pcg-random-bytes.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <unistd.h>

/*
PCG code taken from:
https://github.com/imneme/pcg-c-basic/blob/bc39cd76ac3d541e618606bcc6e1e5ba5e5e6aa3/pcg_basic.c
*/

#define BUFSIZE 1024 * 1024
typedef unsigned char byte;
typedef struct { uint64_t state; uint64_t inc; size_t bufsz; byte buf[0]; } pcg32_random_t;

void printBits(size_t const size, void const * const ptr)
{
    unsigned char *b = (unsigned char*) ptr;
    unsigned char bb;
    int i, j;
    for (i = size-1; i >= 0; i--) {
        // printf("i = %d\n", i);
        for (j = 7; j >= 0; j--) {
            bb = (b[i] >> j) & 1;
            printf("%u", bb);
        }
    }
    puts("");
}

size_t pcg32_random_r(pcg32_random_t* rng)
{
    size_t i;
    for (i = 0 ; i < rng->bufsz ; i += 4) {
        uint64_t oldstate = rng->state;
        // Advance internal state
        rng->state = oldstate * 6364136223846793005ULL + (rng->inc|1);
        // Calculate output function (XSH RR), uses old state for max ILP
        uint32_t xorshifted = ((oldstate >> 18u) ^ oldstate) >> 27u;
        uint32_t rot = oldstate >> 59u;
        uint32_t res = (xorshifted >> rot) | (xorshifted << ((-rot) & 31));
        //printBits(sizeof(uint32_t), &res);
        rng->buf[i+3]   = res >> 24;
        rng->buf[i+2]   = res >> 16;
        rng->buf[i+1]   = res >> 8;
        rng->buf[i]     = res;
    }
    return i;
}

int main(void) {
    pcg32_random_t *rng = malloc(sizeof(*rng) + (sizeof(byte) * BUFSIZE));
    rng->bufsz = BUFSIZE;
    rng->state = 12u;
    rng->inc = 3u;

    if (isatty(STDOUT_FILENO)) {
        fprintf(stderr, "You need to redirect to a file!\n");
    }
    pcg32_random_r(rng);
    ssize_t nwritten = write(STDOUT_FILENO, rng->buf, rng->bufsz);
    if (nwritten == -1) {
        perror("write()");
        return EXIT_FAILURE;
    }
    fprintf(stderr, "Written %zu bytes\n", nwritten);
    return EXIT_SUCCESS;
}