[minuit2] Consider case when variable parameters=0

In this case we cdon;t need to start creating a seed and a Variable Metric Builder. Just return a result containing only the function value

Author: lmoneta

math/minuit2/inc/Minuit2/MinimumParameters.h

@@ -21,11 +21,23 @@ namespace Minuit2 {
 class MinimumParameters {
 
 public:
+
+    enum Status {
+      MnValid,
+      MnInvalid
+   };
+
    MinimumParameters(unsigned int n, double fval = 0)
       : fPtr{new Data{MnAlgebraicVector(n), MnAlgebraicVector(n), fval, false, false}}
    {
    }
 
+   // constructor for case when function has no parameter and only function value
+   MinimumParameters(double fval, Status status)
+      : fPtr{ new Data{MnAlgebraicVector(0), MnAlgebraicVector(0), fval, status == MnValid, false}}
+   {}
+
+
    /** takes the Parameter vector */
    MinimumParameters(const MnAlgebraicVector &avec, double fval)
       : fPtr{new Data{avec, MnAlgebraicVector(avec.size()), fval, true, false}}

math/minuit2/src/MnApplication.cxx

@@ -37,7 +37,18 @@ FunctionMinimum MnApplication::operator()(unsigned int maxfcn, double toler)
 
    const FCNBase &fcn = Fcnbase();
 
+
+   if (npar == 0) {
+      double fval = fcn(fState.Params());
+      print.Info("Function has zero parameters - returning current function value - ",fval);
+      // create a valid Minuit-Parameter objects with just the function value
+      MinimumParameters mparams(fval,MinimumParameters::MnValid);
+      MinimumState mstate(mparams, 0., 1 );
+      return FunctionMinimum( MinimumSeed(mstate, fState.Trafo()), fcn.Up());
+   }
+
    FunctionMinimum min = Minimizer().Minimize(fcn, fState, fStrategy, maxfcn, toler);
+
    fNumCall += min.NFcn();
    fState = min.UserState();