Vulkan-Utility-Libraries/include/vulkan/utility/vk_concurrent_unordered_map.hpp

/* Copyright (c) 2015-2017, 2019-2024 The Khronos Group Inc.
 * Copyright (c) 2015-2017, 2019-2024 Valve Corporation
 * Copyright (c) 2015-2017, 2019-2024 LunarG, Inc.
 * Modifications Copyright (C) 2022 RasterGrid Kft.
 *
 * SPDX-License-Identifier: Apache-2.0
 *
 */

#pragma once

#include <stdint.h>

#include <array>
#include <functional>
#include <mutex>
#include <shared_mutex>
#include <unordered_map>
#include <vector>

namespace vku {
namespace concurrent {
// https://en.cppreference.com/w/cpp/thread/hardware_destructive_interference_size
// https://en.wikipedia.org/wiki/False_sharing
// TODO use C++20 to check for std::hardware_destructive_interference_size feature support.
constexpr std::size_t get_hardware_destructive_interference_size() { return 64; }

// Limited concurrent unordered_map that supports internally-synchronized
// insert/erase/access. Splits locking across N buckets and uses shared_mutex
// for read/write locking. Iterators are not supported. The following
// operations are supported:
//
// insert_or_assign: Insert a new element or update an existing element.
// insert: Insert a new element and return whether it was inserted.
// erase: Remove an element.
// contains: Returns true if the key is in the map.
// find: Returns != end() if found, value is in ret->second.
// pop: Erases and returns the erased value if found.
//
// find/end: find returns a vaguely iterator-like type that can be compared to
// end and can use iter->second to retrieve the reference. This is to ease porting
// for existing code that combines the existence check and lookup in a single
// operation (and thus a single lock). i.e.:
//
//      auto iter = map.find(key);
//      if (iter != map.end()) {
//          T t = iter->second;
//          ...
//
// snapshot: Return an array of elements (key, value pairs) that satisfy an optional
// predicate. This can be used as a substitute for iterators in exceptional cases.
template <typename Key, typename T, int BUCKETSLOG2 = 2, typename Map = std::unordered_map<Key, T>>
class unordered_map {
    // Aliases to avoid excessive typing. We can't easily auto these away because
    // there are virtual methods in ValidationObject which return lock guards
    // and those cannot use return type deduction.
    using ReadLockGuard = std::shared_lock<std::shared_mutex>;
    using WriteLockGuard = std::unique_lock<std::shared_mutex>;

  public:
    template <typename... Args>
    void insert_or_assign(const Key &key, Args &&...args) {
        uint32_t h = ConcurrentMapHashObject(key);
        WriteLockGuard lock(locks[h].lock);
        maps[h][key] = {std::forward<Args>(args)...};
    }

    template <typename... Args>
    bool insert(const Key &key, Args &&...args) {
        uint32_t h = ConcurrentMapHashObject(key);
        WriteLockGuard lock(locks[h].lock);
        auto ret = maps[h].emplace(key, std::forward<Args>(args)...);
        return ret.second;
    }

    // returns size_type
    size_t erase(const Key &key) {
        uint32_t h = ConcurrentMapHashObject(key);
        WriteLockGuard lock(locks[h].lock);
        return maps[h].erase(key);
    }

    bool contains(const Key &key) const {
        uint32_t h = ConcurrentMapHashObject(key);
        ReadLockGuard lock(locks[h].lock);
        return maps[h].count(key) != 0;
    }

    // type returned by find() and end().
    class FindResult {
      public:
        FindResult(bool a, T b) : result(a, std::move(b)) {}

        // == and != only support comparing against end()
        bool operator==(const FindResult &other) const {
            if (result.first == false && other.result.first == false) {
                return true;
            }
            return false;
        }
        bool operator!=(const FindResult &other) const { return !(*this == other); }

        // Make -> act kind of like an iterator.
        std::pair<bool, T> *operator->() { return &result; }
        const std::pair<bool, T> *operator->() const { return &result; }

      private:
        // (found, reference to element)
        std::pair<bool, T> result;
    };

    // find()/end() return a FindResult containing a copy of the value. For end(),
    // return a default value.
    FindResult end() const { return FindResult(false, T()); }
    FindResult cend() const { return end(); }

    FindResult find(const Key &key) const {
        uint32_t h = ConcurrentMapHashObject(key);
        ReadLockGuard lock(locks[h].lock);

        auto itr = maps[h].find(key);
        const bool found = itr != maps[h].end();

        if (found) {
            return FindResult(true, itr->second);
        } else {
            return end();
        }
    }

    FindResult pop(const Key &key) {
        uint32_t h = ConcurrentMapHashObject(key);
        WriteLockGuard lock(locks[h].lock);

        auto itr = maps[h].find(key);
        const bool found = itr != maps[h].end();

        if (found) {
            auto ret = FindResult(true, itr->second);
            maps[h].erase(itr);
            return ret;
        } else {
            return end();
        }
    }

    std::vector<std::pair<const Key, T>> snapshot(std::function<bool(T)> f = nullptr) const {
        std::vector<std::pair<const Key, T>> ret;
        for (int h = 0; h < BUCKETS; ++h) {
            ReadLockGuard lock(locks[h].lock);
            for (const auto &j : maps[h]) {
                if (!f || f(j.second)) {
                    ret.emplace_back(j.first, j.second);
                }
            }
        }
        return ret;
    }

    void clear() {
        for (int h = 0; h < BUCKETS; ++h) {
            WriteLockGuard lock(locks[h].lock);
            maps[h].clear();
        }
    }

    size_t size() const {
        size_t result = 0;
        for (int h = 0; h < BUCKETS; ++h) {
            ReadLockGuard lock(locks[h].lock);
            result += maps[h].size();
        }
        return result;
    }

    bool empty() const {
        bool result = 0;
        for (int h = 0; h < BUCKETS; ++h) {
            ReadLockGuard lock(locks[h].lock);
            result |= maps[h].empty();
        }
        return result;
    }

  private:
    static const int BUCKETS = (1 << BUCKETSLOG2);

    Map maps[BUCKETS];
    struct alignas(get_hardware_destructive_interference_size()) AlignedSharedMutex {
        std::shared_mutex lock;
    };
    mutable std::array<AlignedSharedMutex, BUCKETS> locks;

    uint32_t ConcurrentMapHashObject(const Key &object) const {
        uint64_t u64 = (uint64_t)(uintptr_t)object;
        uint32_t hash = (uint32_t)(u64 >> 32) + (uint32_t)u64;
        hash ^= (hash >> BUCKETSLOG2) ^ (hash >> (2 * BUCKETSLOG2));
        hash &= (BUCKETS - 1);
        return hash;
    }
};
}  // namespace concurrent
}  // namespace vku