From 06769099e4d1b5f9620c0d451b43f1a0f62e28ba Mon Sep 17 00:00:00 2001
From: Pim Nelissen <pim.nelissen@protonmail.com>
Date: Mon, 27 Oct 2025 15:40:46 +0100
Subject: [PATCH] task 2 solution

---
 II.py | 91 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 91 insertions(+)
 create mode 100644 II.py

diff --git a/II.py b/II.py
new file mode 100644
index 0000000..bd0d680
--- /dev/null
+++ b/II.py
@@ -0,0 +1,91 @@
+import numpy as np
+import scipy as scp
+
+from matplotlib import pyplot as plt
+
+from RubberBand import RubberBand
+
+# load
+lengths = np.load("data/lengths.npy")
+NUM_BANDS = len(lengths)
+N = 100
+
+dummy_band = RubberBand(N=N) # create a dummy band to access w
+
+def calculate_mu_eff(w):
+    w_tilde = w / np.sum(w)
+    return 1/np.sum(w_tilde**2)
+
+def P_weighted(L_over_a, f, dummy_band):
+    """
+    Weighted true probablity density function.
+    """
+    N = dummy_band.N
+    weights = dummy_band.w(f, L_over_a)
+    z = scp.special.binom(N, (N+L_over_a)/2) * weights
+    return z / np.sum(z)
+
+fig_dist, axes_dist = plt.subplots(3, 2, figsize=(12, 10))
+fig_ratio, axes_ratio = plt.subplots(3, 2, figsize=(12, 10))
+
+axes_dist = axes_dist.flatten()
+axes_ratio = axes_ratio.flatten()
+
+mu_effs = []
+forces = [+0.01, +0.05, +0.1, +0.25, +0.5, +1.0]
+
+for i, f in enumerate(forces):
+
+    bin_edges = np.arange(-N-1, N+1, 2)
+    values, _ = np.histogram(lengths, bins=bin_edges)
+    p_mc = values / values.sum() # normalise histogram
+    
+    bin_centers = bin_edges[:-1] + np.diff(bin_edges)/2
+    
+    # weighted histogram
+    weights = dummy_band.w(f, bin_centers)
+    values_w = values * weights
+    p_mc_w = values_w / values_w.sum()
+    
+    p_true_w = P_weighted(bin_centers, f, dummy_band)
+    
+    # compute effective samples due to reweighting
+    mu_eff = calculate_mu_eff(weights)
+    mu_effs.append(mu_eff)
+    
+    # plot stuff
+    ax_dist = axes_dist[i]
+    ax_dist.plot(bin_centers, p_true_w, label="$P(L)$ (with $f$)")
+    ax_dist.bar(bin_centers, p_mc, color='g', linestyle='--', alpha = 0.2, label="$\hat{P}(L)$ (no $f$)")
+    ax_dist.bar(bin_centers, p_mc_w, color='r', alpha = 0.5, label="$\hat{{P}}(L)$ (with $f$)")
+    if i == 0:
+        ax_dist.legend()
+      
+    ax_dist.set_xlabel("$L/a$")
+    ax_dist.set_ylabel("P($L/a$)")
+    
+    ax_dist.text(0.03,0.83, f"$f={f}$\n$\mu_{{eff}}={round(mu_eff, 1)}$ %",
+                 transform=ax_dist.transAxes)
+    
+    xlim = (-50, 50)
+
+    # ratios
+    p_ratio = p_mc_w / p_true_w
+    
+    ax_ratio = axes_ratio[i]
+    ax_ratio.text(0.03,0.83, f"$f={f}$\n$\mu_{{eff}}={round(mu_eff, 1)}$ %",
+                 transform=ax_ratio.transAxes)
+    
+    ax_ratio.scatter(bin_centers, p_ratio, marker='x')
+    ax_ratio.hlines(1, *xlim, color='r', linestyle='--')
+    ax_ratio.set_xlim(xlim)
+    ax_ratio.set_xlabel("$L/a$")
+    ax_ratio.set_ylabel("$\hat{P}(L)/P(L)$")
+
+plt.tight_layout()
+
+plt.figure()
+plt.plot(forces, mu_effs)
+plt.xlabel("$f$ [arb.]")
+plt.ylabel("$\mu_{eff}$ [%]")
+plt.show()