mct20/include/lfhmap.hpp

#pragma once
#include <cstddef>
#include <array>
#include <utility>
#include <atomic>
#include <new>
#include <memory>
#include <cassert>
#include <optional>

/**
Pensé en un mundo sin memoria, sin tiempo; consideré la posibilidad de un lenguaje 
que ignorara los sustantivos, un lenguaje de verbos impersonales y de indeclinables 
epítetos. Así fueron muriendo los días y con los días los años, pero algo parecido 
a la felicidad ocurrió una mañana. Llovió, con lentitud poderosa.
**/

namespace mct20 {

	template<typename T>
	class accessor {
		public:
		accessor(const T* ptr, std::atomic<unsigned int>& incremented_ref) 
		: pointer(ptr)
		, reference_cnt(incremented_ref)
		{
			assert(reference_cnt.load() != 0);
		}

		accessor(const accessor& a)
		: pointer(a.pointer)
		, reference_cnt(a.reference_cnt) 
		{
			reference_cnt.fetch_add(1);
		}

		accessor(const accessor&& a)
		: pointer(a.pointer)
		, reference_cnt(a.reference_cnt) 
		{
			reference_cnt.fetch_add(1);
		}

		operator const T&() {
			return *pointer;
		}
		~accessor() {
			reference_cnt.fetch_sub(1);
		}
		private:
		const T* pointer;
		std::atomic<unsigned int>& reference_cnt;
	};

	namespace _details_ {
		#ifdef __cpp_lib_hardware_interference_size
			constexpr size_t predictable_padding = std::hardware_constructive_interference_size;
		#else
			// Wild guess, may be suboptimal or plain wrong
			constexpr size_t predictable_padding = 128;
		#endif

		size_t rotl(size_t a, uint8_t b) {
			b%=sizeof(size_t)*8;
			return a << b | a >> (sizeof(size_t)*8 - b);
		}

		template<size_t against>
		constexpr size_t alignment = 
			(against%predictable_padding!=0)*predictable_padding
			+ (against/predictable_padding)*predictable_padding;

		template<typename K, typename V>
		class bucket {
		public:
			bucket()
			: start{nullptr}
			{}

			void push(size_t hash, const K& key, const V& value) {
				auto t = new node{
					.contents = node_contents{
						.key{key},
						.ptr{new V{value}},
						.hash{hash},
						.references{1}
					}
				};
				t->contents.next.store(t);
				node* expect;
				do {
					expect = start.load();
					t->contents.next.store(expect);
				} while(
					!std::atomic_compare_exchange_strong(
						&start,
						&expect,
						t
					)
				);
			}

			std::optional<accessor<V>> get(const size_t hash, const K& key) {
				auto v = start.load();
				while(v) {
					if(v->contents.references.fetch_add(1)!=0)
					{
						if(v->contents.hash == hash) {
							if(v->contents.key == key) {
								return accessor<V>(
									v->contents.ptr,
									v->contents.references
								);
							} else {
								auto n = reinterpret_cast<node*>(v->contents.next.load());
								v->contents.references.fetch_sub(1);
								v = n;
							}
						} else {
							auto n = reinterpret_cast<node*>(v->contents.next.load());
							v->contents.references.fetch_sub(1);
							v = n;
						}
					}
					else
					{
						auto n = reinterpret_cast<node*>(v->contents.next.load());
						v->contents.references.fetch_sub(1);
						v = n;
					}
				}
				return std::nullopt;
			}
			
			struct node_contents{
				std::atomic<void*> next;
				const K key;
				const V* ptr;
				size_t hash;
				std::atomic<unsigned int> references;
			};

			using node = union {
				alignas(alignment<sizeof(node_contents)>) node_contents contents;
			};

			using node_ptr = std::atomic<node*>;

			node_ptr start;
		};
	}

template<typename K, typename V, size_t bucket_count, typename hash = std::hash<K>>
class lfhmap {
	using bucket = _details_::bucket<K, V>;

public:
	std::optional<accessor<V>> get(const K& key) {
		auto l = hash{}(key);
		auto ret = buckets[l%bucket_count].get(l, key);
		if(ret) return ret;
		l = _details_::rotl(l, sizeof(size_t)*4);
		return buckets[l%bucket_count].get(l, key);
	}

	void set(const K& key, const V& value) {
		const auto l = hash{}(key);
		auto& ref = buckets[l%bucket_count];
		if(ref.start.load() == nullptr)
		{
			ref.push(l, key, value);
			return;
		}
		const auto l2 = _details_::rotl(l, sizeof(size_t)*4);
		auto& ref2 = buckets[l2%bucket_count];
		if(ref2.start.load() == nullptr)
		{
			ref2.push(l2, key, value);
			return;
		}
		if((l^l2)&1) {
			ref.push(l, key, value);
		} else {
			ref2.push(l2, key, value);
		}
		return;
	}

	lfhmap() {
		for(auto& a : buckets) {
			a.start = nullptr;
		}
	}
private:
	std::array<bucket, bucket_count> buckets;
};

}