/*global define*/
define([
'../Core/defined'
], function(
defined) {
'use strict';
/**
* A priority queue of tiles to be replaced, if necessary, to make room for new tiles. The queue
* is implemented as a linked list.
*
* @alias TileReplacementQueue
* @private
*/
function TileReplacementQueue() {
this.head = undefined;
this.tail = undefined;
this.count = 0;
this._lastBeforeStartOfFrame = undefined;
}
/**
* Marks the start of the render frame. Tiles before (closer to the head) this tile in the
* list were used last frame and must not be unloaded.
*/
TileReplacementQueue.prototype.markStartOfRenderFrame = function() {
this._lastBeforeStartOfFrame = this.head;
};
/**
* Reduces the size of the queue to a specified size by unloading the least-recently used
* tiles. Tiles that were used last frame will not be unloaded, even if that puts the number
* of tiles above the specified maximum.
*
* @param {Number} maximumTiles The maximum number of tiles in the queue.
*/
TileReplacementQueue.prototype.trimTiles = function(maximumTiles) {
var tileToTrim = this.tail;
var keepTrimming = true;
while (keepTrimming &&
defined(this._lastBeforeStartOfFrame) &&
this.count > maximumTiles &&
defined(tileToTrim)) {
// Stop trimming after we process the last tile not used in the
// current frame.
keepTrimming = tileToTrim !== this._lastBeforeStartOfFrame;
var previous = tileToTrim.replacementPrevious;
if (tileToTrim.eligibleForUnloading) {
tileToTrim.freeResources();
remove(this, tileToTrim);
}
tileToTrim = previous;
}
};
function remove(tileReplacementQueue, item) {
var previous = item.replacementPrevious;
var next = item.replacementNext;
if (item === tileReplacementQueue._lastBeforeStartOfFrame) {
tileReplacementQueue._lastBeforeStartOfFrame = next;
}
if (item === tileReplacementQueue.head) {
tileReplacementQueue.head = next;
} else {
previous.replacementNext = next;
}
if (item === tileReplacementQueue.tail) {
tileReplacementQueue.tail = previous;
} else {
next.replacementPrevious = previous;
}
item.replacementPrevious = undefined;
item.replacementNext = undefined;
--tileReplacementQueue.count;
}
/**
* Marks a tile as rendered this frame and moves it before the first tile that was not rendered
* this frame.
*
* @param {TileReplacementQueue} item The tile that was rendered.
*/
TileReplacementQueue.prototype.markTileRendered = function(item) {
var head = this.head;
if (head === item) {
if (item === this._lastBeforeStartOfFrame) {
this._lastBeforeStartOfFrame = item.replacementNext;
}
return;
}
++this.count;
if (!defined(head)) {
// no other tiles in the list
item.replacementPrevious = undefined;
item.replacementNext = undefined;
this.head = item;
this.tail = item;
return;
}
if (defined(item.replacementPrevious) || defined(item.replacementNext)) {
// tile already in the list, remove from its current location
remove(this, item);
}
item.replacementPrevious = undefined;
item.replacementNext = head;
head.replacementPrevious = item;
this.head = item;
};
return TileReplacementQueue;
});