devilutionX/Source/engine/random.hpp

/**
 * @file random.hpp
 *
 * Contains convenience functions for random number generation
 *
 * This includes specific engine/distribution functions for logic that needs to be compatible with the base game.
 */
#pragma once

#include <algorithm>
#include <cstdint>
#include <initializer_list>

namespace devilution {

/**
 * @brief Set the state of the RandomNumberEngine used by the base game to the specific seed
 * @param seed New engine state
 */
void SetRndSeed(uint32_t seed);

/**
 * @brief Returns the current state of the RandomNumberEngine used by the base game
 *
 * This is only exposed to allow for debugging vanilla code and testing. Using this engine for new code is discouraged
 * due to the poor randomness and bugs in the implementation that need to be retained for compatibility.
 *
 * @return The current engine state
 */
uint32_t GetLCGEngineState();

/**
 * @brief Generates a random non-negative integer (most of the time) using the vanilla RNG
 *
 * This advances the engine state then interprets the new engine state as a signed value and calls std::abs to try
 * discard the high bit of the result. This usually returns a positive number but may very rarely return -2^31.
 *
 * This function is only used when the base game wants to store the seed used to generate an item or level, however
 * as the returned value is transformed about 50% of values do not reflect the actual engine state. It would be more
 * appropriate to use GetLCGEngineState() in these cases but that may break compatibility with the base game.
 *
 * @return A random number in the range [0,2^31) or -2^31
 */
int32_t AdvanceRndSeed();

/**
 * @brief Generates a random integer less than the given limit using the vanilla RNG
 *
 * If v is not a positive number this function returns 0 without calling the RNG.
 *
 * Limits between 32768 and 65534 should be avoided as a bug in vanilla means this function always returns a value
 * less than 32768 for limits in that range.
 *
 * This can very rarely return a negative value in the range (-v, -1] due to the bug in AdvanceRndSeed()
 *
 * @see AdvanceRndSeed()
 * @param v The upper limit for the return value
 * @return A random number in the range [0, v) or rarely a negative value in (-v, -1]
 */
int32_t GenerateRnd(int32_t v);

/**
 * @brief Picks one of the elements in the list randomly.
 *
 * @param values The values to pick from
 * @return A random value from the 'values' list.
 */
template <typename T>
const T PickRandomlyAmong(const std::initializer_list<T> &values)
{
	const auto index { std::max<int32_t>(GenerateRnd(values.size()), 0) };

	return *(values.begin() + index);
}

} // namespace devilution