66fbde3050
The basic hashtable is intended to be used to support a variety of different datastructures such as map, set, multimap, multiset, linkedmap, generationcache, etc. Consequently its interface is fairly primitive. The basic hashtable supports copy-on-write style functionality using SharedBuffer. The change introduces a simple generic function in TypeHelpers for specifying hash functions. The idea is to add template specializations of hash_type<T> next to the relevant data structures such as String8, String16, sp<T>, etc. Change-Id: I2c479229e9d4527b4fbfe3b8b04776a2fd32c973
394 lines
15 KiB
C++
394 lines
15 KiB
C++
/*
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* Copyright (C) 2011 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#ifndef ANDROID_BASIC_HASHTABLE_H
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#define ANDROID_BASIC_HASHTABLE_H
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#include <stdint.h>
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#include <sys/types.h>
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#include <utils/SharedBuffer.h>
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#include <utils/TypeHelpers.h>
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namespace android {
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/* Implementation type. Nothing to see here. */
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class BasicHashtableImpl {
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protected:
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struct Bucket {
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// The collision flag indicates that the bucket is part of a collision chain
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// such that at least two entries both hash to this bucket. When true, we
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// may need to seek further along the chain to find the entry.
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static const uint32_t COLLISION = 0x80000000UL;
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// The present flag indicates that the bucket contains an initialized entry value.
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static const uint32_t PRESENT = 0x40000000UL;
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// Mask for 30 bits worth of the hash code that are stored within the bucket to
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// speed up lookups and rehashing by eliminating the need to recalculate the
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// hash code of the entry's key.
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static const uint32_t HASH_MASK = 0x3fffffffUL;
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// Combined value that stores the collision and present flags as well as
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// a 30 bit hash code.
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uint32_t cookie;
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// Storage for the entry begins here.
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char entry[0];
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};
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BasicHashtableImpl(size_t entrySize, bool hasTrivialDestructor,
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size_t minimumInitialCapacity, float loadFactor);
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BasicHashtableImpl(const BasicHashtableImpl& other);
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void dispose();
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inline void edit() {
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if (mBuckets && !SharedBuffer::bufferFromData(mBuckets)->onlyOwner()) {
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clone();
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}
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}
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void setTo(const BasicHashtableImpl& other);
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void clear();
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ssize_t next(ssize_t index) const;
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ssize_t find(ssize_t index, hash_t hash, const void* __restrict__ key) const;
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size_t add(hash_t hash, const void* __restrict__ entry);
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void removeAt(size_t index);
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void rehash(size_t minimumCapacity, float loadFactor);
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const size_t mBucketSize; // number of bytes per bucket including the entry
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const bool mHasTrivialDestructor; // true if the entry type does not require destruction
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size_t mCapacity; // number of buckets that can be filled before exceeding load factor
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float mLoadFactor; // load factor
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size_t mSize; // number of elements actually in the table
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size_t mFilledBuckets; // number of buckets for which collision or present is true
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size_t mBucketCount; // number of slots in the mBuckets array
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void* mBuckets; // array of buckets, as a SharedBuffer
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inline const Bucket& bucketAt(const void* __restrict__ buckets, size_t index) const {
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return *reinterpret_cast<const Bucket*>(
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static_cast<const uint8_t*>(buckets) + index * mBucketSize);
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}
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inline Bucket& bucketAt(void* __restrict__ buckets, size_t index) const {
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return *reinterpret_cast<Bucket*>(static_cast<uint8_t*>(buckets) + index * mBucketSize);
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}
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virtual bool compareBucketKey(const Bucket& bucket, const void* __restrict__ key) const = 0;
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virtual void initializeBucketEntry(Bucket& bucket, const void* __restrict__ entry) const = 0;
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virtual void destroyBucketEntry(Bucket& bucket) const = 0;
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private:
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void clone();
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// Allocates a bucket array as a SharedBuffer.
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void* allocateBuckets(size_t count) const;
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// Releases a bucket array's associated SharedBuffer.
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void releaseBuckets(void* __restrict__ buckets, size_t count) const;
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// Destroys the contents of buckets (invokes destroyBucketEntry for each
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// populated bucket if needed).
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void destroyBuckets(void* __restrict__ buckets, size_t count) const;
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// Copies the content of buckets (copies the cookie and invokes copyBucketEntry
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// for each populated bucket if needed).
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void copyBuckets(const void* __restrict__ fromBuckets,
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void* __restrict__ toBuckets, size_t count) const;
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// Determines the appropriate size of a bucket array to store a certain minimum
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// number of entries and returns its effective capacity.
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static void determineCapacity(size_t minimumCapacity, float loadFactor,
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size_t* __restrict__ outBucketCount, size_t* __restrict__ outCapacity);
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// Trim a hash code to 30 bits to match what we store in the bucket's cookie.
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inline static hash_t trimHash(hash_t hash) {
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return (hash & Bucket::HASH_MASK) ^ (hash >> 30);
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}
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// Returns the index of the first bucket that is in the collision chain
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// for the specified hash code, given the total number of buckets.
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// (Primary hash)
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inline static size_t chainStart(hash_t hash, size_t count) {
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return hash % count;
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}
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// Returns the increment to add to a bucket index to seek to the next bucket
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// in the collision chain for the specified hash code, given the total number of buckets.
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// (Secondary hash)
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inline static size_t chainIncrement(hash_t hash, size_t count) {
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return ((hash >> 7) | (hash << 25)) % (count - 1) + 1;
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}
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// Returns the index of the next bucket that is in the collision chain
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// that is defined by the specified increment, given the total number of buckets.
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inline static size_t chainSeek(size_t index, size_t increment, size_t count) {
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return (index + increment) % count;
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}
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};
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/*
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* A BasicHashtable stores entries that are indexed by hash code in place
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* within an array. The basic operations are finding entries by key,
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* adding new entries and removing existing entries.
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*
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* This class provides a very limited set of operations with simple semantics.
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* It is intended to be used as a building block to construct more complex
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* and interesting data structures such as HashMap. Think very hard before
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* adding anything extra to BasicHashtable, it probably belongs at a
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* higher level of abstraction.
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*
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* TKey: The key type.
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* TEntry: The entry type which is what is actually stored in the array.
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*
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* TKey must support the following contract:
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* bool operator==(const TKey& other) const; // return true if equal
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* bool operator!=(const TKey& other) const; // return true if unequal
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*
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* TEntry must support the following contract:
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* const TKey& getKey() const; // get the key from the entry
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*
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* This class supports storing entries with duplicate keys. Of course, it can't
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* tell them apart during removal so only the first entry will be removed.
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* We do this because it means that operations like add() can't fail.
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*/
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template <typename TKey, typename TEntry>
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class BasicHashtable : private BasicHashtableImpl {
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public:
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/* Creates a hashtable with the specified minimum initial capacity.
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* The underlying array will be created when the first entry is added.
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*
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* minimumInitialCapacity: The minimum initial capacity for the hashtable.
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* Default is 0.
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* loadFactor: The desired load factor for the hashtable, between 0 and 1.
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* Default is 0.75.
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*/
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BasicHashtable(size_t minimumInitialCapacity = 0, float loadFactor = 0.75f);
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/* Copies a hashtable.
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* The underlying storage is shared copy-on-write.
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*/
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BasicHashtable(const BasicHashtable& other);
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/* Clears and destroys the hashtable.
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*/
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virtual ~BasicHashtable();
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/* Making this hashtable a copy of the other hashtable.
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* The underlying storage is shared copy-on-write.
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*
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* other: The hashtable to copy.
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*/
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inline BasicHashtable<TKey, TEntry>& operator =(const BasicHashtable<TKey, TEntry> & other) {
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setTo(other);
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return *this;
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}
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/* Returns the number of entries in the hashtable.
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*/
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inline size_t size() const {
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return mSize;
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}
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/* Returns the capacity of the hashtable, which is the number of elements that can
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* added to the hashtable without requiring it to be grown.
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*/
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inline size_t capacity() const {
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return mCapacity;
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}
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/* Returns the number of buckets that the hashtable has, which is the size of its
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* underlying array.
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*/
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inline size_t bucketCount() const {
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return mBucketCount;
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}
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/* Returns the load factor of the hashtable. */
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inline float loadFactor() const {
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return mLoadFactor;
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};
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/* Returns a const reference to the entry at the specified index.
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*
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* index: The index of the entry to retrieve. Must be a valid index within
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* the bounds of the hashtable.
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*/
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inline const TEntry& entryAt(size_t index) const {
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return entryFor(bucketAt(mBuckets, index));
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}
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/* Returns a non-const reference to the entry at the specified index.
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*
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* index: The index of the entry to edit. Must be a valid index within
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* the bounds of the hashtable.
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*/
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inline TEntry& editEntryAt(size_t index) {
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edit();
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return entryFor(bucketAt(mBuckets, index));
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}
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/* Clears the hashtable.
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* All entries in the hashtable are destroyed immediately.
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* If you need to do something special with the entries in the hashtable then iterate
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* over them and do what you need before clearing the hashtable.
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*/
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inline void clear() {
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BasicHashtableImpl::clear();
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}
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/* Returns the index of the next entry in the hashtable given the index of a previous entry.
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* If the given index is -1, then returns the index of the first entry in the hashtable,
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* if there is one, or -1 otherwise.
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* If the given index is not -1, then returns the index of the next entry in the hashtable,
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* in strictly increasing order, or -1 if there are none left.
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*
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* index: The index of the previous entry that was iterated, or -1 to begin
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* iteration at the beginning of the hashtable.
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*/
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inline ssize_t next(ssize_t index) const {
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return BasicHashtableImpl::next(index);
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}
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/* Finds the index of an entry with the specified key.
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* If the given index is -1, then returns the index of the first matching entry,
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* otherwise returns the index of the next matching entry.
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* If the hashtable contains multiple entries with keys that match the requested
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* key, then the sequence of entries returned is arbitrary.
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* Returns -1 if no entry was found.
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*
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* index: The index of the previous entry with the specified key, or -1 to
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* find the first matching entry.
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* hash: The hashcode of the key.
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* key: The key.
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*/
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inline ssize_t find(ssize_t index, hash_t hash, const TKey& key) const {
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return BasicHashtableImpl::find(index, hash, &key);
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}
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/* Adds the entry to the hashtable.
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* Returns the index of the newly added entry.
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* If an entry with the same key already exists, then a duplicate entry is added.
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* If the entry will not fit, then the hashtable's capacity is increased and
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* its contents are rehashed. See rehash().
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*
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* hash: The hashcode of the key.
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* entry: The entry to add.
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*/
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inline size_t add(hash_t hash, const TEntry& entry) {
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return BasicHashtableImpl::add(hash, &entry);
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}
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/* Removes the entry with the specified index from the hashtable.
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* The entry is destroyed immediately.
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* The index must be valid.
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*
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* The hashtable is not compacted after an item is removed, so it is legal
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* to continue iterating over the hashtable using next() or find().
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*
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* index: The index of the entry to remove. Must be a valid index within the
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* bounds of the hashtable, and it must refer to an existing entry.
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*/
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inline void removeAt(size_t index) {
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BasicHashtableImpl::removeAt(index);
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}
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/* Rehashes the contents of the hashtable.
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* Grows the hashtable to at least the specified minimum capacity or the
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* current number of elements, whichever is larger.
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*
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* Rehashing causes all entries to be copied and the entry indices may change.
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* Although the hash codes are cached by the hashtable, rehashing can be an
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* expensive operation and should be avoided unless the hashtable's size
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* needs to be changed.
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*
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* Rehashing is the only way to change the capacity or load factor of the
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* hashtable once it has been created. It can be used to compact the
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* hashtable by choosing a minimum capacity that is smaller than the current
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* capacity (such as 0).
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*
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* minimumCapacity: The desired minimum capacity after rehashing.
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* loadFactor: The desired load factor after rehashing.
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*/
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inline void rehash(size_t minimumCapacity, float loadFactor) {
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BasicHashtableImpl::rehash(minimumCapacity, loadFactor);
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}
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protected:
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static inline const TEntry& entryFor(const Bucket& bucket) {
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return reinterpret_cast<const TEntry&>(bucket.entry);
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}
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static inline TEntry& entryFor(Bucket& bucket) {
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return reinterpret_cast<TEntry&>(bucket.entry);
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}
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virtual bool compareBucketKey(const Bucket& bucket, const void* __restrict__ key) const;
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virtual void initializeBucketEntry(Bucket& bucket, const void* __restrict__ entry) const;
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virtual void destroyBucketEntry(Bucket& bucket) const;
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private:
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// For dumping the raw contents of a hashtable during testing.
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friend class BasicHashtableTest;
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inline uint32_t cookieAt(size_t index) const {
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return bucketAt(mBuckets, index).cookie;
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}
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};
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template <typename TKey, typename TEntry>
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BasicHashtable<TKey, TEntry>::BasicHashtable(size_t minimumInitialCapacity, float loadFactor) :
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BasicHashtableImpl(sizeof(TEntry), traits<TEntry>::has_trivial_dtor,
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minimumInitialCapacity, loadFactor) {
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}
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template <typename TKey, typename TEntry>
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BasicHashtable<TKey, TEntry>::BasicHashtable(const BasicHashtable<TKey, TEntry>& other) :
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BasicHashtableImpl(other) {
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}
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template <typename TKey, typename TEntry>
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BasicHashtable<TKey, TEntry>::~BasicHashtable() {
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dispose();
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}
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template <typename TKey, typename TEntry>
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bool BasicHashtable<TKey, TEntry>::compareBucketKey(const Bucket& bucket,
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const void* __restrict__ key) const {
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return entryFor(bucket).getKey() == *static_cast<const TKey*>(key);
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}
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template <typename TKey, typename TEntry>
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void BasicHashtable<TKey, TEntry>::initializeBucketEntry(Bucket& bucket,
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const void* __restrict__ entry) const {
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if (!traits<TEntry>::has_trivial_copy) {
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new (&entryFor(bucket)) TEntry(*(static_cast<const TEntry*>(entry)));
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} else {
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memcpy(&entryFor(bucket), entry, sizeof(TEntry));
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}
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}
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template <typename TKey, typename TEntry>
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void BasicHashtable<TKey, TEntry>::destroyBucketEntry(Bucket& bucket) const {
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if (!traits<TEntry>::has_trivial_dtor) {
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entryFor(bucket).~TEntry();
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}
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}
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}; // namespace android
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#endif // ANDROID_BASIC_HASHTABLE_H
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