MATH-1244

Utility method "cosAngle".

src/changes/changes.xml

@@ -73,6 +73,9 @@ If the output is not quite correct, check for invisible trailing spaces!
         2-sample Kolmogorov-Smirnov tests. Error was causing incorrect p-values returned 
         by exactP and monteCarloP methods (used by default for small, mid-size samples).
       </action>
+      <action dev="erans" type="add" issue="MATH-1244">
+        Method "cosAngle" in "o.a.c.m.util.MathArrays".
+      </action>
       <action dev="tn" type="fix" issue="MATH-1240">
         "KolmogorovSmirnovTest#ksSum(...)" returned wrong result in case the provided
         t-parameters was zero. This affected the calculation of "approximateP(...)" for

src/main/java/org/apache/commons/math3/util/MathArrays.java

@@ -242,6 +242,17 @@ public class MathArrays {
         return FastMath.sqrt(sum);
     }
 
+    /**
+     * Calculates the cosine of the angle between two vectors.
+     *
+     * @param v1 Cartesian coordinates of the first vector.
+     * @param v2 Cartesian coordinates of the second vector.
+     * @return the cosine of the angle between the vectors.
+     */
+    public static double cosAngle(double[] v1, double[] v2) {
+        return linearCombination(v1, v2) / (safeNorm(v1) * safeNorm(v2));
+    }
+
     /**
      * Calculates the L<sub>2</sub> (Euclidean) distance between two points.
      *

src/test/java/org/apache/commons/math3/util/MathArraysTest.java

@@ -172,6 +172,61 @@ public class MathArraysTest {
         Assert.assertEquals(4, MathArrays.distanceInf(p1, p2));
     }
 
+    @Test
+    public void testCosAngle2D() {
+        double expected;
+
+        final double[] v1 = { 1, 0 };
+        expected = 1;
+        Assert.assertEquals(expected, MathArrays.cosAngle(v1, v1), 0d);
+
+        final double[] v2 = { 0, 1 };
+        expected = 0;
+        Assert.assertEquals(expected, MathArrays.cosAngle(v1, v2), 0d);
+
+        final double[] v3 = { 7, 7 };
+        expected = Math.sqrt(2) / 2;
+        Assert.assertEquals(expected, MathArrays.cosAngle(v1, v3), 1e-15);
+        Assert.assertEquals(expected, MathArrays.cosAngle(v3, v2), 1e-15);
+
+        final double[] v4 = { -5, 0 };
+        expected = -1;
+        Assert.assertEquals(expected, MathArrays.cosAngle(v1, v4), 0);
+
+        final double[] v5 = { -100, 100 };
+        expected = 0;
+        Assert.assertEquals(expected, MathArrays.cosAngle(v3, v5), 0);
+    }
+
+    @Test
+    public void testCosAngle3D() {
+        double expected;
+
+        final double[] v1 = { 1, 1, 0 };
+        expected = 1;
+        Assert.assertEquals(expected, MathArrays.cosAngle(v1, v1), 1e-15);
+
+        final double[] v2 = { 1, 1, 1 };
+        expected = Math.sqrt(2) / Math.sqrt(3);
+        Assert.assertEquals(expected, MathArrays.cosAngle(v1, v2), 1e-15);
+    }
+
+    @Test
+    public void testCosAngleExtreme() {
+        double expected;
+
+        final double tiny = 1e-200;
+        final double[] v1 = { tiny, tiny };
+        final double big = 1e200;
+        final double[] v2 = { -big, -big };
+        expected = -1;
+        Assert.assertEquals(expected, MathArrays.cosAngle(v1, v2), 1e-15);
+
+        final double[] v3 = { big, -big };
+        expected = 0;
+        Assert.assertEquals(expected, MathArrays.cosAngle(v1, v3), 1e-15);
+    }
+
     @Test
     public void testCheckOrder() {
         MathArrays.checkOrder(new double[] {-15, -5.5, -1, 2, 15},