From fdf5ef27fec1368a33801a98bbc5ed3556476979 Mon Sep 17 00:00:00 2001
From: Douglas Rumbaugh <dbr4@psu.edu>
Date: Mon, 8 May 2023 12:59:46 -0400
Subject: Began porting source files over from other repository

---
 include/ds/Alias.h | 72 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)
 create mode 100644 include/ds/Alias.h

(limited to 'include/ds/Alias.h')

diff --git a/include/ds/Alias.h b/include/ds/Alias.h
new file mode 100644
index 0000000..855dc75
--- /dev/null
+++ b/include/ds/Alias.h
@@ -0,0 +1,72 @@
+/*
+ * include/ds/Alias.h
+ *
+ * Copyright (C) 2023 Douglas Rumbaugh <drumbaugh@psu.edu> 
+ *                    Dong Xie <dongx@psu.edu>
+ *
+ * All rights reserved. Published under the Modified BSD License.
+ *
+ */
+#pragma once
+
+#include <gsl/gsl_rng.h>
+#include <vector>
+
+namespace de {
+
+/*
+ * An implementation of Walker's Alias Structure for Weighted Set Sampling. Requires
+ * that the input weight vector is already normalized.
+ */
+class Alias {
+public:
+    Alias(const std::vector<double>& weights)
+    : m_alias(weights.size()), m_cutoff(weights.size()) {
+        size_t n = weights.size();
+        auto overfull = std::vector<size_t>();
+        auto underfull = std::vector<size_t>();
+        overfull.reserve(n);
+        underfull.reserve(n);
+
+        // initialize the probability_table with n*p(i) as well as the overfull and
+        // underfull lists.
+        for (size_t i = 0; i < n; i++) {
+            m_cutoff[i] = (double) n * weights[i];
+            if (m_cutoff[i] > 1) {
+                overfull.emplace_back(i);
+            } else if (m_cutoff[i] < 1) {
+                underfull.emplace_back(i);
+            } else {
+                m_alias[i] = i;
+            }
+        }
+
+        while (overfull.size() > 0 && underfull.size() > 0) {
+            auto i = overfull.back(); overfull.pop_back();
+            auto j = underfull.back(); underfull.pop_back();
+
+            m_alias[j] = i;
+            m_cutoff[i] = m_cutoff[i] + m_cutoff[j] - 1.0;
+
+            if (m_cutoff[i] > 1.0) {
+                overfull.emplace_back(i);
+            } else if (m_cutoff[i] < 1.0) {
+                underfull.emplace_back(i);
+            }
+        }
+    }
+
+    size_t get(const gsl_rng* rng) {
+        double coin1 = gsl_rng_uniform(rng);
+        double coin2 = gsl_rng_uniform(rng);
+
+        size_t k = ((double) m_alias.size()) * coin1;
+        return coin2 < m_cutoff[k] ? k : m_alias[k];
+    }
+
+private:
+    std::vector<size_t> m_alias;
+    std::vector<double> m_cutoff;
+};
+
+}
-- 
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