Use detector grouping from ExperimentInfo

For workspaces with grouped detectors it now only considers the first one
from the group in the main loop but calculates the intersections using
the whole group.
Refs #10470

Code/Mantid/Framework/MDAlgorithms/inc/MantidMDAlgorithms/SXDMDNorm.h

@@ -56,6 +56,11 @@ namespace Mantid
       void cacheDimensionXValues();
       void calculateNormalization(const std::vector<coord_t> &otherValues,
                                   const Kernel::Matrix<coord_t> &affineTrans);
+      std::vector<detid_t> removeGroupedIDs(const API::ExperimentInfo & exptInfo,
+                                            const std::vector<detid_t> &detIDs);
+      Geometry::IDetector_const_sptr getThetaPhi(const detid_t detID,
+                       const API::ExperimentInfo & exptInfo,
+                       double &theta, double &phi);
       std::vector<Kernel::VMD> calculateIntersections(const double theta, const double phi);
 
       /// number of MD dimensions

Code/Mantid/Framework/MDAlgorithms/src/SXDMDNorm.cpp

@@ -382,9 +382,12 @@ namespace Mantid
       const double protonCharge = exptInfoZero.run().getProtonCharge();
 
       auto instrument = exptInfoZero.getInstrument();
-      std::vector<detid_t> detIDS = instrument->getDetectorIDs(true);
-      const int64_t ndets = static_cast<int64_t>(detIDS.size());
-      // detector->workspace index map
+      std::vector<detid_t> detIDs = instrument->getDetectorIDs(true);
+      // Prune out those that are part of a group and simply leave the head of the group
+      detIDs = removeGroupedIDs(exptInfoZero, detIDs);
+
+      // Mappings
+      const int64_t ndets = static_cast<int64_t>(detIDs.size());
       const detid2index_map fluxDetToIdx = fluxEventWS->getDetectorIDToWorkspaceIndexMap();
       const detid2index_map solidAngDetToIdx = solidAngleWS->getDetectorIDToWorkspaceIndexMap();
 
@@ -394,12 +397,11 @@ namespace Mantid
       {
         PARALLEL_START_INTERUPT_REGION
 
-        const auto detID = detIDS[i];
-        auto detector = instrument->getDetector(detID);
-        if(detector->isMonitor() || detector->isMasked()) continue;
-
-        const double theta = detector->getTwoTheta(m_samplePos, m_beamDir);
-        const double phi = detector->getPhi();
+        const auto detID = detIDs[i];
+        double theta(0.0), phi(0.0);
+        auto spectrum = getThetaPhi(detID, exptInfoZero, theta, phi);
+        if(spectrum->isMonitor() || spectrum->isMasked()) continue;
+        // Intersections
         auto intersections = calculateIntersections(theta, phi);
         if(intersections.empty()) continue;
 
@@ -458,7 +460,50 @@ namespace Mantid
       }
       PARALLEL_CHECK_INTERUPT_REGION
 
-     delete prog;
+      delete prog;
+    }
+
+    /**
+     * Checks for IDs that are actually part of the same group and just keeps one from the group.
+     * For a 1:1 map, none will be removed.
+     * @param exptInfo An ExperimentInfo object that defines the grouping
+     * @param detIDs A list of existing IDs
+     * @return A new list of IDs
+     */
+    std::vector<detid_t> SXDMDNorm::removeGroupedIDs(const ExperimentInfo &exptInfo, const std::vector<detid_t> &detIDs)
+    {
+      const size_t ntotal = detIDs.size();
+      std::vector<detid_t> singleIDs;
+      singleIDs.reserve(ntotal/2); // reserve half. In the case of 1:1 it will double to the correct size once
+      
+      for(auto iter = detIDs.begin(); iter != detIDs.end(); ++iter)
+      {
+        const auto & members = exptInfo.getGroupMembers(*iter);
+        singleIDs.push_back(members.front());
+      }
+      
+      return singleIDs;
+    }
+
+    /**
+     * Get the theta and phi angles for the given ID. If the detector was part of a group,
+     * as defined in the ExperimentInfo object, then the theta/phi are for the whole set.
+     * @param detID A reference to a single ID
+     * @param exptInfo A reference to the ExperimentInfo that defines that spectrum->detector mapping
+     * @param theta [Output] Set to the theta angle for the detector (set)
+     * @param phi [Output] Set to the phi angle for the detector (set)
+     * @return A poiner to the Detector object for this spectrum as a whole
+     *         (may be a single pixel or group)
+     */
+    Geometry::IDetector_const_sptr
+    SXDMDNorm::getThetaPhi(const detid_t detID,
+                           const ExperimentInfo &exptInfo,
+                           double &theta, double &phi)
+    {
+      const auto spectrum = exptInfo.getDetectorByID(detID);
+      theta = spectrum->getTwoTheta(m_samplePos, m_beamDir);
+      phi = spectrum->getPhi();
+      return spectrum;
     }
 
     /**