Add ol.TileGrid.getZForResolution

src/ol/tilegrid.js

@@ -259,6 +259,30 @@ ol.TileGrid.prototype.getTileCoordResolution = function(tileCoord) {
 };
 
 
+/**
+ * @param {number} resolution Resolution.
+ * @return {number} Z.
+ */
+ol.TileGrid.prototype.getZForResolution = function(resolution) {
+  var z;
+  for (z = 0; z < this.numResolutions_; ++z) {
+    if (this.resolutions_[z] == resolution) {
+      return z;
+    } else if (this.resolutions_[z] < resolution) {
+      if (z === 0) {
+        return z;
+      } else if (resolution - this.resolutions_[z] <=
+          this.resolutions_[z - 1] - resolution) {
+        return z;
+      } else {
+        return z - 1;
+      }
+    }
+  }
+  return this.numResolutions_ - 1;
+};
+
+
 /**
  * @param {ol.TileCoord} tileCoord Tile coordinate.
  * @param {function(number, ol.TileBounds): boolean} callback Callback.

src/ol/tilegrid_test.js

@@ -264,3 +264,38 @@ function testForEachTileCoordParent() {
   assertEquals(0, tileBoundss[2].left);
 
 }
+
+
+function testGetZForResolutionExact() {
+
+  var tileGrid =
+      new ol.TileGrid(resolutions, extent, origin, xEast, ySouth, tileSize);
+
+  assertEquals(0, tileGrid.getZForResolution(1000));
+  assertEquals(1, tileGrid.getZForResolution(500));
+  assertEquals(2, tileGrid.getZForResolution(250));
+  assertEquals(3, tileGrid.getZForResolution(100));
+
+}
+
+
+function testGetZForResolutionApproximate() {
+
+  var tileGrid =
+      new ol.TileGrid(resolutions, extent, origin, xEast, ySouth, tileSize);
+
+  assertEquals(0, tileGrid.getZForResolution(2000));
+  assertEquals(0, tileGrid.getZForResolution(1000));
+  assertEquals(0, tileGrid.getZForResolution(900));
+  assertEquals(1, tileGrid.getZForResolution(750));
+  assertEquals(1, tileGrid.getZForResolution(625));
+  assertEquals(1, tileGrid.getZForResolution(500));
+  assertEquals(1, tileGrid.getZForResolution(475));
+  assertEquals(2, tileGrid.getZForResolution(375));
+  assertEquals(2, tileGrid.getZForResolution(250));
+  assertEquals(2, tileGrid.getZForResolution(200));
+  assertEquals(3, tileGrid.getZForResolution(125));
+  assertEquals(3, tileGrid.getZForResolution(100));
+  assertEquals(3, tileGrid.getZForResolution(50));
+
+}