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