mirror of
https://github.com/SerenityOS/serenity
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c08d137fcd
Instead of rehashing on collisions, we use Robin Hood hashing: a simple linear probe where we keep track of the distance between the bucket and its ideal position. On insertion, we allow a new bucket to "steal" the position of "rich" buckets (those near their ideal position) and move them further down. On removal, we shift buckets back up into the freed slot, decrementing their distance while doing so. This behavior automatically optimizes the number of required probes for any value, and removes the need for periodic rehashing (except when expanding the capacity).
704 lines
23 KiB
C++
704 lines
23 KiB
C++
/*
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* Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
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* Copyright (c) 2023, Jelle Raaijmakers <jelle@gmta.nl>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include <AK/Concepts.h>
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#include <AK/Error.h>
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#include <AK/StdLibExtras.h>
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#include <AK/Traits.h>
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#include <AK/Types.h>
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#include <AK/kmalloc.h>
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namespace AK {
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enum class HashSetResult {
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InsertedNewEntry,
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ReplacedExistingEntry,
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KeptExistingEntry,
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};
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enum class HashSetExistingEntryBehavior {
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Keep,
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Replace,
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};
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// BucketState doubles as both an enum and a probe length value.
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// - Free: empty bucket
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// - Used (implicit, values 1..254): value-1 represents probe length
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// - CalculateLength: same as Used but probe length > 253, so we calculate the actual probe length
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enum class BucketState : u8 {
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Free = 0,
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CalculateLength = 255,
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};
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template<typename HashTableType, typename T, typename BucketType>
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class HashTableIterator {
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friend HashTableType;
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public:
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bool operator==(HashTableIterator const& other) const { return m_bucket == other.m_bucket; }
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bool operator!=(HashTableIterator const& other) const { return m_bucket != other.m_bucket; }
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T& operator*() { return *m_bucket->slot(); }
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T* operator->() { return m_bucket->slot(); }
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void operator++() { skip_to_next(); }
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private:
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void skip_to_next()
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{
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if (!m_bucket)
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return;
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do {
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++m_bucket;
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if (m_bucket == m_end_bucket) {
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m_bucket = nullptr;
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return;
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}
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} while (m_bucket->state == BucketState::Free);
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}
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HashTableIterator(BucketType* bucket, BucketType* end_bucket)
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: m_bucket(bucket)
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, m_end_bucket(end_bucket)
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{
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}
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BucketType* m_bucket { nullptr };
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BucketType* m_end_bucket { nullptr };
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};
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template<typename OrderedHashTableType, typename T, typename BucketType>
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class OrderedHashTableIterator {
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friend OrderedHashTableType;
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public:
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bool operator==(OrderedHashTableIterator const& other) const { return m_bucket == other.m_bucket; }
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bool operator!=(OrderedHashTableIterator const& other) const { return m_bucket != other.m_bucket; }
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T& operator*() { return *m_bucket->slot(); }
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T* operator->() { return m_bucket->slot(); }
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void operator++() { m_bucket = m_bucket->next; }
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void operator--() { m_bucket = m_bucket->previous; }
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private:
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OrderedHashTableIterator(BucketType* bucket, BucketType*)
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: m_bucket(bucket)
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{
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}
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BucketType* m_bucket { nullptr };
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};
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template<typename T, typename TraitsForT, bool IsOrdered>
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class HashTable {
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static constexpr size_t grow_capacity_at_least = 8;
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static constexpr size_t grow_at_load_factor_percent = 80;
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static constexpr size_t grow_capacity_increase_percent = 60;
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struct Bucket {
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BucketState state;
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alignas(T) u8 storage[sizeof(T)];
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T* slot() { return reinterpret_cast<T*>(storage); }
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T const* slot() const { return reinterpret_cast<T const*>(storage); }
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};
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struct OrderedBucket {
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OrderedBucket* previous;
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OrderedBucket* next;
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BucketState state;
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alignas(T) u8 storage[sizeof(T)];
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T* slot() { return reinterpret_cast<T*>(storage); }
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T const* slot() const { return reinterpret_cast<T const*>(storage); }
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};
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using BucketType = Conditional<IsOrdered, OrderedBucket, Bucket>;
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struct CollectionData {
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};
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struct OrderedCollectionData {
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BucketType* head { nullptr };
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BucketType* tail { nullptr };
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};
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using CollectionDataType = Conditional<IsOrdered, OrderedCollectionData, CollectionData>;
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public:
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HashTable() = default;
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explicit HashTable(size_t capacity) { rehash(capacity); }
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~HashTable()
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{
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if (!m_buckets)
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return;
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if constexpr (!IsTriviallyDestructible<T>) {
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for (size_t i = 0; i < m_capacity; ++i) {
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if (m_buckets[i].state != BucketState::Free)
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m_buckets[i].slot()->~T();
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}
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}
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kfree_sized(m_buckets, size_in_bytes(m_capacity));
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}
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HashTable(HashTable const& other)
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{
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rehash(other.capacity());
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for (auto& it : other)
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set(it);
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}
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HashTable& operator=(HashTable const& other)
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{
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HashTable temporary(other);
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swap(*this, temporary);
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return *this;
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}
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HashTable(HashTable&& other) noexcept
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: m_buckets(other.m_buckets)
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, m_collection_data(other.m_collection_data)
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, m_size(other.m_size)
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, m_capacity(other.m_capacity)
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{
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other.m_size = 0;
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other.m_capacity = 0;
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other.m_buckets = nullptr;
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if constexpr (IsOrdered)
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other.m_collection_data = { nullptr, nullptr };
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}
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HashTable& operator=(HashTable&& other) noexcept
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{
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HashTable temporary { move(other) };
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swap(*this, temporary);
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return *this;
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}
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friend void swap(HashTable& a, HashTable& b) noexcept
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{
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swap(a.m_buckets, b.m_buckets);
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swap(a.m_size, b.m_size);
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swap(a.m_capacity, b.m_capacity);
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if constexpr (IsOrdered)
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swap(a.m_collection_data, b.m_collection_data);
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}
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[[nodiscard]] bool is_empty() const { return m_size == 0; }
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[[nodiscard]] size_t size() const { return m_size; }
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[[nodiscard]] size_t capacity() const { return m_capacity; }
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template<typename U, size_t N>
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ErrorOr<void> try_set_from(U (&from_array)[N])
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{
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for (size_t i = 0; i < N; ++i)
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TRY(try_set(from_array[i]));
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return {};
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}
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template<typename U, size_t N>
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void set_from(U (&from_array)[N])
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{
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MUST(try_set_from(from_array));
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}
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ErrorOr<void> try_ensure_capacity(size_t capacity)
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{
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// The user usually expects "capacity" to mean the number of values that can be stored in a
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// container without it needing to reallocate. Our definition of "capacity" is the number of
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// buckets we can store, but we reallocate earlier because of `grow_at_load_factor_percent`.
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// This calculates the required internal capacity to store `capacity` number of values.
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size_t required_capacity = capacity * 100 / grow_at_load_factor_percent + 1;
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if (required_capacity <= m_capacity)
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return {};
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return try_rehash(required_capacity);
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}
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void ensure_capacity(size_t capacity)
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{
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MUST(try_ensure_capacity(capacity));
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}
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[[nodiscard]] bool contains(T const& value) const
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{
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return find(value) != end();
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}
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template<Concepts::HashCompatible<T> K>
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requires(IsSame<TraitsForT, Traits<T>>) [[nodiscard]] bool contains(K const& value) const
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{
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return find(value) != end();
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}
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using Iterator = Conditional<IsOrdered,
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OrderedHashTableIterator<HashTable, T, BucketType>,
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HashTableIterator<HashTable, T, BucketType>>;
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[[nodiscard]] Iterator begin()
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{
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if constexpr (IsOrdered)
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return Iterator(m_collection_data.head, end_bucket());
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for (size_t i = 0; i < m_capacity; ++i) {
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if (m_buckets[i].state != BucketState::Free)
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return Iterator(&m_buckets[i], end_bucket());
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}
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return end();
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}
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[[nodiscard]] Iterator end()
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{
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return Iterator(nullptr, nullptr);
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}
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using ConstIterator = Conditional<IsOrdered,
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OrderedHashTableIterator<const HashTable, const T, BucketType const>,
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HashTableIterator<const HashTable, const T, BucketType const>>;
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[[nodiscard]] ConstIterator begin() const
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{
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if constexpr (IsOrdered)
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return ConstIterator(m_collection_data.head, end_bucket());
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for (size_t i = 0; i < m_capacity; ++i) {
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if (m_buckets[i].state != BucketState::Free)
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return ConstIterator(&m_buckets[i], end_bucket());
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}
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return end();
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}
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[[nodiscard]] ConstIterator end() const
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{
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return ConstIterator(nullptr, nullptr);
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}
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void clear()
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{
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*this = HashTable();
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}
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void clear_with_capacity()
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{
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if (m_capacity == 0)
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return;
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if constexpr (!IsTriviallyDestructible<T>) {
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for (auto* bucket : *this)
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bucket->~T();
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}
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__builtin_memset(m_buckets, 0, size_in_bytes(m_capacity));
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m_size = 0;
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if constexpr (IsOrdered)
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m_collection_data = { nullptr, nullptr };
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}
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template<typename U = T>
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ErrorOr<HashSetResult> try_set(U&& value, HashSetExistingEntryBehavior existing_entry_behavior = HashSetExistingEntryBehavior::Replace)
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{
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if (should_grow())
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TRY(try_rehash(m_capacity * (100 + grow_capacity_increase_percent) / 100));
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return write_value(forward<U>(value), existing_entry_behavior);
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}
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template<typename U = T>
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HashSetResult set(U&& value, HashSetExistingEntryBehavior existing_entry_behaviour = HashSetExistingEntryBehavior::Replace)
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{
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return MUST(try_set(forward<U>(value), existing_entry_behaviour));
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}
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template<typename TUnaryPredicate>
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[[nodiscard]] Iterator find(unsigned hash, TUnaryPredicate predicate)
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{
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return Iterator(lookup_with_hash(hash, move(predicate)), end_bucket());
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}
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[[nodiscard]] Iterator find(T const& value)
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{
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return find(TraitsForT::hash(value), [&](auto& other) { return TraitsForT::equals(value, other); });
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}
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template<typename TUnaryPredicate>
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[[nodiscard]] ConstIterator find(unsigned hash, TUnaryPredicate predicate) const
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{
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return ConstIterator(lookup_with_hash(hash, move(predicate)), end_bucket());
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}
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[[nodiscard]] ConstIterator find(T const& value) const
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{
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return find(TraitsForT::hash(value), [&](auto& other) { return TraitsForT::equals(value, other); });
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}
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// FIXME: Support for predicates, while guaranteeing that the predicate call
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// does not call a non trivial constructor each time invoked
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template<Concepts::HashCompatible<T> K>
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requires(IsSame<TraitsForT, Traits<T>>) [[nodiscard]] Iterator find(K const& value)
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{
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return find(Traits<K>::hash(value), [&](auto& other) { return Traits<T>::equals(other, value); });
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}
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template<Concepts::HashCompatible<T> K, typename TUnaryPredicate>
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requires(IsSame<TraitsForT, Traits<T>>) [[nodiscard]] Iterator find(K const& value, TUnaryPredicate predicate)
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{
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return find(Traits<K>::hash(value), move(predicate));
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}
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template<Concepts::HashCompatible<T> K>
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requires(IsSame<TraitsForT, Traits<T>>) [[nodiscard]] ConstIterator find(K const& value) const
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{
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return find(Traits<K>::hash(value), [&](auto& other) { return Traits<T>::equals(other, value); });
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}
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template<Concepts::HashCompatible<T> K, typename TUnaryPredicate>
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requires(IsSame<TraitsForT, Traits<T>>) [[nodiscard]] ConstIterator find(K const& value, TUnaryPredicate predicate) const
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{
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return find(Traits<K>::hash(value), move(predicate));
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}
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bool remove(T const& value)
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{
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auto it = find(value);
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if (it != end()) {
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remove(it);
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return true;
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}
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return false;
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}
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template<Concepts::HashCompatible<T> K>
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requires(IsSame<TraitsForT, Traits<T>>) bool remove(K const& value)
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{
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auto it = find(value);
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if (it != end()) {
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remove(it);
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return true;
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}
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return false;
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}
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// This invalidates the iterator
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void remove(Iterator& iterator)
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{
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auto* bucket = iterator.m_bucket;
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VERIFY(bucket);
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delete_bucket(*bucket);
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iterator.m_bucket = nullptr;
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}
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template<typename TUnaryPredicate>
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bool remove_all_matching(TUnaryPredicate const& predicate)
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{
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bool has_removed_anything = false;
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for (size_t i = 0; i < m_capacity; ++i) {
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auto& bucket = m_buckets[i];
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if (bucket.state == BucketState::Free || !predicate(*bucket.slot()))
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continue;
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delete_bucket(bucket);
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has_removed_anything = true;
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// If a bucket was shifted up, reevaluate this bucket index
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if (bucket.state != BucketState::Free)
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--i;
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}
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return has_removed_anything;
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}
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T pop()
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requires(IsOrdered)
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{
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VERIFY(!is_empty());
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T element = *m_collection_data.tail->slot();
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remove(element);
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return element;
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}
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private:
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bool should_grow() const { return ((m_size + 1) * 100) >= (m_capacity * grow_at_load_factor_percent); }
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static constexpr size_t size_in_bytes(size_t capacity) { return sizeof(BucketType) * capacity; }
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BucketType* end_bucket()
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{
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if constexpr (IsOrdered)
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return m_collection_data.tail;
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else
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return &m_buckets[m_capacity];
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}
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BucketType const* end_bucket() const
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{
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return const_cast<HashTable*>(this)->end_bucket();
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}
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ErrorOr<void> try_rehash(size_t new_capacity)
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{
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new_capacity = max(new_capacity, m_capacity + grow_capacity_at_least);
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new_capacity = kmalloc_good_size(size_in_bytes(new_capacity)) / sizeof(BucketType);
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VERIFY(new_capacity >= size());
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auto* old_buckets = m_buckets;
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auto old_buckets_size = size_in_bytes(m_capacity);
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Iterator old_iter = begin();
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auto* new_buckets = kcalloc(1, size_in_bytes(new_capacity));
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if (!new_buckets)
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return Error::from_errno(ENOMEM);
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m_buckets = static_cast<BucketType*>(new_buckets);
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m_capacity = new_capacity;
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if constexpr (IsOrdered)
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m_collection_data = { nullptr, nullptr };
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if (!old_buckets)
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return {};
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m_size = 0;
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for (auto it = move(old_iter); it != end(); ++it) {
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write_value(move(*it), HashSetExistingEntryBehavior::Keep);
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it->~T();
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}
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kfree_sized(old_buckets, old_buckets_size);
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return {};
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}
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void rehash(size_t new_capacity)
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{
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MUST(try_rehash(new_capacity));
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}
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template<typename TUnaryPredicate>
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[[nodiscard]] BucketType* lookup_with_hash(unsigned hash, TUnaryPredicate predicate) const
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{
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if (is_empty())
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return nullptr;
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hash %= m_capacity;
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for (;;) {
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auto* bucket = &m_buckets[hash];
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if (bucket->state == BucketState::Free)
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return nullptr;
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if (predicate(*bucket->slot()))
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return bucket;
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if (++hash == m_capacity) [[unlikely]]
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hash = 0;
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}
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}
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size_t used_bucket_probe_length(BucketType const& bucket) const
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{
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VERIFY(bucket.state != BucketState::Free);
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if (bucket.state == BucketState::CalculateLength) {
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size_t ideal_bucket_index = TraitsForT::hash(*bucket.slot()) % m_capacity;
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VERIFY(&bucket >= m_buckets);
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size_t actual_bucket_index = &bucket - m_buckets;
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if (actual_bucket_index < ideal_bucket_index)
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return m_capacity + actual_bucket_index - ideal_bucket_index;
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return actual_bucket_index - ideal_bucket_index;
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}
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return static_cast<u8>(bucket.state) - 1;
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}
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ALWAYS_INLINE constexpr BucketState bucket_state_for_probe_length(size_t probe_length)
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{
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if (probe_length > 253)
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return BucketState::CalculateLength;
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return static_cast<BucketState>(probe_length + 1);
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}
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template<typename U = T>
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HashSetResult write_value(U&& value, HashSetExistingEntryBehavior existing_entry_behavior)
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{
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auto update_collection_for_new_bucket = [&](BucketType& bucket) {
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if constexpr (IsOrdered) {
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if (!m_collection_data.head) [[unlikely]] {
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m_collection_data.head = &bucket;
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|
} else {
|
|
bucket.previous = m_collection_data.tail;
|
|
m_collection_data.tail->next = &bucket;
|
|
}
|
|
m_collection_data.tail = &bucket;
|
|
}
|
|
};
|
|
auto update_collection_for_swapped_buckets = [&](BucketType* left_bucket, BucketType* right_bucket) {
|
|
if constexpr (IsOrdered) {
|
|
if (m_collection_data.head == left_bucket)
|
|
m_collection_data.head = right_bucket;
|
|
else if (m_collection_data.head == right_bucket)
|
|
m_collection_data.head = left_bucket;
|
|
if (m_collection_data.tail == left_bucket)
|
|
m_collection_data.tail = right_bucket;
|
|
else if (m_collection_data.tail == right_bucket)
|
|
m_collection_data.tail = left_bucket;
|
|
|
|
if (left_bucket->previous) {
|
|
if (left_bucket->previous == left_bucket)
|
|
left_bucket->previous = right_bucket;
|
|
left_bucket->previous->next = left_bucket;
|
|
}
|
|
if (left_bucket->next) {
|
|
if (left_bucket->next == left_bucket)
|
|
left_bucket->next = right_bucket;
|
|
left_bucket->next->previous = left_bucket;
|
|
}
|
|
|
|
if (right_bucket->previous && right_bucket->previous != left_bucket)
|
|
right_bucket->previous->next = right_bucket;
|
|
if (right_bucket->next && right_bucket->next != left_bucket)
|
|
right_bucket->next->previous = right_bucket;
|
|
}
|
|
};
|
|
|
|
auto bucket_index = TraitsForT::hash(value) % m_capacity;
|
|
size_t probe_length = 0;
|
|
for (;;) {
|
|
auto* bucket = &m_buckets[bucket_index];
|
|
|
|
// We found a free bucket, write to it and stop
|
|
if (bucket->state == BucketState::Free) {
|
|
new (bucket->slot()) T(forward<U>(value));
|
|
bucket->state = bucket_state_for_probe_length(probe_length);
|
|
update_collection_for_new_bucket(*bucket);
|
|
++m_size;
|
|
return HashSetResult::InsertedNewEntry;
|
|
}
|
|
|
|
// The bucket is already used, does it have an identical value?
|
|
if (TraitsForT::equals(*bucket->slot(), static_cast<T const&>(value))) {
|
|
if (existing_entry_behavior == HashSetExistingEntryBehavior::Replace) {
|
|
(*bucket->slot()) = forward<U>(value);
|
|
return HashSetResult::ReplacedExistingEntry;
|
|
}
|
|
return HashSetResult::KeptExistingEntry;
|
|
}
|
|
|
|
// Robin hood: if our probe length is larger (poor) than this bucket's (rich), steal its position!
|
|
// This ensures that we will always traverse buckets in order of probe length.
|
|
auto target_probe_length = used_bucket_probe_length(*bucket);
|
|
if (probe_length > target_probe_length) {
|
|
// Copy out bucket
|
|
BucketType bucket_to_move = move(*bucket);
|
|
update_collection_for_swapped_buckets(bucket, &bucket_to_move);
|
|
|
|
// Write new bucket
|
|
new (bucket->slot()) T(forward<U>(value));
|
|
bucket->state = bucket_state_for_probe_length(probe_length);
|
|
probe_length = target_probe_length;
|
|
if constexpr (IsOrdered)
|
|
bucket->next = nullptr;
|
|
update_collection_for_new_bucket(*bucket);
|
|
++m_size;
|
|
|
|
// Find a free bucket, swapping with smaller probe length buckets along the way
|
|
for (;;) {
|
|
if (++bucket_index == m_capacity) [[unlikely]]
|
|
bucket_index = 0;
|
|
bucket = &m_buckets[bucket_index];
|
|
++probe_length;
|
|
|
|
if (bucket->state == BucketState::Free) {
|
|
*bucket = move(bucket_to_move);
|
|
bucket->state = bucket_state_for_probe_length(probe_length);
|
|
update_collection_for_swapped_buckets(&bucket_to_move, bucket);
|
|
break;
|
|
}
|
|
|
|
target_probe_length = used_bucket_probe_length(*bucket);
|
|
if (probe_length > target_probe_length) {
|
|
swap(bucket_to_move, *bucket);
|
|
bucket->state = bucket_state_for_probe_length(probe_length);
|
|
probe_length = target_probe_length;
|
|
update_collection_for_swapped_buckets(&bucket_to_move, bucket);
|
|
}
|
|
}
|
|
|
|
return HashSetResult::InsertedNewEntry;
|
|
}
|
|
|
|
// Try next bucket
|
|
if (++bucket_index == m_capacity) [[unlikely]]
|
|
bucket_index = 0;
|
|
++probe_length;
|
|
}
|
|
}
|
|
|
|
void delete_bucket(auto& bucket)
|
|
{
|
|
VERIFY(bucket.state != BucketState::Free);
|
|
|
|
// Delete the bucket
|
|
bucket.slot()->~T();
|
|
if constexpr (IsOrdered) {
|
|
if (bucket.previous)
|
|
bucket.previous->next = bucket.next;
|
|
else
|
|
m_collection_data.head = bucket.next;
|
|
if (bucket.next)
|
|
bucket.next->previous = bucket.previous;
|
|
else
|
|
m_collection_data.tail = bucket.previous;
|
|
bucket.previous = nullptr;
|
|
bucket.next = nullptr;
|
|
}
|
|
--m_size;
|
|
|
|
// If we deleted a bucket, we need to make sure to shift up all buckets after it to ensure
|
|
// that we can still probe for buckets with collisions, and we automatically optimize the
|
|
// probe lengths. To do so, we shift the following buckets up until we reach a free bucket,
|
|
// or a bucket with a probe length of 0 (the ideal index for that bucket).
|
|
auto update_bucket_neighbours = [&](BucketType* bucket) {
|
|
if constexpr (IsOrdered) {
|
|
if (bucket->previous)
|
|
bucket->previous->next = bucket;
|
|
if (bucket->next)
|
|
bucket->next->previous = bucket;
|
|
}
|
|
};
|
|
|
|
VERIFY(&bucket >= m_buckets);
|
|
size_t shift_to_index = &bucket - m_buckets;
|
|
VERIFY(shift_to_index < m_capacity);
|
|
size_t shift_from_index = shift_to_index;
|
|
for (;;) {
|
|
if (++shift_from_index == m_capacity) [[unlikely]]
|
|
shift_from_index = 0;
|
|
|
|
auto* shift_from_bucket = &m_buckets[shift_from_index];
|
|
if (shift_from_bucket->state == BucketState::Free)
|
|
break;
|
|
|
|
auto shift_from_probe_length = used_bucket_probe_length(*shift_from_bucket);
|
|
if (shift_from_probe_length == 0)
|
|
break;
|
|
|
|
auto* shift_to_bucket = &m_buckets[shift_to_index];
|
|
*shift_to_bucket = move(*shift_from_bucket);
|
|
shift_to_bucket->state = bucket_state_for_probe_length(shift_from_probe_length - 1);
|
|
update_bucket_neighbours(shift_to_bucket);
|
|
|
|
if (++shift_to_index == m_capacity) [[unlikely]]
|
|
shift_to_index = 0;
|
|
}
|
|
|
|
// Mark last bucket as free
|
|
m_buckets[shift_to_index].state = BucketState::Free;
|
|
}
|
|
|
|
BucketType* m_buckets { nullptr };
|
|
|
|
[[no_unique_address]] CollectionDataType m_collection_data;
|
|
size_t m_size { 0 };
|
|
size_t m_capacity { 0 };
|
|
};
|
|
}
|
|
|
|
#if USING_AK_GLOBALLY
|
|
using AK::HashSetResult;
|
|
using AK::HashTable;
|
|
using AK::OrderedHashTable;
|
|
#endif
|