added new zoom method as inverse of existing scale method

src/proj4leaflet.js

@@ -124,6 +124,33 @@
 				return baseScale + scaleDiff * zDiff;
 			}
 		},
+
+		zoom: function(scale) {
+			// Find closest number in this._scales, down 
+			var downScale = this._closestElement(this._scales, scale),
+				downZoom = this._scales.indexOf(downScale),
+				nextZoom,
+				scaleDiff;
+			// Check if scale is downScale => return array index
+			if (scale === downScale) {
+				return downZoom;
+			}
+			// Interpolate
+			nextZoom = downZoom + 1;
+			scaleDiff = this._scales[nextZoom] - downScale;
+			return (scale - downScale) / scaleDiff + downZoom;
+		},
+
+		/* Get the closest lowest element in an array */
+		_closestElement: function(array, element) {
+			var low;
+			for (var i = array.length; i--;) {
+				if (array[i] <= element && (low === undefined || low < array[i])) {
+					low = array[i];
+				}
+			}
+			return low;
+		}
 	});
 
 	L.Proj.GeoJSON = L.GeoJSON.extend({

test/specs.js

@@ -27,7 +27,7 @@ describe('L.Proj.CRS', function() {
 			'+towgs84=414.1,41.3,603.1,-0.855,2.141,-7.023,0 +no_defs');
 
 		var pp = crs.project(new L.LatLng(55.723337, 14.194313));
-		expect(pp.x).toBeCloseTo(1398776, 0)
+		expect(pp.x).toBeCloseTo(1398776, 0);
 		expect(pp.y).toBeCloseTo(6178304, 0);
 	});
 
@@ -135,4 +135,20 @@ describe('L.Proj.CRS', function() {
 			worldSize *= 2;
 		}
 	});
+	it('convert zoom to scale and viceversa and return the same values', function () {
+		 var crs = new L.Proj.CRS('EPSG:3006',
+				'+proj=utm +zone=33 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs',
+		{
+			resolutions: [
+				8192, 4096, 2048, 1024, 512, 256, 128,
+				64, 32, 16, 8, 4, 2, 1, 0.5
+			]
+		});
+
+		expect(crs.zoom(crs.scale(8.9578457485))).toBe(8.9578457485);
+		expect(crs.zoom(crs.scale(8))).toBe(8);
+		expect(crs.zoom(crs.scale(1/8191))).toBeCloseTo(1/8191, 6);
+		expect(crs.zoom(crs.scale(0.5))).toBe(0.5);
+		expect(crs.zoom(crs.scale(0.51))).toBe(0.51);
+	});
 });