composition_hash.h

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#pragma once
 
#include <cstring>
#include <vector>
#include <bit>
 
#include "xxhash64.h"
#include "fourdst/composition/composition.h"
#include "fourdst/composition/composition_abstract.h"
 
namespace fourdst::composition::utils {
    // Make a concept which checks if a type inherits from CompositionAbstract
    template <typename CompT>
    concept CompositionType = std::is_base_of_v<CompositionAbstract, CompT>;
 
    struct CompositionHash {
        template <CompositionType CompositionT>
        static uint64_t hash_exact(const CompositionT& comp) {
            uint64_t h0 = kSeed;
            uint64_t h1 = kSeed ^ kPrime1;
            uint64_t h2 = kSeed ^ kPrime2;
            uint64_t h3 = kSeed ^ kPrime3;
 
            auto it = comp.begin();
            size_t remaining = comp.size();
 
            while (remaining >= 4) {
                const auto& p0 = *it;
                ++it;
                h0 ^= pack_species_id(p0.first);
                h0 = mum(h0, kPrime1);
                h0 ^= normalize_double_bits(p0.second);
                h0 = mum(h0, kPrime2);
 
                const auto& p1 = *it;
                ++it;
                h1 ^= pack_species_id(p1.first);
                h1 = mum(h1, kPrime1);
                h1 ^= normalize_double_bits(p1.second);
                h1 = mum(h1, kPrime2);
 
                const auto& p2 = *it;
                ++it;
                h2 ^= pack_species_id(p2.first);
                h2 = mum(h2, kPrime1);
                h2 ^= normalize_double_bits(p2.second);
                h2 = mum(h2, kPrime2);
 
                const auto& p3 = *it;
                ++it;
                h3 ^= pack_species_id(p3.first);
                h3 = mum(h3, kPrime1);
                h3 ^= normalize_double_bits(p3.second);
                h3 = mum(h3, kPrime2);
 
                remaining -= 4;
            }
 
            while (remaining > 0) {
                const auto& p = *it;
                ++it;
                h0 ^= pack_species_id(p.first);
                h0 = mum(h0, kPrime1);
                h0 ^= normalize_double_bits(p.second);
                h0 = mum(h0, kPrime2);
                --remaining;
            }
 
            return mum(h0 ^ h1 ^ h2 ^ h3, kPrime3);
        }
 
    private:
        static constexpr uint64_t kSeed = 0xC04D5EEDBEEFull;
        static constexpr uint64_t kPrime1 = 0xa0761d6478bd642fULL;
        static constexpr uint64_t kPrime2 = 0xe7037ed1a0b428dbULL;
        static constexpr uint64_t kPrime3 = 0x8ebc6af09c88c6e3ULL;
 
        // --- Helper: Fast integer mixing ---
        static inline uint64_t mum(const uint64_t a, const uint64_t b) noexcept {
            const unsigned __int128 r = static_cast<unsigned __int128>(a) * static_cast<unsigned __int128>(b);
            return static_cast<uint64_t>(r) ^ static_cast<uint64_t>(r >> 64);
        }
 
        static inline uint64_t mix(const uint64_t h) noexcept {
            return mum(h, kPrime1);
        }
 
        // --- Normalization Logic ---
        static inline uint64_t normalize_double_bits(double v) noexcept {
            if (v == 0.0) v = 0.0; // fold -0.0 -> +0.0
            if (std::isnan(v)) {
                return 0x7ff8000000000000ULL; // canonical quiet NaN
            }
            return std::bit_cast<uint64_t>(v);
        }
 
        static inline uint32_t pack_species_id(const auto& s) noexcept {
            const auto z = static_cast<uint16_t>(s.z());
            const auto a = static_cast<uint16_t>(s.a());
            return (static_cast<uint32_t>(z) << 16) | static_cast<uint32_t>(a);
        }
    };
}
 
template<>
struct std::hash<fourdst::composition::CompositionAbstract> {
    std::size_t operator()(const fourdst::composition::CompositionAbstract& c) const noexcept {
        return static_cast<std::size_t>(
            fourdst::composition::utils::CompositionHash::hash_exact(c)
        );
    }
};
template<>
struct std::hash<fourdst::composition::Composition> {
    std::size_t operator()(const fourdst::composition::Composition& c) const noexcept {
        return static_cast<std::size_t>(
            fourdst::composition::utils::CompositionHash::hash_exact(c)
        );
    }
};