Birthday collision calculator

main.rs

use num::{
    bigint::{BigUint, ToBigUint},
    traits::One,
    ToPrimitive,
};

fn factorial(num: u32) -> BigUint {
    let mut result = BigUint::one();
    for i in 1..=num {
        result *= i;
    }
    result
}

fn birthday_probability(num_days: u32, num_people: u32) -> f32 {
    let numerator = factorial(num_days);
    let denominator =
        num_days.to_biguint().unwrap().pow(num_people) * factorial(num_days - num_people);

    let probability = numerator * 1000.to_biguint().unwrap() / denominator;
    1.0 - probability.to_f32().unwrap() / 1000.0
}

fn main() {
    let num_days = 365;
    let num_people = 23;

    println!(
        "Probability of at least two people sharing a birthday with {} people: {}",
        num_people,
        birthday_probability(num_days, num_people)
    );
}