Get closest resolution.

src/ol/renderer/TileLayerRenderer.js

@@ -26,6 +26,12 @@ ol.renderer.TileLayerRenderer = function(container, layer) {
      * @private
      */
     this.rendererdBounds_ = null;
+    
+    /**
+     * @type {Array.<number>}
+     */
+    this.layerResolutions_ = layer.getResolutions();
+
 
     /**
      * @type {number|undefined}
@@ -43,7 +49,27 @@ ol.renderer.TileLayerRenderer = function(container, layer) {
 
 goog.inherits(ol.renderer.TileLayerRenderer, ol.renderer.LayerRenderer);
 
-
+/**
+ * @param {number} resolution
+ * @return {Array.<number>}
+ */
+ol.renderer.TileLayerRenderer.prototype.getPreferredResAndZ_ = function(resolution) {
+    var minDiff = Number.POSITIVE_INFINITY;
+    var candidate, diff, z, r;
+    for (var i=0, ii=this.layerResolutions_.length; i<ii; ++i) {
+        // assumes sorted resolutions
+        candidate = this.layerResolutions_[i];
+        diff = Math.abs(resolution - candidate);
+        if (diff < minDiff) {
+            z = i;
+            r = candidate;
+            minDiff = diff;
+        } else {
+            break;
+        }
+    }
+    return [r, z];
+};
 
 /**
  * @return {goog.math.Size}

test/spec/ol/renderer/TileLayerRenderer.test.js

@@ -15,5 +15,61 @@ describe("ol.renderer.TileLayerRenderer", function() {
         expect(ol.renderer.TileLayerRenderer.canRender(layer)).toBe(false);
     });
     
+    describe("getPreferredResAndZ_", function() {
+        var layer = new ol.layer.TileLayer();
+        var resolutions = [100, 80, 50, 10, 1, 0.1];
+        layer.setResolutions(resolutions);
+        
+        var container = document.createElement("div");
+        var renderer = new ol.renderer.TileLayerRenderer(container, layer);
+
+        it("gets the max resolution", function() {
+            var pair = renderer.getPreferredResAndZ_(100);
+            expect(pair[0]).toBe(100);
+            expect(pair[1]).toBe(0);
+        });
+
+        it("gets the min resolution", function() {
+            var pair = renderer.getPreferredResAndZ_(0.1);
+            expect(pair[0]).toBe(0.1);
+            expect(pair[1]).toBe(5);
+        });
+
+        it("gets the max when bigger", function() {
+            var pair = renderer.getPreferredResAndZ_(200);
+            expect(pair[0]).toBe(100);
+            expect(pair[1]).toBe(0);
+        });
+
+        it("gets the min when smaller", function() {
+            var pair = renderer.getPreferredResAndZ_(0.01);
+            expect(pair[0]).toBe(0.1);
+            expect(pair[1]).toBe(5);
+        });
+
+        it("gets the first when in the middle", function() {
+            var pair = renderer.getPreferredResAndZ_(90);
+            expect(pair[0]).toBe(100);
+            expect(pair[1]).toBe(0);
+        });
+
+        it("gets the closer when elsewhere", function() {
+            var pair = renderer.getPreferredResAndZ_(89);
+            expect(pair[0]).toBe(80);
+            expect(pair[1]).toBe(1);
+            
+            pair = renderer.getPreferredResAndZ_(49);
+            expect(pair[0]).toBe(50);
+            expect(pair[1]).toBe(2);
+
+            pair = renderer.getPreferredResAndZ_(0.5);
+            expect(pair[0]).toBe(0.1);
+            expect(pair[1]).toBe(5);
+        });
+        
+    });
+    
+    
+    
 });