;
  this.$document = $document;
  this.$$rAF = $$rAF;

  /** @type {boolean} Whether we are in on-demand mode. */
  this.onDemand = $attrs.hasOwnProperty('mdOnDemand');
  /** @type {!Function} Backup reference to $browser.$$checkUrlChange */
  this.browserCheckUrlChange = $browser.$$checkUrlChange;
  /** @type {number} Most recent starting repeat index (based on scroll offset) */
  this.newStartIndex = 0;
  /** @type {number} Most recent ending repeat index (based on scroll offset) */
  this.newEndIndex = 0;
  /** @type {number} Most recent end visible index (based on scroll offset) */
  this.newVisibleEnd = 0;
  /** @type {number} Previous starting repeat index (based on scroll offset) */
  this.startIndex = 0;
  /** @type {number} Previous ending repeat index (based on scroll offset) */
  this.endIndex = 0;
  // TODO: measure width/height of first element from dom if not provided.
  // getComputedStyle?
  /** @type {?number} Height/width of repeated elements. */
  this.itemSize = $scope.$eval($attrs.mdItemSize) || null;

  /** @type {boolean} Whether this is the first time that items are rendered. */
  this.isFirstRender = true;

  /** @type {number} Most recently seen length of items. */
  this.itemsLength = 0;

  /**
   * @type {!Function} Unwatch callback for item size (when md-items-size is
   *     not specified), or angular.noop otherwise.
   */
  this.unwatchItemSize_ = angular.noop;

  /**
   * Presently rendered blocks by repeat index.
   * @type {Object<number, !VirtualRepeatController.Block}
   */
  this.blocks = {};
  /** @type {Array<!VirtualRepeatController.Block>} A pool of presently unused blocks. */
  this.pooledBlocks = [];
}


/**
 * An object representing a repeated item.
 * @typedef {{element: !jqLite, new: boolean, scope: !angular.Scope}}
 */
VirtualRepeatController.Block;


/**
 * Called at startup by the md-virtual-repeat postLink function.
 * @param {!VirtualRepeatContainerController} container The container's controller.
 * @param {!Function} transclude The repeated element's bound transclude function.
 * @param {string} repeatName The left hand side of the repeat expression, indicating
 *     the name for each item in the array.
 * @param {!Function} repeatListExpression A compiled expression based on the right hand side
 *     of the repeat expression. Points to the array to repeat over.
 * @param {string|undefined} extraName The optional extra repeatName.
 */
VirtualRepeatController.prototype.link_ =
    function(container, transclude, repeatName, repeatListExpression, extraName) {
  this.container = container;
  this.transclude = transclude;
  this.repeatName = repeatName;
  this.rawRepeatListExpression = repeatListExpression;
  this.extraName = extraName;
  this.sized = false;

  this.repeatListExpression = angular.bind(this, this.repeatListExpression_);

  this.container.register(this);
};


/** @private Attempts to set itemSize by measuring a repeated element in the dom */
VirtualRepeatController.prototype.readItemSize_ = function() {
  if (this.itemSize) {
    // itemSize was successfully read in a different asynchronous call.
    return;
  }

  this.items = this.repeatListExpression(this.$scope);
  this.parentNode = this.$element[0].parentNode;
  var block = this.getBlock_(0);
  if (!block.element[0].parentNode) {
    this.parentNode.appendChild(block.element[0]);
  }

  this.itemSize = block.element[0][
      this.container.isHorizontal() ? 'offsetWidth' : 'offsetHeight'] || null;

  this.blocks[0] = block;
  this.poolBlock_(0);

  if (this.itemSize) {
    this.containerUpdated();
  }
};


/**
 * Returns the user-specified repeat list, transforming it into an array-like
 * object in the case of infinite scroll/dynamic load mode.
 * @param {!angular.Scope} The scope.
 * @return {!Array|!Object} An array or array-like object for iteration.
 */
VirtualRepeatController.prototype.repeatListExpression_ = function(scope) {
  var repeatList = this.rawRepeatListExpression(scope);

  if (this.onDemand && repeatList) {
    var virtualList = new VirtualRepeatModelArrayLike(repeatList);
    virtualList.$$includeIndexes(this.newStartIndex, this.newVisibleEnd);
    return virtualList;
  } else {
    return repeatList;
  }
};


/**
 * Called by the container. Informs us that the containers scroll or size has
 * changed.
 */
VirtualRepeatController.prototype.containerUpdated = function() {
  // If itemSize is unknown, attempt to measure it.
  if (!this.itemSize) {
    this.unwatchItemSize_ = this.$scope.$watchCollection(
        this.repeatListExpression,
        angular.bind(this, function(items) {
          if (items && items.length) {
            this.$$rAF(angular.bind(this, this.readItemSize_));
          }
        }));
    if (!this.$scope.$root.$$phase) this.$scope.$digest();

    return;
  } else if (!this.sized) {
    this.items = this.repeatListExpression(this.$scope);
  }

  if (!this.sized) {
    this.unwatchItemSize_();
    this.sized = true;
    this.$scope.$watchCollection(this.repeatListExpression,
        angular.bind(this, this.virtualRepeatUpdate_));
  }

  this.updateIndexes_();

  if (this.newStartIndex !== this.startIndex ||
      this.newEndIndex !== this.endIndex ||
      this.container.getScrollOffset() > this.container.getScrollSize()) {
    if (this.items instanceof VirtualRepeatModelArrayLike) {
      this.items.$$includeIndexes(this.newStartIndex, this.newEndIndex);
    }
    this.virtualRepeatUpdate_(this.items, this.items);
  }
};


/**
 * Called by the container. Returns the size of a single repeated item.
 * @return {?number} Size of a repeated item.
 */
VirtualRepeatController.prototype.getItemSize = function() {
  return this.itemSize;
};


/**
 * Updates the order and visible offset of repeated blocks in response to scrolling
 * or items updates.
 * @private
 */
VirtualRepeatController.prototype.virtualRepeatUpdate_ = function(items, oldItems) {
  var itemsLength = items ? items.length : 0;
  var lengthChanged = false;

  if (itemsLength !== this.itemsLength) {
    lengthChanged = true;
    this.itemsLength = itemsLength;
  }

  // If the number of items shrank, scroll up to the top.
  if (this.items && itemsLength < this.items.length && this.container.getScrollOffset() !== 0) {
    this.items = items;
    this.container.resetScroll();
    return;
  }

  this.items = items;
  if (items !== oldItems || lengthChanged) {
    this.updateIndexes_();
  }

  this.parentNode = this.$element[0].parentNode;

  if (lengthChanged) {
    this.container.setScrollSize(itemsLength * this.itemSize);
  }

  if (this.isFirstRender) {
    this.isFirstRender = false;
    var startIndex = this.$attrs.mdStartIndex ? this.$scope.$eval(this.$attrs.mdStartIndex) : 0;
    this.container.scrollTo(startIndex * this.itemSize);
  }

  // Detach and pool any blocks that are no longer in the viewport.
  Object.keys(this.blocks).forEach(function(blockIndex) {
    var index = parseInt(blockIndex, 10);
    if (index < this.newStartIndex || index >= this.newEndIndex) {
      this.poolBlock_(index);
    }
  }, this);

  // Add needed blocks.
  // For performance reasons, temporarily block browser url checks as we digest
  // the restored block scopes ($$checkUrlChange reads window.location to
  // check for changes and trigger route change, etc, which we don't need when
  // trying to scroll at 60fps).
  this.$browser.$$checkUrlChange = angular.noop;

  var i, block,
      newStartBlocks = [],
      newEndBlocks = [];

  // Collect blocks at the top.
  for (i = this.newStartIndex; i < this.newEndIndex && this.blocks[i] == null; i++) {
    block = this.getBlock_(i);
    this.updateBlock_(block, i);
    newStartBlocks.push(block);
  }

  // Update blocks that are already rendered.
  for (; this.blocks[i] != null; i++) {
    this.updateBlock_(this.blocks[i], i);
  }
  var maxIndex = i - 1;

  // Collect blocks at the end.
  for (; i < this.newEndIndex; i++) {
    block = this.getBlock_(i);
    this.updateBlock_(block, i);
    newEndBlocks.push(block);
  }

  // Attach collected blocks to the document.
  if (newStartBlocks.length) {
    this.parentNode.insertBefore(
        this.domFragmentFromBlocks_(newStartBlocks),
        this.$element[0].nextSibling);
  }
  if (newEndBlocks.length) {
    this.parentNode.insertBefore(
        this.domFragmentFromBlocks_(newEndBlocks),
        this.blocks[maxIndex] && this.blocks[maxIndex].element[0].nextSibling);
  }

  // Restore $$checkUrlChange.
  this.$browser.$$checkUrlChange = this.browserCheckUrlChange;

  this.startIndex = this.newStartIndex;
  this.endIndex = this.newEndIndex;
};


/**
 * @param {number} index Where the block is to be in the repeated list.
 * @return {!VirtualRepeatController.Block} A new or pooled block to place at the specified index.
 * @private
 */
VirtualRepeatController.prototype.getBlock_ = function(index) {
  if (this.pooledBlocks.length) {
    return this.pooledBlocks.pop();
  }

  var block;
  this.transclude(angular.bind(this, function(clone, scope) {
    block = {
      element: clone,
      new: true,
      scope: scope
    };

    this.updateScope_(scope, index);
    this.parentNode.appendChild(clone[0]);
  }));

  return block;
};


/**
 * Updates and if not in a digest cycle, digests the specified block's scope to the data
 * at the specified index.
 * @param {!VirtualRepeatController.Block} block The block whose scope should be updated.
 * @param {number} index The index to set.
 * @private
 */
VirtualRepeatController.prototype.updateBlock_ = function(block, index) {
  this.blocks[index] = block;

  if (!block.new &&
      (block.scope.$index === index && block.scope[this.repeatName] === this.items[index])) {
    return;
  }
  block.new = false;

  // Update and digest the block's scope.
  this.updateScope_(block.scope, index);

  // Perform digest before reattaching the block.
  // Any resulting synchronous dom mutations should be much faster as a result.
  // This might break some directives, but I'm going to try it for now.
  if (!this.$scope.$root.$$phase) {
    block.scope.$digest();
  }
};


/**
 * Updates scope to the data at the specified index.
 * @param {!angular.Scope} scope The scope which should be updated.
 * @param {number} index The index to set.
 * @private
 */
VirtualRepeatController.prototype.updateScope_ = function(scope, index) {
  scope.$index = index;
  scope[this.repeatName] = this.items && this.items[index];
  if (this.extraName) scope[this.extraName(this.$scope)] = this.items[index];
};


/**
 * Pools the block at the specified index (Pulls its element out of the dom and stores it).
 * @param {number} index The index at which the block to pool is stored.
 * @private
 */
VirtualRepeatController.prototype.poolBlock_ = function(index) {
  this.pooledBlocks.push(this.blocks[index]);
  this.parentNode.removeChild(this.blocks[index].element[0]);
  delete this.blocks[index];
};


/**
 * Produces a dom fragment containing the elements from the list of blocks.
 * @param {!Array<!VirtualRepeatController.Block>} blocks The blocks whose elements
 *     should be added to the document fragment.
 * @return {DocumentFragment}
 * @private
 */
VirtualRepeatController.prototype.domFragmentFromBlocks_ = function(blocks) {
  var fragment = this.$document[0].createDocumentFragment();
  blocks.forEach(function(block) {
    fragment.appendChild(block.element[0]);
  });
  return fragment;
};


/**
 * Updates start and end indexes based on length of repeated items and container size.
 * @private
 */
VirtualRepeatController.prototype.updateIndexes_ = function() {
  var itemsLength = this.items ? this.items.length : 0;
  var containerLength = Math.ceil(this.container.getSize() / this.itemSize);

  this.newStartIndex = Math.max(0, Math.min(
      itemsLength - containerLength,
      Math.floor(this.container.getScrollOffset() / this.itemSize)));
  this.newVisibleEnd = this.newStartIndex + containerLength + NUM_EXTRA;
  this.newEndIndex = Math.min(itemsLength, this.newVisibleEnd);
  this.newStartIndex = Math.max(0, this.newStartIndex - NUM_EXTRA);
};

/**
 * This VirtualRepeatModelArrayLike class enforces the interface requirements
 * for infinite scrolling within a mdVirtualRepeatContainer. An object with this
 * interface must implement the following interface with two (2) methods:
 *
 * getItemAtIndex: function(index) -> item at that index or null if it is not yet
 *     loaded (It should start downloading the item in that case).
 *
 * getLength: function() -> number The data legnth to which the repeater container
 *     should be sized. Ideally, when the count is known, this method should return it.
 *     Otherwise, return a higher number than the currently loaded items to produce an
 *     infinite-scroll behavior.
 *
 * @usage
 * <hljs lang="html">
 *  <md-virtual-repeat-container md-orient-horizontal>
 *    <div md-virtual-repeat="i in items" md-on-demand>
 *      Hello {{i}}!
 *    </div>
 *  </md-virtual-repeat-container>
 * </hljs>
 *
 */
function VirtualRepeatModelArrayLike(model) {
  if (!angular.isFunction(model.getItemAtIndex) ||
      !angular.isFunction(model.getLength)) {
    throw Error('When md-on-demand is enabled, the Object passed to md-virtual-repeat must implement ' +
        'functions getItemAtIndex() and getLength() ');
  }

  this.model = model;
}


VirtualRepeatModelArrayLike.prototype.$$includeIndexes = function(start, end) {
  for (var i = start; i < end; i++) {
    if (!this.hasOwnProperty(i)) {
      this[i] = this.model.getItemAtIndex(i);
    }
  }
  this.length = this.model.getLength();
};


function abstractMethod() {
  throw Error('Non-overridden abstract method called.');
}
