added logic to look into the current zone which has been used by the...

added logic to look into the current zone which has been used by the AtlasFieldCache to cache the field description for one cell. The current zone has multiple cells, e.g. for all of the solenoid. This avoids re-looking up the same zone as one navigates out of a cell

MagneticField/MagFieldElements/MagFieldElements/AtlasFieldCache.h

@@ -86,6 +86,9 @@ private:
   /// Full 3d field
   BFieldCache m_cache3d;
 
+  /// zone of full 3d field cache  
+  const BFieldZone* m_zone3d { 0 };
+
   /// Fast 2d field
   BFieldCacheZR m_cacheZR;
 

MagneticField/MagFieldElements/MagFieldElements/AtlasFieldCache.icc

@@ -22,23 +22,26 @@ inline MagField::AtlasFieldCache::AtlasFieldCache(double solFieldScale,
 inline bool
 MagField::AtlasFieldCache::fillFieldCache(double z, double r, double phi)
 {
-  // search for the zone
-  const BFieldZone* zone =
-    m_fieldMap ? m_fieldMap->findBFieldZone(z, r, phi) : nullptr;
-
-  if (!zone) {
+  // Get a new zone from the map ONLY if we are no longer in the current zone
+  if (!m_zone3d || !m_zone3d->inside( z, r, phi )) {
+    
+    // search for the zone
+      m_zone3d = m_fieldMap ? m_fieldMap->findBFieldZone(z, r, phi) : nullptr;
+  }
+  
+  if (!m_zone3d) {
     // outsize all zones
     return false;
   }
 
   // set scale for field
-  m_scaleToUse = (zone->id() == m_solZoneId) ? m_solScale : m_torScale;
+  m_scaleToUse = (m_zone3d->id() == m_solZoneId) ? m_solScale : m_torScale;
 
   // fill the cache, pass in current scale factor
-  zone->getCache(z, r, phi, m_cache3d, m_scaleToUse);
+  m_zone3d->getCache(z, r, phi, m_cache3d, m_scaleToUse);
 
   // save pointer to the conductors in the zone
-  m_cond = zone->condVector();
+  m_cond = m_zone3d->condVector();
 
   return true;
 }