Added signum to DerivativeStructure.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1373781 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/math3/analysis/differentiation/DerivativeStructure.java

@@ -402,6 +402,17 @@ public class DerivativeStructure implements FieldElement<DerivativeStructure>, S
         return FastMath.round(data[0]);
     }
 
+    /** Compute the signum of the instance.
+     * The signum is -1 for negative numbers, +1 for positive numbers and 0 otherwise
+     * @param a number on which evaluation is done
+     * @return -1.0, -0.0, +0.0, +1.0 or NaN depending on sign of a
+     */
+    public DerivativeStructure signum() {
+        return new DerivativeStructure(compiler.getFreeParameters(),
+                                       compiler.getOrder(),
+                                       FastMath.signum(data[0]));
+    }
+
     /**
      * Returns the instance with the sign of the argument.
      * A NaN {@code sign} argument is treated as positive.

src/test/java/org/apache/commons/math3/analysis/differentiation/DerivativeStructureTest.java

@@ -837,6 +837,29 @@ public class DerivativeStructureTest {
 
     }
 
+    @Test
+    public void testSignum() {
+
+        DerivativeStructure minusOne = new DerivativeStructure(1, 1, 0, -1.0);
+        Assert.assertEquals(-1.0, minusOne.signum().getPartialDerivative(0), 1.0e-15);
+        Assert.assertEquals( 0.0, minusOne.signum().getPartialDerivative(1), 1.0e-15);
+
+        DerivativeStructure plusOne = new DerivativeStructure(1, 1, 0, +1.0);
+        Assert.assertEquals(+1.0, plusOne.signum().getPartialDerivative(0), 1.0e-15);
+        Assert.assertEquals( 0.0, plusOne.signum().getPartialDerivative(1), 1.0e-15);
+
+        DerivativeStructure minusZero = new DerivativeStructure(1, 1, 0, -0.0);
+        Assert.assertEquals(-0.0, minusZero.signum().getPartialDerivative(0), 1.0e-15);
+        Assert.assertTrue(Double.doubleToLongBits(minusZero.signum().getValue()) < 0);
+        Assert.assertEquals( 0.0, minusZero.signum().getPartialDerivative(1), 1.0e-15);
+
+        DerivativeStructure plusZero = new DerivativeStructure(1, 1, 0, +0.0);
+        Assert.assertEquals(+0.0, plusZero.signum().getPartialDerivative(0), 1.0e-15);
+        Assert.assertTrue(Double.doubleToLongBits(plusZero.signum().getValue()) == 0);
+        Assert.assertEquals( 0.0, plusZero.signum().getPartialDerivative(1), 1.0e-15);
+
+    }
+
     @Test
     public void testCeilFloorRintLong() {