Add a wrapAngle utility function.

This takes a value in radians and returns a wrapped value in the range
of [-PI, PI).

src/mapInteractor.js

@@ -1156,10 +1156,8 @@ var mapInteractor = function (args) {
     }
     if (m_state.zoomrotateAllowRotation) {
       var theta = m_state.initialRotation + deltaTheta;
-      /* Compute the delta in the range of [-PI, PI).  This is involed to work
-       * around modulo returning a signed value. */
-      deltaTheta = ((theta - m_this.map().rotation()) % (Math.PI * 2) +
-                    Math.PI * 3) % (Math.PI * 2) - Math.PI;
+      /* Compute the delta in the range of [-PI, PI). */
+      deltaTheta = util.wrapAngle(theta - m_this.map().rotation());
       /* If we reverse direction, don't rotate until some threshold is
        * exceeded.  This helps prevent rotation bouncing while panning. */
       if (deltaTheta && (deltaTheta * (m_state.lastRotationDelta || 0) >= 0 ||

src/util/index.js

@@ -1020,6 +1020,23 @@ var util = module.exports = {
     return {min: min, max: max};
   },
 
+  /**
+   * Given a value in radians, return a value wrapped to the range [-PI, PI).
+   *
+   * @param {number} value A value in radians.
+   * @returns {number} The wrapped value.
+   */
+  wrapAngle: function (value) {
+    /* Module will only ensure that this is between [-2 PI, 2 PI). */
+    value = value % (Math.PI * 2);
+    if (value < -Math.PI) {
+      value += Math.PI * 2;
+    } else if (value >= Math.PI) {
+      value -= Math.PI * 2;
+    }
+    return value;
+  },
+
   /**
    * Escape any character in a string that has a code point >= 127.
    *

tests/cases/util.js

@@ -124,4 +124,16 @@ describe('geo.util', function () {
     expect(util.getMinMaxValues(values, 200, 300, true)).toEqual({min: 200, max: 300});
     expect(util.getMinMaxValues(values, 100, 400, true)).toEqual({min: 181, max: 303});
   });
+
+  it('wrapAngle', function () {
+    expect(util.wrapAngle(0)).toBe(0);
+    expect(util.wrapAngle(2)).toBe(2);
+    expect(util.wrapAngle(4)).toBe(4 - Math.PI * 2);
+    expect(util.wrapAngle(7)).toBe(7 - Math.PI * 2);
+    expect(util.wrapAngle(17)).toBe(17 - Math.PI * 6);
+    expect(util.wrapAngle(-2)).toBe(-2);
+    expect(util.wrapAngle(-4)).toBe(-4 + Math.PI * 2);
+    expect(util.wrapAngle(-7)).toBe(-7 + Math.PI * 2);
+    expect(util.wrapAngle(-17)).toBe(-17 + Math.PI * 6);
+  });
 });