gecko/mobile/chrome/content/TileManager.js.in

// -*- Mode: js2; tab-width: 2; indent-tabs-mode: nil; js2-basic-offset: 2; js2-skip-preprocessor-directives: t; -*-
/*
 * ***** BEGIN LICENSE BLOCK *****
 * Version: MPL 1.1/GPL 2.0/LGPL 2.1
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 * http://www.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is Mozilla Mobile Browser.
 *
 * The Initial Developer of the Original Code is
 * Mozilla Corporation.
 * Portions created by the Initial Developer are Copyright (C) 2009
 * the Initial Developer. All Rights Reserved.
 *
 * Contributor(s):
 *   Roy Frostig <rfrostig@mozilla.com>
 *   Stuart Parmenter <stuart@mozilla.com>
 *
 * Alternatively, the contents of this file may be used under the terms of
 * either the GNU General Public License Version 2 or later (the "GPL"), or
 * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
 * in which case the provisions of the GPL or the LGPL are applicable instead
 * of those above. If you wish to allow use of your version of this file only
 * under the terms of either the GPL or the LGPL, and not to allow others to
 * use your version of this file under the terms of the MPL, indicate your
 * decision by deleting the provisions above and replace them with the notice
 * and other provisions required by the GPL or the LGPL. If you do not delete
 * the provisions above, a recipient may use your version of this file under
 * the terms of any one of the MPL, the GPL or the LGPL.
 *
 * ***** END LICENSE BLOCK ***** */

#define BEGIN_FOREACH_IN_RECT(rect, tilecache, tile)                           \
  {                                                                            \
    let __starti = (rect).left  >> kTileExponentWidth;                         \
    let __endi   = (rect).right >> kTileExponentWidth;                         \
    let __startj = (rect).top    >> kTileExponentHeight;                       \
    let __endj   = (rect).bottom >> kTileExponentHeight;                       \
                                                                               \
    let tile = null;                                                           \
    let __i, __j;                                                              \
    for (__j = __startj; __j <= __endj; ++__j) {                               \
      for (__i = __starti; __i <= __endi; ++__i) {                             \
        tile = (tilecache).getTile(__i, __j, false, null);                     \
        if (tile) {

#define END_FOREACH_IN_RECT                                                    \
        }                                                                      \
      }                                                                        \
    }                                                                          \
  }

#define BEGIN_FORCREATE_IN_RECT(rect, tilecache, tile)                         \
  {                                                                            \
    let __starti = (rect).left  >> kTileExponentWidth;                         \
    let __endi   = (rect).right >> kTileExponentWidth;                         \
    let __startj = (rect).top    >> kTileExponentHeight;                       \
    let __endj   = (rect).bottom >> kTileExponentHeight;                       \
                                                                               \
    let tile = null;                                                           \
    let __i, __j;                                                              \
    for (__j = __startj; __j <= __endj; ++__j) {                               \
      for (__i = __starti; __i <= __endi; ++__i) {                             \
        tile = (tilecache).getTile(__i, __j, true, null);                      \
        if (tile) {                                                            \

#define END_FORCREATE_IN_RECT                                                  \
        }                                                                      \
      }                                                                        \
    }                                                                          \
  }

#define FOREACH_IN_RECT(rect, tilecache, fn, thisObj)                          \
  BEGIN_FOREACH_IN_RECT(rect, tilecache, tile)                                 \
  (fn).call((thisObj), tile);                                                  \
  END_FOREACH_IN_RECT

#define FORCREATE_IN_RECT(rect, tilecache, fn, thisObj)                        \
  BEGIN_FORCREATE_IN_RECT(rect, tilecache, tile)                               \
  (fn).call((thisObj), tile);                                                  \
  END_FORCREATE_IN_RECT


const kXHTMLNamespaceURI  = "http://www.w3.org/1999/xhtml";

// base-2 exponent for width, height of a single tile.
const kTileExponentWidth  = 9;
const kTileExponentHeight = 9;
const kTileWidth  = Math.pow(2, kTileExponentWidth);   // 2^9 = 512
const kTileHeight = Math.pow(2, kTileExponentHeight);  // 2^9 = 512
const kTileCrawlTimeCap = 100;    // millis
const kTileCrawlComeAgain = 0;    // millis

/**
 * The Tile Manager!
 *
 * @param appendTile The function the tile manager should call in order to
 * "display" a tile (e.g. append it to the DOM).  The argument to this
 * function is a TileManager.Tile object.
 * @param removeTile The function the tile manager should call in order to
 * "undisplay" a tile (e.g. remove it from the DOM).  The argument to this
 * function is a TileManager.Tile object.
 * @param fakeWidth The width of the widest possible visible rectangle, e.g.
 * the width of the screen.  This is used in setting the zoomLevel.
 */
function TileManager(appendTile, removeTile, browserView, cacheSize) {
  /* backref to the BrowserView object that owns us */
  this._browserView = browserView;

  /* callbacks to append / remove a tile to / from the parent */
  this._appendTile = appendTile;
  this._removeTile = removeTile;

  /* tile cache holds tile objects and pools them under a given capacity */
  let self = this;
  this._tileCache = new TileManager.TileCache(function(tile) { self._removeTileSafe(tile); },
                                              -1, -1, cacheSize);

  /* Rectangle within the viewport that is visible to the user.  It is "critical"
   * in the sense that it must be rendered as soon as it becomes dirty, and tiles
   * within this rectangle should not be evicted for use elsewhere. */
  this._criticalRect = new Rect(0, 0, 0, 0);

  /* timeout of the non-visible-tiles-crawler to cache renders from the browser */
  this._idleTileCrawlerTimeout = 0;

  /* object that keeps state on our current prefetch crawl */
  this._crawler = null;

  /* remember these values to reduce the recenterEvictionQueue cost */
  this._ctr = new Point(0, 0);
}

TileManager.prototype = {

  /**
   * Entry point by which the BrowserView informs of changes to the viewport or
   * critical rect.
   */
  viewportChangeHandler: function viewportChangeHandler(viewportRect,
                                                        criticalRect,
                                                        boundsSizeChanged,
                                                        dirtyAll) {
    let tc = this._tileCache;

    tc.iBound = Math.ceil(viewportRect.right / kTileWidth);
    tc.jBound = Math.ceil(viewportRect.bottom / kTileHeight);

    if (criticalRect.isEmpty() || !criticalRect.equals(this._criticalRect)) {
      this.beginCriticalMove(criticalRect);
      this.endCriticalMove(criticalRect, !boundsSizeChanged);
    }

    if (boundsSizeChanged) {
      // TODO fastpath if !dirtyAll
      this.dirtyRects([viewportRect.clone()], true);
    }
  },

  dirtyRects: function dirtyRects(rects, doCriticalRender) {
    let criticalIsDirty = false;
    let criticalRect = this._criticalRect;
    let tc = this._tileCache;
    let crawler = this._crawler;

    for (let i = 0, len = rects.length; i < len; ++i) {
      let rect = rects[i];

      BEGIN_FOREACH_IN_RECT(rect, tc, tile)

      if (!tile.boundRect.intersects(criticalRect))
        this._removeTileSafe(tile);
      else
        criticalIsDirty = true;

      let intersection = tile.boundRect.intersect(rects[i]);
      tile.markDirty(intersection);

      if (crawler)
        crawler.enqueue(tile.i, tile.j);

      END_FOREACH_IN_RECT
    }

    if (criticalIsDirty && doCriticalRender)
      this.criticalRectPaint();
  },

  criticalRectPaint: function criticalRectPaint() {
    let cr = this._criticalRect;

    //let start = Date.now();

    if (!cr.isEmpty()) {
      let ctr = cr.center().map(Math.round);

      if (!this._ctr.equals(ctr)) {
        this._ctr.set(ctr);
        this.recenterEvictionQueue(ctr);
      }

      //let start = Date.now();

      this._renderAppendHoldRect(cr);

      //dump("  render, append, hold: " + (Date.now() - start) + "\n");
    }

    //dump(" paint: " + (Date.now() - start) + "\n");
  },

  beginCriticalMove: function beginCriticalMove(destCriticalRect) {
    if (destCriticalRect) {
      let tc = this._tileCache;

      BEGIN_FOREACH_IN_RECT(destCriticalRect, tc, tile)

      if (!tile.isDirty())
        this._appendTileSafe(tile);

      END_FOREACH_IN_RECT
    }
  },

  endCriticalMove: function endCriticalMove(destCriticalRect, doCriticalPaint) {
    let tc = this._tileCache;
    let cr = this._criticalRect;

    //let start = Date.now();

    if (!cr.isEmpty()) {
      BEGIN_FOREACH_IN_RECT(cr, tc, tile)
	    tc.releaseTile(tile);
      END_FOREACH_IN_RECT
    }

    //dump(" release: " + (Date.now() - start) + "\n");
    //start = Date.now();

    // XXX the conjunction with doCriticalPaint may cause tiles to disappear
    // (be evicted) during a (relatively slow) move as no tiles will be "held"
    // until a critical paint is requested.  Also, while we have this
    // && doCriticalPaint then we don't need this loop altogether, as
    // criticalRectPaint will hold everything for us (called below)
    //if (destCriticalRect && doCriticalPaint) {
    //  BEGIN_FOREACH_IN_RECT(destCriticalRect, tc, tile)
    //  tc.holdTile(tile);
    //  END_FOREACH_IN_RECT
    //}

    //dump(" hold: " + (Date.now() - start) + "\n");

    cr.copyFrom(destCriticalRect);

    if (doCriticalPaint)
      this.criticalRectPaint();
  },

  restartPrefetchCrawl: function restartPrefetchCrawl(startRectOrQueue) {
    if (startRectOrQueue instanceof Array) {
      this._crawler = new TileManager.CrawlIterator(this._tileCache);

      if (startRectOrQueue) {
        for (let k = 0, len = startRectOrQueue.length; k < len; ++k)
          this._crawler.enqueue(startRectOrQueue[k].i, startRectOrQueue[k].j);
      }
    } else {
      let cr = this._criticalRect;
      this._crawler = new TileManager.CrawlIterator(this._tileCache,
                                                    startRectOrQueue || (cr ? cr.clone() : null));
    }

    if (!this._idleTileCrawlerTimeout)
      this._idleTileCrawlerTimeout = setTimeout(this._idleTileCrawler, kTileCrawlComeAgain, this);
  },

  stopPrefetchCrawl: function stopPrefetchCrawl(skipRecenter) {
    if (this._idleTileCrawlerTimeout)
      clearTimeout(this._idleTileCrawlerTimeout);
    delete this._idleTileCrawlerTimeout;
    this._crawler = null;

    if (!skipRecenter) {
      let cr = this._criticalRect;
      if (!cr.isEmpty()) {
        let ctr = cr.center().map(Math.round);
        this.recenterEvictionQueue(ctr);
      }
    }
  },

  recenterEvictionQueue: function recenterEvictionQueue(ctr) {
    if (this._crawler) {
      this.restartPrefetchCrawl();
    }

    let ctri = ctr.x >> kTileExponentWidth;
    let ctrj = ctr.y >> kTileExponentHeight;

    this._tileCache.sortEvictionQueue(function evictFarTiles(a, b) {
      let dista = Math.max(Math.abs(a.i - ctri), Math.abs(a.j - ctrj));
      let distb = Math.max(Math.abs(b.i - ctri), Math.abs(b.j - ctrj));
      return dista - distb;
    });
  },

  /**
   * Render a rect to the canvas under the given scale.  We attempt to avoid a
   * drawWindow() by copying the image (via drawImage()) from  cached tiles, we
   * may have.  If we find that we're missing a necessary tile, we fall back on
   * drawWindow() directly to the destination canvas.
   */
  renderRectToCanvas: function renderRectToCanvas(srcRect, destCanvas, scalex, scaley) {
    let tc = this._tileCache;
    let ctx = destCanvas.getContext("2d");

    let completed = (function breakableLoop() {
      BEGIN_FOREACH_IN_RECT(srcRect, tc, tile)

      if (tile.isDirty())
        return false;

      let dx = Math.round(scalex * (tile.boundRect.left - srcRect.left));
      let dy = Math.round(scaley * (tile.boundRect.top - srcRect.top));

      ctx.drawImage(tile._canvas, dx, dy,
                    Math.round(scalex * kTileWidth),
                    Math.round(scaley * kTileHeight));

      END_FOREACH_IN_RECT

      return true;
    })();

    if (!completed) {
      let bv = this._browserView;

      bv.viewportToBrowserRect(srcRect);

      ctx.save();

      bv.browserToViewportCanvasContext(ctx);
      ctx.scale(scalex, scaley);
      ctx.drawWindow(bv._contentWindow,
                     0, 0, srcRect.width, srcRect.height,
                     "white",
                     (ctx.DRAWWINDOW_DO_NOT_FLUSH | ctx.DRAWWINDOW_DRAW_CARET));

      ctx.restore();
    }
  },

  _renderTile: function _renderTile(tile) {
    if (tile.isDirty()) {
/*
      let ctx = tile._canvas.getContext("2d");
      ctx.save();
      ctx.fillStyle = "rgba(0,255,0,.5)";
      ctx.translate(-tile.boundRect.left, -tile.boundRect.top);
      ctx.fillRect(tile._dirtyTileCanvasRect.left, tile._dirtyTileCanvasRect.top,
        tile._dirtyTileCanvasRect.width, tile._dirtyTileCanvasRect.height);
      ctx.restore();
      window.setTimeout(function(bv) {
        tile.render(bv);
      }, 1000, this._browserView);
 */
      tile.render(this._browserView);
    }
  },

  _appendTileSafe: function _appendTileSafe(tile) {
    if (!tile.appended) {
      this._appendTile(tile);
      tile.appended = true;
    }
  },

  _removeTileSafe: function _removeTileSafe(tile) {
    if (tile.appended) {
      this._removeTile(tile);
      tile.appended = false;
    }
  },

  _renderAppendHoldRect: function _renderAppendHoldRect(rect) {
    let tc = this._tileCache;

    BEGIN_FORCREATE_IN_RECT(rect, tc, tile)

    if (tile.isDirty())
      this._renderTile(tile);

    this._appendTileSafe(tile);
    this._tileCache.holdTile(tile);

    END_FORCREATE_IN_RECT
  },

  _idleTileCrawler: function _idleTileCrawler(self) {
    if (!self) self = this;

    let start = Date.now();
    let comeAgain = true;

    while ((Date.now() - start) <= kTileCrawlTimeCap) {
      let tile = self._crawler.next();
      if (!tile) {
        comeAgain = false;
        break;
      }
      if (tile.isDirty()) {
        self._renderTile(tile);
      }
    }

    if (comeAgain) {
      self._idleTileCrawlerTimeout = setTimeout(self._idleTileCrawler, kTileCrawlComeAgain, self);
    } else {
      self.stopPrefetchCrawl();
    }
  }
};

/**
 * The tile cache used by the tile manager to hold and index all
 * tiles.  Also responsible for pooling tiles and maintaining the
 * number of tiles under given capacity.
 *
 * @param onBeforeTileDetach callback set by the TileManager to call before
 * we must "detach" a tile from a tileholder due to needing it elsewhere or
 * having to discard it on capacity decrease
 * @param capacity the initial capacity of the tile cache, i.e. the max number
 * of tiles the cache can have allocated
 */
TileManager.TileCache = function TileCache(onBeforeTileDetach, iBound, jBound, capacity) {
  if (arguments.length <= 3 || capacity < 0)
    capacity = Infinity;

  // We track all pooled tiles in a 2D array (row, column) ordered as
  // they "appear on screen".  The array is a grid that functions for
  // storage of the tiles and as a lookup map.  Each array entry is
  // a reference to the tile occupying that space ("tileholder").  Entries
  // are not unique, so a tile could be referenced by multiple array entries,
  // i.e. a tile could "span" many tile placeholders (e.g. if we merge
  // neighbouring tiles).
  this._tileMap = [];

  // holds the same tiles that _tileMap holds, but as contiguous array
  // elements, for pooling tiles for reuse under finite capacity
  this._tilePool = [];
  this._pos = -1;

  this._capacity = capacity;

  this._onBeforeTileDetach = onBeforeTileDetach;

  this.iBound = iBound;
  this.jBound = jBound;
};

TileManager.TileCache.prototype = {

  get size() { return this._tilePool.length; },

  /**
   * The default tile comparison function used to order the tile pool such that
   * tiles most eligible for eviction have higher index.
   *
   * Unless reset, this is a comparison function that will compare free tiles
   * as greater than all non-free tiles.  Useful, for instance, to shrink
   * the tile pool when capacity is lowered, as we want to remove all tiles
   * at the new cap and beyond, favoring removal of free tiles first.
   */
  evictionCmp: function evictionCmp(a, b) {
    if (a.free == b.free) return (a.j == b.j) ? b.i - a.i : b.j - a.j;
    return (a.free) ? 1 : -1;
  },

  lookup: function lookup(i, j) {
    let tile = null;
    if (this._tileMap[i])
      tile = this._tileMap[i][j] || null;

    return tile;
  },

  getCapacity: function getCapacity() { return this._capacity; },

  setCapacity: function setCapacity(newCap, skipEvictionQueueSort) {
    if (newCap < 0)
      throw "Cannot set a negative tile cache capacity";

    if (newCap == Infinity) {
      this._capacity = Infinity;
      return;
    } else if (this._capacity == Infinity) {
      // pretend we had a finite capacity all along and proceed normally
      this._capacity = this._tilePool.length;
    }

    if (newCap < this._capacity) {
      // This case is obnoxious.  We're decreasing our capacity which means
      // we may have to get rid of tiles.  Note that "throwing out" means the
      // cache won't keep them, and they'll get GC'ed as soon as all other
      // refholders let go of their refs to the tile.

      if (!skipEvictionQueueSort)
        this.sortEvictionQueue();

      let rem = this._tilePool.splice(newCap);
      for (let k = 0, len = rem.length; k < len; ++k)
        this._detachTile(rem[k].i, rem[k].j);
    }

    this._capacity = newCap;
  },

  inBounds: function inBounds(i, j) {
    return (0 <= i && 0 <= j && i <= this.iBound && j <= this.jBound);
  },

  sortEvictionQueue: function sortEvictionQueue(cmp) {
    let pool = this._tilePool;

    pool.sort(cmp ? cmp : this.evictionCmp);
    this._pos = pool.length - 1;
  },

  /**
   * Get a tile by its indices
   *
   * @param i Column
   * @param j Row
   * @param create Flag true if the tile should be created in case there is no
   * tile at (i, j)
   * @param reuseCondition Boolean-valued function to restrict conditions under
   * which an old tile may be reused for creating this one.  This can happen if
   * the cache has reached its capacity and must reuse existing tiles in order to
   * create this one.  The function is given a Tile object as its argument and
   * returns true if the tile is OK for reuse. This argument has no effect if the
   * create argument is false.
   */
  getTile: function getTile(i, j, create, evictionGuard) {
    if (!this.inBounds(i, j))
      return null;

    let tile = this.lookup(i, j);
    if (!tile && create) {
      tile = this._createTile(i, j, evictionGuard);
      if (tile) tile.markDirty();
    }

    return tile;
  },

  /**
   * Look up (possibly creating) a tile from its viewport coordinates.
   *
   * @param x
   * @param y
   * @param create Flag true if the tile should be created in case it doesn't
   * already exist at the tileholder corresponding to (x, y)
   */
  tileFromPoint: function tileFromPoint(x, y, create) {
    let i = x >> kTileExponentWidth;
    let j = y >> kTileExponentHeight;

    return this.getTile(i, j, create);
  },

  /**
   * Hold a tile (i.e. mark it non-free).
   */
  holdTile: function holdTile(tile) {
    if (tile) tile.free = false;
  },

  /**
   * Release a tile (i.e. mark it free).
   */
  releaseTile: function releaseTile(tile) {
    if (tile) tile.free = true;
  },

  _detachTile: function _detachTile(i, j) {
    let tile = this.lookup(i, j);
    if (tile) {
      if (this._onBeforeTileDetach)
        this._onBeforeTileDetach(tile);

      this.releaseTile(tile);
      delete this._tileMap[i][j];
    }
    return tile;
  },

  /**
   * Pluck tile from its current tileMap position and drop it in position
   * given by (i, j).
   */
  _reassignTile: function _reassignTile(tile, i, j) {
    this._detachTile(tile.i, tile.j);    // detach
    tile.init(i, j);                     // re-init
    this._tileMap[i][j] = tile;          // attach
    return tile;
  },

  _evictTile: function _evictTile(evictionGuard) {
    let k = this._pos;
    let pool = this._tilePool;
    let victim = null;

    for (; k >= 0; --k) {
      if (pool[k].free && ( !evictionGuard || evictionGuard(pool[k]) )) {
        victim = pool[k];
        --k;
        break;
      }
    }

    this._pos = k;

    return victim;
  },

  _createTile: function _createTile(i, j, evictionGuard) {
    if (!this._tileMap[i])
      this._tileMap[i] = [];

    let tile = null;
    if (this._tilePool.length < this._capacity) { // either capacity is
      tile = new TileManager.Tile(i, j);          // infinite, or we have
      this._tileMap[i][j] = tile;                 // room to allocate more
      this._tilePool.push(tile);
    } else {
      tile = this._evictTile(evictionGuard);
      if (tile)
        this._reassignTile(tile, i, j);
    }

    return tile;
  }

};


/**
 * A tile is a little object with an <html:canvas> DOM element that it used for
 * caching renders from drawWindow().
 *
 * Supports the dirtying of only a subrectangle of the bounding rectangle, as
 * well as just dirtying the entire tile.
 */
TileManager.Tile = function Tile(i, j) {
  this._canvas = document.createElementNS(kXHTMLNamespaceURI, "canvas");
  this._canvas.setAttribute("width", String(kTileWidth));
  this._canvas.setAttribute("height", String(kTileHeight));
  this._canvas.setAttribute("moz-opaque", "true");

  this.init(i, j);  // defines more properties, cf below
};

TileManager.Tile.prototype = {
  init: function init(i, j) {
    if (!this.boundRect)
      this.boundRect = new Rect(i * kTileWidth, j * kTileHeight, kTileWidth, kTileHeight);
    else
      this.boundRect.setRect(i * kTileWidth, j * kTileHeight, kTileWidth, kTileHeight);

    /* indices of this tile in the tile cache's tile grid map */
    this.i = i;    // row
    this.j = j;    // column

    /* flag used by TileManager to avoid re-appending tiles that have already
     * been appended */
    this.appended = false;

    /* flag used by the TileCache to mark tiles as ineligible for eviction,
     * usually because they are fixed in some critical position */
    this.free = true;

    /* flags true if we need to repaint our own local canvas */
    this._dirtyTileCanvas = false;

    /* keep a dirty rectangle (i.e. only part of the tile is dirty) */
    this._dirtyTileCanvasRect = new Rect(0, 0, 0, 0);
  },

  get x() { return this.boundRect.left; },
  get y() { return this.boundRect.top; },

  /**
   * Get the <html:canvas> DOM element with this tile's cached paint.
   */
  getContentImage: function getContentImage() { return this._canvas; },

  isDirty: function isDirty() { return this._dirtyTileCanvas; },

  /**
   * This will mark dirty at least everything in dirtyRect (which must be
   * specified in canvas coordinates).  If dirtyRect is not given then
   * the entire tile is marked dirty (i.e. the whole tile needs to be rendered
   * on next render).
   */
  markDirty: function markDirty(dirtyRect) {
    if (!dirtyRect) {
      this._dirtyTileCanvasRect.copyFrom(this.boundRect);
    } else {
      this._dirtyTileCanvasRect.expandToContain(dirtyRect.intersect(this.boundRect)).expandToIntegers();
    }

    // XXX if, after the above, the dirty rectangle takes up a large enough
    // portion of the boundRect, we should probably just mark the entire tile
    // dirty and fastpath for future calls.
    if (!this._dirtyTileCanvasRect.isEmpty())
      this._dirtyTileCanvas = true;
  },

  unmarkDirty: function unmarkDirty() {
    this._dirtyTileCanvasRect.setRect(0, 0, 0, 0);
    this._dirtyTileCanvas = false;
  },

  /**
   * Actually draw the browser content into the dirty region of this tile.  This
   * requires us to actually draw with nsIDOMCanvasRenderingContext2D::
   * drawWindow(), which we expect to be a heavy operation.
   *
   * You likely want to check if the tile isDirty() before asking it
   * to render, as this will cause the entire tile to re-render in the
   * case that it is not dirty.
   */
  render: function render(browserView) {
    if (!this.isDirty())
      this.markDirty();

    let rect = this._dirtyTileCanvasRect;
    let x = rect.left - this.boundRect.left;
    let y = rect.top - this.boundRect.top;

    browserView.viewportToBrowserRect(rect);

    let ctx = this._canvas.getContext("2d");
    ctx.save();

    ctx.translate(x, y);
    browserView.browserToViewportCanvasContext(ctx);
    ctx.drawWindow(browserView._contentWindow,
                   rect.left, rect.top,
                   rect.right - rect.left, rect.bottom - rect.top,
                   "white",
                   (ctx.DRAWWINDOW_DRAW_CARET));

    ctx.restore();

    this.unmarkDirty();
  },

  /**
   * Standard toString prints "Tile(<row>, <column>)", but if `more' flags true
   * then some extra information is printed.
   */
  toString: function toString(more) {
    if (more) {
      return 'Tile(' + this.i                                   + ', '
                     + this.j                                   + ', '
                     + 'dirty=' + this.isDirty()                + ', '
                     + 'boundRect=' + this.boundRect.toString() + ')';
    }

    return 'Tile(' + this.i + ', ' + this.j + ')';
  }

};


/**
 * A CrawlIterator is in charge of creating and returning subsequent tiles "crawled"
 * over as we render tiles lazily.
 *
 * Currently the CrawlIterator is built to expand a rectangle iteratively and return
 * subsequent tiles that intersect the boundary of the rectangle.  Each expansion of
 * the rectangle is one unit of tile dimensions in each direction.  This is repeated
 * until all tiles from elsewhere have been reused (assuming the cache has finite
 * capacity) in this crawl, so that we don't start reusing tiles from the beginning
 * of our crawl.  Afterward, the CrawlIterator enters a state where it operates as a
 * FIFO queue, and calls to next() simply dequeue elements, which must be added with
 * enqueue().
 *
 * @param tileCache The TileCache over whose tiles this CrawlIterator will crawl
 * @param startRect [optional] The rectangle that we grow in the first (rectangle
 * expansion) iteration state.
 */
TileManager.CrawlIterator = function CrawlIterator(tileCache, startRect) {
  this._tileCache = tileCache;
  this._stepRect = startRect;

  // used to remember tiles that we've reused during this crawl
  this._visited = {};

  // filters the tiles we've already reused once from being considered victims
  // for reuse when we ask the tile cache to create a new tile
  let visited = this._visited;
  this._notVisited = function(tile) { return !visited[tile.toString()]; };

  // a generator that generates tile indices corresponding to tiles intersecting
  // the boundary of an expanding rectangle
  this._crawlIndices = !startRect ? null : (function indicesGenerator(rect, tc) {
    let outOfBounds = false;
    while (!outOfBounds) {
      rect.left   -= kTileWidth;   // expand the rect
      rect.right  += kTileWidth;
      rect.top    -= kTileHeight;
      rect.bottom += kTileHeight;

      let starti = rect.left  >> kTileExponentWidth;
      let endi   = rect.right >> kTileExponentWidth;
      let startj = rect.top    >> kTileExponentHeight;
      let endj   = rect.bottom >> kTileExponentHeight;
      let i, j;

      outOfBounds = true;

      // top, bottom rect borders
      for each (let y in [rect.top, rect.bottom]) {
        j = y >> kTileExponentHeight;

        for (i = starti; i <= endi; ++i) {
          if (tc.inBounds(i, j)) {
            outOfBounds = false;
            yield [i, j];
          }
        }
      }

      // left, right rect borders
      for each (let x in [rect.left, rect.right]) {
        i = x >> kTileExponentWidth;

        for (j = startj; j <= endj; ++j) {
          if (tc.inBounds(i, j)) {
            outOfBounds = false;
            yield [i, j];
          }
        }
      }
    }
  })(this._stepRect, this._tileCache),    // instantiate the generator

  // after we finish the rectangle iteration state, we enter the FIFO queue state
  this._queueState = !startRect;
  this._queue = [];

  // used to prevent tiles from being enqueued twice --- "patience, we'll get to
  // it in a moment"
  this._enqueued = {};
};

TileManager.CrawlIterator.prototype = {
  __iterator__: function() {
    while (true) {
      let tile = this.next();
      if (!tile) break;
      yield tile;
    }
  },

  becomeQueue: function becomeQueue() {
    this._queueState = true;
  },

  unbecomeQueue: function unbecomeQueue() {
    this._queueState = false;
  },

  next: function next() {
    if (this._queueState)
      return this.dequeue();

    let tile = null;

    if (this._crawlIndices) {
      try {
        let [i, j] = this._crawlIndices.next();
        tile = this._tileCache.getTile(i, j, true, this._notVisited);
      } catch (e) {
        if (!(e instanceof StopIteration))
          throw e;
      }
    }

    if (tile) {
      this._visited[tile.toString()] = true;
    } else {
      this.becomeQueue();
      return this.next();
    }

    return tile;
  },

  dequeue: function dequeue() {
    let tile = null;
    do {
      let idx = this._queue.shift();
      if (!idx)
      return null;

      delete this._enqueued[idx];
      let [i, j]  = this._unstrIndices(idx);
      tile = this._tileCache.getTile(i, j, false);

    } while (!tile);

    return tile;
  },

  enqueue: function enqueue(i, j) {
    let idx = this._strIndices(i, j);
    if (!this._enqueued[idx]) {
      this._queue.push(idx);
      this._enqueued[idx] = true;
    }
  },

  _strIndices: function _strIndices(i, j) {
    return i + "," + j;
  },

  _unstrIndices: function _unstrIndices(str) {
    return str.split(',');
  }

};