tdf104532 handle constraints for NEGBINOM.DIST correctly.

Also changed variable names for easier understanding of their meaning.

Change-Id: Iab558d7d1d9533f2a0c42e3d5f4acecead2e818e
Reviewed-on: https://gerrit.libreoffice.org/31807Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>

sc/source/core/tool/interpr3.cxx

@@ -1514,21 +1514,21 @@ void ScInterpreter::ScNegBinomDist_MS()
     if ( MustHaveParamCount( GetByte(), 4 ) )
     {
         bool bCumulative = GetBool();
-        double p      = GetDouble();                    // p
-        double r      = GetDouble();                    // r
-        double x      = GetDouble();                    // x
-        if ( r < 0.0 || x < 0.0 || p < 0.0 || p > 1.0 )
+        double p = GetDouble();                            // probability
+        double s = ::rtl::math::approxFloor(GetDouble());  // No of successes
+        double f = ::rtl::math::approxFloor(GetDouble());  // No of failures
+        if ( s < 1.0 || f < 0.0 || p < 0.0 || p > 1.0 )
             PushIllegalArgument();
         else
         {
             double q = 1.0 - p;
             if ( bCumulative )
-                PushDouble( 1.0 - GetBetaDist( q, x + 1, r ) );
+                PushDouble( 1.0 - GetBetaDist( q, f + 1, s ) );
             else
             {
-                double fFactor = pow( p, r );
-                for ( double i = 0.0; i < x; i++ )
-                    fFactor *= ( i + r ) / ( i + 1.0 ) * q;
+                double fFactor = pow( p, s );
+                for ( double i = 0.0; i < f; i++ )
+                    fFactor *= ( i + s ) / ( i + 1.0 ) * q;
                 PushDouble( fFactor );
             }
         }