add Chapter18

Chapter18/cib/libs/flow/step.hpp

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+#pragma once
+
+#include <cib/detail/runtime_conditional.hpp>
+#include <cib/func_decl.hpp>
+#include <flow/common.hpp>
+#include <flow/log.hpp>
+#include <flow/subgraph_identity.hpp>
+#include <sc/string_constant.hpp>
+
+#include <stdx/compiler.hpp>
+#include <stdx/ct_string.hpp>
+#include <stdx/type_traits.hpp>
+
+#include <type_traits>
+
+namespace flow {
+template <stdx::ct_string Type, stdx::ct_string Name,
+          subgraph_identity Identity, typename Cond, typename F>
+struct ct_node {
+    using is_subgraph = void;
+    using type_t =
+        decltype(stdx::ct_string_to_type<Type, sc::string_constant>());
+    using name_t =
+        decltype(stdx::ct_string_to_type<Name, sc::string_constant>());
+    using func_t = F;
+
+    constexpr static auto ct_name = Name;
+    constexpr static auto identity = Identity;
+    constexpr static auto condition = Cond{};
+
+    constexpr auto operator*() const {
+        if constexpr (Identity == subgraph_identity::REFERENCE) {
+            return ct_node<Type, Name, subgraph_identity::VALUE, Cond, F>{};
+        } else {
+            return ct_node{};
+        }
+    }
+};
+
+namespace detail {
+template <stdx::ct_string Type, stdx::ct_string Name, typename F>
+[[nodiscard]] constexpr auto make_node() {
+    return ct_node<Type, Name, subgraph_identity::REFERENCE,
+                   cib::detail::always_condition_t, F>{};
+}
+
+constexpr auto empty_func = []() {};
+} // namespace detail
+
+template <stdx::ct_string Name, typename F>
+    requires(stdx::is_function_object_v<F> and std::is_empty_v<F>)
+[[nodiscard]] constexpr auto action(F const &) {
+    return detail::make_node<"action", Name, F>();
+}
+
+template <stdx::ct_string Name> [[nodiscard]] constexpr auto action() {
+    return action<Name>(cib::func_decl<Name>);
+}
+
+template <stdx::ct_string Name> [[nodiscard]] constexpr auto step() {
+    return action<Name>(cib::func_decl<Name>);
+}
+
+template <stdx::ct_string Name> [[nodiscard]] constexpr auto milestone() {
+    return detail::make_node<"milestone", Name, decltype(detail::empty_func)>();
+}
+
+inline namespace literals {
+template <stdx::ct_string S> [[nodiscard]] constexpr auto operator""_action() {
+    return action<S>();
+}
+
+template <stdx::ct_string S> [[nodiscard]] constexpr auto operator""_step() {
+    return action<S>();
+}
+
+template <stdx::ct_string S>
+[[nodiscard]] constexpr auto operator""_milestone() {
+    return milestone<S>();
+}
+} // namespace literals
+
+template <typename Cond, stdx::ct_string Type, stdx::ct_string Name,
+          subgraph_identity Identity, typename NodeCond, typename F>
+constexpr auto
+make_runtime_conditional(Cond, ct_node<Type, Name, Identity, NodeCond, F>) {
+    if constexpr (Identity == subgraph_identity::VALUE) {
+        return ct_node<Type, Name, Identity, decltype(NodeCond{} and Cond{}),
+                       F>{};
+    } else {
+        return ct_node<Type, Name, Identity, NodeCond, F>{};
+    }
+}
+
+} // namespace flow