siberian.c

#include <cpuid.h>   // for __get_cpuid intrinsic
#include <stdio.h>
#include <string.h>
#include <immintrin.h>
#include <math.h>

int has_rdrand() {
    const unsigned int flag_RDRAND = (1 << 30);

    unsigned int eax, ebx, ecx, edx;
    __get_cpuid(1, &eax, &ebx, &ecx, &edx);
    return ((ecx & flag_RDRAND) == flag_RDRAND);
}

void translate_sequence(int s, char out[4]) {
    for (int i=0; i<3; i++) {
        out[i] = s & (1 << i) ? 'H' : 'T';
    }
}

int main(int argc, char **argv) {
    if (!has_rdrand()) {
        printf("This CPU does not support RDRAND, exiting\n");
        return 1;
    }

    if (argc < 3) {
        printf("Requires 2 arguments: numiters and doprint\n");
        return 1;
    }

    uint64_t iterations = atoi(argv[1]), wins=0;
    uint8_t doprint = atoi(argv[2]);

    // through this program, H is a binary 1 and T is a binary 0
    int winmap[8] = {
        0, // TTT you bet T on the 4th
        1, // TTH you bet H on the 4th
        1, // THT you bet H on the 4th
        0, // THH you bet T on the 4th
        1, // HTT you bet H on the 4th
        0, // HTH you bet T on the 4th
        0, // HHT you bet T on the 4th
        1, // HHH you bet H on the 4th
    };

    char outseq[4] = {0, 0, 0, 0};
    uint8_t sequence, final, win;
    uint32_t randblock;
    for (int i=0; i<iterations; i++) {
        _rdrand32_step(&randblock);
        for (int s1=0; s1<8; s1++) {
            sequence = randblock & 7; // first 3 bits
            final = (randblock & 8) == 8; // 4th bit
            translate_sequence(sequence, outseq); // to make it look nice
            win = final == winmap[sequence];  // check whether we won
            wins += win;
            if (doprint)
                printf("sequence = %s, bet = %c, final = %c, win = %c\n", outseq, winmap[sequence]?'H':'T', final?'H':'T', win?'Y':'N');
            randblock = randblock >> 4;  // shift 4 bits out of our block to get a fresh game
        }

        if (i % 10000000 == 0) printf("%0.1f%% done\n", 100.0*i/iterations);
    }

    printf("won %lld / %lld = %0.6f", wins, iterations*8, 100.0*wins/(iterations*8));

    return 0;
}