5land.c

#include <errno.h>
#include <fcntl.h>
#include <stdbool.h>
#include <stdint.h>
#include <string.h>
#include <stdio.h>
#include <unistd.h>

struct rng {
    uint64_t state;
    uint64_t inc;
};

struct rng rngState;

bool seed_rng(void)
{
    int fd = open("/dev/urandom", O_RDONLY);
    if (fd == -1) {
        printf("Failed to open /dev/urandom: %s\n", strerror(errno));
        return false;
    }
    uint64_t buf[2];
    ssize_t bytes = read(fd, buf, 16);
    if (bytes != 16) {
        if (bytes == -1) {
            printf("Failed to read from /dev/urandom: %s\n", strerror(errno));
        } else {
            printf("16 bytes expected. Read %zd\n", bytes);
        }
        close(fd);
        return false;
    }
    rngState.state = buf[0];
    rngState.inc = buf[1] | 1;
    close(fd);
    return true;
}

static uint32_t pcg_random(void)
{
    uint64_t oldstate = rngState.state;
    rngState.state = oldstate * 6364136223846793005ULL + rngState.inc;
    uint32_t xorshifted = ((oldstate >> 18u) ^ oldstate) >> 27u;
    uint32_t rot = oldstate >> 59u;
    return (xorshifted >> rot) | (xorshifted << ((0 - rot) & 31));
}

static uint32_t pcg_ranged_random(uint32_t range)
{
    uint32_t x = pcg_random();
    uint64_t m = (uint64_t)x * (uint64_t)range;
    uint32_t l = (uint32_t)m;
    if (l < range) {
        uint32_t t = (0 - range) % range;
        while (l < t) {
            x = pcg_random();
            m = (uint64_t)x * (uint64_t)range;
            l = (uint32_t)m;
        }
    }
    return m >> 32;
}

void shuffle_deck(bool *deck)
{
    for (uint32_t i = 59; i > 0; i--) {
        uint32_t random = pcg_ranged_random(i + 1);
        bool temp = deck[i];
        deck[i] = deck[random];
        deck[random] = temp;
    }
}

int main(void)
{
    if (!seed_rng()) {
        puts("Failed to seed RNG");
        return 1;
    }
    bool deck[60]; // true = land, false = non-land
    for (int i = 0; i < 24; i++) {
        deck[i] = true;
    }
    for (int i = 24; i < 60; i++) {
        deck[i] = false;
    }
    int floods = 0;
    int games = 100000;
    for (int i = 0; i < games; i++) {
        shuffle_deck(deck);
        int lands = 0;
        for (int i = 0; i < 60; i++) {
            if (deck[i]) {
                lands++;
                if (lands >= 5) {
                    floods++;
                    break;
                }
            } else {
                lands = 0;
            }
        }
    }
    double percent = ((double)floods / (double)games) * 100.0;
    printf("Out of %d games, 5 lands in a row happened in %d\n%f%% chance\n", games, floods, percent);
    return 0;
}