Added XY, made IPoint.subtract return Point.

XY provides a common super-interface for IPoint and IVector for APIs that don't
care if you have a point or vector, and just want it's x/y coordinates.

Returning a vector when subtracting two points was clever and all, but turns
out to be annoying. More often, you're just adjusting a point and don't want to
switch to vector-baesd math. If we want an easy way to obtain a vector that is
the difference of two points we can add Vectors.subtract() or a new
constructor.

src/main/java/pythagoras/d/AbstractPoint.java

@@ -58,17 +58,17 @@ public abstract class AbstractPoint implements IPoint
     }
 
     @Override // from IPoint
-    public Vector subtract (double x, double y) {
-        return subtract(x, y, new Vector());
+    public Point subtract (double x, double y) {
+        return subtract(x, y, new Point());
     }
 
     @Override
-    public Vector subtract (double x, double y, Vector result) {
+    public Point subtract (double x, double y, Point result) {
         return result.set(x() - x, y() - y);
     }
 
     @Override
-    public Vector subtract (IPoint other, Vector result) {
+    public Point subtract (IPoint other, Point result) {
         return subtract(other.x(), other.y(), result);
     }
 

src/main/java/pythagoras/d/IPoint.java

@@ -7,14 +7,8 @@ package pythagoras.d;
 /**
  * Provides read-only access to a {@link Point}.
  */
-public interface IPoint extends Cloneable
+public interface IPoint extends XY, Cloneable
 {
-    /** Returns this point's x-coordinate. */
-    double x ();
-
-    /** Returns this point's y-coordinate. */
-    double y ();
-
     /** Returns the squared Euclidian distance between this point and the specified point. */
     double distanceSq (double px, double py);
 
@@ -48,16 +42,16 @@ public interface IPoint extends Cloneable
     Point add (double x, double y, Point result);
 
     /** Subtracts the supplied point from {@code this}.
-     * @return a new vector containing the result. */
-    Vector subtract (double x, double y);
+     * @return a new point containing the result. */
+    Point subtract (double x, double y);
 
     /** Subtracts the supplied point from {@code this} and stores the result in {@code result}.
      * @return a reference to the result, for chaining. */
-    Vector subtract (double x, double y, Vector result);
+    Point subtract (double x, double y, Point result);
 
     /** Subtracts the supplied point from {@code this} and stores the result in {@code result}.
      * @return a reference to the result, for chaining. */
-    Vector subtract (IPoint other, Vector result);
+    Point subtract (IPoint other, Point result);
 
     /** Rotates this point around the origin by the specified angle.
      * @return a new point containing the result. */

src/main/java/pythagoras/d/IVector.java

@@ -7,14 +7,8 @@ package pythagoras.d;
 /**
  * Provides read-only access to a {@link Vector}.
  */
-public interface IVector
+public interface IVector extends XY
 {
-    /** Returns the x-component of this vector. */
-    double x ();
-
-    /** Returns the y-component of this vector. */
-    double y ();
-
     /** Computes and returns the dot product of this and the specified other vector. */
     double dot (IVector other);
 

src/main/java/pythagoras/d/Point.java

@@ -69,12 +69,18 @@ public class Point extends AbstractPoint implements Serializable
         return rotate(angle, this);
     }
 
-    @Override // from interface IPoint
+    /** Subtracts the supplied x/y from this point.
+     * @return a reference to this point, for chaining. */
+    public Point subtractLocal (double x, double y) {
+        return subtract(x, y, this);
+    }
+
+    @Override // from XY
     public double x () {
         return x;
     }
 
-    @Override // from interface IPoint
+    @Override // from XY
     public double y () {
         return y;
     }

src/main/java/pythagoras/d/Vector.java

@@ -139,12 +139,12 @@ public class Vector extends AbstractVector
         return normalizeLocal().scaleLocal(length);
     }
 
-    @Override // from AbstractVector
+    @Override // from XY
     public double x () {
         return x;
     }
 
-    @Override // from AbstractVector
+    @Override // from XY
     public double y () {
         return y;
     }

src/main/java/pythagoras/d/XY.java

@@ -0,0 +1,19 @@
+//
+// Pythagoras - a collection of geometry classes
+// http://github.com/samskivert/pythagoras
+
+package pythagoras.d;
+
+/**
+ * Defines an x/y coordinate. This is implemented by both {@code Point} and {@code Vector} so that
+ * APIs which require an x/y coordinate, but don't really want to mak the distinction between a
+ * translation vector versus a point in 2D space, can simply accept both.
+ */
+public interface XY
+{
+    /** The x coordinate. */
+    double x ();
+
+    /** The y coordinate. */
+    double y ();
+}

src/main/java/pythagoras/f/AbstractPoint.java

@@ -58,17 +58,17 @@ public abstract class AbstractPoint implements IPoint
     }
 
     @Override // from IPoint
-    public Vector subtract (float x, float y) {
-        return subtract(x, y, new Vector());
+    public Point subtract (float x, float y) {
+        return subtract(x, y, new Point());
     }
 
     @Override
-    public Vector subtract (float x, float y, Vector result) {
+    public Point subtract (float x, float y, Point result) {
         return result.set(x() - x, y() - y);
     }
 
     @Override
-    public Vector subtract (IPoint other, Vector result) {
+    public Point subtract (IPoint other, Point result) {
         return subtract(other.x(), other.y(), result);
     }
 

src/main/java/pythagoras/f/IPoint.java

@@ -7,14 +7,8 @@ package pythagoras.f;
 /**
  * Provides read-only access to a {@link Point}.
  */
-public interface IPoint extends Cloneable
+public interface IPoint extends XY, Cloneable
 {
-    /** Returns this point's x-coordinate. */
-    float x ();
-
-    /** Returns this point's y-coordinate. */
-    float y ();
-
     /** Returns the squared Euclidian distance between this point and the specified point. */
     float distanceSq (float px, float py);
 
@@ -48,16 +42,16 @@ public interface IPoint extends Cloneable
     Point add (float x, float y, Point result);
 
     /** Subtracts the supplied point from {@code this}.
-     * @return a new vector containing the result. */
-    Vector subtract (float x, float y);
+     * @return a new point containing the result. */
+    Point subtract (float x, float y);
 
     /** Subtracts the supplied point from {@code this} and stores the result in {@code result}.
      * @return a reference to the result, for chaining. */
-    Vector subtract (float x, float y, Vector result);
+    Point subtract (float x, float y, Point result);
 
     /** Subtracts the supplied point from {@code this} and stores the result in {@code result}.
      * @return a reference to the result, for chaining. */
-    Vector subtract (IPoint other, Vector result);
+    Point subtract (IPoint other, Point result);
 
     /** Rotates this point around the origin by the specified angle.
      * @return a new point containing the result. */

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

@@ -7,14 +7,8 @@ package pythagoras.f;
 /**
  * Provides read-only access to a {@link Vector}.
  */
-public interface IVector
+public interface IVector extends XY
 {
-    /** Returns the x-component of this vector. */
-    float x ();
-
-    /** Returns the y-component of this vector. */
-    float y ();
-
     /** Computes and returns the dot product of this and the specified other vector. */
     float dot (IVector other);
 

src/main/java/pythagoras/f/Point.java

@@ -69,12 +69,18 @@ public class Point extends AbstractPoint implements Serializable
         return rotate(angle, this);
     }
 
-    @Override // from interface IPoint
+    /** Subtracts the supplied x/y from this point.
+     * @return a reference to this point, for chaining. */
+    public Point subtractLocal (float x, float y) {
+        return subtract(x, y, this);
+    }
+
+    @Override // from XY
     public float x () {
         return x;
     }
 
-    @Override // from interface IPoint
+    @Override // from XY
     public float y () {
         return y;
     }

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

@@ -139,12 +139,12 @@ public class Vector extends AbstractVector
         return normalizeLocal().scaleLocal(length);
     }
 
-    @Override // from AbstractVector
+    @Override // from XY
     public float x () {
         return x;
     }
 
-    @Override // from AbstractVector
+    @Override // from XY
     public float y () {
         return y;
     }

src/main/java/pythagoras/f/XY.java

@@ -0,0 +1,19 @@
+//
+// Pythagoras - a collection of geometry classes
+// http://github.com/samskivert/pythagoras
+
+package pythagoras.f;
+
+/**
+ * Defines an x/y coordinate. This is implemented by both {@code Point} and {@code Vector} so that
+ * APIs which require an x/y coordinate, but don't really want to mak the distinction between a
+ * translation vector versus a point in 2D space, can simply accept both.
+ */
+public interface XY
+{
+    /** The x coordinate. */
+    float x ();
+
+    /** The y coordinate. */
+    float y ();
+}