fnv1a.hpp

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#pragma once
 
#if defined(DEBUG_FNV1A)
#include DEBUG_FNV1A
#else
#include "ndebug.hpp"
#endif
 
#include <stddef.h>
#include <stdint.h>
 
#include <span>
 
namespace Fnv1a
{
    constexpr uint32_t initialHash = 0x811c9dc5;
    constexpr size_t size = sizeof(initialHash);

    constexpr uint32_t feed(uint32_t hash, uint8_t byte)
    {
        hash ^= byte;
        hash *= 0x01000193;
        return hash;
    }

    template<size_t Size>
    constexpr uint32_t checksum(std::span<uint8_t, Size> span, uint32_t hash = initialHash)
    {
        for (auto c : span)
        {
            hash = feed(hash, c);
        }
        return hash;
    }

    template<size_t Size>
    constexpr void putAtEnd(std::span<uint8_t, Size> span, uint32_t hash = initialHash)
    {
        size_t end = span.size();
        if (end < 4)
        {
            return;
        }
        const uint32_t got = checksum(span.first(end - 4), hash);
        span[end - 4] = static_cast<uint8_t>(got >>  0);
        span[end - 3] = static_cast<uint8_t>(got >>  8);
        span[end - 2] = static_cast<uint8_t>(got >> 16);
        span[end - 1] = static_cast<uint8_t>(got >> 24);
    }

    template<size_t Size>
    constexpr bool checkAtEnd(std::span<uint8_t, Size> span, uint32_t hash = initialHash)
    {
        size_t end = span.size();
        if (end < 4)
        {
            return false;
        }
        const uint32_t got = checksum(span.first(end - 4), hash);
        const uint32_t expected =
                (static_cast<uint32_t>(span[end - 4]) <<  0) |
                (static_cast<uint32_t>(span[end - 3]) <<  8) |
                (static_cast<uint32_t>(span[end - 2]) << 16) |
                (static_cast<uint32_t>(span[end - 1]) << 24);
        if (got == expected)
        {
            debugf(INFO "Checksum OK" END LOGLEVEL_ARGS);
        }
        else
        {
            debugf(INFO "Checksum got %08X expected %08X" END LOGLEVEL_ARGS, got, expected);
        }
        return got == expected;
    }
};
 
// This is a header, undefine the debugf macro
#include "debug_end.hpp"