Added copySign to DerivativeStructure.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1373777 13f79535-47bb-0310-9956-ffa450edef68

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

@@ -386,6 +386,22 @@ public class DerivativeStructure implements FieldElement<DerivativeStructure>, S
                                        FastMath.floor(data[0]));
     }
 
+    /**
+     * Returns the instance with the sign of the argument.
+     * A NaN {@code sign} argument is treated as positive.
+     *
+     * @param sign the sign for the returned value
+     * @return the instance with the same sign as the {@code sign} argument
+     */
+    public DerivativeStructure copySign(final double sign){
+        long m = Double.doubleToLongBits(data[0]);
+        long s = Double.doubleToLongBits(sign);
+        if ((m >= 0 && s >= 0) || (m < 0 && s < 0)) { // Sign is currently OK
+            return this;
+        }
+        return negate(); // flip sign
+    }
+
     /** {@inheritDoc} */
     public DerivativeStructure reciprocal() {
         final DerivativeStructure result = new DerivativeStructure(compiler);

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

@@ -765,6 +765,21 @@ public class DerivativeStructureTest {
 
     }
 
+    @Test
+    public void testCopySign() {
+        DerivativeStructure minusOne = new DerivativeStructure(1, 1, 0, -1.0);
+        Assert.assertEquals(+1.0, minusOne.copySign(+1.0).getPartialDerivative(0), 1.0e-15);
+        Assert.assertEquals(-1.0, minusOne.copySign(+1.0).getPartialDerivative(1), 1.0e-15);
+        Assert.assertEquals(-1.0, minusOne.copySign(-1.0).getPartialDerivative(0), 1.0e-15);
+        Assert.assertEquals(+1.0, minusOne.copySign(-1.0).getPartialDerivative(1), 1.0e-15);
+        Assert.assertEquals(+1.0, minusOne.copySign(+0.0).getPartialDerivative(0), 1.0e-15);
+        Assert.assertEquals(-1.0, minusOne.copySign(+0.0).getPartialDerivative(1), 1.0e-15);
+        Assert.assertEquals(-1.0, minusOne.copySign(-0.0).getPartialDerivative(0), 1.0e-15);
+        Assert.assertEquals(+1.0, minusOne.copySign(-0.0).getPartialDerivative(1), 1.0e-15);
+        Assert.assertEquals(+1.0, minusOne.copySign(Double.NaN).getPartialDerivative(0), 1.0e-15);
+        Assert.assertEquals(-1.0, minusOne.copySign(Double.NaN).getPartialDerivative(1), 1.0e-15);
+    }
+
     @Test
     public void testField() {
         for (int maxOrder = 1; maxOrder < 5; ++maxOrder) {