fix(mass_balance): avoid div-by-zero, don't check mass balance with frequency 0

Author: hellkite500

src/utilities/bmi/mass_balance.cpp

@@ -37,24 +37,27 @@ NgenMassBalance::NgenMassBalance() : check(false) {}
 NgenMassBalance::~NgenMassBalance() = default;
 
 auto NgenMassBalance::run(const ModelPtr& model, const Context& ctx) const -> expected<void, ProtocolError> {
-    bool check_step = false;
-    //if frequency was set to -1 (or any negative), only check at the end
-    if( frequency < 0 ){
-        if(ctx.current_time_step == ctx.total_steps){
-            check_step = true;
-        }
-    }
-    else{
-        check_step = (ctx.current_time_step % frequency) == 0;
-    }
     if( model == nullptr ) {
         return make_unexpected<ProtocolError>( ProtocolError(
             Error::UNITIALIZED_MODEL,
             "Cannot run mass balance protocol with null model."
             )
         );
+    } else if( !check || !supported ) {
+        return {};
     }
-    if(model && supported && check && check_step) {
+    bool check_step = false;
+    //if frequency was set to -1 (or any negative), only check at the end
+    //use <= to avoid a potential divide by zero should frequency be 0
+    //(though frequency 0 should have been caught during initialization and check disabled)
+    if( frequency > 0 ){
+        check_step = (ctx.current_time_step % frequency) == 0;
+    }
+    else if(ctx.current_time_step == ctx.total_steps){
+        check_step = true;
+    }
+
+    if(check_step) {
         double mass_in, mass_out, mass_stored, mass_leaked, mass_balance;
         model->GetValue(INPUT_MASS_NAME, &mass_in);
         model->GetValue(OUTPUT_MASS_NAME, &mass_out);
@@ -174,6 +177,9 @@ auto NgenMassBalance::initialize(const ModelPtr& model, const Properties& proper
         } else {
             frequency = 1; //default, check every timestep
         }
+        if ( frequency == 0 ) {
+            check = false; // can't check at frequency 0, disable mass balance checking
+        }
     } else{
         //no mass balance requested, or not supported, so don't check it
         check = false;

test/utils/bmi/mass_balance_Test.cpp

@@ -369,6 +369,16 @@ TEST_F(Bmi_Mass_Balance_Test, frequency) {
     ASSERT_THROW(protocols.run(models::bmi::protocols::Protocol::MASS_BALANCE, make_context(2, 2, time, model_name)), ProtocolError);
 }
 
+TEST_F(Bmi_Mass_Balance_Test, frequency_zero) {
+    auto properties = MassBalanceMock(true, 1e-5, 0).as_json_property();
+    auto context = make_context(0, 2, time, model_name);
+    auto protocols = NgenBmiProtocols(model, properties);
+    double mass_error = 10; // Force a mass balance error above tolerance
+    model->SetValue(OUTPUT_MASS_NAME, &mass_error);
+    auto result = protocols.run(models::bmi::protocols::Protocol::MASS_BALANCE, make_context(0, 2, time, model_name));
+    EXPECT_TRUE( result.has_value() ); // should pass, frequency 0 means never check
+}
+
 TEST_F(Bmi_Mass_Balance_Test, frequency_end) {
     auto properties = MassBalanceMock(true, 1e-5, -1).as_json_property();
     auto context = make_context(0, 2, time, model_name);