MATH-1269: fixed FastMath.exp returning NaN for non-NaN arguments

src/changes/changes.xml

@@ -54,6 +54,9 @@ If the output is not quite correct, check for invisible trailing spaces!
     </release>
 
     <release version="4.0" date="XXXX-XX-XX" description="">
+      <action dev="oertl" type="fix" issue="MATH-1269"> <!-- backported to 3.6 -->
+        Fixed FastMath.exp that potentially returned NaN for non-NaN argument.
+      </action>
      <action dev="luc" type="add"> <!-- backported to 3.6 -->
        Added a nth order Brent solver for general real fields, replacing the former
        solver that was restricted to Dfp fields only.

src/main/java/org/apache/commons/math4/util/FastMath.java

@@ -968,6 +968,13 @@ public class FastMath {
        much larger than the others.  If there are extra bits specified from the
        pow() function, use them. */
         final double tempC = tempB + tempA;
+
+        // If tempC is positive infinite, the evaluation below could result in NaN,
+        // because z could be negative at the same time.
+        if (tempC == Double.POSITIVE_INFINITY) {
+            return Double.POSITIVE_INFINITY;
+        }
+
         final double result;
         if (extra != 0.0) {
             result = tempC*extra*z + tempC*extra + tempC*z + tempB + tempA;

src/test/java/org/apache/commons/math4/util/FastMathTest.java

@@ -217,6 +217,15 @@ public class FastMathTest {
         }
         Assert.assertEquals(0, maxErr, 3);
     }
+    
+    @Test
+    public void testMath1269() {
+        final double arg = 709.8125;
+        final double vM = Math.exp(arg);
+        final double vFM = FastMath.exp(arg);
+        Assert.assertTrue("exp(" + arg + ") is " + vFM + " instead of " + vM,
+                          Precision.equalsIncludingNaN(vM, vFM));
+    }
 
     @Test
     public void testHyperbolicInverses() {