From 1d7e8389eff142b836fb816d3d79adf63e4c4a46 Mon Sep 17 00:00:00 2001
From: Amrita Goswami <amrita16thaug646@gmail.com>
Date: Sat, 23 Mar 2024 16:19:32 +0000
Subject: [PATCH] InertialModel: Implemented update rule

Implemented a Velocity Verlet algorithm for the Inertial model.

Co-authored-by: Moritz Sallermann <moritzsallermann@gmail.com>
---
 include/models/InertialModel.hpp |  5 +++
 src/models/InertialModel.cpp     | 62 +++++++++++++++++++++-----------
 2 files changed, 47 insertions(+), 20 deletions(-)

diff --git a/include/models/InertialModel.hpp b/include/models/InertialModel.hpp
index 89adb93..8f5577d 100644
--- a/include/models/InertialModel.hpp
+++ b/include/models/InertialModel.hpp
@@ -28,6 +28,11 @@ class InertialModel : public ActivityDrivenModelAbstract<InertialAgent>
 
 private:
     double friction_coefficient = 1.0;
+    std::vector<double> drift_t_buffer{};
+    std::vector<double> drift_next_t_buffer{};
+
+    void calc_velocity();
+    void calc_position();
 };
 
 } // namespace Seldon
\ No newline at end of file
diff --git a/src/models/InertialModel.cpp b/src/models/InertialModel.cpp
index aaf825b..62a5795 100644
--- a/src/models/InertialModel.cpp
+++ b/src/models/InertialModel.cpp
@@ -9,32 +9,54 @@
 namespace Seldon
 {
 
+// X(t+dt)
+// Also updates the position
+void InertialModel::calc_position()
+{
+    // Calculating 'drift' = a(t)-friction
+    get_euler_slopes( drift_t_buffer, [this]( size_t i ) { return network.agents[i].data.opinion; } );
+
+    for( size_t idx_agent = 0; idx_agent < network.n_agents(); idx_agent++ )
+    {
+        auto & agent_data    = network.agents[idx_agent].data;
+        auto accleration     = drift_t_buffer[idx_agent] - friction_coefficient * agent_data.velocity;
+        double next_position = agent_data.opinion + agent_data.velocity * dt + 0.5 * (accleration)*dt * dt;
+
+        // Update the position to the new position
+        agent_data.opinion = next_position;
+    }
+}
+
+// V(t+dt)
+// Also updates the velocity
+void InertialModel::calc_velocity()
+{
+    // Calculating new 'drift'
+    get_euler_slopes( drift_next_t_buffer, [this]( size_t i ) { return network.agents[i].data.opinion; } );
+
+    for( size_t idx_agent = 0; idx_agent < network.n_agents(); idx_agent++ )
+    {
+        auto & agent_data    = network.agents[idx_agent].data;
+        double next_velocity = agent_data.velocity
+                               + 0.5 * dt
+                                     * ( drift_t_buffer[idx_agent] - friction_coefficient * agent_data.velocity
+                                         + drift_next_t_buffer[idx_agent] );
+        next_velocity /= 1.0 + 0.5 * friction_coefficient * dt;
+
+        // Update velocity
+        agent_data.velocity = next_velocity;
+    }
+}
+
 void InertialModel::iteration()
 {
     Model<AgentT>::iteration();
 
     update_network();
 
-    // Integrate the ODE using 4th order Runge-Kutta
-    // k_1 =   hf(x_n,y_n)
-    get_euler_slopes( k1_buffer, [this]( size_t i ) { return network.agents[i].data.opinion; } );
-    // k_2  =   hf(x_n+1/2h,y_n+1/2k_1)
-    get_euler_slopes(
-        k2_buffer, [this]( size_t i ) { return network.agents[i].data.opinion + 0.5 * this->k1_buffer[i]; } );
-    // k_3  =   hf(x_n+1/2h,y_n+1/2k_2)
-    get_euler_slopes(
-        k3_buffer, [this]( size_t i ) { return network.agents[i].data.opinion + 0.5 * this->k2_buffer[i]; } );
-    // k_4  =   hf(x_n+h,y_n+k_3)
-    get_euler_slopes( k4_buffer, [this]( size_t i ) { return network.agents[i].data.opinion + this->k3_buffer[i]; } );
-
-    // Update the agent opinions
-    for( size_t idx_agent = 0; idx_agent < network.n_agents(); ++idx_agent )
-    {
-        // y_(n+1) =   y_n+1/6k_1+1/3k_2+1/3k_3+1/6k_4+O(h^5)
-        network.agents[idx_agent].data.opinion
-            += ( k1_buffer[idx_agent] + 2 * k2_buffer[idx_agent] + 2 * k3_buffer[idx_agent] + k4_buffer[idx_agent] )
-               / 6.0;
-    }
+    // Use Velocity Verlet algorithm
+    calc_position();
+    calc_velocity();
 
     if( bot_present() )
     {