maintain first visible page

TODO

@@ -1,3 +1,4 @@
+split js
 position history stack (popstate)
 scale limits
 onresize

share/pdf2htmlEX.js.in

@@ -244,11 +244,20 @@ function Viewer(config) {
 
 Viewer.prototype = {
   scale : 1,
-  // index of the active page (most visible one)
+  /* 
+   * index of the active page (the one with largest visible area)
+   * which estimates the page currently being viewed
+   */
   cur_page_idx : 0,
 
+  /*
+   * index of the first visible page
+   * used when determining current view
+   */
+  first_page_idx : 0,
+
   init_before_loading_content : function() {
-    /*hide all pages before loading, will reveal only visible ones later */
+    /* hide all pages before loading, will reveal only visible ones later */
     this.pre_hide_pages();
   },
 
@@ -275,6 +284,8 @@ Viewer.prototype = {
     }
 
     this.find_pages();
+    // do nothing if there's nothing
+    if(this.pages.length == 0) return;
 
     // disable dragging of background images
     disable_dragstart(document.getElementsByClassName(CSS_CLASS_NAMES.background_image));
@@ -293,6 +304,7 @@ Viewer.prototype = {
     // register schedule rendering
     // renew old schedules since scroll() may be called frequently
     this.container.addEventListener('scroll', function() {
+      self.update_page_idx();
       self.schedule_render(true);
     }, false);
 
@@ -434,7 +446,6 @@ Viewer.prototype = {
 
   /*
    * show visible pages and hide invisible pages
-   * update current page number
    */
   render : function () {
     var container = this.container;
@@ -469,27 +480,80 @@ Viewer.prototype = {
         } else {
           this.load_page(i);
         }
-
-        if (!cur_page_fully_visible) {
-          // check the visible fraction of the page
-          var page_visible_ratio = (Math.min(container_max_y, page_max_y) - Math.max(container_min_y, page_min_y)) / page_height;
-          if ((i === cur_page_idx) && (Math.abs(page_visible_ratio - 1.0) <= EPS)) {
-            cur_page_fully_visible = true;
-          } else if (page_visible_ratio > max_visible_ratio) {
-            max_visible_ratio = page_visible_ratio;
-            max_visible_page_idx = i;
-          }
-        }
       } else {
         cur_page.hide();
       }
     }
+  },
+  /*
+   * update cur_page_idx and first_page_idx
+   * normally called upon scrolling
+   */
+  update_page_idx: function () {
+    var pages = this.pages;
+    var pages_len = pages.length;
+    // there is no chance that cur_page_idx or first_page_idx is modified
+    if (pages_len < 2) return;
+   
+    var container = this.container;
+    var container_min_y = container.scrollTop;
+    var container_max_y = container_min_y + container.clientHeight;
+
+    // binary search for the first page
+    // whose bottom border is below the top border of the container
+    var first_idx = -1;
+    var last_idx = pages_len;
+    var rest_len = last_idx - first_idx;
+    // TODO: use current first_page_idx as a hint?
+    while(rest_len > 1) {
+      var idx = first_idx + Math.floor(rest_len / 2);
+      var cur_page_ele = pages[idx].page;
+      if (cur_page_ele.offsetTop + cur_page_ele.clientTop + cur_page_ele.clientHeight >= container_min_y) {
+        last_idx = idx;
+      } else {
+        first_idx = idx;
+      }
+      rest_len = last_idx - first_idx;
+    }
+    
     /*
-     * update current page number to the maximum visible page
-     * do not update it when current page is still fully visible
+     * with malformed settings it is possible that no page is visible, e.g.
+     * - the container is to thin, which lies in the margin between two pages
+     * - all pages are completely above or below the container
+     * but we just assume that they won't happen.
      */
-    if (!cur_page_fully_visible) 
-      this.cur_page_idx = max_visible_page_idx;
+    this.first_page_idx = last_idx;
+
+    // find the page with largest visible area
+    var cur_page_idx = this.cur_page_idx;
+    var max_visible_page_idx = cur_page_idx;
+    var max_visible_ratio = 0.0;
+
+    for(var i = last_idx; i < pages_len; ++i) {
+      var cur_page_ele = pages[i].page;
+      var page_min_y = cur_page_ele.offsetTop + cur_page_ele.clientTop;
+      var page_height = cur_page_ele.clientHeight;
+      var page_max_y = page_min_y + page_height;
+      if (page_min_y > container_max_y) break;
+
+      // check the visible fraction of the page
+      var page_visible_ratio = ( Math.min(container_max_y, page_max_y) 
+                                 - Math.max(container_min_y, page_min_y)
+                               ) / page_height;
+
+      // stay with the current page if it is still fully visible
+      if ((i === cur_page_idx) && (Math.abs(page_visible_ratio - 1.0) <= EPS)) {
+        max_visible_page_idx = cur_page_idx;
+        break;
+      }
+
+      if (page_visible_ratio > max_visible_ratio) {
+        max_visible_ratio = page_visible_ratio;
+        max_visible_page_idx = i;
+      }
+    }
+
+    this.cur_page_idx = max_visible_page_idx;
   },
 
   /**