Added the ability to apply a custom "filter" to the conjugate momentum in the Integrator classes, applied both after refresh and after applying the forces

Added a conjugate momentum "filter" that applies a phase to each site. With sites set to 1.0 or 0.0 this acts as a mask and enables, for example, the freezing of inactive gauge links in DDHMC
Added tests/forces/Test_momentum_filter demonstrating the use of the filter to freeze boundary links

Grid/qcd/hmc/integrators/Integrator.h

@@ -33,6 +33,7 @@ directory
 #define INTEGRATOR_INCLUDED
 
 #include <memory>
+#include "MomentumFilter.h"
 
 NAMESPACE_BEGIN(Grid);
 
@@ -78,8 +79,19 @@ protected:
   RepresentationPolicy Representations;
   IntegratorParameters Params;
 
+  //Filters allow the user to manipulate the conjugate momentum, for example to freeze links in DDHMC
+  //It is applied whenever the momentum is updated / refreshed
+  //The default filter does nothing
+  MomentumFilterBase<MomentaField> const* MomFilter;
+
   const ActionSet<Field, RepresentationPolicy> as;
 
+  //Get a pointer to a shared static instance of the "do-nothing" momentum filter to serve as a default
+  static MomentumFilterBase<MomentaField> const* getDefaultMomFilter(){ 
+    static MomentumFilterNone<MomentaField> filter;
+    return &filter;
+  }
+
   void update_P(Field& U, int level, double ep) 
   {
     t_P[level] += ep;
@@ -135,6 +147,8 @@ protected:
 
     // Force from the other representations
     as[level].apply(update_P_hireps, Representations, Mom, U, ep);
+
+    MomFilter->applyFilter(Mom);
   }
 
   void update_U(Field& U, double ep) 
@@ -174,11 +188,23 @@ public:
     t_P.resize(levels, 0.0);
     t_U = 0.0;
     // initialization of smearer delegated outside of Integrator
+
+    //Default the momentum filter to "do-nothing"
+    MomFilter = getDefaultMomFilter();
   };
 
   virtual ~Integrator() {}
 
   virtual std::string integrator_name() = 0;
+  
+  //Set the momentum filter allowing for manipulation of the conjugate momentum
+  void setMomentumFilter(const MomentumFilterBase<MomentaField> &filter){
+    MomFilter = &filter;
+  }
+
+  //Access the conjugate momentum
+  const MomentaField & getMomentum() const{ return P; }
+  
 
   void print_parameters()
   {
@@ -249,6 +275,8 @@ public:
       // Refresh the higher representation actions
       as[level].apply(refresh_hireps, Representations, pRNG);
     }
+
+    MomFilter->applyFilter(P);
   }
 
   // to be used by the actionlevel class to iterate

Grid/qcd/hmc/integrators/MomentumFilter.h

@@ -0,0 +1,94 @@
+/*************************************************************************************
+
+Grid physics library, www.github.com/paboyle/Grid
+
+Source file: ./lib/qcd/hmc/integrators/MomentumFilter.h
+
+Copyright (C) 2015
+
+Author: Christopher Kelly <ckelly@bnl.gov>
+Author: Peter Boyle <paboyle@ph.ed.ac.uk>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+See the full license in the file "LICENSE" in the top level distribution
+directory
+*************************************************************************************/
+/*  END LEGAL */
+//--------------------------------------------------------------------
+#ifndef MOMENTUM_FILTER
+#define MOMENTUM_FILTER
+
+NAMESPACE_BEGIN(Grid);
+
+//These filter objects allow the user to manipulate the conjugate momentum as part of the update / refresh
+
+template<typename MomentaField>
+struct MomentumFilterBase{
+  virtual void applyFilter(MomentaField &P) const;
+};
+
+//Do nothing
+template<typename MomentaField>
+struct MomentumFilterNone: public MomentumFilterBase<MomentaField>{
+  void applyFilter(MomentaField &P) const override{}
+};
+
+//Multiply each site/direction by a Lorentz vector complex number field
+//Can be used to implement a mask, zeroing out sites
+template<typename MomentaField>
+struct MomentumFilterApplyPhase: public MomentumFilterBase<MomentaField>{
+  typedef typename MomentaField::vector_type vector_type; //SIMD-vectorized complex type
+  typedef typename MomentaField::scalar_type scalar_type; //scalar complex type
+  typedef iVector<iScalar<iScalar<vector_type> >, Nd > LorentzScalarType; //complex phase for each site/direction
+  typedef Lattice<LorentzScalarType> LatticeLorentzScalarType;
+  
+  LatticeLorentzScalarType phase;
+ 
+  MomentumFilterApplyPhase(const LatticeLorentzScalarType _phase): phase(_phase){}
+
+  //Default to uniform field of (1,0)
+  MomentumFilterApplyPhase(GridBase* _grid): phase(_grid){
+    LorentzScalarType one;
+    for(int mu=0;mu<Nd;mu++)
+      one(mu)()() = scalar_type(1.);
+    
+    phase = one;
+  }
+
+  void applyFilter(MomentaField &P) const override{
+    conformable(P,phase);
+    autoView( P_v , P, AcceleratorWrite);
+    autoView( phase_v , phase, AcceleratorRead);
+
+    accelerator_for(ss,P_v.size(),MomentaField::vector_type::Nsimd(),{
+    	auto site_mom = P_v(ss);
+    	auto site_phase = phase_v(ss);
+	for(int mu=0;mu<Nd;mu++)
+	  site_mom(mu) = site_mom(mu) * site_phase(mu);
+    	coalescedWrite(P_v[ss], site_mom);
+      });
+    
+  }
+
+
+};
+
+
+
+
+NAMESPACE_END(Grid);
+
+#endif