From ce829f7356a69d4110e81c2e762bc27d876917a9 Mon Sep 17 00:00:00 2001
From: lisyarus <lisyarus@gmail.com>
Date: Sat, 12 Jul 2025 01:28:40 +0300
Subject: [PATCH] Add permutation groups & implement converting cyclic &
 dihedral groups into permutations

---
 libs/group/include/psemek/group/cyclic.hpp    |   9 +
 libs/group/include/psemek/group/dihedral.hpp  |  20 +++
 .../include/psemek/group/permutation.hpp      | 154 ++++++++++++++++++
 3 files changed, 183 insertions(+)
 create mode 100644 libs/group/include/psemek/group/permutation.hpp

diff --git a/libs/group/include/psemek/group/cyclic.hpp b/libs/group/include/psemek/group/cyclic.hpp
index aad2a145..132c66e2 100644
--- a/libs/group/include/psemek/group/cyclic.hpp
+++ b/libs/group/include/psemek/group/cyclic.hpp
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <psemek/group/permutation.hpp>
 #include <psemek/util/range.hpp>
 
 #include <cstddef>
@@ -55,6 +56,14 @@ namespace psemek::group
 			return repr_;
 		}
 
+		group::permutation<N, Repr> permutation() const
+		{
+			typename group::permutation<N, Repr>::repr_type repr;
+			for (Repr i = 0; i < N; ++i)
+				repr[i] = (i + repr_) % N;
+			return group::permutation<N, Repr>::from_repr(repr);
+		}
+
 		struct value_iterator
 		{
 			using difference_type = Repr;
diff --git a/libs/group/include/psemek/group/dihedral.hpp b/libs/group/include/psemek/group/dihedral.hpp
index a1a5e364..cc199a35 100644
--- a/libs/group/include/psemek/group/dihedral.hpp
+++ b/libs/group/include/psemek/group/dihedral.hpp
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <psemek/group/cyclic.hpp>
+#include <psemek/group/permutation.hpp>
 
 #include <cstddef>
 #include <type_traits>
@@ -87,6 +88,25 @@ namespace psemek::group
 			return repr_ >= N;
 		}
 
+		group::permutation<N, Repr> permutation() const
+		{
+			if (repr_ < N)
+			{
+				typename group::permutation<N, Repr>::repr_type repr;
+				for (Repr i = 0; i < N; ++i)
+					repr[i] = (i + repr_) % N;
+				return group::permutation<N, Repr>::from_repr(repr);
+			}
+			else
+			{
+				typename group::permutation<N, Repr>::repr_type repr;
+				for (Repr i = 0; i < N; ++i)
+					repr[i] = (N - i) % N;
+
+				return rotation(repr_ % N).permutation() * group::permutation<N, Repr>::from_repr(repr);
+			}
+		}
+
 		struct value_iterator
 		{
 			using difference_type = Repr;
diff --git a/libs/group/include/psemek/group/permutation.hpp b/libs/group/include/psemek/group/permutation.hpp
new file mode 100644
index 00000000..f8efffa1
--- /dev/null
+++ b/libs/group/include/psemek/group/permutation.hpp
@@ -0,0 +1,154 @@
+#pragma once
+
+#include <psemek/util/range.hpp>
+
+#include <cstddef>
+#include <type_traits>
+#include <limits>
+#include <array>
+#include <algorithm>
+#include <numeric>
+
+namespace psemek::group
+{
+
+	template <std::size_t N, typename Repr = std::size_t>
+	struct permutation
+	{
+		static_assert(std::is_integral_v<Repr> && std::is_unsigned_v<Repr>);
+		static_assert(std::numeric_limits<Repr>::max() > N);
+
+		using repr_type = std::array<Repr, N>;
+
+		static constexpr std::size_t size()
+		{
+			return N;
+		}
+
+		permutation() = default;
+
+		static permutation identity()
+		{
+			return permutation{};
+		}
+
+		static permutation from_repr(repr_type const & value)
+		{
+			return permutation{value};
+		}
+
+		repr_type const & value() const
+		{
+			return repr_;
+		}
+
+		repr_type operator() (repr_type const & value) const
+		{
+			repr_type result;
+			for (Repr i = 0; i < N; ++i)
+				result[repr_[i]] = value[i];
+			return result;
+		}
+
+		struct value_iterator
+		{
+			using difference_type = Repr;
+			using value_type = permutation<N, Repr>;
+			using pointer = value_type *;
+			using reference	= value_type &;
+			using iterator_category = std::forward_iterator_tag;
+
+			repr_type repr;
+			bool end;
+
+			permutation<N, Repr> operator *() const
+			{
+				return permutation<N, Repr>{repr};
+			}
+
+			value_iterator & operator++()
+			{
+				end = std::next_permutation(repr.begin(), repr.end());
+				return *this;
+			}
+
+			friend bool operator == (value_iterator const & it1, value_iterator const & it2)
+			{
+				return it1.repr == it2.repr && it1.end == it2.end;
+			}
+		};
+
+		static auto values()
+		{
+			repr_type repr;
+			std::iota(repr.begin(), repr.end(), Repr{0});
+			return util::range{value_iterator{repr, false}, value_iterator{repr, true}};
+		}
+
+	private:
+		repr_type repr_{0};
+
+		explicit permutation(repr_type const & repr)
+			: repr_(repr)
+		{}
+	};
+
+	template <std::size_t N, typename Repr>
+	bool operator == (permutation<N, Repr> const & g1, permutation<N, Repr> const & g2)
+	{
+		return g1.value() == g2.value();
+	}
+
+	template <std::size_t N, typename Repr>
+	auto operator <=> (permutation<N, Repr> const & g1, permutation<N, Repr> const & g2)
+	{
+		return g1.value() <=> g2.value();
+	}
+
+	template <std::size_t N, typename Repr>
+	permutation<N, Repr> operator * (permutation<N, Repr> const & g1, permutation<N, Repr> const & g2)
+	{
+		typename permutation<N, Repr>::repr_type repr;
+		for (Repr i = 0; i < N; ++i)
+			repr[i] = g1.value()[g2.value()[i]];
+		return permutation<N, Repr>::from_repr(repr);
+	}
+
+	template <std::size_t N, typename Repr>
+	permutation<N, Repr> inverse(permutation<N, Repr> const & g)
+	{
+		typename permutation<N, Repr>::repr_type repr;
+		for (Repr i = 0; i < N; ++i)
+			repr[g.value()[i]] = i;
+		return permutation<N, Repr>::from_repr(repr);
+	}
+
+	template <typename OStream, std::size_t N, typename Repr>
+	OStream & operator << (OStream & os, permutation<N, Repr> const & g)
+	{
+		os << 'S' << N << '(';
+		for (Repr i = 0; i < N; ++i)
+		{
+			if (i > 0)
+				os << ", ";
+			os << g.value()[i];
+		}
+		os << ')';
+		return os;
+	}
+
+	template <typename OStream, std::size_t N, typename Repr>
+	void write(OStream & out, permutation<N, Repr> const & g)
+	{
+		write(out, g.value());
+	}
+
+	template <typename IStream, std::size_t N, typename Repr>
+	void read(IStream & in, permutation<N, Repr> & g)
+	{
+		typename permutation<N, Repr>::repr_type value;
+		read(in, value);
+		g = permutation<N, Repr>::from_repr(value);
+	}
+
+}