statichash.h

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/**********************************************************************
 * LeechCraft - modular cross-platform feature rich internet client.
 * Copyright (C) 2006-2014  Georg Rudoy
 *
 * Distributed under the Boost Software License, Version 1.0.
 * (See accompanying file LICENSE or copy at https://www.boost.org/LICENSE_1_0.txt)
 **********************************************************************/
 
#pragma once
 
#include <string_view>
 
namespace LC::Util
{
    namespace
    {
        template<typename K, typename V>
        struct KVPair
        {
            const K Key_;
            const V Val_;
 
            consteval KVPair (K name, V val)
            : Key_ { name }
            , Val_ { val }
            {
            }
        };
 
        template<size_t N, typename V>
        KVPair (const char (&) [N], V) -> KVPair<std::string_view, V>;
    }
 
    template<typename K>
    constexpr uint64_t DefaultHashImpl (K);
 
    template<>
    constexpr uint64_t DefaultHashImpl (std::string_view name)
    {
        uint64_t res = 0;
        for (auto ch : name)
            res = (res << 8) + ch;
        return res;
    }
 
    template<std::convertible_to<uint64_t> K>
    constexpr uint64_t DefaultHashImpl (K val)
    {
        return static_cast<uint64_t> (val);
    }
 
    constexpr uint64_t DefaultHash (auto val)
    {
        return DefaultHashImpl (val);
    }
 
    template<typename K, typename V, auto Hasher = DefaultHash<K>>
    consteval auto MakeHash (auto&&... commands)
    {
        const std::initializer_list<KVPair<K, V>> commandsList { commands... };
        for (auto i = commandsList.begin (); i != std::prev (commandsList.end ()); ++i)
            for (auto j = std::next (i); j != commandsList.end (); ++j)
                if (Hasher (i->Key_) == Hasher (j->Key_))
                    throw "duplicate hashes";
 
        return [=] (K key, V defValue = V {})
        {
            const auto keyHash = Hasher (key);
            V result = defValue;
            K foundKey {};
            (void) ((Hasher (commands.Key_) == keyHash && (result = commands.Val_, foundKey = commands.Key_, true)) || ...);
            if (foundKey != key)
                return defValue;
            return result;
        };
    }
 
    template<typename V>
    consteval auto MakeStringHash (auto&&... commands)
    {
        return MakeHash<std::string_view, V> (std::forward<decltype (commands)> (commands)...);
    }
}