concepts: add initial clock concept

CMakeLists.txt

@@ -281,6 +281,7 @@ list (
     colour.cpp
     colour.hpp
     concepts.hpp
+    concepts/clock.hpp
     container.hpp
     coord.hpp
     coord/fwd.hpp

concepts/clock.hpp

@@ -0,0 +1,73 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * Copyright 2020, Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include "../concepts.hpp"
+
+#include <chrono>
+#include <ratio>
+
+#include <cstdint>
+
+
+namespace cruft::concepts {
+    namespace detail {
+        template <typename ValueT>
+        struct is_ratio : public std::false_type {};
+
+
+        template <std::intmax_t Num, std::intmax_t Denom>
+        struct is_ratio<
+            std::ratio<Num, Denom>
+        > : public std::true_type { };
+    };
+
+    template <typename T>
+    concept clock =
+        numeric<typename T::rep> &&
+        detail::is_ratio<typename T::period>::value &&
+        same_as<typename T::duration, std::chrono::duration<typename T::rep, typename T::period>> &&
+        //same_as<typename T::time_point, std::chrono::time_point<typename T::rep, typename T::period>> &&
+        same_as<decltype(T::is_steady), bool const> &&
+        requires (T t)
+    {
+        typename T::rep;
+        typename T::period;
+        typename T::time_point;
+        typename T::duration;
+
+        { T::is_steady  } -> convertible_to<bool>;
+        { t.now () } -> same_as<typename T::time_point>;
+    };
+
+    namespace detail {
+        template <typename T>
+        concept trivial_clock_inner_type =
+            equality_comparable<T> &&
+            is_less_than_comparable_v<T> &&
+            default_constructible<T> &&
+            copy_constructible<T> &&
+            copy_assignable<T> &&
+            destructible<T> &&
+            numeric<T> &&
+            swappable<T>;
+
+        template <typename T>
+        concept trivial_clock =
+            trivial_clock_inner_type<typename T::rep> &&
+            trivial_clock_inner_type<typename T::duration> &&
+            trivial_clock_inner_type<typename T::time_point> &&
+            noexcept (std::declval<T> ().now());
+    }
+
+    template <typename T>
+    concept trivial_clock =
+        detail::trivial_clock<T> &&
+        detail::trivial_clock<typename T::time_point::clock>;
+}