cuckoocache.h

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// Copyright (c) 2016 Jeremy Rubin
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#ifndef BITCOIN_CUCKOOCACHE_H
#define BITCOIN_CUCKOOCACHE_H

#include <algorithm> // std::find
#include <array>
#include <atomic>
#include <cmath>
#include <cstring>
#include <memory>
#include <utility>
#include <vector>


namespace CuckooCache
{
class bit_packed_atomic_flags
{
    std::unique_ptr<std::atomic<uint8_t>[]> mem;

public:
    bit_packed_atomic_flags() = delete;

    explicit bit_packed_atomic_flags(uint32_t size)
    {
        // pad out the size if needed
        size = (size + 7) / 8;
        mem.reset(new std::atomic<uint8_t>[size]);
        for (uint32_t i = 0; i < size; ++i)
            mem[i].store(0xFF);
    };

    inline void setup(uint32_t b)
    {
        bit_packed_atomic_flags d(b);
        std::swap(mem, d.mem);
    }

    inline void bit_set(uint32_t s)
    {
        mem[s >> 3].fetch_or(1 << (s & 7), std::memory_order_relaxed);
    }

    inline void bit_unset(uint32_t s)
    {
        mem[s >> 3].fetch_and(~(1 << (s & 7)), std::memory_order_relaxed);
    }

    inline bool bit_is_set(uint32_t s) const
    {
        return (1 << (s & 7)) & mem[s >> 3].load(std::memory_order_relaxed);
    }
};

template <typename Element, typename Hash>
class cache
{
private:
    std::vector<Element> table;

    uint32_t size;

    mutable bit_packed_atomic_flags collection_flags;

    mutable std::vector<bool> epoch_flags;

    uint32_t epoch_heuristic_counter;

    uint32_t epoch_size;

    uint8_t depth_limit;

    const Hash hash_function;

    inline std::array<uint32_t, 8> compute_hashes(const Element& e) const
    {
        return {{(uint32_t)(((uint64_t)hash_function.template operator()<0>(e) * (uint64_t)size) >> 32),
                 (uint32_t)(((uint64_t)hash_function.template operator()<1>(e) * (uint64_t)size) >> 32),
                 (uint32_t)(((uint64_t)hash_function.template operator()<2>(e) * (uint64_t)size) >> 32),
                 (uint32_t)(((uint64_t)hash_function.template operator()<3>(e) * (uint64_t)size) >> 32),
                 (uint32_t)(((uint64_t)hash_function.template operator()<4>(e) * (uint64_t)size) >> 32),
                 (uint32_t)(((uint64_t)hash_function.template operator()<5>(e) * (uint64_t)size) >> 32),
                 (uint32_t)(((uint64_t)hash_function.template operator()<6>(e) * (uint64_t)size) >> 32),
                 (uint32_t)(((uint64_t)hash_function.template operator()<7>(e) * (uint64_t)size) >> 32)}};
    }

    constexpr uint32_t invalid() const
    {
        return ~(uint32_t)0;
    }

    inline void allow_erase(uint32_t n) const
    {
        collection_flags.bit_set(n);
    }

    inline void please_keep(uint32_t n) const
    {
        collection_flags.bit_unset(n);
    }

    void epoch_check()
    {
        if (epoch_heuristic_counter != 0) {
            --epoch_heuristic_counter;
            return;
        }
        // count the number of elements from the latest epoch which
        // have not been erased.
        uint32_t epoch_unused_count = 0;
        for (uint32_t i = 0; i < size; ++i)
            epoch_unused_count += epoch_flags[i] &&
                                  !collection_flags.bit_is_set(i);
        // If there are more non-deleted entries in the current epoch than the
        // epoch size, then allow_erase on all elements in the old epoch (marked
        // false) and move all elements in the current epoch to the old epoch
        // but do not call allow_erase on their indices.
        if (epoch_unused_count >= epoch_size) {
            for (uint32_t i = 0; i < size; ++i)
                if (epoch_flags[i])
                    epoch_flags[i] = false;
                else
                    allow_erase(i);
            epoch_heuristic_counter = epoch_size;
        } else
            // reset the epoch_heuristic_counter to next do a scan when worst
            // case behavior (no intermittent erases) would exceed epoch size,
            // with a reasonable minimum scan size.
            // Ordinarily, we would have to sanity check std::min(epoch_size,
            // epoch_unused_count), but we already know that `epoch_unused_count
            // < epoch_size` in this branch
            epoch_heuristic_counter = std::max(1u, std::max(epoch_size / 16,
                        epoch_size - epoch_unused_count));
    }

public:
    cache() : table(), size(), collection_flags(0), epoch_flags(),
    epoch_heuristic_counter(), epoch_size(), depth_limit(0), hash_function()
    {
    }

    uint32_t setup(uint32_t new_size)
    {
        // depth_limit must be at least one otherwise errors can occur.
        depth_limit = static_cast<uint8_t>(std::log2(static_cast<float>(std::max((uint32_t)2, new_size))));
        size = std::max<uint32_t>(2, new_size);
        table.resize(size);
        collection_flags.setup(size);
        epoch_flags.resize(size);
        // Set to 45% as described above
        epoch_size = std::max((uint32_t)1, (45 * size) / 100);
        // Initially set to wait for a whole epoch
        epoch_heuristic_counter = epoch_size;
        return size;
    }

    uint32_t setup_bytes(size_t bytes)
    {
        return setup(bytes/sizeof(Element));
    }

    inline void insert(Element e)
    {
        epoch_check();
        uint32_t last_loc = invalid();
        bool last_epoch = true;
        std::array<uint32_t, 8> locs = compute_hashes(e);
        // Make sure we have not already inserted this element
        // If we have, make sure that it does not get deleted
        for (const uint32_t loc : locs)
            if (table[loc] == e) {
                please_keep(loc);
                epoch_flags[loc] = last_epoch;
                return;
            }
        for (uint8_t depth = 0; depth < depth_limit; ++depth) {
            // First try to insert to an empty slot, if one exists
            for (const uint32_t loc : locs) {
                if (!collection_flags.bit_is_set(loc))
                    continue;
                table[loc] = std::move(e);
                please_keep(loc);
                epoch_flags[loc] = last_epoch;
                return;
            }
            last_loc = locs[(1 + (std::find(locs.begin(), locs.end(), last_loc) - locs.begin())) & 7];
            std::swap(table[last_loc], e);
            // Can't std::swap a std::vector<bool>::reference and a bool&.
            bool epoch = last_epoch;
            last_epoch = epoch_flags[last_loc];
            epoch_flags[last_loc] = epoch;

            // Recompute the locs -- unfortunately happens one too many times!
            locs = compute_hashes(e);
        }
    }

    inline bool contains(const Element& e, const bool erase) const
    {
        std::array<uint32_t, 8> locs = compute_hashes(e);
        for (const uint32_t loc : locs)
            if (table[loc] == e) {
                if (erase)
                    allow_erase(loc);
                return true;
            }
        return false;
    }
};
} // namespace CuckooCache

#endif // BITCOIN_CUCKOOCACHE_H