Rack/include/random.hpp

#pragma once
#include <common.hpp>
#include <random>
#include <vector>


namespace rack {
/** Random number generation */
namespace random {


/** xoroshiro128+. Very fast, not-cryptographic random number generator.
From https://prng.di.unimi.it/
Example:

	std::random_device rd;
	random::Xoroshiro128Plus rng(rd(), rd());
	uint64_t r = rng();

	std::uniform_real_distribution<float> uniform(0.f, 1.f);
	float r = uniform(rng);

	std::normal_distribution<> normal(0.0, 1.0);
	double r = normal(rng);
*/
struct Xoroshiro128Plus {
	uint64_t state[2] = {};

	void seed(uint64_t s0, uint64_t s1) {
		state[0] = s0;
		state[1] = s1;
		// A bad seed will give a bad first result, so shift the state
		operator()();
	}

	bool isSeeded() {
		return state[0] || state[1];
	}

	static uint64_t rotl(uint64_t x, int k) {
		return (x << k) | (x >> (64 - k));
	}

	uint64_t operator()() {
		uint64_t s0 = state[0];
		uint64_t s1 = state[1];
		uint64_t result = s0 + s1;

		s1 ^= s0;
		state[0] = rotl(s0, 55) ^ s1 ^ (s1 << 14);
		state[1] = rotl(s1, 36);

		return result;
	}
	constexpr uint64_t min() const {
		return 0;
	}
	constexpr uint64_t max() const {
		return UINT64_MAX;
	}
};


// Simple thread-local API

/** Initializes the thread-local RNG state.
Must call when creating a thread, otherwise random state is undefined (might always return 0).
*/
void init();

/** Returns the thread-local generator.
*/
Xoroshiro128Plus& local();

template <typename T>
T get() {
	// Call operator()() and cast by default
	return local()();
}

template <>
inline uint32_t get() {
	// Take top 32 bits which has better randomness properties.
	return get<uint64_t>() >> 32;
}

template <>
inline uint16_t get() {
	return get<uint64_t>() >> 48;
}

template <>
inline uint8_t get() {
	return get<uint64_t>() >> 56;
}

template <>
inline bool get() {
	return get<uint64_t>() >> 63;
}

template <>
inline float get() {
	// The multiplier is 2f7fffff in hex. This gives maximum precision of uint32_t -> float conversion and its image is [0, 1).
	return get<uint32_t>() * 2.32830629e-10f;
}

template <>
inline double get() {
	return get<uint64_t>() * 5.421010862427522e-20;
}


/** Returns a uniform random uint64_t from 0 to UINT64_MAX */
inline uint64_t u64() {return get<uint64_t>();}
/** Returns a uniform random uint32_t from 0 to UINT32_MAX */
inline uint32_t u32() {return get<uint32_t>();}
/** Returns a uniform random float in the interval [0.0, 1.0) */
inline float uniform() {return get<float>();}

/** Returns a normal random number with mean 0 and standard deviation 1 */
inline float normal() {
	// Box-Muller transform
	float radius = std::sqrt(-2.f * std::log(1.f - get<float>()));
	float theta = 2.f * M_PI * get<float>();
	return radius * std::sin(theta);

	// // Central Limit Theorem
	// const int n = 8;
	// float sum = 0.0;
	// for (int i = 0; i < n; i++) {
	// 	sum += get<float>();
	// }
	// return (sum - n / 2.f) / std::sqrt(n / 12.f);
}

/** Fills an array with random bytes. */
inline void buffer(uint8_t* out, size_t len) {
	Xoroshiro128Plus& rng = local();
	for (size_t i = 0; i < len; i += 4) {
		uint64_t r = rng();
		out[i] = r;
		if (i + 1 < len)
			out[i + 1] = r >> 8;
		if (i + 2 < len)
			out[i + 2] = r >> 16;
		if (i + 3 < len)
			out[i + 3] = r >> 24;
	}
}

/** Creates a vector of random bytes. */
inline std::vector<uint8_t> vector(size_t len) {
	std::vector<uint8_t> v(len);
	buffer(v.data(), len);
	return v;
}


} // namespace random
} // namespace rack