feat: get_common_base() added

src/core/CMakeLists.txt

@@ -44,6 +44,7 @@ add_library(
     include/mp_units/bits/dimension_concepts.h
     include/mp_units/bits/expression_template.h
     include/mp_units/bits/get_associated_quantity.h
+    include/mp_units/bits/get_common_base.h
     include/mp_units/bits/magnitude.h
     include/mp_units/bits/math_concepts.h
     include/mp_units/bits/prime.h

src/core/include/mp_units/bits/get_common_base.h

@@ -0,0 +1,91 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Mateusz Pusz
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#pragma once
+
+#include <mp_units/bits/quantity_spec_concepts.h>
+
+namespace mp_units::detail {
+
+[[nodiscard]] consteval std::size_t hierarchy_path_length(QuantitySpec auto q)
+{
+  if constexpr (requires { q._parent_; })
+    return hierarchy_path_length(q._parent_) + 1;
+  else
+    return 1;
+}
+
+template<std::size_t Offset>
+  requires(Offset >= 0)
+[[nodiscard]] consteval QuantitySpec auto hierarchy_path_advance(QuantitySpec auto q)
+{
+  if constexpr (Offset == 0)
+    return q;
+  else if constexpr (requires { q._parent_; })
+    return hierarchy_path_advance<Offset - 1>(q._parent_);
+}
+
+template<QuantitySpec A, QuantitySpec B>
+[[nodiscard]] consteval bool have_common_base_in_hierarchy_of_equal_length(A a, B b)
+{
+  if constexpr (is_same_v<A, B>)
+    return true;
+  else if constexpr (requires { a._parent_; })
+    return have_common_base_in_hierarchy_of_equal_length(a._parent_, b._parent_);
+  else
+    return false;
+}
+
+template<QuantitySpec A, QuantitySpec B>
+[[nodiscard]] consteval auto have_common_base(A a, B b)
+{
+  constexpr int a_length = hierarchy_path_length(A{});
+  constexpr int b_length = hierarchy_path_length(B{});
+  if constexpr (a_length > b_length)
+    return have_common_base_in_hierarchy_of_equal_length(hierarchy_path_advance<a_length - b_length>(a), b);
+  else
+    return have_common_base_in_hierarchy_of_equal_length(a, hierarchy_path_advance<b_length - a_length>(b));
+}
+
+template<QuantitySpec A, QuantitySpec B>
+  requires(have_common_base_in_hierarchy_of_equal_length(A{}, B{}))
+[[nodiscard]] consteval auto get_common_base_for_hierarchy_of_equal_length(A a, B b)
+{
+  if constexpr (is_same_v<A, B>)
+    return a;
+  else
+    return get_common_base_for_hierarchy_of_equal_length(a._parent_, b._parent_);
+}
+
+template<QuantitySpec A, QuantitySpec B>
+  requires(have_common_base(A{}, B{}))
+[[nodiscard]] consteval auto get_common_base(A a, B b)
+{
+  constexpr int a_length = hierarchy_path_length(A{});
+  constexpr int b_length = hierarchy_path_length(B{});
+  if constexpr (a_length > b_length)
+    return get_common_base_for_hierarchy_of_equal_length(hierarchy_path_advance<a_length - b_length>(a), b);
+  else
+    return get_common_base_for_hierarchy_of_equal_length(a, hierarchy_path_advance<b_length - a_length>(b));
+}
+
+}  // namespace mp_units::detail