Reflection table performance improvement (dials#2718)

Eliminate a double loop in selections from reflection tables. Speedup of 12 min per call
for a table containing 165k experiment identifiers.

Co-authored-by: David Mittan-Moreau <dwmoreau@lbl.gov>
Co-authored-by: Aaron Brewster <asbrewster@lbl.gov>

newsfragments/2718.bugfix

@@ -0,0 +1,3 @@
+Performance improvement for selections from large reflection tables. For a
+table containing 165k experiment identifiers the speedup is 1000x (12 minutes
+per call).

src/dials/array_family/boost_python/reflection_table_suite.h

@@ -32,12 +32,9 @@ namespace dials { namespace af { namespace boost_python {
         // Copy across identifiers for ids in new table
         typedef typename T::experiment_map_type::const_iterator const_iterator;
         for (std::set<int>::iterator i = new_ids.begin(); i != new_ids.end(); ++i) {
-          for (const_iterator it = self.experiment_identifiers()->begin();
-               it != self.experiment_identifiers()->end();
-               ++it) {
-            if (it->first == *i) {
-              (*new_table.experiment_identifiers())[it->first] = it->second;
-            }
+          const_iterator found = self.experiment_identifiers()->find(*i);
+          if (found != self.experiment_identifiers()->end()) {
+            (*new_table.experiment_identifiers())[found->first] = found->second;
           }
         }
       }
@@ -262,4 +259,4 @@ namespace dials { namespace af { namespace boost_python {
 
 }}}}  // namespace dials::af::boost_python::reflection_table_suite
 
-#endif  // DIALS_ARRAY_FAMILY_BOOST_PYTHON_REFLECTION_TABLE_SUITE_H
+#endif  // DIALS_ARRAY_FAMILY_BOOST_PYTHON_REFLECTION_TABLE_SUITE_H