eqemu-server/common/additive_lagged_fibonacci_engine.h

/*	EQEmu: EQEmulator

	Copyright (C) 2001-2026 EQEmu Development Team

	This program is free software; you can redistribute it and/or modify
	it under the terms of the GNU General Public License as published by
	the Free Software Foundation; either version 3 of the License, or
	(at your option) any later version.

	This program is distributed in the hope that it will be useful,
	but WITHOUT ANY WARRANTY; without even the implied warranty of
	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	GNU General Public License for more details.

	You should have received a copy of the GNU General Public License
	along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#pragma once

#include <algorithm>
#include <cstdint>
#include <iostream>
#include <limits>
#include <type_traits>

/*
 * This is an additive lagged fibonacci generator as seen in The Art of Computer Programming, Vol. 2
 * This should roughly match the implementation that EQ's client uses and be compatible with our Random class
 *
 * EQ's rand looks like it was from an example implementation that as posted on pscode.com
 *
 * You might also want to consider defining BIASED_INT_DIST as well to more closely match EQ
 */

namespace EQ {
	template<typename UIntType, size_t w, size_t j, size_t k>
	class additive_lagged_fibonacci_engine {
		static_assert(std::is_unsigned<UIntType>::value, "result_type must be an unsigned integral type");
		static_assert(0u < j && j < k, "0 < j < k");
		static_assert(0u < w && w <= std::numeric_limits<UIntType>::digits,
			      "template argument substituting w out of bounds");

	public:
		using result_type = UIntType;
		static constexpr size_t word_size = w;
		static constexpr size_t short_lag = j;
		static constexpr size_t long_lag = k;
		static constexpr result_type default_seed = 19780503u; // default for subtract_with_carry_engine

		additive_lagged_fibonacci_engine() : additive_lagged_fibonacci_engine(default_seed) {}

		explicit additive_lagged_fibonacci_engine(result_type sd) { seed(sd); }

		void seed(result_type seed = default_seed)
		{
			state1 = long_lag - long_lag;
			state2 = long_lag - short_lag;
			state[0] = static_cast<int>(seed) & ((1u << word_size) - 1);
			state[1] = 1;
			for (int i = 2; i < long_lag; ++i)
				state[i] = (state[i - 1] + state[i - 2]) & ((1u << word_size) - 1);
			return;
			}
			// TODO: seed via seed_seq

			static constexpr result_type min() { return 0; }
			static constexpr result_type max() { return ((1u << word_size) - 1) >> 6; }

			void discard(unsigned long long z) {
				for (; z != 0ULL; --z)
					(*this)();
			}

			result_type operator()() {
				result_type rand = (state[state1] + state[state2]) & ((1u << word_size) - 1);
				state[state1] = rand;
				if (++state1 == long_lag)
					state1 = 0;
				if (++state2 == long_lag)
					state2 = 0;

				return rand >> 6;
			}

		private:
			result_type state1;
			result_type state2;
			result_type state[long_lag];

		public:
			template<typename UInt, size_t W, size_t J, size_t K>
			friend bool operator==(const additive_lagged_fibonacci_engine<UInt, W, J, K> &x,
					const additive_lagged_fibonacci_engine<UInt, W, J, K> &y)
			{
				return std::equal(x.state, x.state + long_lag, y.state) && x.state1 == y.state1 &&
				       x.state2 == y.state2;
			}

			template<typename UInt, size_t W, size_t J, size_t K>
			friend bool operator!=(const additive_lagged_fibonacci_engine<UInt, W, J, K> &x,
					const additive_lagged_fibonacci_engine<UInt, W, J, K> &y)
			{ return !(x == y); }

			template<typename UInt, size_t W, size_t J, size_t K, typename CharT, typename Traits>
			friend std::basic_ostream<CharT, Traits> &
			operator<<(std::basic_istream<CharT, Traits> &os, additive_lagged_fibonacci_engine<UInt, W, J, K> &x)
			{
				using ios_base = typename std::basic_istream<CharT, Traits>::ios_base;

				const typename ios_base::fmtflags flags = os.flags();
				const CharT fill = os.fill();
				const CharT space = os.widen(' ');
				os.flags(ios_base::dec | ios_base::fixed | ios_base::left);
				os.fill(space);

				for (size_t i = 0; i < long_lag; ++i)
					os << x.state[i] << space;
				os << x.state1 << space << x.state2;

				os.flags(flags);
				os.fill(fill);
				return os;
			}

			template<typename UInt, size_t W, size_t J, size_t K, typename CharT, typename Traits>
			friend std::basic_istream<CharT, Traits> &
			operator>>(std::basic_istream<CharT, Traits> &is, additive_lagged_fibonacci_engine<UInt, W, J, K> &x)
			{
				using ios_base = typename std::basic_istream<CharT, Traits>::ios_base;

				const typename ios_base::fmtflags flags = is.flags();
				is.flags(ios_base::dec | ios_base::skipws);

				for (size_t i = 0; i < long_lag; ++i)
					is >> x.state[i];
				is >> x.state1;
				is >> x.state2;

				is.flags(flags);
				return is;
			}
	};

	using EQRand = additive_lagged_fibonacci_engine<uint32_t, 30, 24, 55>;
};