Add leapfrog integration method in integrators library

lib/integrator.py

@@ -2,7 +2,35 @@
 # -*- coding:utf-8 -*-
 """
 Implementation of the various integrators for numerical integration.
-"""
 
-def leapfrog():
+Comes from the assumption that the problem is analytically defined in position-momentum (q-p) space for a given hamiltonian H.
-    return 0
+"""
+import numpy as np
+
+def dp_dt(m_array, q_array):
+    """
+    Time derivative of the momentum, given by the position derivative of the Hamiltonian.
+    dp/dt = -dH/dq
+    """
+    dp_array = np.zeros((q_array.shape[0],3))
+    for i in range(q_array.shape[0]):
+        m_j = np.delete(m_array)
+        q_j = np.delete(q_array)
+        dp_array[i] = m_array[i]*np.sum(m_j*/(q_j-q_array[i])**2, axis=0)
+    return dp_array
+
+def leapfrog(duration, step, m_array, q_array, p_array):
+    """
+    Leapfrog integrator for first order partial differential equations.
+    iteration : half-step drift -> full-step kick -> half-step drift
+    """
+    N = np.ceil(duration/step).astype(int)
+    for _ in range(N):
+        # half-step drift
+        q_array, p_array = q_array + step/2*p_array/m_array , p_array
+        # full-step kick
+        q_array, p_array = q_array , p_array - step*dp_dt(m_array, q_array)
+        # half-step drift
+        q_array, p_array = q_array + step/2*p_array/m_array , p_array
+
+    return q_array, p_array