RFC 8682: TinyMT32 Pseudorandom Number Generator (PRNG)

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/**
 * Tiny Mersenne Twister: only 127-bit internal state.
 * Derived from the reference implementation version 1.1 (2015/04/24)
 * by Mutsuo Saito (Hiroshima University) and Makoto Matsumoto
 * (Hiroshima University).
 */
#include <stdint.h>

/**
 * tinymt32 internal state vector and parameters
 */
typedef struct {
    uint32_t status[4];
    uint32_t mat1;
    uint32_t mat2;
    uint32_t tmat;
} tinymt32_t;

static void tinymt32_next_state (tinymt32_t* s);
static uint32_t tinymt32_temper (tinymt32_t* s);

/**
 * Parameter set to use for this IETF specification. Don't change.
 * This parameter set is the first entry of the precalculated
 * parameter sets in tinymt32dc/tinymt32dc.0.1048576.txt by
 * Kenji Rikitake, available at:
 *    https://github.com/jj1bdx/tinymtdc-longbatch/.
 * It is also the parameter set used in:
 *    Rikitake, K., "TinyMT pseudo random number generator for
 *    Erlang", Proceedings of the 11th ACM SIGPLAN Erlang Workshop,
 *    September 2012.
 */
const uint32_t  TINYMT32_MAT1_PARAM = UINT32_C(0x8f7011ee);
const uint32_t  TINYMT32_MAT2_PARAM = UINT32_C(0xfc78ff1f);
const uint32_t  TINYMT32_TMAT_PARAM = UINT32_C(0x3793fdff);

/**
 * This function initializes the internal state array with a
 * 32-bit unsigned integer seed.
 * @param s     pointer to tinymt internal state.
 * @param seed  a 32-bit unsigned integer used as a seed.
 */
void tinymt32_init (tinymt32_t* s, uint32_t seed)
{
    const uint32_t    MIN_LOOP = 8;
    const uint32_t    PRE_LOOP = 8;
    s->status[0] = seed;
    s->status[1] = s->mat1 = TINYMT32_MAT1_PARAM;
    s->status[2] = s->mat2 = TINYMT32_MAT2_PARAM;
    s->status[3] = s->tmat = TINYMT32_TMAT_PARAM;
    for (int i = 1; i < MIN_LOOP; i++) {
        s->status[i & 3] ^= i + UINT32_C(1812433253)
            * (s->status[(i - 1) & 3]
               ^ (s->status[(i - 1) & 3] >> 30));
    }
    /*
     * NB: The parameter set of this specification warrants
     * that none of the possible 2^^32 seeds leads to an
     * all-zero 127-bit internal state. Therefore, the
     * period_certification() function of the original
     * TinyMT32 source code has been safely removed. If
     * another parameter set is used, this function will
     * have to be reintroduced here.
     */
    for (int i = 0; i < PRE_LOOP; i++) {
        tinymt32_next_state(s);
    }
}

/**
 * This function outputs a 32-bit unsigned integer from
 * the internal state.
 * @param s     pointer to tinymt internal state.
 * @return      32-bit unsigned integer r (0 <= r < 2^32).
 */
uint32_t tinymt32_generate_uint32 (tinymt32_t* s)
{
    tinymt32_next_state(s);
    return tinymt32_temper(s);
}

/**
 * Internal tinymt32 constants and functions.
 * Users should not call these functions directly.
 */
const uint32_t  TINYMT32_SH0 = 1;
const uint32_t  TINYMT32_SH1 = 10;
const uint32_t  TINYMT32_SH8 = 8;
const uint32_t  TINYMT32_MASK = UINT32_C(0x7fffffff);

/**
 * This function changes the internal state of tinymt32.
 * @param s     pointer to tinymt internal state.
 */
static void tinymt32_next_state (tinymt32_t* s)
{
    uint32_t x;
    uint32_t y;

    y = s->status[3];
    x = (s->status[0] & TINYMT32_MASK)
        ^ s->status[1]
        ^ s->status[2];
    x ^= (x << TINYMT32_SH0);
    y ^= (y >> TINYMT32_SH0) ^ x;
    s->status[0] = s->status[1];
    s->status[1] = s->status[2];
    s->status[2] = x ^ (y << TINYMT32_SH1);
    s->status[3] = y;
    /*
     * The if (y & 1) {...} block below replaces:
     *     s->status[1] ^= -((int32_t)(y & 1)) & s->mat1;
     *     s->status[2] ^= -((int32_t)(y & 1)) & s->mat2;
     * The adopted code is equivalent to the original code
     * but does not depend on the representation of negative
     * integers by 2's complements. It is therefore more
     * portable but includes an if branch, which may slow
     * down the generation speed.
     */
    if (y & 1) {
         s->status[1] ^= s->mat1;
         s->status[2] ^= s->mat2;
     }
}

/**
 * This function outputs a 32-bit unsigned integer from
 * the internal state.
 * @param s     pointer to tinymt internal state.
 * @return      32-bit unsigned pseudorandom number.
 */
static uint32_t tinymt32_temper (tinymt32_t* s)
{
    uint32_t t0, t1;
    t0 = s->status[3];
    t1 = s->status[0] + (s->status[2] >> TINYMT32_SH8);
    t0 ^= t1;
    /*
     * The if (t1 & 1) {...} block below replaces:
     *     t0 ^= -((int32_t)(t1 & 1)) & s->tmat;
     * The adopted code is equivalent to the original code
     * but does not depend on the representation of negative
     * integers by 2's complements. It is therefore more
     * portable but includes an if branch, which may slow
     * down the generation speed.
     */
    if (t1 & 1) {
        t0 ^= s->tmat;
    }
    return t0;
}

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