mirror of
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129 lines
3.7 KiB
C++
129 lines
3.7 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
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* vim: set ts=8 sts=4 et sw=4 tw=99:
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* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#ifndef jsfixedsizehash_h_
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#define jsfixedsizehash_h_
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#include "ds/LifoAlloc.h"
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namespace js {
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/*
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* Class representing a hash set with fixed capacity, with newer entries
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* evicting older entries. Each entry has several hashes and can be stored in
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* different buckets, with the choice of which to evict on insertion being
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* managed via LRU. For tables with a relatively small size, using different
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* hashes increases utilization and makes it less likely that entries will keep
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* evicting each other due to wanting to use the same bucket.
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*
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* T indicates the type of hash elements, HashPolicy must have the following
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* contents:
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*
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* Lookup - As for HashMap / HashSet.
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*
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* bool match(T, Lookup) - As for HashMap / HashSet.
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*
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* NumHashes - Number of different hashes generated for each entry.
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*
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* void hash(Lookup, HashNumber[NumHashes]) - Compute all hashes for an entry.
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*
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* void clear(T*) - Clear an entry, such that isCleared() holds afterwards.
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*
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* bool isCleared(T) - Test whether an entry has been cleared.
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*/
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template <class T, class HashPolicy, size_t Capacity>
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class FixedSizeHashSet
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{
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T entries[Capacity];
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uint32_t lastOperations[Capacity];
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uint32_t numOperations;
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static const size_t NumHashes = HashPolicy::NumHashes;
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static_assert(Capacity > 0, "an empty fixed-size hash set is meaningless");
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public:
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typedef typename HashPolicy::Lookup Lookup;
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FixedSizeHashSet()
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: entries(), lastOperations(), numOperations(0)
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{
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MOZ_ASSERT(HashPolicy::isCleared(entries[0]));
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}
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bool lookup(const Lookup& lookup, T* pentry)
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{
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size_t bucket;
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if (lookupReference(lookup, &bucket)) {
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*pentry = entries[bucket];
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lastOperations[bucket] = numOperations++;
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return true;
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}
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return false;
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}
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void insert(const Lookup& lookup, const T& entry)
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{
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size_t buckets[NumHashes];
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getBuckets(lookup, buckets);
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size_t min = buckets[0];
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for (size_t i = 0; i < NumHashes; i++) {
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const T& entry = entries[buckets[i]];
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if (HashPolicy::isCleared(entry)) {
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entries[buckets[i]] = entry;
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lastOperations[buckets[i]] = numOperations++;
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return;
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}
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if (i && lastOperations[min] > lastOperations[buckets[i]])
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min = buckets[i];
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}
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entries[min] = entry;
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lastOperations[min] = numOperations++;
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}
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template <typename S>
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void remove(const S& s)
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{
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size_t bucket;
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if (lookupReference(s, &bucket))
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HashPolicy::clear(&entries[bucket]);
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}
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private:
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template <typename S>
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bool lookupReference(const S& s, size_t* pbucket)
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{
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size_t buckets[NumHashes];
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getBuckets(s, buckets);
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for (size_t i = 0; i < NumHashes; i++) {
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const T& entry = entries[buckets[i]];
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if (!HashPolicy::isCleared(entry) && HashPolicy::match(entry, s)) {
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*pbucket = buckets[i];
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return true;
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}
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}
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return false;
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}
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template <typename S>
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void getBuckets(const S& s, size_t buckets[NumHashes])
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{
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HashNumber hashes[NumHashes];
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HashPolicy::hash(s, hashes);
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for (size_t i = 0; i < NumHashes; i++)
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buckets[i] = hashes[i] % Capacity;
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}
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};
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} /* namespace js */
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#endif /* jsfixedsizehash_h_ */
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