Implement Curve#getOffsetAtTime()

CHANGELOG.md

@@ -169,6 +169,8 @@ contribute to the code.
   matrix (#972).
 - Allow `Item#position` to be selected via `Item#position.selected` (#980).
 - Add `tolerance` argument to `Path#join(path, tolerance)`.
+- Add `Curve#getOffsetAtTime(time)`, as the reverse of
+  `Curve#getTimeAt(offset)`.
 
 ### Fixed
 - Fix calculations of `Item#strokeBounds` for all possible combinations of

src/path/Curve.js

@@ -1050,6 +1050,17 @@ statics: /** @lends Curve */{
      */
     getParameterAt: '#getTimeAt',
 
+    /**
+     * Calculates the curve offset at the specified curve-time parameter on
+     * the curve.
+     *
+     * @param {Number} time the curve-time parameter on the curve
+     * @return {Number} the curve offset at the specified the location
+     */
+    getOffsetAtTime: function(t) {
+        return this.getPartLength(0, t);
+    },
+
     /**
      * Returns the curve location of the specified point if it lies on the
      * curve, `null` otherwise.

test/tests/Curve.js

@@ -160,19 +160,22 @@ test('Curve#getTimeAt()', function() {
     ]).firstCurve;
 
     for (var f = 0; f <= 1; f += 0.1) {
-        var o1 = curve.length * f;
-        var o2 = -curve.length * (1 - f);
+        var o1 = curve.length * f,
+            o2 = -curve.length * (1 - f),
+            t1 = curve.getTimeAt(o1),
+            t2 = curve.getTimeAt(o2);
         var message = 'Curve-time parameter at offset ' + o1
                 + ' should be the same value as at offset ' + o2;
-        equals(curve.getTimeAt(o1), curve.getTimeAt(o2), message,
-                Numerical.CURVETIME_EPSILON);
+        equals(t1, t2, message, Numerical.CURVETIME_EPSILON);
+        equals(function() { return curve.getOffsetAtTime(t1); }, o1);
+        equals(function() { return curve.getOffsetAtTime(t2); }, curve.length + o2);
         // Legacy version:
         equals(curve.getParameterAt(o1), curve.getParameterAt(o2),
                 'Legacy: ' + message, Numerical.CURVETIME_EPSILON);
+        // Test other methods with negatives offsets
         equals(curve.getTangentAt(o1), curve.getTangentAt(o2),
                 'Tangent at offset ' + o1
-                + ' should be the same value as at offset ' + o2,
-                Numerical.CURVETIME_EPSILON);
+                + ' should be the same value as at offset ' + o2);
     }
 
     equals(curve.getTimeAt(curve.length + 1), null,