Added abs to DerivativeStructure.

We handle +0.0/-0.0 correctly, i.e. we flip derivatives sign when value
is negative zero.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1373775 13f79535-47bb-0310-9956-ffa450edef68

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

@@ -355,6 +355,18 @@ public class DerivativeStructure implements FieldElement<DerivativeStructure>, S
         return ds;
     }
 
+    /** absolute value.
+     * @return abs(this)
+     */
+    public DerivativeStructure abs() {
+        if (Double.doubleToLongBits(data[0]) < 0) {
+            // we use the bits representation to also handle -0.0
+            return negate();
+        } else {
+            return this;
+        }
+    }
+
     /** {@inheritDoc} */
     public DerivativeStructure reciprocal() {
         final DerivativeStructure result = new DerivativeStructure(compiler);

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

@@ -731,6 +731,27 @@ public class DerivativeStructureTest {
         }
     }
 
+    @Test
+    public void testAbs() {
+
+        DerivativeStructure minusOne = new DerivativeStructure(1, 1, 0, -1.0);
+        Assert.assertEquals(+1.0, minusOne.abs().getPartialDerivative(0), 1.0e-15);
+        Assert.assertEquals(-1.0, minusOne.abs().getPartialDerivative(1), 1.0e-15);
+
+        DerivativeStructure plusOne = new DerivativeStructure(1, 1, 0, +1.0);
+        Assert.assertEquals(+1.0, plusOne.abs().getPartialDerivative(0), 1.0e-15);
+        Assert.assertEquals(+1.0, plusOne.abs().getPartialDerivative(1), 1.0e-15);
+
+        DerivativeStructure minusZero = new DerivativeStructure(1, 1, 0, -0.0);
+        Assert.assertEquals(+0.0, minusZero.abs().getPartialDerivative(0), 1.0e-15);
+        Assert.assertEquals(-1.0, minusZero.abs().getPartialDerivative(1), 1.0e-15);
+
+        DerivativeStructure plusZero = new DerivativeStructure(1, 1, 0, +0.0);
+        Assert.assertEquals(+0.0, plusZero.abs().getPartialDerivative(0), 1.0e-15);
+        Assert.assertEquals(+1.0, plusZero.abs().getPartialDerivative(1), 1.0e-15);
+
+    }
+
     @Test
     public void testField() {
         for (int maxOrder = 1; maxOrder < 5; ++maxOrder) {