/* Single-header LCG random generator and faster-than-modulo rand_until, rand_between */
/* LICENCE: Feel free to use it but please mention MagosIT Kft or its youtube channel */
#ifndef FAST_RAND_H
#define FAST_RAND_H

#include <stdint.h>
#ifndef NO_CSTDLIB
#include <stdlib.h>
#endif /* NO_CSTDLIB */

#ifdef __cplusplus
  // C++-specific logic
  #if defined(__GNUC__) || defined(__clang__)
    #define restrict __restrict__  // GCC/Clang
  #elif defined(_MSC_VER)
    #define restrict __restrict    // MSVC
  #else
    #error "Compiler not supported for 'restrict' keyword in C++"
  #endif
#endif

/* Currently a single integer is enough */
typedef uint32_t rand_state;

/** Creates a random number generator state with given seed */
static inline rand_state init_rand_with(uint32_t seed) {
	return seed;
}

#ifndef NO_CSTDLIB
/** Creates a random number generator state with arc4random() which does not need seeding as it uses system etropy */
static inline rand_state init_rand() {
	return arc4random();
}
#endif /* NO_CSTDLIB */

// 32-bit LCG
static inline uint32_t lcg(rand_state *state) {
	*state = *state * 1664525u + 1013904223u;
	return *state;
}

/** Pick a "reasonably random" number in [0, until-1] without modulus */
static inline uint32_t rand_until(rand_state *restrict state, uint32_t until) {
	uint32_t rand = lcg(state);
	// Multiply by "until", take the upper 32 bits of the 64-bit result
	return (uint32_t)(((uint64_t)rand * until) >> 32);
}

/**
 * Pick a "reasonably random" number in [from, to) without modulus.
 *
 * @param state The random state - See: init_rand() and init_rand_with(seed)
 * @param from The smallest possible value
 * @param to The biggest possible value + 1
 * @returns A value in [from, to) interval
 */
static inline uint32_t rand_between(rand_state *restrict state, uint32_t from, uint32_t to) {
	return from + rand_until(state, to - from);
}

#endif /* FAST_RAND_H */