constexpr_hash.hpp

#ifndef UT_CONSTEXPR_HASH_HPP
#define UT_CONSTEXPR_HASH_HPP
#include <string_view>

/* Usage:
 *
 * int main(int argc, char const **argv) {
 *     constexpr auto hash = ut::fnv_1a<>; // defaults to uint64_t
 *
 *     for (int i = 1; i < argc; i++) {
 *         switch (hash(argv[i])) {
 *             case hash("-h"): std::cout << argv[0] << ": Usage: -hv [ARG...]\n"; return 0;
 *             case hash("-v"): std::cout << "1.0.0\n"; return 0;
 *             default: std::cout << argv[i] << '\n';
 *         }
 *     }
 *
 *  static_assert(ut::fnv_1a<uint64_t, ut::CHIgnoreCase::Yes>("hello")
 *                == ut::fnv_1a<uint64_t, ut::CHIgnoreCase::Yes>("helLO"));
 * }
 *
 *
 *
 */

/**
 * constexpr fnv-1a hash for 32 or 64 bit hashes
 * requires minimum c++17
 * tested on gcc 11.1.0 and clang 13.0.0
 * ---------
 * using uint64_t calculates hashes for ~400'000 strings (between 1 and 29 characters) in ~300ms with no collisions
 *
 * algorithm from from: https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function#FNV-1a_hash
 * ---------
 * NOTE on CHIgnoreCase: This only supports ASCII characters and converts everything to lower case
 */
namespace ut {
enum struct CHIgnoreCase : bool {
    No = false,
    Yes = true,
};

namespace detail {
    constexpr char asciiToLower(char c) {
        switch (c) {
                // clang-format off
            case 'A': case 'B': case 'C': case 'D': case 'E': case 'F': case 'G':
            case 'H': case 'I': case 'J': case 'K': case 'L': case 'M': case 'N':
            case 'O': case 'P': case 'Q': case 'R': case 'S': case 'T': case 'U':
            case 'V': case 'W': case 'X': case 'Y': case 'Z': return c + ('a' - 'A');
                // clang-format on
            default: return c;
        }
    }

}  // namespace detail

template<typename Size = std::size_t, CHIgnoreCase IgnoreCase = CHIgnoreCase::No>
[[nodiscard]]
constexpr Size fnv_1a(std::string_view data) {
    static_assert(std::is_unsigned_v<Size>, "Cannot use signed integers in hash. Signed integer overflow is UB.");
    static_assert(sizeof(Size) == 4 || sizeof(Size) == 8, "Size has to be of size 4 or 8");
    Size hash = sizeof(Size) == 4 ? 0x811c9dc5 : sizeof(Size) == 8 ? 0xcbf29ce484222325 : 0;
    constexpr Size prime = sizeof(Size) == 4 ? 0x01000193 : sizeof(Size) == 8 ? 0x00000100000001B3 : 0;

    for (auto chr : data) {
        if constexpr (IgnoreCase == CHIgnoreCase::Yes) chr = detail::asciiToLower(chr);
        hash ^= static_cast<Size>(chr);
        hash *= prime;
    }
    return hash;
}

}  // namespace ut

/*
This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or
distribute this software, either in source code form or as a compiled
binary, for any purpose, commercial or non-commercial, and by any
means.

In jurisdictions that recognize copyright laws, the author or authors
of this software dedicate any and all copyright interest in the
software to the public domain. We make this dedication for the benefit
of the public at large and to the detriment of our heirs and
successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to <https://unlicense.org>
*/
#endif  // UT_CONSTEXPR_HASH_HPP

zzz_description

a hashing function usable in `constexpr` context