Keep reactant-less v0 photolysis as an emission

src/v0/photolysis_parser.cpp

@@ -31,10 +31,19 @@ namespace mechanism_configuration::v0
       errors.insert(errors.end(), parse_error.begin(), parse_error.end());
 
       double scaling_factor = object[keys::SCALING_FACTOR] ? object[keys::SCALING_FACTOR].as<double>() : 1.0;
+      std::string name = object[keys::MUSICA_NAME].as<std::string>();
 
-      if (!reactants.empty())
+      if (reactants.empty())
+      {
+        // A photolysis reaction with no reactants is a pure production term --
+        // Music Box Interactive encodes emissions this way (as a photolysis so
+        // they can carry an irr product). Represent it as an emission, which has
+        // no reactant, so it is not silently dropped.
+        types::Emission emission = { .scaling_factor = scaling_factor, .products = products, .name = name };
+        mechanism.reactions.emission.push_back(emission);
+      }
+      else
       {
-        std::string name = object[keys::MUSICA_NAME].as<std::string>();
         types::Photolysis user_defined = {
           .scaling_factor = scaling_factor, .reactants = reactants[0], .products = products, .name = name
         };

test/unit/v0/test_photolysis_config.cpp

@@ -86,6 +86,15 @@ TEST(PhotolysisConfig, ParseConfig)
       EXPECT_EQ(process_vector[1].name, "jbar");
       EXPECT_EQ(process_vector[1].scaling_factor, 2.5);
     }
+
+    // A photolysis reaction with no reactants (an emission encoded as photolysis)
+    // is kept as an emission rather than dropped.
+    auto& emissions = mechanism.reactions.emission;
+    ASSERT_EQ(emissions.size(), 1);
+    EXPECT_EQ(emissions[0].name, "jemis");
+    ASSERT_EQ(emissions[0].products.size(), 1);
+    EXPECT_EQ(emissions[0].products[0].name, "foo");
+    EXPECT_EQ(emissions[0].products[0].coefficient, 1.0);
   }
 }
 

test/unit/v0/v0_unit_configs/photolysis/valid/reactions.json

@@ -26,6 +26,14 @@
           },
           "MUSICA name": "jbar",
           "scaling factor": 2.5
+        },
+        {
+          "type": "PHOTOLYSIS",
+          "reactants": { },
+          "products": {
+            "foo": { "yield": 1.0 }
+          },
+          "MUSICA name": "jemis"
         }
       ]
     }

test/unit/v0/v0_unit_configs/photolysis/valid/reactions.yaml

@@ -19,4 +19,10 @@ camp-data:
       bar: {}
     scaling factor: 2.5
     type: PHOTOLYSIS
+  - MUSICA name: jemis
+    products:
+      foo:
+        yield: 1.0
+    reactants: {}
+    type: PHOTOLYSIS
   type: MECHANISM