View / refactor how zoom and resolution are computed

This commit aims to simplify the computation of zoom and resolution in the
View class.

Previously zoom levels and resolution computations were mixed in different places,
ie resolution constraints, initial values, etc.

Now the View class only has the `getZoomForResolution` and `getResolutionForZoom`
methods to convert from one system to another.

Other than that, most computations use the resolution system internally.

The `constrainResolution` method also does not exist anymore, and is replaced
by `getValidResolution` and `getValidZoomLevel` public methods.

test/spec/ol/view.test.js

@@ -1104,7 +1104,9 @@ describe('ol.View', function() {
       const min = view.getMinZoom();
 
       expect(view.getResolutionForZoom(max)).to.be(view.getMinResolution());
+      expect(view.getResolutionForZoom(max + 1)).to.be(view.getMinResolution());
       expect(view.getResolutionForZoom(min)).to.be(view.getMaxResolution());
+      expect(view.getResolutionForZoom(min - 1)).to.be(view.getMaxResolution());
     });
 
     it('returns correct zoom levels for specifically configured resolutions', function() {
@@ -1112,11 +1114,30 @@ describe('ol.View', function() {
         resolutions: [10, 8, 6, 4, 2]
       });
 
+      expect(view.getResolutionForZoom(-1)).to.be(10);
       expect(view.getResolutionForZoom(0)).to.be(10);
       expect(view.getResolutionForZoom(1)).to.be(8);
       expect(view.getResolutionForZoom(2)).to.be(6);
       expect(view.getResolutionForZoom(3)).to.be(4);
       expect(view.getResolutionForZoom(4)).to.be(2);
+      expect(view.getResolutionForZoom(5)).to.be(2);
+    });
+
+    it('returns correct zoom levels for resolutions with variable zoom levels', function() {
+      const view = new View({
+        resolutions: [50, 10, 5, 2.5, 1.25, 0.625]
+      });
+
+      expect(view.getResolutionForZoom(-1)).to.be(50);
+      expect(view.getResolutionForZoom(0)).to.be(50);
+      expect(view.getResolutionForZoom(0.5)).to.be(50 / Math.pow(5, 0.5));
+      expect(view.getResolutionForZoom(1)).to.be(10);
+      expect(view.getResolutionForZoom(2)).to.be(5);
+      expect(view.getResolutionForZoom(2.75)).to.be(5 / Math.pow(2, 0.75));
+      expect(view.getResolutionForZoom(3)).to.be(2.5);
+      expect(view.getResolutionForZoom(4)).to.be(1.25);
+      expect(view.getResolutionForZoom(5)).to.be(0.625);
+      expect(view.getResolutionForZoom(6)).to.be(0.625);
     });
 
   });
@@ -1470,6 +1491,41 @@ describe('ol.View', function() {
       expect(view.getValidZoomLevel(8)).to.be(6);
     });
   });
+
+  describe('#getValidResolution()', function() {
+    let view;
+    const defaultMaxRes = 156543.03392804097;
+
+    it('works correctly by snapping to power of 2', function() {
+      view = new View();
+      expect(view.getValidResolution(1000000)).to.be(defaultMaxRes);
+      expect(view.getValidResolution(defaultMaxRes / 8)).to.be(defaultMaxRes / 8);
+    });
+    it('works correctly by snapping to a custom zoom factor', function() {
+      view = new View({
+        maxResolution: 2500,
+        zoomFactor: 5,
+        maxZoom: 4
+      });
+      expect(view.getValidResolution(90, 1)).to.be(100);
+      expect(view.getValidResolution(90, -1)).to.be(20);
+      expect(view.getValidResolution(20)).to.be(20);
+      expect(view.getValidResolution(5)).to.be(4);
+      expect(view.getValidResolution(1)).to.be(4);
+    });
+    it('works correctly with a specific resolution set', function() {
+      view = new View({
+        zoom: 0,
+        resolutions: [512, 256, 128, 64, 32, 16, 8]
+      });
+      expect(view.getValidResolution(1000, 1)).to.be(512);
+      expect(view.getValidResolution(260, 1)).to.be(512);
+      expect(view.getValidResolution(260)).to.be(256);
+      expect(view.getValidResolution(30)).to.be(32);
+      expect(view.getValidResolution(30, -1)).to.be(16);
+      expect(view.getValidResolution(4, -1)).to.be(8);
+    });
+  });
 });
 
 describe('ol.View.isNoopAnimation()', function() {