Convert search for most visible page in the document to a binary search.

chrome/browser/resources/pdf/viewport.js

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * Predefined zoom factors to be used when zooming in/out. These are in
  * ascending order.
  */
 var ZOOM_FACTORS = [0.25, 0.333, 0.5, 0.666, 0.75, 0.9, 1.0,
                     1.1, 1.25, 1.5, 1.75, 2.0, 2.5, 3.0, 4.0, 5.0];
@@ skipping 136 matching lines @@
   getCurrentViewportRect_: function() {
     return {
       x: this.window_.pageXOffset / this.zoom_,
       y: this.window_.pageYOffset / this.zoom_,
       width: this.window_.innerWidth / this.zoom_,
       height: this.window_.innerHeight / this.zoom_,
     };
   },
 
   /**
+   * @private
+   * @param {integer} y the y-coordinate to get the page at.
+   * @return {integer} the index of a page overlapping the given y-coordinate.
+   */
+  getPageAtY_: function(y) {
+    var min = 0;
+    var max = this.pageDimensions_.length - 1;
+    while (max >= min) {
+      var page = Math.floor(min + ((max - min) / 2));
+      var top = this.pageDimensions_[page].y;
+      var bottom = top + this.pageDimensions_[page].height;
+      if (top <= y && bottom > y)
+        return page;
+      else if (top > y)
+        max = page - 1;
+      else
+        min = page + 1;
+    }
+    return 0;
+  },
+
+  /**
    * Returns the page with the most pixels in the current viewport.
    * @return {int} the index of the most visible page.
    */
   getMostVisiblePage: function() {
-    // TODO(raymes): Do a binary search here.
+    var firstVisiblePage = this.getPageAtY_(this.getCurrentViewportRect_().y);
     var mostVisiblePage = {'number': 0, 'area': 0};
-    for (var i = 0; i < this.pageDimensions_.length; i++) {
+    for (var i = firstVisiblePage; i < this.pageDimensions_.length; i++) {
       var area = getIntersectionArea(this.pageDimensions_[i],
                                      this.getCurrentViewportRect_());
+      // If we hit a page with 0 area overlap, we must have gone past the
+      // pages visible in the viewport so we can break.
+      if (area == 0)
+        break;
       if (area > mostVisiblePage.area) {
         mostVisiblePage.area = area;
         mostVisiblePage.number = i;
       }
     }
     return mostVisiblePage.number;
   },
 
   /**
    * @private
@@ skipping 123 matching lines @@
    * Set the dimensions of the document.
    * @param {Object} documentDimensions the dimensions of the document
    */
   setDocumentDimensions: function(documentDimensions) {
     this.documentDimensions_ = documentDimensions;
     this.pageDimensions_ = this.documentDimensions_.pageDimensions;
     this.contentSizeChanged_();
     this.updateViewport_();
   }
 };