Move headers into include folder and add single header to include all reachability algorithms

include/algorithm/tarjan.h

@@ -0,0 +1,85 @@
+#ifndef TARJAN_H_
+#define TARJAN_H_
+
+#include "graph/scc.h"
+
+#include <stack>
+#include <vector>
+#include <ranges>
+
+using namespace graph;
+
+namespace algo {
+
+template<typename T>
+class Tarjan {
+public:
+  Tarjan() = default;
+
+  Tarjan(std::unordered_map<T, std::unordered_set<T>> adjList) : adjList(adjList) {}
+
+  //
+  auto execute();
+
+  //
+  void strongConnect(const T& u);
+private:
+  std::unordered_map<T, std::unordered_set<T>> adjList;
+  std::stack<T> S;
+  std::int16_t index = 0;
+  std::vector<SCC<T>> SCCs;
+  T cid;
+
+  struct Vertex {
+    int index = -1;
+    int lowlink = -1;
+    bool onStack = false;
+  };
+  std::unordered_map<T, Vertex> vmap;
+};
+
+template<typename T>
+void Tarjan<T>::strongConnect(const T& u) {
+  vmap[u].index = vmap[u].lowlink = index++;
+  S.push(u);
+  vmap[u].onStack = true;
+
+  for (const auto& w : adjList[u]) {
+    if (vmap[w].index == -1) {
+      strongConnect(w);
+      vmap[u].lowlink = std::min(vmap[u].lowlink, vmap[w].lowlink);
+    } else if (vmap[w].onStack) {
+      vmap[u].lowlink = std::min(vmap[u].lowlink, vmap[w].index);
+    }
+  }
+
+  // If u is a root node, pop the stack and generate an SCC
+  if (vmap[u].lowlink == vmap[u].index) {
+    std::unordered_map<T, std::unordered_set<T>> scc;
+    bool finished = false;
+    cid = S.top();
+
+    do {
+      const auto w = S.top();
+      S.pop();
+      vmap[w].onStack = false;
+      scc[w] = adjList[w];
+      finished = (w == u);
+    } while (!finished);
+    
+    SCCs.push_back({ scc, static_cast<T>(cid) });
+  }
+}
+
+template<typename T>
+auto Tarjan<T>::execute() {
+  for (const auto& u : std::views::keys(adjList)) {
+    if (vmap[u].index == -1)
+      strongConnect(u);
+  }
+  return SCCs;
+}
+
+} // namespace algo
+
+#endif