#include "openssl/sha.h"

#include <time.h>
#include <stdio.h>
#include <string.h>

static int estimate_entropy(long l) {
    int bits = 1; /* for the sign bit */
    if (l < 0) l = -l;
    while (l > 0) {
        ++bits;
        l >>= 1;
    }
    return bits;
}

int main() {
    clockid_t clockid = CLOCK_REALTIME;

    struct timespec ts;
    if (clock_getres(clockid, &ts) != 0) {
        perror("clock_getres");
        return 1;
    }
    printf("Clock resolution: %ld.%09ld\n", (long) ts.tv_sec, (long) ts.tv_nsec);

    unsigned char hash[SHA256_DIGEST_LENGTH];
    memset(hash, 0, sizeof(hash));

    #define N 50  /* number of samples */
    struct timespec samples[N];
    memset(samples, 0, sizeof(samples));
    for (int i = 0; i < N; ++i) {
        clock_gettime(clockid, &samples[i]);
        SHA256_CTX ctx;
        SHA256_Init(&ctx);
        SHA256_Update(&ctx, &hash, sizeof(hash));
        SHA256_Update(&ctx, &samples[i], sizeof(samples[i]));
        SHA256_Final(hash, &ctx);
    }

    printf("Hash: ");
    for (int i = 0; i < SHA256_DIGEST_LENGTH; ++i) printf("%02x", hash[i]);
    printf("\n");

    printf("Deltas (ns):");
    long deltas[N - 1];
    for (int i = 0; i < N - 1; ++i) {
        deltas[i] =
            (samples[i + 1].tv_sec - samples[i].tv_sec)*1000000000L
            + (samples[i + 1].tv_nsec - samples[i].tv_nsec);
        printf(" %4ld", deltas[i]);
    }
    printf("\n");
    long entropy = 0;
    printf("Deltas of deltas: ");
    for (int i = 0; i < N - 2; ++i) {
        long dd = deltas[i + 1] - deltas[i];
        printf(" %4ld", dd);
        entropy += estimate_entropy(dd);
    }
    printf("\n");
    printf("Maximum entropy: %lld\n", entropy);
}