Resolves: tdf#46119 implement GeneralFunction_VAR, VARP, STDEV and STDEVP

These were never implemented. Likely because they aren't used
internally by Calc, which for formula expressions in the
interpreter and for DataPilot / pivot table uses a different
approach, but they are needed for
css::sheet::XSheetOperation::computeFunction()

Change-Id: I1af038bf9db8d0c04d69598b992b827b083e2248
Reviewed-on: https://gerrit.libreoffice.org/64957Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jenkins

sc/inc/subtotal.hxx

@@ -30,11 +30,32 @@ public:
     static bool SafeDiv( double& fVal1, double fVal2);
 };
 
+/** Implements the Welford Online one-pass algorithm.
+    See https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_Online_algorithm
+    and Donald E. Knuth, TAoCP vol.2, 3rd edn., p. 232
+ */
+class WelfordRunner
+{
+public:
+    WelfordRunner() : fMean(0.0), fM2(0.0), nCount(0) {}
+    void        update( double fVal );
+    sal_uInt64  getCount() const                { return nCount; }
+    double      getMean() const                 { return fMean; }
+    double      getVarianceSample() const       { return nCount > 1 ? fM2 / (nCount-1) : 0.0; }
+    double      getVariancePopulation() const   { return nCount > 0 ? fM2 / nCount : 0.0; }
+
+private:
+    double      fMean;
+    double      fM2;
+    sal_Int64   nCount;
+};
+
 struct ScFunctionData                   // to calculate single functions
 {
+    WelfordRunner   maWelford;
     ScSubTotalFunc const  eFunc;
     double          nVal;
-    long            nCount;
+    sal_uInt64      nCount;
     bool            bError;
 
     ScFunctionData( ScSubTotalFunc eFn ) :

sc/source/core/data/column2.cxx

@@ -3423,10 +3423,20 @@ class UpdateSubTotalHandler
                     mrData.nVal = fVal;
             }
             break;
-            default:
+            case SUBTOTAL_FUNC_VAR:
+            case SUBTOTAL_FUNC_VARP:
+            case SUBTOTAL_FUNC_STD:
+            case SUBTOTAL_FUNC_STDP:
             {
-                // added to avoid warnings
+                if (!bVal)
+                    return;
+
+                mrData.maWelford.update( fVal);
             }
+            break;
+            default:
+                // unhandled unknown
+                mrData.bError = true;
         }
     }
 

sc/source/core/data/documen4.cxx

@@ -642,10 +642,43 @@ bool ScDocument::GetSelectionFunction( ScSubTotalFunc eFunc,
                 else
                     aData.bError = true;
                 break;
-            default:
+            case SUBTOTAL_FUNC_VAR:
+            case SUBTOTAL_FUNC_STD:
+                if (aData.maWelford.getCount() < 2)
+                    aData.bError = true;
+                else
+                {
+                    rResult = aData.maWelford.getVarianceSample();
+                    if (eFunc == SUBTOTAL_FUNC_STD)
+                    {
+                        if (rResult < 0.0)
+                            aData.bError = true;
+                        else
+                            rResult = sqrt( rResult);
+                    }
+                }
+                break;
+            case SUBTOTAL_FUNC_VARP:
+            case SUBTOTAL_FUNC_STDP:
+                if (aData.maWelford.getCount() < 1)
+                    aData.bError = true;
+                else if (aData.maWelford.getCount() == 1)
+                    rResult = 0.0;
+                else
                 {
-                // added to avoid warnings
+                    rResult = aData.maWelford.getVariancePopulation();
+                    if (eFunc == SUBTOTAL_FUNC_STDP)
+                    {
+                        if (rResult < 0.0)
+                            aData.bError = true;
+                        else
+                            rResult = sqrt( rResult);
+                    }
                 }
+                break;
+            default:
+                // unhandled unknown
+                aData.bError = true;
         }
 
     if (aData.bError)

sc/source/core/tool/subtotal.cxx

@@ -62,4 +62,12 @@ bool SubTotal::SafeDiv(double& fVal1, double fVal2)
     return bOk;
 }
 
+void WelfordRunner::update( double fVal )
+{
+    ++nCount;
+    const double fDelta = fVal - fMean;
+    fMean += fDelta / nCount;
+    fM2 += fDelta * (fVal - fMean);
+}
+
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */