Simulates the Monty Hall problem

monty-hall.rs

// Run with two arguments: strategy and simulations
//
// eg.
// cargo run stay 500
// cargo run switch 10000

extern crate rand;

use rand::Rng;
use std::env;

#[derive(Debug)]
enum Strategy {
    Stay,
    Switch,
}

fn main() {
    let arguments: Vec<String> = env::args().skip(1).take(2).collect();
    let strategy = match &*arguments[0] {
        "stay"   => Strategy::Stay,
        "switch" => Strategy::Switch,
        arg      => panic!("Invalid strategy {:?}", arg)
    };
    let simulations = arguments[1].parse::<i32>().unwrap();

    let mut wins   = 0;
    let mut games  = 0;

    for _ in 0..simulations {
        let mut rng = rand::thread_rng();

        let prize_slot   = rng.gen_range::<i32>(0, 3);
        let first_choice = rng.gen_range::<i32>(0, 3);
        let second_choice;

        let revealed = match prize_slot {
            0 => match first_choice {
                0 => rng.gen_range::<i32>(1, 3),
                1 => 2,
                2 => 1,
                _ => unreachable!()
            },
            1 => match first_choice {
                0 => 2,
                1 => rng.gen_range::<i32>(0, 2) * 2, // 0 or 1 times 2
                2 => 0,
                _ => unreachable!()
            },
            2 => match first_choice {
                0 => 1,
                1 => 0,
                2 => rng.gen_range::<i32>(0, 2),
                _ => unreachable!()
            },
            _ => unreachable!()
        };

        let left_over = 3 - first_choice - revealed;

        assert_ne!(revealed, first_choice);
        assert_ne!(revealed, prize_slot);

        assert_ne!(left_over, first_choice);
        assert_ne!(left_over, revealed);

        match strategy {
            Strategy::Stay   => second_choice = first_choice,
            Strategy::Switch => second_choice = left_over,
        }

        if second_choice == prize_slot {
            wins += 1;
        }

        games += 1;
    }

    println!("Results:");
    println!("  Wins:         {}", wins);
    println!("  Total games:  {}", games);
}