src/libANGLE/SizedMRUCache.h

//
// Copyright 2017 The ANGLE Project Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
//
// SizedMRUCache.h: A hashing map that stores blobs of sized, untyped data.

#ifndef LIBANGLE_SIZED_MRU_CACHE_H_
#define LIBANGLE_SIZED_MRU_CACHE_H_

#include <anglebase/containers/mru_cache.h>
#include "common/third_party/smhasher/src/PMurHash.h"

namespace angle
{

template <typename Key, typename Value>
class SizedMRUCache final : angle::NonCopyable
{
  public:
    SizedMRUCache(size_t maximumTotalSize)
        : mMaximumTotalSize(maximumTotalSize),
          mCurrentSize(0),
          mStore(SizedMRUCacheStore::NO_AUTO_EVICT)
    {
    }

    // Returns nullptr on failure.
    const Value *put(const Key &key, Value &&value, size_t size)
    {
        if (size > mMaximumTotalSize)
        {
            return nullptr;
        }

        // Check for existing key.
        eraseByKey(key);

        auto retVal = mStore.Put(key, ValueAndSize(std::move(value), size));
        mCurrentSize += size;

        shrinkToSize(mMaximumTotalSize);

        return &retVal->second.value;
    }

    bool get(const Key &key, const Value **valueOut)
    {
        const auto &iter = mStore.Get(key);
        if (iter == mStore.end())
        {
            return false;
        }
        *valueOut = &iter->second.value;
        return true;
    }

    bool getAt(size_t index, Key *keyOut, const Value **valueOut)
    {
        if (index < mStore.size())
        {
            auto it = mStore.begin();
            std::advance(it, index);
            *keyOut   = it->first;
            *valueOut = &it->second.value;
            return true;
        }
        *valueOut = nullptr;
        return false;
    }

    bool empty() const { return mStore.empty(); }

    void clear()
    {
        mStore.Clear();
        mCurrentSize = 0;
    }

    bool eraseByKey(const Key &key)
    {
        // Check for existing key.
        auto existing = mStore.Peek(key);
        if (existing != mStore.end())
        {
            mCurrentSize -= existing->second.size;
            mStore.Erase(existing);
            return true;
        }

        return false;
    }

    size_t entryCount() const { return mStore.size(); }

    size_t size() const { return mCurrentSize; }

    // Also discards the cache contents.
    void resize(size_t maximumTotalSize)
    {
        clear();
        mMaximumTotalSize = maximumTotalSize;
    }

    // Reduce current memory usage.
    size_t shrinkToSize(size_t limit)
    {
        size_t initialSize = mCurrentSize;

        while (mCurrentSize > limit)
        {
            ASSERT(!mStore.empty());
            auto iter = mStore.rbegin();
            mCurrentSize -= iter->second.size;
            mStore.Erase(iter);
        }

        return (initialSize - mCurrentSize);
    }

    size_t maxSize() const { return mMaximumTotalSize; }

  private:
    struct ValueAndSize
    {
        ValueAndSize() : value(), size(0) {}
        ValueAndSize(Value &&value, size_t size) : value(std::move(value)), size(size) {}
        ValueAndSize(ValueAndSize &&other) : ValueAndSize() { *this = std::move(other); }
        ValueAndSize &operator=(ValueAndSize &&other)
        {
            std::swap(value, other.value);
            std::swap(size, other.size);
            return *this;
        }

        Value value;
        size_t size;
    };

    using SizedMRUCacheStore = base::HashingMRUCache<Key, ValueAndSize>;

    size_t mMaximumTotalSize;
    size_t mCurrentSize;
    SizedMRUCacheStore mStore;
};

// Helper function used in a few places.
template <typename T>
void TrimCache(size_t maxStates, size_t gcLimit, const char *name, T *cache)
{
    const size_t kGarbageCollectionLimit = maxStates / 2 + gcLimit;

    if (cache->size() >= kGarbageCollectionLimit)
    {
        WARN() << "Overflowed the " << name << " cache limit of " << (maxStates / 2)
               << " elements, removing the least recently used to make room.";
        cache->ShrinkToSize(maxStates / 2);
    }
}

// Computes a hash of struct "key". Any structs passed to this function must be multiples of
// 4 bytes, since the PMurhHas32 method can only operate increments of 4-byte words.
template <typename T>
std::size_t ComputeGenericHash(const T &key)
{
    static const unsigned int seed = 0xABCDEF98;

    // We can't support "odd" alignments.
    static_assert(sizeof(key) % 4 == 0, "ComputeGenericHash requires aligned types");
    return PMurHash32(seed, &key, sizeof(T));
}

}  // namespace angle
#endif  // LIBANGLE_SIZED_MRU_CACHE_H_