mirrorAngle(HALF_PI) should be -HALFPI rather than HALF_PI

src/main/java/pythagoras/f/MathUtil.java

@@ -169,7 +169,7 @@ public class MathUtil
      * Returns the mirror angle of the specified angle (assumed to be in [-pi, +pi]).
      */
     public static float mirrorAngle (float a) {
-        return (a > 0f ? FloatMath.PI : -FloatMath.PI) - a;
+        return a + (a > 0f ? -FloatMath.PI : FloatMath.PI);
     }
 
     /**

src/test/java/pythagoras/f/MathUtilTest.java

@@ -0,0 +1,19 @@
+//
+// Pythagoras - a collection of geometry classes
+// http://github.com/samskivert/pythagoras
+
+package pythagoras.f;
+
+import org.junit.*;
+import static org.junit.Assert.*;
+
+/**
+ * Tests parts of the {@link MathUtil} class.
+ */
+public class MathUtilTest
+{
+    @Test public void testMirrorAngle() {
+        assertEquals(-MathUtil.HALF_PI, MathUtil.mirrorAngle(MathUtil.HALF_PI), MathUtil.EPSILON);
+        assertEquals(MathUtil.HALF_PI, MathUtil.mirrorAngle(-MathUtil.HALF_PI), MathUtil.EPSILON);
+    }
+}