hash.hpp

struct FilenameHash
{
	using transparent_key_equal = FilenameEqual;

	std::size_t operator()(std::wstring_view k) const
	{
		std::size_t hash = Util::INITIAL_HASH_VALUE;
		for (std::size_t i = 0; i < k.length(); ++i)
		{
			if (Util::IsAscii(k[i]))
			{
				// if the string is all ascii characters, avoid API calls
				// this is much faster due to being branchless code
				Util::HashCharacter(hash, Util::AsciiToUpper(k[i]));
			}
			else
			{
				// when we encounter a non-ascii character, call an API to hash the rest and break out
				SlowHash(hash, k.substr(i));
				break;
			}
		}

		return hash;
	}

private:
	inline static void SlowHash(std::size_t &hash, std::wstring_view k)
	{
		fmt::wmemory_buffer buf;
		buf.resize(k.length());

		const int result = LCMapStringEx(
			LOCALE_NAME_INVARIANT, LCMAP_UPPERCASE,
			k.data(), wil::safe_cast<int>(k.length()),
			buf.data(), wil::safe_cast<int>(buf.size()),
			nullptr, nullptr, 0
		);

		if (result)
		{
			buf.resize(result);

			for (const wchar_t &c : buf)
			{
				Util::HashCharacter(hash, c);
			}
		}
		else
		{
			throw std::system_error(static_cast<int>(GetLastError()), std::system_category(), "Failed to hash string");
		}
	}
};