From 4a2ce7eaca70deaa05a53a1032559f67126a0bcd Mon Sep 17 00:00:00 2001
From: Dhandeep <dd11singh10@gmail.com>
Date: Sun, 1 Oct 2023 17:55:11 +0530
Subject: [PATCH] added kahn algorithm

---
 README.md                                     |  1 +
 .../algorithm/topological_sorting/kahn.cpp    | 51 +++++++++++++++++++
 .../algorithm/topological_sorting/kahn.h      | 19 +++++++
 3 files changed, 71 insertions(+)
 create mode 100644 include/graaflib/algorithm/topological_sorting/kahn.cpp
 create mode 100644 include/graaflib/algorithm/topological_sorting/kahn.h

diff --git a/README.md b/README.md
index 5a714804..aec71938 100644
--- a/README.md
+++ b/README.md
@@ -107,6 +107,7 @@ take a look at the [docs](https://bobluppes.github.io/graaf/docs/algorithms/intr
    - Tarjan's Strongly Connected Components
 6. [**Topological Sorting Algorithms**](https://bobluppes.github.io/graaf/docs/category/topological-sorting):
    - Topological sorting DFS-based
+   - Kahn's Algorithm
 7. [**Traversal Algorithms**](https://bobluppes.github.io/graaf/docs/category/traversal-algorithms):
    - Breadth-First Search (BFS)
    - Depth-First Search (DFS)
diff --git a/include/graaflib/algorithm/topological_sorting/kahn.cpp b/include/graaflib/algorithm/topological_sorting/kahn.cpp
new file mode 100644
index 00000000..9fad8526
--- /dev/null
+++ b/include/graaflib/algorithm/topological_sorting/kahn.cpp
@@ -0,0 +1,51 @@
+// graaflib/algorithm/topological_sorting/kahn.cpp
+#include "kahn.h"
+
+namespace graaflib::algorithm::topological_sorting {
+
+    template <typename V, typename E>
+    std::optional<std::vector<vertex_id_t>> kahns_topological_sort(
+        const graph<V, E, graph_type::DIRECTED>& graph
+    ) {
+        std::vector<vertex_id_t> topological_order;
+        std::vector<vertex_id_t> in_degree(graph.num_vertices(), 0);
+
+        //calculate in-degrees for all vertices
+        for (const auto& edge : graph.edges()) {
+            in_degree[edge.target()]++;
+        }
+
+        //queue to store vertices with in-degree 0
+        std::queue<vertex_id_t> zero_in_degree_vertices;
+
+        //initialize the queue with vertices having in-degree 0
+        for (vertex_id_t vertex = 0; vertex < graph.num_vertices(); ++vertex) {
+            if (in_degree[vertex] == 0) {
+                zero_in_degree_vertices.push(vertex);
+            }
+        }
+
+        while (!zero_in_degree_vertices.empty()) {
+            vertex_id_t current_vertex = zero_in_degree_vertices.front();
+            zero_in_degree_vertices.pop();
+
+            topological_order.push_back(current_vertex);
+
+            //remove the current vertex and its outgoing edges
+            for (const auto& edge : graph.edges_from(current_vertex)) {
+                vertex_id_t neighbor = edge.target();
+                if (--in_degree[neighbor] == 0) {
+                    zero_in_degree_vertices.push(neighbor);
+                }
+            }
+        }
+
+        //check for the presence of a cycle
+        if (topological_order.size() != graph.num_vertices()) {
+            return std::nullopt; //graph contains a cycle
+        }
+
+        return topological_order;
+    }
+
+}
diff --git a/include/graaflib/algorithm/topological_sorting/kahn.h b/include/graaflib/algorithm/topological_sorting/kahn.h
new file mode 100644
index 00000000..3bdfeaea
--- /dev/null
+++ b/include/graaflib/algorithm/topological_sorting/kahn.h
@@ -0,0 +1,19 @@
+// graaflib/algorithm/topological_sorting/kahn.h
+#ifndef GRAAF_KAHN_H
+#define GRAAF_KAHN_H
+
+#include <vector>
+#include <optional>
+#include <graaf/include/graaflib/graph.h> // Include graph header
+#include "graaf/properties/vertex_properties.h" // Include vertex properties header
+
+namespace graaflib::algorithm::topological_sorting {
+
+    template <typename V, typename E>
+    std::optional<std::vector<vertex_id_t>> kahns_topological_sort(
+        const graph<V, E, graph_type::DIRECTED>& graph
+    );
+
+} 
+
+#endif