add vec2.setLength. fixes #393

spec/gl-matrix/vec2-spec.js

@@ -337,6 +337,16 @@ describe("vec2", function() {
         it("should return the squared length", function() { expect(result).toEqual(5); });
     });
 
+    describe("setLength", function() {
+        const vecA = [0, 0];
+        const vecB = [0.5, 0];
+        const result = vec2.setLength(vecA, vecB, 10);
+
+        it("should return the updated vector", function() { expect(result).toBeEqualish([10, 0]); });
+        it("should modify vecA", function() { expect(vecA).toBeEqualish([10, 0]); });
+        it("should not modify vecB", function() { expect(vecB).toBeEqualish([0.5, 0]); });
+    });
+
     describe("negate", function() {
         describe("with a separate output vector", function() {
             beforeEach(function() { result = vec2.negate(out, vecA); });

src/vec2.js

@@ -275,6 +275,20 @@ export function squaredLength(a) {
   return x * x + y * y;
 }
 
+/**
+ * Set the length of a 2D vector
+ * @param {vec2} out The receiving vec2
+ * @param {vec2} a the vector to lengthen
+ * @param {Number} length of the resulting vector
+ * @returns {vec3} out
+ */
+export function setLength(out, a, length) {
+  const angle = Math.atan2(a[1], a[0]);
+  out[0] = Math.cos(angle) * length;
+  out[1] = Math.sin(angle) * length;
+  return out;
+}
+
 /**
  * Negates the components of a vec2
  *