Dedicated function for inverting a transform without modifying the source

src/ol/transform.js

@@ -203,30 +203,40 @@ export function compose(transform, dx1, dy1, sx, sy, angle, dx2, dy2) {
 
 /**
  * Invert the given transform.
- * @param {!Transform} transform Transform.
+ * @param {!Transform} source The source transform to invert.
- * @return {!Transform} Inverse of the transform.
+ * @return {!Transform} The inverted (source) transform.
  */
-export function invert(transform) {
+export function invert(source) {
-  const det = determinant(transform);
+  return makeInverse(source, source);
-  assert(det !== 0, 32); // Transformation matrix cannot be inverted
-
-  const a = transform[0];
-  const b = transform[1];
-  const c = transform[2];
-  const d = transform[3];
-  const e = transform[4];
-  const f = transform[5];
-
-  transform[0] = d / det;
-  transform[1] = -b / det;
-  transform[2] = -c / det;
-  transform[3] = a / det;
-  transform[4] = (c * f - d * e) / det;
-  transform[5] = -(a * f - b * e) / det;
-
-  return transform;
 }
 
+/**
+ * Invert the given transform.
+ * @param {!Transform} source The source transform to invert.
+ * @param {!Transform} target Transform to be set as the inverse of
+ *     the source transform.
+ * @return {!Transform} The inverted (target) transform.
+ */
+export function makeInverse(source, target) {
+  const det = determinant(source);
+  assert(det !== 0, 32); // Transformation matrix cannot be inverted
+
+  const a = source[0];
+  const b = source[1];
+  const c = source[2];
+  const d = source[3];
+  const e = source[4];
+  const f = source[5];
+
+  target[0] = d / det;
+  target[1] = -b / det;
+  target[2] = -c / det;
+  target[3] = a / det;
+  target[4] = (c * f - d * e) / det;
+  target[5] = -(a * f - b * e) / det;
+
+  return target;
+}
 
 /**
  * Returns the determinant of the given matrix.

test/spec/ol/transform.test.js

@@ -9,6 +9,7 @@ import {
   multiply,
   compose,
   invert,
+  makeInverse,
   apply
 } from '../../../src/ol/transform.js';
 
@@ -110,13 +111,39 @@ describe('ol.transform', function() {
 
   describe('invert()', function() {
     it('inverts a transform', function() {
-      let transform = [1, 0, 1, 0, 1, 0];
+      const transform = [1, 1, 1, 2, 2, 0];
-      expect(function() {
-        invert(transform);
-      }).to.throwException();
-      transform = [1, 1, 1, 2, 2, 0];
       expect(invert(transform)).to.eql([2, -1, -1, 1, -4, 2]);
-      expect(transform).to.eql([2, -1, -1, 1, -4, 2]);
+    });
+
+    it('throws if the transform cannot be inverted', function() {
+      const indeterminant = [1, 0, 1, 0, 1, 0];
+      expect(function() {
+        invert(indeterminant);
+      }).to.throwException();
+    });
+
+    it('modifies the source', function() {
+      const source = [1, 1, 1, 2, 2, 0];
+      const inverted = invert(source);
+      expect(inverted).to.eql([2, -1, -1, 1, -4, 2]);
+      expect(source).to.be(inverted);
+    });
+  });
+
+  describe('makeInverse()', function() {
+    it('makes the target the inverse of the source', function() {
+      const source = [1, 1, 1, 2, 2, 0];
+      const target = [1, 0, 0, 1, 0, 0];
+      makeInverse(source, target);
+      expect(source).to.eql([1, 1, 1, 2, 2, 0]);
+      expect(target).to.eql([2, -1, -1, 1, -4, 2]);
+    });
+
+    it('returns the target', function() {
+      const source = [1, 1, 1, 2, 2, 0];
+      const target = [1, 0, 0, 1, 0, 0];
+      const inverted = makeInverse(source, target);
+      expect(target).to.be(inverted);
     });
   });