Added Vector.cross() & co.

Closes #10.

src/main/java/pythagoras/f/AbstractVector.java

@@ -17,6 +17,17 @@ public abstract class AbstractVector implements IVector
         return x()*other.x() + y()*other.y();
     }
 
+    @Override // from interface IVector
+    public Vector cross (IVector other) {
+        return cross(other, new Vector());
+    }
+
+    @Override // from interface IVector
+    public Vector cross (IVector other, Vector result) {
+        float x = x(), y = y(), ox = other.x(), oy = other.y();
+        return result.set(y*ox - x*oy, x*oy - y*ox);
+    }
+
     @Override // from interface IVector
     public Vector negate () {
         return negate(new Vector());

src/main/java/pythagoras/f/IVector.java

@@ -18,6 +18,15 @@ public interface IVector
     /** Computes and returns the dot product of this and the specified other vector. */
     float dot (IVector other);
 
+    /** Computes the cross product of this and the specified other vector.
+     * @return a new vector containing the result. */
+    Vector cross (IVector other);
+
+    /** Computes the cross product of this and the specified other vector, placing the result in
+     * the object supplied.
+     * @return a reference to the result, for chaining. */
+    Vector cross (IVector other, Vector result);
+
     /** Negates this vector.
      * @return a new vector containing the result. */
     Vector negate ();

src/main/java/pythagoras/f/Vector.java

@@ -29,6 +29,13 @@ public class Vector extends AbstractVector
     public Vector () {
     }
 
+    /** Computes the cross product of this and the specified other vector, storing the result in
+     * this vector.
+     * @return a reference to this vector, for chaining. */
+    public Vector crossLocal (IVector other) {
+        return cross(other, this);
+    }
+
     /** Negates this vector in-place.
      * @return a reference to this vector, for chaining. */
     public Vector negateLocal () {