xo-unit: adopt dim-checking unit impl from xo-observable

include/xo/unit/dim_util.hpp

@@ -0,0 +1,59 @@
+/* @file dim_util.hpp */
+
+#pragma once
+
+#include "stringliteral.hpp"
+
+namespace xo {
+    namespace obs {
+        enum class dim {
+            /** weight.  native unit = 1 gram **/
+            mass,
+            /** distance.  native unit = 1 meter **/
+            distance,
+            /** time. native unit = 1 second **/
+            time,
+            /** a currency amount. native unit depends on actual currency.
+             *  For USD: one US dollar.
+             *
+             *  NOTE: unit system isn't suitable for multicurrency work:
+             *        (1usd + 1eur) is well-defined,  but (1sec + 1m) is not.
+             **/
+            currency,
+            /** a screen price. dimensionless **/
+            price,
+        };
+
+        enum class native_unit_id {
+            gram,
+            meter,
+            second,
+            currency,
+            price
+        };
+
+        template <dim Dim>
+        struct native_unit_for;
+
+        template <>
+        struct native_unit_for<dim::mass> { static constexpr auto value = native_unit_id::gram; };
+
+        template <>
+        struct native_unit_for<dim::distance> { static constexpr auto value = native_unit_id::meter; };
+
+        template <>
+        struct native_unit_for<dim::time> { static constexpr auto value = native_unit_id::second; };
+
+        template <>
+        struct native_unit_for<dim::currency> { static constexpr auto value = native_unit_id::currency; };
+
+        template <>
+        struct native_unit_for<dim::price> { static constexpr auto value = native_unit_id::price; };
+
+        template <dim Dim>
+        constexpr auto native_unit_for_v = native_unit_for<Dim>::value;
+    } /*namespace obs*/
+} /*namespace xo*/
+
+
+/* end dim_util.hpp */