From eb31b75caa11bce95f0382139f604d61694ac07c Mon Sep 17 00:00:00 2001
From: Tim Schaub <tim.schaub@gmail.com>
Date: Sat, 2 Sep 2017 15:38:25 -0600
Subject: [PATCH 1/2] Fast path for child range when zoom factor is 2

---
 src/ol/tilegrid/tilegrid.js            | 32 ++++++++++++++++++++++++--
 test/spec/ol/tilegrid/tilegrid.test.js | 20 ++++++++++++++++
 2 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/src/ol/tilegrid/tilegrid.js b/src/ol/tilegrid/tilegrid.js
index 93b49ee7e7..f5a099eaf9 100644
--- a/src/ol/tilegrid/tilegrid.js
+++ b/src/ol/tilegrid/tilegrid.js
@@ -37,6 +37,30 @@ ol.tilegrid.TileGrid = function(options) {
     return b - a;
   }, true), 17); // `resolutions` must be sorted in descending order
 
+
+  // check if we've got a consistent zoom factor and origin
+  var zoomFactor;
+  if (!options.origins) {
+    for (var i = 0, ii = this.resolutions_.length - 1; i < ii; ++i) {
+      if (!zoomFactor) {
+        zoomFactor = this.resolutions_[i] / this.resolutions_[i + 1];
+      } else {
+        if (this.resolutions_[i] / this.resolutions_[i + 1] !== zoomFactor) {
+          zoomFactor = undefined;
+          break;
+        }
+      }
+    }
+  }
+
+
+  /**
+   * @private
+   * @type {number|undefined}
+   */
+  this.zoomFactor_ = zoomFactor;
+
+
   /**
    * @protected
    * @type {number}
@@ -248,12 +272,16 @@ ol.tilegrid.TileGrid.prototype.getResolutions = function() {
  */
 ol.tilegrid.TileGrid.prototype.getTileCoordChildTileRange = function(tileCoord, opt_tileRange, opt_extent) {
   if (tileCoord[0] < this.maxZoom) {
+    if (this.zoomFactor_ === 2) {
+      var minX = tileCoord[1] * 2;
+      var minY = tileCoord[2] * 2;
+      return ol.TileRange.createOrUpdate(minX, minX + 1, minY, minY + 1, opt_tileRange);
+    }
     var tileCoordExtent = this.getTileCoordExtent(tileCoord, opt_extent);
     return this.getTileRangeForExtentAndZ(
         tileCoordExtent, tileCoord[0] + 1, opt_tileRange);
-  } else {
-    return null;
   }
+  return null;
 };
 
 
diff --git a/test/spec/ol/tilegrid/tilegrid.test.js b/test/spec/ol/tilegrid/tilegrid.test.js
index 650653a0ea..21ac5e381b 100644
--- a/test/spec/ol/tilegrid/tilegrid.test.js
+++ b/test/spec/ol/tilegrid/tilegrid.test.js
@@ -287,6 +287,26 @@ describe('ol.tilegrid.TileGrid', function() {
     });
   });
 
+  describe('#zoomFactor_', function() {
+    it('is set for a consistent zoom factor', function() {
+      var grid = new ol.tilegrid.TileGrid({
+        resolutions: [10, 5, 2.5, 1.25],
+        origin: origin,
+        tileSize: tileSize
+      });
+      expect(grid.zoomFactor_).to.be(2);
+    });
+
+    it('is not set for an inconsistent zoom factor', function() {
+      var grid = new ol.tilegrid.TileGrid({
+        resolutions: [10, 5, 3, 1.25],
+        origin: origin,
+        tileSize: tileSize
+      });
+      expect(grid.zoomFactor_).to.be(undefined);
+    });
+  });
+
   describe('createForProjection', function() {
 
     it('allows easier creation of a tile grid', function() {

From d777edcc052b2695fdad3fa70d46d32759246d60 Mon Sep 17 00:00:00 2001
From: Tim Schaub <tim.schaub@gmail.com>
Date: Sat, 2 Sep 2017 20:36:38 -0600
Subject: [PATCH 2/2] Fast path for parent range when zoom factor is 2

---
 src/ol/tilegrid/tilegrid.js | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/src/ol/tilegrid/tilegrid.js b/src/ol/tilegrid/tilegrid.js
index f5a099eaf9..bc933cf28c 100644
--- a/src/ol/tilegrid/tilegrid.js
+++ b/src/ol/tilegrid/tilegrid.js
@@ -186,11 +186,24 @@ ol.tilegrid.TileGrid.prototype.forEachTileCoord = function(extent, zoom, callbac
  * @template T
  */
 ol.tilegrid.TileGrid.prototype.forEachTileCoordParentTileRange = function(tileCoord, callback, opt_this, opt_tileRange, opt_extent) {
-  var tileCoordExtent = this.getTileCoordExtent(tileCoord, opt_extent);
+  var tileRange, x, y;
+  var tileCoordExtent = null;
   var z = tileCoord[0] - 1;
+  if (this.zoomFactor_ === 2) {
+    x = tileCoord[1];
+    y = tileCoord[2];
+  } else {
+    tileCoordExtent = this.getTileCoordExtent(tileCoord, opt_extent);
+  }
   while (z >= this.minZoom) {
-    if (callback.call(opt_this, z,
-        this.getTileRangeForExtentAndZ(tileCoordExtent, z, opt_tileRange))) {
+    if (this.zoomFactor_ === 2) {
+      x = Math.floor(x / 2);
+      y = Math.floor(y / 2);
+      tileRange = ol.TileRange.createOrUpdate(x, x, y, y, opt_tileRange);
+    } else {
+      tileRange = this.getTileRangeForExtentAndZ(tileCoordExtent, z, opt_tileRange);
+    }
+    if (callback.call(opt_this, z, tileRange)) {
       return true;
     }
     --z;