fnv1a.h

#pragma once
 
#include <cstddef>
#include <cstdint>
#include <string_view>
 
namespace Vkm::Engine {
 
// 64-bit FNV-1a: non-cryptographic, deterministic across runs and platforms.
constexpr uint64_t FNV1A_OFFSET_BASIS = 14695981039346656037ull;
constexpr uint64_t FNV1A_PRIME        = 1099511628211ull;

inline uint64_t fnv1a64Bytes(const void* data, std::size_t size, uint64_t seed = FNV1A_OFFSET_BASIS) {
    const auto* bytes = static_cast<const unsigned char*>(data);
    uint64_t hash = seed;
    for (std::size_t i = 0; i < size; ++i) {
        hash ^= bytes[i];
        hash *= FNV1A_PRIME;
    }
    return hash;
}

inline uint64_t fnv1a64(std::string_view s, uint64_t seed = FNV1A_OFFSET_BASIS) {
    return fnv1a64Bytes(s.data(), s.size(), seed);
}
 
// 32-bit FNV-1a, for a hash that has to fit a 32-bit field.
constexpr uint32_t FNV1A32_OFFSET_BASIS = 2166136261u;
constexpr uint32_t FNV1A32_PRIME        = 16777619u;

inline uint32_t fnv1a32(std::string_view s, uint32_t seed = FNV1A32_OFFSET_BASIS) {
    uint32_t hash = seed;
    for (const char c : s) {
        hash ^= static_cast<unsigned char>(c);
        hash *= FNV1A32_PRIME;
    }
    return hash;
}
 
} // namespace Vkm::Engine