Keep track of whether demographic_model_state pointer was null

at start of simulation.  If not, then we do NOT call the "model
updating" code path prior to entering the main loop.

CANDIDATE fix for #775

fwdpy11/headers/fwdpy11/discrete_demography/simulation/demographic_model_state.hpp

@@ -103,15 +103,19 @@ namespace fwdpy11
             }
         };
 
+        /// model_needs_update added in 0.16.0
+        /// in relation to GitHub issue 775
         template <typename METADATATYPE>
         inline demographic_model_state_pointer
         initialize_model_state(std::uint32_t generation,
                                const std::vector<METADATATYPE>& metadata,
-                               DiscreteDemography& demography)
+                               DiscreteDemography& demography, bool* model_needs_update)
         {
             // "steal" pointer from input
             auto rv = demography.get_model_state();
 
+            *model_needs_update = (rv == nullptr);
+
             if (rv == nullptr || generation == 0)
                 // If there is no state, then we need to make
                 // one.  If there is a state, but the generation
@@ -121,6 +125,7 @@ namespace fwdpy11
                 {
                     demography.update_event_times(generation);
                     rv.reset(new demographic_model_state(metadata, demography));
+                    *model_needs_update = true;
                 }
             return rv;
         }

fwdpy11/src/evolve_population/evolvets.cc

@@ -222,8 +222,12 @@ evolve_with_tree_sequences(
         }
 
     // Set up discrete demography types. New in 0.6.0
-    auto current_demographic_state = ddemog::initialize_model_state(
-        pop.generation, pop.diploid_metadata, demography);
+    /// model_needs_update added in 0.16.0
+    /// in relation to GitHub issue 775
+    bool demographic_model_needs_update = true;
+    auto current_demographic_state
+        = ddemog::initialize_model_state(pop.generation, pop.diploid_metadata,
+                                         demography, &demographic_model_needs_update);
     if (gvalue_pointers.genetic_values.size()
         > static_cast<std::size_t>(current_demographic_state->maxdemes))
         {
@@ -285,8 +289,11 @@ evolve_with_tree_sequences(
                               record_genotype_matrix);
     pop.genetic_value_matrix.swap(new_diploid_gvalues);
     pop.diploid_metadata.swap(offspring_metadata);
-    ddemog::update_demography_manager(rng, pop.generation, pop.diploid_metadata,
-                                      demography, *current_demographic_state);
+    if (demographic_model_needs_update == true)
+        {
+            ddemog::update_demography_manager(rng, pop.generation, pop.diploid_metadata,
+                                              demography, *current_demographic_state);
+        }
     if (current_demographic_state->will_go_globally_extinct() == true)
         {
             std::ostringstream o;