From 18e13d6e4e00d3569671ab94ef24c7cc159756a5 Mon Sep 17 00:00:00 2001
From: Tom Hodson <thomas.c.hodson@gmail.com>
Date: Fri, 27 May 2022 11:15:54 +0200
Subject: [PATCH] Add initial code to the package

---
 code/src/MCFF/ising_model.py | 83 ++++++++++++++++++++++++++++++++++++
 code/src/MCFF/mcmc.py        | 41 ++++++++++++++++++
 2 files changed, 124 insertions(+)
 create mode 100644 code/src/MCFF/ising_model.py
 create mode 100644 code/src/MCFF/mcmc.py

diff --git a/code/src/MCFF/ising_model.py b/code/src/MCFF/ising_model.py
new file mode 100644
index 0000000..4c37cfc
--- /dev/null
+++ b/code/src/MCFF/ising_model.py
@@ -0,0 +1,83 @@
+"""ising_model provides functions specific to the 2D ising model such as computing its energy and generating special states.
+
+States are represented by NxM numpy arrays with values +/-1. These functions are intended to be used with a markov chain monte carlo routine to sample from the thermal ensemble of the 2d Ising model.
+
+"""
+
+import numpy as np
+
+def all_up_state(N):
+    "The NxN all up state of the Ising model."
+    return np.ones([N, N])
+
+def all_down_state(N):
+    "The NxN all down state of the Ising model."
+    return -np.ones([N, N])
+
+def random_state(N, magnetisation = 0.5, rng = np.random.default_rng())
+    r"""Return the a random NxN state of the Ising model.
+
+    Parameters
+    ----------
+    N : int
+        The returned state will have shape (N,N)
+    magnetisation : int, default = 0.5
+        What proportion of the values will be +1
+    rng : numpy.random._generator.Generator, default = np.random.default_rng()
+        Optionally pass in the rng to be used, useful to get reproducable answers.
+
+    Returns
+    -------
+    np.array
+        A random NxN state.
+    """
+    return np.random.choice([+1, -1], size = (N,N), p = magnetisation)
+
+
+# nopython=True instructs numba that we want all use of the python interpreter to removed from this function
+# numba will throw and error if it cannot achieve this.
+# nogil = True says that numba can release python's Global Interpreter Lock, read more about that here: https://numba.pydata.org/numba-doc/latest/user/jit.html#nogil
+@jit(nopython=True, nogil = True) 
+def energy(state):
+    r"""Compute the energy of a state of the Ising Model with open boundary conditions.
+
+    Parameters
+    ----------
+    state : array_like
+        A NxM array containing only the values +1 and -1
+    Returns
+    -------
+    float64
+        The interaction energy per site.
+    """
+    E = 0
+    N, M = state.shape
+    for i in range(N):
+        for j in range(M):
+            # handle the north and south neighbours
+            if 0 <= (i + 1) < N:
+                E -= state[i,j] * state[i+1, j]
+            
+            # handle the east and west neighbours
+            if 0 <= (j + 1) < M:
+                E -= state[i,j] * state[i, j+1]
+            
+    return 2*E / (N*M)
+
+def energy_numpy(state):
+    r"""Compute the energy of a state of the Ising Model with open boundary conditions.
+    
+    An alternate implementation of `energy` using Numpy array operations, used for comparison and correctness checks.
+
+    Parameters
+    ----------
+    state : array_like ()
+        A 2D array containing only the values +1 and -1
+    Returns
+    -------
+    float64
+        The interaction energy per site.
+    """
+    E = - np.sum(state[:-1, :] * state[1:, :]) - np.sum(state[:, :-1] * state[:, 1:]) 
+    return 2*E / np.product(state.shape)
+
diff --git a/code/src/MCFF/mcmc.py b/code/src/MCFF/mcmc.py
new file mode 100644
index 0000000..27ce015
--- /dev/null
+++ b/code/src/MCFF/mcmc.py
@@ -0,0 +1,41 @@
+"""mcmc provides routines for performin Markov Chain Monte Carlo
+
+
+"""
+
+def mcmc(initial_state, steps, T, rng = np.random.default_rng()):
+    r"""Return the state after performing `steps` monte carlo steps starting from `initial_state` at temperature `T`.
+
+    Parameters
+    ----------
+    initial_state : array_like
+        Numpy array of shape (N,N) with values +1,-1
+    steps : int
+        The number of steps to take.
+    T : float
+        The temperature at which to perform the simulation.
+    rng : numpy.random._generator.Generator, default = np.random.default_rng()
+        Optionally pass in the rng to be used, useful to get reproducable answers.
+
+    Returns
+    -------
+    np.array
+        The final state.
+    """
+    N, M = initial_state.shape
+    assert N == M
+    
+    current_state = initial_state.copy()
+    E = N**2 * energy(state)
+    for i in range(steps):
+        i, j = rng.integers(N, size = 2)
+        
+        new_state = current_state.copy()
+        new_state[i,j] *= -1
+        new_E = N**2 * energy(new_state)
+        
+        if (new_E < E) or np.exp(-(new_E - E) / T) > rng.uniform():
+            current_state = new_state
+            E = new_E
+            
+    return current_state 
\ No newline at end of file