src/nnue/nnue_accumulator.h

/*
  Stockfish, a UCI chess playing engine derived from Glaurung 2.1
  Copyright (C) 2004-2026 The Stockfish developers (see AUTHORS file)

  Stockfish is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.

  Stockfish is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

// Class for difference calculation of NNUE evaluation function

#ifndef NNUE_ACCUMULATOR_H_INCLUDED
#define NNUE_ACCUMULATOR_H_INCLUDED

#include <array>
#include <cstddef>
#include <cstdint>
#include <cstring>
#include <utility>

#include "../types.h"
#include "nnue_architecture.h"
#include "nnue_common.h"

namespace Stockfish {
class Position;
}

namespace Stockfish::Eval::NNUE {

template<IndexType Size>
struct alignas(CacheLineSize) Accumulator;

template<IndexType TransformedFeatureDimensions>
class FeatureTransformer;

// Class that holds the result of affine transformation of input features
template<IndexType Size>
struct alignas(CacheLineSize) Accumulator {
    std::array<std::array<std::int16_t, Size>, COLOR_NB>        accumulation;
    std::array<std::array<std::int32_t, PSQTBuckets>, COLOR_NB> psqtAccumulation;
    std::array<bool, COLOR_NB>                                  computed = {};
};


// AccumulatorCaches struct provides per-thread accumulator caches, where each
// cache contains multiple entries for each of the possible king squares.
// When the accumulator needs to be refreshed, the cached entry is used to more
// efficiently update the accumulator, instead of rebuilding it from scratch.
// This idea, was first described by Luecx (author of Koivisto) and
// is commonly referred to as "Finny Tables".
struct AccumulatorCaches {

    template<typename Networks>
    AccumulatorCaches(const Networks& networks) {
        clear(networks);
    }

    template<IndexType Size>
    struct alignas(CacheLineSize) Cache {

        struct alignas(CacheLineSize) Entry {
            std::array<BiasType, Size>              accumulation;
            std::array<PSQTWeightType, PSQTBuckets> psqtAccumulation;
            std::array<Piece, SQUARE_NB>            pieces;
            Bitboard                                pieceBB;

            // To initialize a refresh entry, we set all its bitboards empty,
            // so we put the biases in the accumulation, without any weights on top
            void clear(const std::array<BiasType, Size>& biases) {
                accumulation = biases;
                std::memset(reinterpret_cast<std::byte*>(this) + offsetof(Entry, psqtAccumulation),
                            0, sizeof(Entry) - offsetof(Entry, psqtAccumulation));
            }
        };

        template<typename Network>
        void clear(const Network& network) {
            for (auto& entries1D : entries)
                for (auto& entry : entries1D)
                    entry.clear(network.featureTransformer.biases);
        }

        std::array<Entry, COLOR_NB>& operator[](Square sq) { return entries[sq]; }

        std::array<std::array<Entry, COLOR_NB>, SQUARE_NB> entries;
    };

    template<typename Networks>
    void clear(const Networks& networks) {
        big.clear(networks.big);
        small.clear(networks.small);
    }

    Cache<TransformedFeatureDimensionsBig>   big;
    Cache<TransformedFeatureDimensionsSmall> small;
};


template<typename FeatureSet>
struct AccumulatorState {
    Accumulator<TransformedFeatureDimensionsBig>   accumulatorBig;
    Accumulator<TransformedFeatureDimensionsSmall> accumulatorSmall;
    typename FeatureSet::DiffType                  diff;

    template<IndexType Size>
    auto& acc() noexcept {
        static_assert(Size == TransformedFeatureDimensionsBig
                        || Size == TransformedFeatureDimensionsSmall,
                      "Invalid size for accumulator");

        if constexpr (Size == TransformedFeatureDimensionsBig)
            return accumulatorBig;
        else if constexpr (Size == TransformedFeatureDimensionsSmall)
            return accumulatorSmall;
    }

    template<IndexType Size>
    const auto& acc() const noexcept {
        static_assert(Size == TransformedFeatureDimensionsBig
                        || Size == TransformedFeatureDimensionsSmall,
                      "Invalid size for accumulator");

        if constexpr (Size == TransformedFeatureDimensionsBig)
            return accumulatorBig;
        else if constexpr (Size == TransformedFeatureDimensionsSmall)
            return accumulatorSmall;
    }

    void reset(const typename FeatureSet::DiffType& dp) noexcept {
        diff = dp;
        accumulatorBig.computed.fill(false);
        accumulatorSmall.computed.fill(false);
    }

    typename FeatureSet::DiffType& reset() noexcept {
        accumulatorBig.computed.fill(false);
        accumulatorSmall.computed.fill(false);
        return diff;
    }
};

class AccumulatorStack {
   public:
    static constexpr std::size_t MaxSize = MAX_PLY + 1;

    template<typename T>
    [[nodiscard]] const AccumulatorState<T>& latest() const noexcept;

    void                                  reset() noexcept;
    std::pair<DirtyPiece&, DirtyThreats&> push() noexcept;
    void                                  pop() noexcept;

    template<IndexType Dimensions>
    void evaluate(const Position&                       pos,
                  const FeatureTransformer<Dimensions>& featureTransformer,
                  AccumulatorCaches::Cache<Dimensions>& cache) noexcept;

   private:
    template<typename T>
    [[nodiscard]] AccumulatorState<T>& mut_latest() noexcept;

    template<typename T>
    [[nodiscard]] const std::array<AccumulatorState<T>, MaxSize>& accumulators() const noexcept;

    template<typename T>
    [[nodiscard]] std::array<AccumulatorState<T>, MaxSize>& mut_accumulators() noexcept;

    template<typename FeatureSet, IndexType Dimensions>
    void evaluate_side(Color                                 perspective,
                       const Position&                       pos,
                       const FeatureTransformer<Dimensions>& featureTransformer,
                       AccumulatorCaches::Cache<Dimensions>& cache) noexcept;

    template<typename FeatureSet, IndexType Dimensions>
    [[nodiscard]] std::size_t find_last_usable_accumulator(Color perspective) const noexcept;

    template<typename FeatureSet, IndexType Dimensions>
    void forward_update_incremental(Color                                 perspective,
                                    const Position&                       pos,
                                    const FeatureTransformer<Dimensions>& featureTransformer,
                                    const std::size_t                     begin) noexcept;

    template<typename FeatureSet, IndexType Dimensions>
    void backward_update_incremental(Color                                 perspective,
                                     const Position&                       pos,
                                     const FeatureTransformer<Dimensions>& featureTransformer,
                                     const std::size_t                     end) noexcept;

    std::array<AccumulatorState<PSQFeatureSet>, MaxSize>    psq_accumulators;
    std::array<AccumulatorState<ThreatFeatureSet>, MaxSize> threat_accumulators;
    std::size_t                                             size = 1;
};

}  // namespace Stockfish::Eval::NNUE

#endif  // NNUE_ACCUMULATOR_H_INCLUDED