checksum.c

// Basic idea is to perturb a PRNG state using each word in a bytestream.

#include <assert.h>
#include <stdio.h>
#include <stddef.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>

#include "pcg_variants.h"

void pcg_checksum_init(pcg64si_random_t *rng) {
    pcg64si_srandom_r(rng, 0u);
}

void pcg_checksum_update(pcg64si_random_t *rng, uint8_t *data, size_t len) {
    assert((len % sizeof(uint64_t)) == 0);
    uint64_t *words = (uint64_t *)data;
    size_t word_count = len / sizeof(uint64_t);
    uint64_t *end = words + word_count;

    while (words != end) {
        (void)pcg64si_random_r(rng);
        rng->state ^= *words;
        words++;
    }
}

uint64_t pcg_checksum_final(pcg64si_random_t *rng) {
    return pcg64si_random_r(rng);
}

int main(int argc, char** argv)
{
    pcg64si_random_t rng;
    pcg_checksum_init(&rng);
    char *one_mb = calloc(1, 1024 * 1024);
    one_mb[0] = '3';
    int i;

    for (i=0; i < 1024; i++) {
        pcg_checksum_update(&rng, (uint8_t *)one_mb, 1024 * 1024);
    }
    printf("Final was: %" PRIx64 "\n", pcg_checksum_final(&rng));
    return 0;
}