feat: Added gale_shapley.cpp in greedy_algorithms

greedy_algorithms/gale_shapley.cpp

@@ -0,0 +1,129 @@
+/**
+ * @file
+ * @brief [Gale Shapley Algorithm](https://en.wikipedia.org/wiki/Gale%E2%80%93Shapley_algorithm)
+ * @details
+ * This implementation utilizes the Gale-Shapley algorithm to find stable matches.
+ *
+ * **Gale Shapley Algorithm** aims to find a stable matching between two equally sized 
+ * sets of elements given an ordinal preference for each element. The algorithm was
+ * introduced by David Gale and Lloyd Shapley in 1962.
+ * 
+ * Reference: 
+ * [Wikipedia](https://en.wikipedia.org/wiki/Gale%E2%80%93Shapley_algorithm)
+ * [Wikipedia](https://en.wikipedia.org/wiki/Stable_matching_problem)
+ *
+ * @author [B Karthik](https://github.com/BKarthik7)
+ */
+
+#include <iostream>
+#include <vector>
+#include <algorithm>
+#include <cassert>
+
+/**
+ * @namespace
+ * @brief Greedy Algorithms
+ */
+namespace greedy_algorithms {
+/**
+ * @namespace
+ * @brief Functions for the Gale-Shapley Algorithm
+ */
+namespace stable_matching {
+/**
+ * @brief The main function that finds the stable matching between two sets of elements 
+ * using the Gale-Shapley Algorithm.
+ * @note This doesn't work on negative preferences. the preferences should be continuous integers starting from 
+ * 0 to number of preferences - 1.
+ * @param primary_preferences the preferences of the primary set should be a 2D vector
+ * @param secondary_preferences the preferences of the secondary set should be a 2D vector
+ * @returns matches the stable matching between the two sets
+ */
+std::vector<unsigned int> gale_shapley(const std::vector<std::vector<unsigned int>>& secondary_preferences, const std::vector<std::vector<unsigned int>>& primary_preferences) {
+    unsigned int num_elements = secondary_preferences.size();
+    std::vector<unsigned int> matches(num_elements, -1);
+    std::vector<bool> is_free_primary(num_elements, true);
+    std::vector<unsigned int> proposal_index(num_elements, 0); // Tracks the next secondary to propose for each primary
+
+    while (true) {
+        int free_primary_index = -1;
+
+        // Find the next free primary
+        for (unsigned int i = 0; i < num_elements; i++) {
+            if (is_free_primary[i]) {
+                free_primary_index = i;
+                break;
+            }
+        }
+
+        // If no free primary is found, break the loop
+        if (free_primary_index == -1) break;
+
+        // Get the next secondary to propose
+        unsigned int secondary_to_propose = primary_preferences[free_primary_index][proposal_index[free_primary_index]];
+        proposal_index[free_primary_index]++;
+
+        // Get the current match of the secondary
+        unsigned int current_match = matches[secondary_to_propose];
+
+        // If the secondary is free, match them
+        if (current_match == -1) {
+            matches[secondary_to_propose] = free_primary_index;
+            is_free_primary[free_primary_index] = false;
+        } else {
+            // Determine if the current match should be replaced
+            auto new_proposer_rank = std::find(secondary_preferences[secondary_to_propose].begin(),
+                                                secondary_preferences[secondary_to_propose].end(),
+                                                free_primary_index);
+            auto current_match_rank = std::find(secondary_preferences[secondary_to_propose].begin(),
+                                                 secondary_preferences[secondary_to_propose].end(),
+                                                 current_match);
+
+            // If the new proposer is preferred over the current match
+            if (new_proposer_rank < current_match_rank) {
+                matches[secondary_to_propose] = free_primary_index;
+                is_free_primary[free_primary_index] = false;
+                is_free_primary[current_match] = true; // Current match is now free
+            }
+        }
+    }
+
+    return matches;
+}
+}  // namespace stable_matching
+}  // namespace greedy_algorithms
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void tests() {
+    // Test Case 1
+    std::vector<std::vector<unsigned int>> primary_preferences = {{0, 1, 2, 3}, {2, 1, 3, 0}, {1, 2, 0, 3}, {3, 0, 1, 2}};
+    std::vector<std::vector<unsigned int>> secondary_preferences = {{1, 0, 2, 3}, {3, 0, 1, 2}, {0, 2, 1, 3}, {1, 2, 0, 3}};
+    assert(greedy_algorithms::stable_matching::gale_shapley(secondary_preferences, primary_preferences) == std::vector<unsigned int>({0, 2, 1, 3}));
+
+    // Test Case 2
+    primary_preferences = {{0, 2, 1, 3}, {2, 3, 0, 1}, {3, 1, 2, 0}, {2, 1, 0, 3}};
+    secondary_preferences = {{1, 0, 2, 3}, {3, 0, 1, 2}, {0, 2, 1, 3}, {1, 2, 0, 3}};
+    assert(greedy_algorithms::stable_matching::gale_shapley(secondary_preferences, primary_preferences) == std::vector<unsigned int>({0, 3, 1, 2}));
+
+    // Test Case 3
+    primary_preferences = {{0, 1, 2}, {2, 1, 0}, {1, 2, 0}};
+    secondary_preferences = {{1, 0, 2}, {2, 0, 1}, {0, 2, 1}};
+    assert(greedy_algorithms::stable_matching::gale_shapley(secondary_preferences, primary_preferences) == std::vector<unsigned int>({0, 2, 1}));
+
+    // Test Case 4
+    primary_preferences = {};
+    secondary_preferences = {};
+    assert(greedy_algorithms::stable_matching::gale_shapley(secondary_preferences, primary_preferences) == std::vector<unsigned int>({}));
+}
+
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
+int main() {
+    tests(); // Run self-test implementations
+    return 0;
+}