hashed_string.h

#pragma once

#include <cstddef>
#include <cstdint>
#include "forwards.h"

namespace escad {

namespace details {

template<typename>
struct fnv1a_traits;

template<>
struct fnv1a_traits<std::uint32_t> {
    using type = std::uint32_t;
    static constexpr std::uint32_t offset = 2166136261;
    static constexpr std::uint32_t prime = 16777619;
};

template<>
struct fnv1a_traits<std::uint64_t> {
    using type = std::uint64_t;
    static constexpr std::uint64_t offset = 14695981039346656037ull;
    static constexpr std::uint64_t prime = 1099511628211ull;
};

template<typename Char>
struct basic_hashed_string {
    using value_type = Char;
    using size_type = std::size_t;
    using hash_type = id_type;

    const value_type *repr;
    size_type length;
    hash_type hash;
};

} // namespace details

template<typename Char>
class basic_hashed_string: details::basic_hashed_string<Char> {
    using base_type = details::basic_hashed_string<Char>;
    using hs_traits = details::fnv1a_traits<id_type>;

    struct const_wrapper {
        // non-explicit constructor on purpose
        constexpr const_wrapper(const Char *str) noexcept
            : repr{str} {}

        const Char *repr;
    };

    // Fowler–Noll–Vo hash function v. 1a - the good
    [[nodiscard]] static constexpr auto helper(const Char *str) noexcept {
        base_type base{str, 0u, hs_traits::offset};

        for(; str[base.length]; ++base.length) {
            base.hash = (base.hash ^ static_cast<hs_traits::type>(str[base.length])) * hs_traits::prime;
        }

        return base;
    }

    // Fowler–Noll–Vo hash function v. 1a - the good
    [[nodiscard]] static constexpr auto helper(const Char *str, const std::size_t len) noexcept {
        base_type base{str, len, hs_traits::offset};

        for(size_type pos{}; pos < len; ++pos) {
            base.hash = (base.hash ^ static_cast<hs_traits::type>(str[pos])) * hs_traits::prime;
        }

        return base;
    }

public:
    using value_type = typename base_type::value_type;
    using size_type = typename base_type::size_type;
    using hash_type = typename base_type::hash_type;

    [[nodiscard]] static constexpr hash_type value(const value_type *str, const size_type len) noexcept {
        return basic_hashed_string{str, len};
    }

    template<std::size_t N>
    [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) noexcept {
        return basic_hashed_string{str};
    }

    [[nodiscard]] static constexpr hash_type value(const_wrapper wrapper) noexcept {
        return basic_hashed_string{wrapper};
    }

    constexpr basic_hashed_string() noexcept
        : base_type{} {}

    constexpr basic_hashed_string(const value_type *str, const size_type len) noexcept
        : base_type{helper(str, len)} {}

    template<std::size_t N>
    constexpr basic_hashed_string(const value_type (&str)[N]) noexcept
        : base_type{helper(str)} {}

    explicit constexpr basic_hashed_string(const_wrapper wrapper) noexcept
        : base_type{helper(wrapper.repr)} {}

    [[nodiscard]] constexpr size_type size() const noexcept {
        return base_type::length;
    }

    [[nodiscard]] constexpr const value_type *data() const noexcept {
        return base_type::repr;
    }

    [[nodiscard]] constexpr hash_type value() const noexcept {
        return base_type::hash;
    }

    [[nodiscard]] constexpr operator const value_type *() const noexcept {
        return data();
    }

    [[nodiscard]] constexpr operator hash_type() const noexcept {
        return value();
    }
};

template<typename Char>
basic_hashed_string(const Char *str, const std::size_t len) -> basic_hashed_string<Char>;

template<typename Char, std::size_t N>
basic_hashed_string(const Char (&str)[N]) -> basic_hashed_string<Char>;

template<typename Char>
[[nodiscard]] constexpr bool operator==(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
    return lhs.value() == rhs.value();
}

template<typename Char>
[[nodiscard]] constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
    return !(lhs == rhs);
}

template<typename Char>
[[nodiscard]] constexpr bool operator<(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
    return lhs.value() < rhs.value();
}

template<typename Char>
[[nodiscard]] constexpr bool operator<=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
    return !(rhs < lhs);
}

template<typename Char>
[[nodiscard]] constexpr bool operator>(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
    return rhs < lhs;
}

template<typename Char>
[[nodiscard]] constexpr bool operator>=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
    return !(lhs < rhs);
}

using hashed_string = basic_hashed_string<char>;

using hashed_wstring = basic_hashed_string<wchar_t>;

inline namespace literals {

[[nodiscard]] constexpr hashed_string operator"" _hs(const char *str, std::size_t) noexcept {
    return hashed_string{str};
}

[[nodiscard]] constexpr hashed_wstring operator"" _hws(const wchar_t *str, std::size_t) noexcept {
    return hashed_wstring{str};
}

} // namespace literals

} // namespace base