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 Advance the global RandomNumberEngine state by the specified number of rounds
 *
 * Only used to maintain vanilla compatibility until logic requiring reproducable random number generation is isolated.
 * @param count How many values to discard
 */
void DiscardRandomValues(unsigned count);

/**
 * @brief Advances the global RandomNumberEngine state and returns the new value
 */
uint32_t GenerateSeed();

/**
 * @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
 */
[[nodiscard]] 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 Generates a random boolean value using the vanilla RNG
 *
 * This function returns true 1 in `frequency` of the time, otherwise false. For example the default frequency of 2
 * represents a 50/50 chance.
 *
 * @param frequency odds of returning a true value
 * @return A random boolean value
 */
bool FlipCoin(unsigned frequency = 2);

/**
 * @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(static_cast<int32_t>(values.size())), 0) };

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

/**
 * @brief Generates a random non-negative integer
 *
 * Effectively the same as GenerateRnd but will never return a negative value
 * @param v upper limit for the return value
 * @return a value between 0 and v-1 inclusive, i.e. the range [0, v)
 */
inline int32_t RandomIntLessThan(int32_t v)
{
	return std::max<int32_t>(GenerateRnd(v), 0);
}

/**
 * @brief Randomly chooses a value somewhere within the given range
 * @param min lower limit, minumum possible value
 * @param max upper limit, either the maximum possible value for a closed range (the default behaviour) or one greater than the maximum value for a half-open range
 * @param halfOpen whether to use the limits as a half-open range or not
 * @return a randomly selected integer
 */
inline int32_t RandomIntBetween(int32_t min, int32_t max, bool halfOpen = false)
{
	return RandomIntLessThan(max - min + (halfOpen ? 0 : 1)) + min;
}

} // namespace devilution