add TileLayerRenderer.getPreferredZ_ plus tests

src/ol/dom/tilelayerrenderer.js

@@ -135,3 +135,39 @@ ol.dom.TileLayerRenderer.prototype.removeInvisibleTiles_ = function(
     }
   }
 };
+
+
+/**
+ * Get the best/preferred z for an arbitrary resolution.
+ * @private
+ * @param {number} resolution Resolution.
+ * @return {number} Z.
+ * TODO: this should probably be shared with the WebGL TileLayerRenderer,
+ * but our current class hierarchy does not allow this sharing.
+*/
+ol.dom.TileLayerRenderer.prototype.getPreferredZ_ = (function() {
+  var cache = {};
+  return function(resolution) {
+    if (resolution in cache) {
+      return cache[resolution];
+    }
+    var tileLayer = /** @type {ol.TileLayer} */ (this.getLayer());
+    var tileStore = tileLayer.getStore();
+    var tileGrid = tileStore.getTileGrid();
+    var gridResolutions = tileGrid.getResolutions();
+    var minDiff = Number.POSITIVE_INFINITY;
+    var candidate, diff, z, r;
+    for (var i = 0, ii = gridResolutions.length; i < ii; ++i) {
+      // assumes sorted resolutions
+      diff = Math.abs(resolution - gridResolutions[i]);
+      if (diff < minDiff) {
+        z = i;
+        minDiff = diff;
+      } else {
+        break;
+      }
+    }
+    cache[resolution] = z;
+    return z;
+  };
+})();