rexylib/include/rexy/cx/hashmap.hpp

/**
	This file is a part of rexy's general purpose library
	Copyright (C) 2020 rexy712

	This program is free software: you can redistribute it and/or modify
	it under the terms of the GNU General Public License as published by
	the Free Software Foundation, either version 3 of the License, or
	(at your option) any later version.

	This program is distributed in the hope that it will be useful,
	but WITHOUT ANY WARRANTY; without even the implied warranty of
	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
	GNU General Public License for more details.

	You should have received a copy of the GNU General Public License
	along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#ifndef REXY_CX_HASHMAP_HPP
#define REXY_CX_HASHMAP_HPP

#include "vector.hpp"
#include "array.hpp"
#include "../algorithm.hpp"
#include "../hash.hpp"

#include <climits> //CHAR_BIT
#include <cstddef> //size_t, ptrdiff_t
#include <utility> //pair
#include <type_traits> //decay
#include <initializer_list>

namespace rexy::cx{

	template<class Key, class Value>
	struct element{
		using key_type = Key;
		using value_type = Value;

		Key key;
		Value value;
	};
	template<class Key, class Value>
	element(Key,Value) -> element<Key,Value>;

	template<class Key, class Value, size_t N, class Hash = hash<Key>>
	class hashmap
	{
	public:
		using key_type = Key;
		using mapped_type = Value;
		using value_type = element<Key,Value>;
		using size_type = size_t;
		using difference_type = ptrdiff_t;
		using hasher = Hash;
		using reference = mapped_type&;
		using const_reference = const mapped_type&;
		using pointer = mapped_type*;
		using const_pointer = const mapped_type*;

		static constexpr size_type single_bucket_bit = size_type{1} << ((sizeof(size_type)*CHAR_BIT) - 1);
		static constexpr size_type max_size = N;

		static_assert((max_size & single_bucket_bit) == 0);


	private:
		array<mapped_type,N>  m_values; //perfect hash table
		array<size_type,N> m_g;      //'salt' values for indexing into the perfect hash table
		array<size_type,N> m_key_hashes; //full hash values for keys to verify good index values

	public:
		constexpr hashmap(const value_type(&elements)[N])
			noexcept(std::is_nothrow_default_constructible<value_type>::value &&
			         std::is_nothrow_copy_constructible<value_type>::value &&
			         std::is_nothrow_move_assignable<mapped_type>::value &&
			         std::is_nothrow_invocable<Hash,Key,size_t>::value);

		//no key checks. give a correct key or get a random answer :)
		template<class U, class UHash = hash<std::decay_t<U>>>
		constexpr reference operator[](U&& u)noexcept;
		template<class U, class UHash = hash<std::decay_t<U>>>
		constexpr const_reference operator[](U&& u)const noexcept;

		template<class U, class UHash = hash<std::decay_t<U>>>
		constexpr bool is_valid(U&& u)const noexcept;

	};

	template<class Key, class Value, size_t N, class Hash>
	constexpr hashmap<Key,Value,N,Hash>::hashmap(const value_type(&elements)[N])
		noexcept(std::is_nothrow_default_constructible<value_type>::value &&
		         std::is_nothrow_copy_constructible<value_type>::value &&
		         std::is_nothrow_move_assignable<mapped_type>::value &&
		         std::is_nothrow_invocable<Hash,Key,size_t>::value)
	{
		array<vector<value_type,N>,N> buckets;
		size_type current_bucket = 0;

		//place all keys into buckets
		for(auto& element : elements){
			buckets[Hash{}(element.key, 0) % max_size].push_back(element);
		}

		//sort the buckets based on size, largest first
		quicksort(buckets.begin(), buckets.end(), [](auto&& left, auto&& right) -> bool{
			return left.size() > right.size();
		});

		//for each bucket, try different values of 'd' to try to find a hash that doesn't collide
		for(current_bucket = 0;current_bucket < buckets.size();++current_bucket){
			auto& bucket = buckets[current_bucket];
			//only handle buckets containing collisions
			if(bucket.size() <= 1)
				break;

			const auto hashval = Hash{}(bucket[0].key, 0);

			array<bool,N> pass_slots_used;
			vector<size_type,N> pass_slots;
			size_type d = 1;

			for(size_type i = 0;i < bucket.size();){
				size_type slot = Hash{}(bucket[i].key, d) % max_size;
				if(pass_slots_used[slot] || m_key_hashes[slot] != 0){
					//slot already in use, try another value for 'd'
					++d;
					i = 0;
					pass_slots_used.fill(false);
					pass_slots.clear();
				}else{
					//slot is good to go
					pass_slots_used[slot] = true;
					pass_slots.push_back(slot);
					++i;
				}
			}
			//store the successful value of 'd' at index of the first hash for this bucket
			m_g[hashval % max_size] = d;

			//take the value from the temporary bucket into the permanent slot
			for(size_type i = 0;i < bucket.size();++i){
				m_values[pass_slots[i]] = std::move(bucket[i].value);
				m_key_hashes[pass_slots[i]] = hashval;
			}
		}

		//Handle remaining single value buckets
		size_type next_free_slot = 0;

		for(;current_bucket < buckets.size();++current_bucket){
			auto& bucket = buckets[current_bucket];
			if(bucket.size() == 0)
				break;

			const auto hashval = Hash{}(bucket[0].key, 0);

			for(;m_key_hashes[next_free_slot] != 0;++next_free_slot);

			m_g[Hash{}(bucket[0].key, 0) % max_size] = (next_free_slot | single_bucket_bit);
			m_values[next_free_slot] = std::move(bucket[0].value);
			m_key_hashes[next_free_slot] = hashval;
		}
	}

	//no key checks. give a correct key or get a random answer :)
	template<class Key, class Value, size_t N, class Hash>
	template<class U, class UHash>
	constexpr auto hashmap<Key,Value,N,Hash>::operator[](U&& key)noexcept -> reference{
		auto d = m_g[UHash{}(std::forward<U>(key), 0) % max_size];
		if(d & single_bucket_bit)
			return m_values[d & ~single_bucket_bit];
		return m_values[UHash{}(std::forward<U>(key), d) % max_size];
	}
	template<class Key, class Value, size_t N, class Hash>
	template<class U, class UHash>
	constexpr auto hashmap<Key,Value,N,Hash>::operator[](U&& key)const noexcept -> const_reference{
		auto d = m_g[UHash{}(std::forward<U>(key), 0) % max_size];
		if(d & single_bucket_bit)
			return m_values[d & ~single_bucket_bit];
		return m_values[UHash{}(std::forward<U>(key), d) % max_size];
	}

	template<class Key, class Value, size_t N, class Hash>
	template<class U, class UHash>
	constexpr bool hashmap<Key,Value,N,Hash>::is_valid(U&& key)const noexcept{
		const auto hashval = UHash{}(std::forward<U>(key), 0);
		const auto d = m_g[hashval % max_size];
		if(d & single_bucket_bit){
			return m_key_hashes[d & ~single_bucket_bit] == hashval;
		}
		return m_key_hashes[UHash{}(std::forward<U>(key), d) % max_size] == hashval;
	}

	template<class Key, class Value, size_t N, class Hash = hash<Key>>
	constexpr auto make_hashmap(const typename hashmap<Key,Value,N,Hash>::value_type(&list)[N]){
		return hashmap<Key,Value,N,Hash>(list);
	}
}

#ifdef REXY_STRING_BASE_HPP
#include "../string_hash.hpp"
#endif

#endif