1/*
2 * Copyright (C) 2011 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#ifndef ANDROID_BASIC_HASHTABLE_H
18#define ANDROID_BASIC_HASHTABLE_H
19
20#include <stdint.h>
21#include <sys/types.h>
22#include <utils/SharedBuffer.h>
23#include <utils/TypeHelpers.h>
24
25namespace android {
26
27/* Implementation type.  Nothing to see here. */
28class BasicHashtableImpl {
29protected:
30    struct Bucket {
31        // The collision flag indicates that the bucket is part of a collision chain
32        // such that at least two entries both hash to this bucket.  When true, we
33        // may need to seek further along the chain to find the entry.
34        static const uint32_t COLLISION = 0x80000000UL;
35
36        // The present flag indicates that the bucket contains an initialized entry value.
37        static const uint32_t PRESENT   = 0x40000000UL;
38
39        // Mask for 30 bits worth of the hash code that are stored within the bucket to
40        // speed up lookups and rehashing by eliminating the need to recalculate the
41        // hash code of the entry's key.
42        static const uint32_t HASH_MASK = 0x3fffffffUL;
43
44        // Combined value that stores the collision and present flags as well as
45        // a 30 bit hash code.
46        uint32_t cookie;
47
48        // Storage for the entry begins here.
49        char entry[0];
50    };
51
52    BasicHashtableImpl(size_t entrySize, bool hasTrivialDestructor,
53            size_t minimumInitialCapacity, float loadFactor);
54    BasicHashtableImpl(const BasicHashtableImpl& other);
55
56    void dispose();
57
58    inline void edit() {
59        if (mBuckets && !SharedBuffer::bufferFromData(mBuckets)->onlyOwner()) {
60            clone();
61        }
62    }
63
64    void setTo(const BasicHashtableImpl& other);
65    void clear();
66
67    ssize_t next(ssize_t index) const;
68    ssize_t find(ssize_t index, hash_t hash, const void* __restrict__ key) const;
69    size_t add(hash_t hash, const void* __restrict__ entry);
70    void removeAt(size_t index);
71    void rehash(size_t minimumCapacity, float loadFactor);
72
73    const size_t mBucketSize; // number of bytes per bucket including the entry
74    const bool mHasTrivialDestructor; // true if the entry type does not require destruction
75    size_t mCapacity;         // number of buckets that can be filled before exceeding load factor
76    float mLoadFactor;        // load factor
77    size_t mSize;             // number of elements actually in the table
78    size_t mFilledBuckets;    // number of buckets for which collision or present is true
79    size_t mBucketCount;      // number of slots in the mBuckets array
80    void* mBuckets;           // array of buckets, as a SharedBuffer
81
82    inline const Bucket& bucketAt(const void* __restrict__ buckets, size_t index) const {
83        return *reinterpret_cast<const Bucket*>(
84                static_cast<const uint8_t*>(buckets) + index * mBucketSize);
85    }
86
87    inline Bucket& bucketAt(void* __restrict__ buckets, size_t index) const {
88        return *reinterpret_cast<Bucket*>(static_cast<uint8_t*>(buckets) + index * mBucketSize);
89    }
90
91    virtual bool compareBucketKey(const Bucket& bucket, const void* __restrict__ key) const = 0;
92    virtual void initializeBucketEntry(Bucket& bucket, const void* __restrict__ entry) const = 0;
93    virtual void destroyBucketEntry(Bucket& bucket) const = 0;
94
95private:
96    void clone();
97
98    // Allocates a bucket array as a SharedBuffer.
99    void* allocateBuckets(size_t count) const;
100
101    // Releases a bucket array's associated SharedBuffer.
102    void releaseBuckets(void* __restrict__ buckets, size_t count) const;
103
104    // Destroys the contents of buckets (invokes destroyBucketEntry for each
105    // populated bucket if needed).
106    void destroyBuckets(void* __restrict__ buckets, size_t count) const;
107
108    // Copies the content of buckets (copies the cookie and invokes copyBucketEntry
109    // for each populated bucket if needed).
110    void copyBuckets(const void* __restrict__ fromBuckets,
111            void* __restrict__ toBuckets, size_t count) const;
112
113    // Determines the appropriate size of a bucket array to store a certain minimum
114    // number of entries and returns its effective capacity.
115    static void determineCapacity(size_t minimumCapacity, float loadFactor,
116            size_t* __restrict__ outBucketCount, size_t* __restrict__ outCapacity);
117
118    // Trim a hash code to 30 bits to match what we store in the bucket's cookie.
119    inline static hash_t trimHash(hash_t hash) {
120        return (hash & Bucket::HASH_MASK) ^ (hash >> 30);
121    }
122
123    // Returns the index of the first bucket that is in the collision chain
124    // for the specified hash code, given the total number of buckets.
125    // (Primary hash)
126    inline static size_t chainStart(hash_t hash, size_t count) {
127        return hash % count;
128    }
129
130    // Returns the increment to add to a bucket index to seek to the next bucket
131    // in the collision chain for the specified hash code, given the total number of buckets.
132    // (Secondary hash)
133    inline static size_t chainIncrement(hash_t hash, size_t count) {
134        return ((hash >> 7) | (hash << 25)) % (count - 1) + 1;
135    }
136
137    // Returns the index of the next bucket that is in the collision chain
138    // that is defined by the specified increment, given the total number of buckets.
139    inline static size_t chainSeek(size_t index, size_t increment, size_t count) {
140        return (index + increment) % count;
141    }
142};
143
144/*
145 * A BasicHashtable stores entries that are indexed by hash code in place
146 * within an array.  The basic operations are finding entries by key,
147 * adding new entries and removing existing entries.
148 *
149 * This class provides a very limited set of operations with simple semantics.
150 * It is intended to be used as a building block to construct more complex
151 * and interesting data structures such as HashMap.  Think very hard before
152 * adding anything extra to BasicHashtable, it probably belongs at a
153 * higher level of abstraction.
154 *
155 * TKey: The key type.
156 * TEntry: The entry type which is what is actually stored in the array.
157 *
158 * TKey must support the following contract:
159 *     bool operator==(const TKey& other) const;  // return true if equal
160 *     bool operator!=(const TKey& other) const;  // return true if unequal
161 *
162 * TEntry must support the following contract:
163 *     const TKey& getKey() const;  // get the key from the entry
164 *
165 * This class supports storing entries with duplicate keys.  Of course, it can't
166 * tell them apart during removal so only the first entry will be removed.
167 * We do this because it means that operations like add() can't fail.
168 */
169template <typename TKey, typename TEntry>
170class BasicHashtable : private BasicHashtableImpl {
171public:
172    /* Creates a hashtable with the specified minimum initial capacity.
173     * The underlying array will be created when the first entry is added.
174     *
175     * minimumInitialCapacity: The minimum initial capacity for the hashtable.
176     *     Default is 0.
177     * loadFactor: The desired load factor for the hashtable, between 0 and 1.
178     *     Default is 0.75.
179     */
180    BasicHashtable(size_t minimumInitialCapacity = 0, float loadFactor = 0.75f);
181
182    /* Copies a hashtable.
183     * The underlying storage is shared copy-on-write.
184     */
185    BasicHashtable(const BasicHashtable& other);
186
187    /* Clears and destroys the hashtable.
188     */
189    virtual ~BasicHashtable();
190
191    /* Making this hashtable a copy of the other hashtable.
192     * The underlying storage is shared copy-on-write.
193     *
194     * other: The hashtable to copy.
195     */
196    inline BasicHashtable<TKey, TEntry>& operator =(const BasicHashtable<TKey, TEntry> & other) {
197        setTo(other);
198        return *this;
199    }
200
201    /* Returns the number of entries in the hashtable.
202     */
203    inline size_t size() const {
204        return mSize;
205    }
206
207    /* Returns the capacity of the hashtable, which is the number of elements that can
208     * added to the hashtable without requiring it to be grown.
209     */
210    inline size_t capacity() const {
211        return mCapacity;
212    }
213
214    /* Returns the number of buckets that the hashtable has, which is the size of its
215     * underlying array.
216     */
217    inline size_t bucketCount() const {
218        return mBucketCount;
219    }
220
221    /* Returns the load factor of the hashtable. */
222    inline float loadFactor() const {
223        return mLoadFactor;
224    };
225
226    /* Returns a const reference to the entry at the specified index.
227     *
228     * index:   The index of the entry to retrieve.  Must be a valid index within
229     *          the bounds of the hashtable.
230     */
231    inline const TEntry& entryAt(size_t index) const {
232        return entryFor(bucketAt(mBuckets, index));
233    }
234
235    /* Returns a non-const reference to the entry at the specified index.
236     *
237     * index: The index of the entry to edit.  Must be a valid index within
238     *        the bounds of the hashtable.
239     */
240    inline TEntry& editEntryAt(size_t index) {
241        edit();
242        return entryFor(bucketAt(mBuckets, index));
243    }
244
245    /* Clears the hashtable.
246     * All entries in the hashtable are destroyed immediately.
247     * If you need to do something special with the entries in the hashtable then iterate
248     * over them and do what you need before clearing the hashtable.
249     */
250    inline void clear() {
251        BasicHashtableImpl::clear();
252    }
253
254    /* Returns the index of the next entry in the hashtable given the index of a previous entry.
255     * If the given index is -1, then returns the index of the first entry in the hashtable,
256     * if there is one, or -1 otherwise.
257     * If the given index is not -1, then returns the index of the next entry in the hashtable,
258     * in strictly increasing order, or -1 if there are none left.
259     *
260     * index:   The index of the previous entry that was iterated, or -1 to begin
261     *          iteration at the beginning of the hashtable.
262     */
263    inline ssize_t next(ssize_t index) const {
264        return BasicHashtableImpl::next(index);
265    }
266
267    /* Finds the index of an entry with the specified key.
268     * If the given index is -1, then returns the index of the first matching entry,
269     * otherwise returns the index of the next matching entry.
270     * If the hashtable contains multiple entries with keys that match the requested
271     * key, then the sequence of entries returned is arbitrary.
272     * Returns -1 if no entry was found.
273     *
274     * index:   The index of the previous entry with the specified key, or -1 to
275     *          find the first matching entry.
276     * hash:    The hashcode of the key.
277     * key:     The key.
278     */
279    inline ssize_t find(ssize_t index, hash_t hash, const TKey& key) const {
280        return BasicHashtableImpl::find(index, hash, &key);
281    }
282
283    /* Adds the entry to the hashtable.
284     * Returns the index of the newly added entry.
285     * If an entry with the same key already exists, then a duplicate entry is added.
286     * If the entry will not fit, then the hashtable's capacity is increased and
287     * its contents are rehashed.  See rehash().
288     *
289     * hash:    The hashcode of the key.
290     * entry:   The entry to add.
291     */
292    inline size_t add(hash_t hash, const TEntry& entry) {
293        return BasicHashtableImpl::add(hash, &entry);
294    }
295
296    /* Removes the entry with the specified index from the hashtable.
297     * The entry is destroyed immediately.
298     * The index must be valid.
299     *
300     * The hashtable is not compacted after an item is removed, so it is legal
301     * to continue iterating over the hashtable using next() or find().
302     *
303     * index:   The index of the entry to remove.  Must be a valid index within the
304     *          bounds of the hashtable, and it must refer to an existing entry.
305     */
306    inline void removeAt(size_t index) {
307        BasicHashtableImpl::removeAt(index);
308    }
309
310    /* Rehashes the contents of the hashtable.
311     * Grows the hashtable to at least the specified minimum capacity or the
312     * current number of elements, whichever is larger.
313     *
314     * Rehashing causes all entries to be copied and the entry indices may change.
315     * Although the hash codes are cached by the hashtable, rehashing can be an
316     * expensive operation and should be avoided unless the hashtable's size
317     * needs to be changed.
318     *
319     * Rehashing is the only way to change the capacity or load factor of the
320     * hashtable once it has been created.  It can be used to compact the
321     * hashtable by choosing a minimum capacity that is smaller than the current
322     * capacity (such as 0).
323     *
324     * minimumCapacity: The desired minimum capacity after rehashing.
325     * loadFactor: The desired load factor after rehashing.
326     */
327    inline void rehash(size_t minimumCapacity, float loadFactor) {
328        BasicHashtableImpl::rehash(minimumCapacity, loadFactor);
329    }
330
331protected:
332    static inline const TEntry& entryFor(const Bucket& bucket) {
333        return reinterpret_cast<const TEntry&>(bucket.entry);
334    }
335
336    static inline TEntry& entryFor(Bucket& bucket) {
337        return reinterpret_cast<TEntry&>(bucket.entry);
338    }
339
340    virtual bool compareBucketKey(const Bucket& bucket, const void* __restrict__ key) const;
341    virtual void initializeBucketEntry(Bucket& bucket, const void* __restrict__ entry) const;
342    virtual void destroyBucketEntry(Bucket& bucket) const;
343
344private:
345    // For dumping the raw contents of a hashtable during testing.
346    friend class BasicHashtableTest;
347    inline uint32_t cookieAt(size_t index) const {
348        return bucketAt(mBuckets, index).cookie;
349    }
350};
351
352template <typename TKey, typename TEntry>
353BasicHashtable<TKey, TEntry>::BasicHashtable(size_t minimumInitialCapacity, float loadFactor) :
354        BasicHashtableImpl(sizeof(TEntry), traits<TEntry>::has_trivial_dtor,
355                minimumInitialCapacity, loadFactor) {
356}
357
358template <typename TKey, typename TEntry>
359BasicHashtable<TKey, TEntry>::BasicHashtable(const BasicHashtable<TKey, TEntry>& other) :
360        BasicHashtableImpl(other) {
361}
362
363template <typename TKey, typename TEntry>
364BasicHashtable<TKey, TEntry>::~BasicHashtable() {
365    dispose();
366}
367
368template <typename TKey, typename TEntry>
369bool BasicHashtable<TKey, TEntry>::compareBucketKey(const Bucket& bucket,
370        const void* __restrict__ key) const {
371    return entryFor(bucket).getKey() == *static_cast<const TKey*>(key);
372}
373
374template <typename TKey, typename TEntry>
375void BasicHashtable<TKey, TEntry>::initializeBucketEntry(Bucket& bucket,
376        const void* __restrict__ entry) const {
377    if (!traits<TEntry>::has_trivial_copy) {
378        new (&entryFor(bucket)) TEntry(*(static_cast<const TEntry*>(entry)));
379    } else {
380        memcpy(&entryFor(bucket), entry, sizeof(TEntry));
381    }
382}
383
384template <typename TKey, typename TEntry>
385void BasicHashtable<TKey, TEntry>::destroyBucketEntry(Bucket& bucket) const {
386    if (!traits<TEntry>::has_trivial_dtor) {
387        entryFor(bucket).~TEntry();
388    }
389}
390
391}; // namespace android
392
393#endif // ANDROID_BASIC_HASHTABLE_H
394