build: add entt library

include/entt/graph/dot.hpp

@@ -0,0 +1,56 @@
+#ifndef ENTT_GRAPH_DOT_HPP
+#define ENTT_GRAPH_DOT_HPP
+
+#include "../stl/concepts.hpp"
+#include "../stl/ostream.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Outputs a graph in dot format.
+ * @tparam Graph Graph type, valid as long as it exposes edges and vertices.
+ * @param out A standard output stream.
+ * @param graph The graph to output.
+ * @param writer Vertex decorator object.
+ */
+template<typename Graph>
+requires stl::derived_from<typename Graph::graph_category, directed_tag>
+void dot(stl::ostream &out, const Graph &graph, stl::invocable<stl::ostream &, typename Graph::vertex_type> auto writer) {
+    if constexpr(stl::same_as<typename Graph::graph_category, undirected_tag>) {
+        out << "graph{";
+    } else {
+        out << "digraph{";
+    }
+
+    for(auto &&vertex: graph.vertices()) {
+        out << vertex << "[";
+        writer(out, vertex);
+        out << "];";
+    }
+
+    for(auto [lhs, rhs]: graph.edges()) {
+        if constexpr(stl::same_as<typename Graph::graph_category, undirected_tag>) {
+            out << lhs << "--" << rhs << ";";
+        } else {
+            out << lhs << "->" << rhs << ";";
+        }
+    }
+
+    out << "}";
+}
+
+/**
+ * @brief Outputs a graph in dot format.
+ * @tparam Graph Graph type, valid as long as it exposes edges and vertices.
+ * @param out A standard output stream.
+ * @param graph The graph to output.
+ */
+template<typename Graph>
+void dot(stl::ostream &out, const Graph &graph) {
+    return dot(out, graph, [](auto &&...) {});
+}
+
+} // namespace entt
+
+#endif