Iterative Poohsticks (https://possiblywrong.wordpress.com/2017/04/09/iterative-poohsticks/)

poohstick.c

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <time.h>

#define A 10
#define B 30
#define C 5
#define N 1
#define RULE_B

#define M 1000000L

static uint64_t
xorshift(uint64_t *state)
{
    uint64_t x = *state;
    x ^= x >> 12;
    x ^= x << 25;
    x ^= x >> 27;
    *state = x;
    return x * UINT64_C(2685821657736338717);
}

static int
stick_time(uint64_t *s)
{
    return xorshift(s) % (B - A + 1) + A + C;
}

static int
simulate(uint64_t *s)
{
    int t = 0;
    int laps_a = 0;
    int laps_b = 0;
    int time_a = stick_time(s) - C;
    int time_b = stick_time(s) - C;

    while (abs(laps_a - laps_b) <= N) {
        if (time_a == time_b) {
            t += time_a;
            time_a = stick_time(s);
            time_b = stick_time(s);
            laps_a++;
            laps_b++;
            #ifdef RULE_B
            if (abs(laps_a - laps_b) == N)
                break;
            #endif
        } else if (time_a < time_b) {
            t += time_a;
            time_b -= time_a;
            laps_a++;
            time_a = stick_time(s);
        } else if (time_b < time_a) {
            t += time_b;
            time_a -= time_b;
            laps_b++;
            time_b = stick_time(s);
        }
    }

    return t;
}

int
main(void)
{
    uint64_t s[1] = {UINT64_C(0xdeadbeefcafebabe)};
    double average = 0.0;

    *s ^= time(0);
    for (long i = 0; i < M; i++)
        average += simulate(s) / (double)M;

    printf("%f\n", average);
    return 0;
}