1185 wrong index in ode seir model (#1186)

-  Fixed wrong index in model.h of ODE SEIR model
-  Added test to test_odeseir.cpp that would have failed with the previous wrong index

Co-authored-by: HenrZu <69154294+HenrZu@users.noreply.github.com>

Author: kilianvolmer

cpp/models/ode_seir/model.h

@@ -80,7 +80,7 @@ class Model
             const size_t Ii = this->populations.get_flat_index({i, InfectionState::Infected});
 
             for (auto j : make_index_range(age_groups)) {
-                const size_t Sj = this->populations.get_flat_index({i, InfectionState::Susceptible});
+                const size_t Sj = this->populations.get_flat_index({j, InfectionState::Susceptible});
                 const size_t Ej = this->populations.get_flat_index({j, InfectionState::Exposed});
                 const size_t Ij = this->populations.get_flat_index({j, InfectionState::Infected});
                 const size_t Rj = this->populations.get_flat_index({j, InfectionState::Recovered});

cpp/tests/test_odeseir.cpp

@@ -362,6 +362,50 @@ TEST(TestSeir, get_flows)
     EXPECT_NEAR(dydt_default[2], 25, 1e-12);
 }
 
+TEST(TestSeir, get_flows_two_agegroups)
+{
+    mio::oseir::Model<double> model(2);
+    auto nb_groups = model.parameters.get_num_groups();
+
+    constexpr double total_first_population  = 400;
+    constexpr double total_second_population = 200;
+
+    auto& params = model.parameters;
+
+    model.populations[{mio::AgeGroup(0), mio::oseir::InfectionState::Exposed}]   = 100;
+    model.populations[{mio::AgeGroup(0), mio::oseir::InfectionState::Infected}]  = 100;
+    model.populations[{mio::AgeGroup(0), mio::oseir::InfectionState::Recovered}] = 100;
+    model.populations.set_difference_from_group_total<mio::AgeGroup>({mio::AgeGroup(0), mio::oseir::InfectionState::Susceptible},
+                                                                     total_first_population);
+    model.populations[{mio::AgeGroup(1), mio::oseir::InfectionState::Exposed}]   = 10;
+    model.populations[{mio::AgeGroup(1), mio::oseir::InfectionState::Infected}]  = 10;
+    model.populations[{mio::AgeGroup(1), mio::oseir::InfectionState::Recovered}] = 10;
+    model.populations.set_difference_from_group_total<mio::AgeGroup>({mio::AgeGroup(1), mio::oseir::InfectionState::Susceptible},
+                                                                     total_second_population);
+
+    for (auto i = mio::AgeGroup(0); i <= mio::AgeGroup(1); i++) {
+        model.parameters.get<mio::oseir::TimeExposed<double>>()[i]                      = 2;
+        model.parameters.get<mio::oseir::TimeInfected<double>>()[i]                     = 4;
+        model.parameters.get<mio::oseir::TransmissionProbabilityOnContact<double>>()[i] = 1;
+    }
+
+    mio::ContactMatrixGroup& contact_matrix = params.get<mio::oseir::ContactPatterns<double>>();
+    contact_matrix[0] = mio::ContactMatrix(Eigen::MatrixXd::Constant((size_t)nb_groups, (size_t)nb_groups, 1.0));
+    model.check_constraints();
+
+    auto dydt_default = Eigen::VectorXd(6);
+    dydt_default.setZero();
+    auto y0 = model.get_initial_values();
+    model.get_flows(y0, y0, 0, dydt_default);
+
+    EXPECT_NEAR(dydt_default[0], 30, 1e-12);
+    EXPECT_NEAR(dydt_default[1], 50, 1e-12);
+    EXPECT_NEAR(dydt_default[2], 25, 1e-12);
+    EXPECT_NEAR(dydt_default[3], 51, 1e-12);
+    EXPECT_NEAR(dydt_default[4], 5, 1e-12);
+    EXPECT_NEAR(dydt_default[5], 2.5, 1e-12);
+}
+
 TEST(TestSeir, Simulation)
 {
     double t0   = 0;