add Chapter18

Chapter18/cib/libs/flow/graphviz_builder.hpp

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+#include <flow/detail/walk.hpp>
+
+#include <stdx/tuple_algorithms.hpp>
+
+#include <set>
+#include <string>
+#include <string_view>
+
+namespace flow {
+using VizFunctionPtr = auto (*)() -> std::string;
+
+struct graphviz_builder {
+    template <typename Graph>
+    [[nodiscard]] static auto build(Graph const &input) {
+        auto const nodes = flow::dsl::get_nodes(input);
+        auto const edges = flow::dsl::get_edges(input);
+
+        using nodes_t = std::set<std::string_view>;
+        nodes_t sources{};
+        nodes_t sinks{};
+        for_each(
+            [&]<typename Node>(Node const &) {
+                sources.insert(Node::name_t::value);
+                sinks.insert(Node::name_t::value);
+            },
+            nodes);
+        for_each(
+            [&]<typename Edge>(Edge const &) {
+                sinks.erase(Edge::source_t::name_t::value);
+                sources.erase(Edge::dest_t::name_t::value);
+            },
+            edges);
+
+        std::string output{"digraph "};
+        output += std::string_view{Graph::name};
+        output += " {\n";
+        for (auto const &node : sources) {
+            output += "start -> " + std::string{node} + '\n';
+        }
+        for_each(
+            [&]<typename Edge>(Edge const &) {
+                output += std::string{Edge::source_t::name_t::value};
+                output += " -> ";
+                output += std::string{Edge::dest_t::name_t::value};
+                output += '\n';
+            },
+            edges);
+        for (auto const &node : sinks) {
+            output += std::string{node} + " -> end\n";
+        }
+        output += "}";
+        return output;
+    }
+
+    template <typename Initialized> class built_flow {
+        static auto built() {
+            auto const v = Initialized::value;
+            return build(v);
+        }
+
+        static auto run() -> std::string { return built(); }
+
+      public:
+        // NOLINTNEXTLINE(google-explicit-constructor)
+        constexpr explicit(false) operator VizFunctionPtr() const {
+            return run;
+        }
+        auto operator()() const -> std::string { return run(); }
+        constexpr static bool active = true;
+    };
+
+    template <typename Initialized>
+    [[nodiscard]] constexpr static auto render() -> built_flow<Initialized> {
+        return {};
+    }
+};
+} // namespace flow