add findPointOnPath geometry2d util function

- to be used to find pt on violin bezier curves

Author: etpinard

src/lib/geometry2d.js

@@ -193,3 +193,52 @@ exports.getVisibleSegment = function getVisibleSegment(path, bounds, buffer) {
             Math.abs(pt0.y - ptTotal.y) < 0.1
     };
 };
+
+/**
+ * Find point on SVG path corresponding to a given constraint coordinate
+ *
+ * @param {SVGPathElement} path
+ * @param {Number} val : constraint coordinate value
+ * @param {String} coord : 'x' or 'y' the constraint coordinate
+ * @param {Object} opts :
+ *  - {Number} pathLength : supply total path length before hand
+ *  - {Number} tolerance
+ *  - {Number} iterationLimit
+ * @return {SVGPoint}
+ */
+exports.findPointOnPath = function findPointOnPath(path, val, coord, opts) {
+    opts = opts || {};
+
+    var pathLength = opts.pathLength || path.getTotalLength();
+    var tolerance = opts.tolerance || 1e-3;
+    var iterationLimit = opts.iterationLimit || 30;
+
+    // if path starts at a val greater than the path tail (like on vertical violins),
+    // we must flip the sign of the compute diff.
+    var mul = path.getPointAtLength(0)[coord] > path.getPointAtLength(pathLength)[coord] ? -1 : 1;
+
+    var i = 0;
+    var b0 = 0;
+    var b1 = pathLength;
+    var mid;
+    var pt;
+    var diff;
+
+    while(i < iterationLimit) {
+        mid = (b0 + b1) / 2;
+        pt = path.getPointAtLength(mid);
+        diff = pt[coord] - val;
+
+        if(Math.abs(diff) < tolerance) {
+            return pt;
+        } else {
+            if(mul * diff > 0) {
+                b1 = mid;
+            } else {
+                b0 = mid;
+            }
+            i++;
+        }
+    }
+    return pt;
+};

src/lib/index.js

@@ -82,6 +82,7 @@ lib.segmentDistance = geom2dModule.segmentDistance;
 lib.getTextLocation = geom2dModule.getTextLocation;
 lib.clearLocationCache = geom2dModule.clearLocationCache;
 lib.getVisibleSegment = geom2dModule.getVisibleSegment;
+lib.findPointOnPath = geom2dModule.findPointOnPath;
 
 var extendModule = require('./extend');
 lib.extendFlat = extendModule.extendFlat;