improved rounding in MathX.mod, see Bugzilla 50033

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1003504 13f79535-47bb-0310-9956-ffa450edef68

src/documentation/content/xdocs/status.xml

@@ -34,6 +34,7 @@
 
     <changes>
         <release version="3.7-beta4" date="2010-??-??">
+           <action dev="poi-developers" type="fix">50033 - Improved rounding in MOD</action>
            <action dev="poi-developers" type="add">Generate SHA1 hashes of distribution files, alongside existing MD5 ones</action>
         </release>
         <release version="3.7-beta3" date="2010-09-24">

src/java/org/apache/poi/hssf/record/formula/functions/MathX.java

@@ -17,6 +17,7 @@
 
 package org.apache.poi.hssf.record.formula.functions;
 
+
 /**
  * @author Amol S. Deshmukh < amolweb at ya hoo dot com >
  * This class is an extension to the standard math library
@@ -349,21 +350,15 @@ final class MathX {
             result = Double.NaN;
         }
         else if (sign(n) == sign(d)) {
-            double t = Math.abs(n / d);
-            t = t - (long) t;
-            result = sign(d) * Math.abs(t * d);
+            result = n % d;
         }
         else {
-            double t = Math.abs(n / d);
-            t = t - (long) t;
-            t = Math.ceil(t) - t;
-            result = sign(d) * Math.abs(t * d);
+            result = ((n % d) + d) % d;
         }
 
         return result;
     }
 
-
     /**
      * inverse hyperbolic cosine
      * @param d

src/testcases/org/apache/poi/hssf/record/formula/functions/TestMathX.java

@@ -292,6 +292,20 @@ public class TestMathX extends AbstractNumericTestCase {
     }
 
     public void testMod() {
+
+        //example from Excel help
+        assertEquals(1.0, MathX.mod(3, 2));
+        assertEquals(1.0, MathX.mod(-3, 2));
+        assertEquals(-1.0, MathX.mod(3, -2));
+        assertEquals(-1.0, MathX.mod(-3, -2));
+
+        assertEquals((double) 1.4, MathX.mod(3.4, 2));
+        assertEquals((double) -1.4, MathX.mod(-3.4, -2));
+        assertEquals((double) 0.6000000000000001, MathX.mod(-3.4, 2.0));// should actually be 0.6
+        assertEquals((double) -0.6000000000000001, MathX.mod(3.4, -2.0));// should actually be -0.6
+
+        // Bugzilla 50033
+        assertEquals(1.0, MathX.mod(13, 12));
     }
 
     public void testNChooseK() {