birthday_paradox.rs

extern crate rand;
use rand::Rng;
use std::env;

const MAX_DAYS: usize = 365;

/// Creates an array with `count` number of birthdays in it represented by numbers
/// if more than one birthday occurs on the same day, then that day's entry will be > 1
fn paradox(count: u16) -> [u16; MAX_DAYS] {
    let mut arr = [0; MAX_DAYS];
    let mut rng = rand::thread_rng();

    for _ in 0 .. count {
        let index = rng.gen_range(0, MAX_DAYS);
        arr[index] += 1;
    }

    arr
}

/// Calculates the percentage based on a number of occurrences (hits) and the total number of trials
fn pct_u32(hits: u32, total: u32) -> f32 {
    hits as f32 / total as f32 * 100.0
}

fn run(count: u16, trials: u32) {
    let mut hits = 0;

    println!("Running {} trials with {} random birthdays, this might take a while.", trials, count);

    for t in 1 ..= trials {
        if t % 100 == 0 {
            let percentage = pct_u32(t, trials);
            print!("\rThe trials are {:3.2}% done.", percentage);
        }

        for count in paradox(count).iter() {
            if *count > 1 {
                hits += 1;
                break;
            }
        }
    }

    println!("\nThe chance of {} people having the same birthday is {:.2}% ({} trials)", 
        count, 
        pct_u32(hits, trials),
        trials
    );
}

fn main() {
    let args: Vec<String> = env::args().collect();
    let count = args.get(1);

    if let Some(count) = count {
        match count.parse::<u16>() {
            Ok(count) => run(count, 2_000_000), // 2 million trials by default
            Err(err)  => println!("{}", err),
        };
    } else {
        println!("Please supply a number as an input value")
    }
}