src/core/SkTDynamicHash.h

/*
 * Copyright 2013 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#ifndef SkTDynamicHash_DEFINED
#define SkTDynamicHash_DEFINED

#include "include/core/SkMath.h"
#include "include/core/SkTypes.h"
#include "include/private/SkTemplates.h"

// Traits requires:
//   static const Key& GetKey(const T&) { ... }
//   static uint32_t Hash(const Key&) { ... }
// We'll look on T for these by default, or you can pass a custom Traits type.
template <typename T,
          typename Key,
          typename Traits = T,
          int kGrowPercent = 75>  // Larger -> more memory efficient, but slower.
class SkTDynamicHash {
public:
    SkTDynamicHash() : fCount(0), fDeleted(0), fCapacity(0), fArray(nullptr) {
        SkASSERT(this->validate());
    }

    ~SkTDynamicHash() {
        sk_free(fArray);
    }

    class Iter {
    public:
        explicit Iter(SkTDynamicHash* hash) : fHash(hash), fCurrentIndex(-1) {
            SkASSERT(hash);
            ++(*this);
        }
        bool done() const {
            SkASSERT(fCurrentIndex <= fHash->fCapacity);
            return fCurrentIndex == fHash->fCapacity;
        }
        T& operator*() const {
            SkASSERT(!this->done());
            return *this->current();
        }
        void operator++() {
            do {
                fCurrentIndex++;
            } while (!this->done() && (this->current() == Empty() || this->current() == Deleted()));
        }

    private:
        T* current() const { return fHash->fArray[fCurrentIndex]; }

        SkTDynamicHash* fHash;
        int fCurrentIndex;
    };

    class ConstIter {
    public:
        explicit ConstIter(const SkTDynamicHash* hash) : fHash(hash), fCurrentIndex(-1) {
            SkASSERT(hash);
            ++(*this);
        }
        bool done() const {
            SkASSERT(fCurrentIndex <= fHash->fCapacity);
            return fCurrentIndex == fHash->fCapacity;
        }
        const T& operator*() const {
            SkASSERT(!this->done());
            return *this->current();
        }
        void operator++() {
            do {
                fCurrentIndex++;
            } while (!this->done() && (this->current() == Empty() || this->current() == Deleted()));
        }

    private:
        const T* current() const { return fHash->fArray[fCurrentIndex]; }

        const SkTDynamicHash* fHash;
        int fCurrentIndex;
    };

    int count() const { return fCount; }

    // Return the entry with this key if we have it, otherwise nullptr.
    T* find(const Key& key) const {
        int index = this->firstIndex(key);
        for (int round = 0; round < fCapacity; round++) {
            SkASSERT(index >= 0 && index < fCapacity);
            T* candidate = fArray[index];
            if (Empty() == candidate) {
                return nullptr;
            }
            if (Deleted() != candidate && GetKey(*candidate) == key) {
                return candidate;
            }
            index = this->nextIndex(index, round);
        }
        SkASSERT(fCapacity == 0);
        return nullptr;
    }

    // Add an entry with this key.  We require that no entry with newEntry's key is already present.
    void add(T* newEntry) {
        SkASSERT(nullptr == this->find(GetKey(*newEntry)));
        this->maybeGrow();
        this->innerAdd(newEntry);
        SkASSERT(this->validate());
    }

    // Remove the entry with this key.  We require that an entry with this key is present.
    void remove(const Key& key) {
        SkASSERT(this->find(key));
        this->innerRemove(key);
        SkASSERT(this->validate());
    }

    void rewind() {
        if (fArray) {
            sk_bzero(fArray, sizeof(T*)* fCapacity);
        }
        fCount = 0;
        fDeleted = 0;
    }

    void reset() {
        fCount = 0;
        fDeleted = 0;
        fCapacity = 0;
        sk_free(fArray);
        fArray = nullptr;
    }

protected:
    // These methods are used by tests only.

    int capacity() const { return fCapacity; }

    // How many collisions do we go through before finding where this entry should be inserted?
    int countCollisions(const Key& key) const {
        int index = this->firstIndex(key);
        for (int round = 0; round < fCapacity; round++) {
            SkASSERT(index >= 0 && index < fCapacity);
            const T* candidate = fArray[index];
            if (Empty() == candidate || Deleted() == candidate || GetKey(*candidate) == key) {
                return round;
            }
            index = this->nextIndex(index, round);
        }
        SkASSERT(fCapacity == 0);
        return 0;
    }

private:
    // We have two special values to indicate an empty or deleted entry.
    static T* Empty()   { return reinterpret_cast<T*>(0); }  // i.e. nullptr
    static T* Deleted() { return reinterpret_cast<T*>(1); }  // Also an invalid pointer.

    bool validate() const {
        #define SKTDYNAMICHASH_CHECK(x) SkASSERT(x); if (!(x)) return false
        static const int kLarge = 50;  // Arbitrary, tweak to suit your patience.

        // O(1) checks, always done.
        // Is capacity sane?
        SKTDYNAMICHASH_CHECK(SkIsPow2(fCapacity));

        // O(N) checks, skipped when very large.
        if (fCount < kLarge * kLarge) {
            // Are fCount and fDeleted correct, and are all elements findable?
            int count = 0, deleted = 0;
            for (int i = 0; i < fCapacity; i++) {
                if (Deleted() == fArray[i]) {
                    deleted++;
                } else if (Empty() != fArray[i]) {
                    count++;
                    SKTDYNAMICHASH_CHECK(this->find(GetKey(*fArray[i])));
                }
            }
            SKTDYNAMICHASH_CHECK(count == fCount);
            SKTDYNAMICHASH_CHECK(deleted == fDeleted);
        }

        // O(N^2) checks, skipped when large.
        if (fCount < kLarge) {
            // Are all entries unique?
            for (int i = 0; i < fCapacity; i++) {
                if (Empty() == fArray[i] || Deleted() == fArray[i]) {
                    continue;
                }
                for (int j = i+1; j < fCapacity; j++) {
                    if (Empty() == fArray[j] || Deleted() == fArray[j]) {
                        continue;
                    }
                    SKTDYNAMICHASH_CHECK(fArray[i] != fArray[j]);
                    SKTDYNAMICHASH_CHECK(!(GetKey(*fArray[i]) == GetKey(*fArray[j])));
                }
            }
        }
        #undef SKTDYNAMICHASH_CHECK
        return true;
    }

    void innerAdd(T* newEntry) {
        const Key& key = GetKey(*newEntry);
        int index = this->firstIndex(key);
        for (int round = 0; round < fCapacity; round++) {
            SkASSERT(index >= 0 && index < fCapacity);
            const T* candidate = fArray[index];
            if (Empty() == candidate || Deleted() == candidate) {
                if (Deleted() == candidate) {
                    fDeleted--;
                }
                fCount++;
                fArray[index] = newEntry;
                return;
            }
            index = this->nextIndex(index, round);
        }
        SkASSERT(fCapacity == 0);
    }

    void innerRemove(const Key& key) {
        const int firstIndex = this->firstIndex(key);
        int index = firstIndex;
        for (int round = 0; round < fCapacity; round++) {
            SkASSERT(index >= 0 && index < fCapacity);
            const T* candidate = fArray[index];
            if (Deleted() != candidate && GetKey(*candidate) == key) {
                fDeleted++;
                fCount--;
                fArray[index] = Deleted();
                return;
            }
            index = this->nextIndex(index, round);
        }
        SkASSERT(fCapacity == 0);
    }

    void maybeGrow() {
        if (100 * (fCount + fDeleted + 1) > fCapacity * kGrowPercent) {
            auto newCapacity = fCapacity > 0 ? fCapacity : 4;

            // Only grow the storage when most non-empty entries are
            // in active use.  Otherwise, just purge the tombstones.
            if (fCount > fDeleted) {
                newCapacity *= 2;
            }
            SkASSERT(newCapacity > fCount + 1);

            this->resize(newCapacity);
        }
    }

    void resize(int newCapacity) {
        SkDEBUGCODE(int oldCount = fCount;)
        int oldCapacity = fCapacity;
        SkAutoTMalloc<T*> oldArray(fArray);

        fCount = fDeleted = 0;
        fCapacity = newCapacity;
        fArray = (T**)sk_calloc_throw(sizeof(T*) * fCapacity);

        for (int i = 0; i < oldCapacity; i++) {
            T* entry = oldArray[i];
            if (Empty() != entry && Deleted() != entry) {
                this->innerAdd(entry);
            }
        }
        SkASSERT(oldCount == fCount);
    }

    // fCapacity is always a power of 2, so this masks the correct low bits to index into our hash.
    uint32_t hashMask() const { return fCapacity - 1; }

    int firstIndex(const Key& key) const {
        return Hash(key) & this->hashMask();
    }

    // Given index at round N, what is the index to check at N+1?  round should start at 0.
    int nextIndex(int index, int round) const {
        // This will search a power-of-two array fully without repeating an index.
        return (index + round + 1) & this->hashMask();
    }

    static const Key& GetKey(const T& t) { return Traits::GetKey(t); }
    static uint32_t Hash(const Key& key) { return Traits::Hash(key); }

    int fCount;     // Number of non Empty(), non Deleted() entries in fArray.
    int fDeleted;   // Number of Deleted() entries in fArray.
    int fCapacity;  // Number of entries in fArray.  Always a power of 2.
    T** fArray;
};

#endif