use std::iter;

// in order to determine if n is prime
// we will use a primality test.
// https://en.wikipedia.org/wiki/Primality_test#Pseudocode
fn is_prime(n: u128) -> bool {
    if n <= 3 {
        n > 1
    } else if n % 2 == 0 || n % 3 == 0 {
        false
    } else {
        for i in (5..=(n as f64).sqrt() as u128).step_by(6) {
            if n % i == 0 || n % (i + 2) == 0 {
                return false;
            }
        }
        true
    }
}

// given a sequence of digits, return the corresponding number
// eg: assert_eq!(1234, digits_to_number(vec![1,2,3,4]))
fn digits_to_number(iter: impl Iterator<Item = usize>) -> u128 {
    iter.fold(0, |acc, b| acc * 10 + b as u128)
}

fn get_min_bound(num_digits: usize) -> u128 {
    let lower_bound_iter =
        iter::once(1usize).chain(iter::repeat(0usize).take(num_digits - 1 as usize));
    digits_to_number(lower_bound_iter)
}

fn get_max_bound(num_digits: usize) -> u128 {
    let lower_bound_iter = iter::once(1usize).chain(iter::repeat(0usize).take(num_digits));
    digits_to_number(lower_bound_iter)
}

fn number_or_panic(number_to_return: u128) -> u128 {
    // Let's roll a dice
    if rand::random::<u8>() % 6 == 0 {
        panic!(format!(
            "I was about to return {} but I chose to panic instead!",
            number_to_return
        ))
    }
    number_to_return
}

fn get_prime(num_digits: usize) -> u128 {
    let min_bound = get_min_bound(num_digits);
    // with num_digits = 4, max_bound == 10000
    let max_bound = get_max_bound(num_digits);
    // maybe_prime is a number in range [1000, 10000)
    // the closing parenthesiss means it won't reach the number.
    // the maximum allowed value for maybe_prime is 9999.
    use rand::Rng;
    let mut maybe_prime = rand::thread_rng().gen_range(min_bound, max_bound);
    loop {
        if is_prime(maybe_prime) {
            return number_or_panic(maybe_prime);
        }
        // for any integer n > 3,
        // there always exists at least one prime number p
        // with n < p < 2n - 2
        maybe_prime += 1;
        // We don't want to return a number
        // that doesn't have the right number of digits
        if maybe_prime == max_bound {
            maybe_prime = min_bound;
        }
    }
}

async fn prime_number(req: tide::Request<()>) -> String {
    use std::time::Instant;
    let d: usize = req.param("digits").unwrap_or(1);
    // Start a stopwatch
    let start = Instant::now();
    // Get a prime number
    let prime_number = get_prime(d);
    // Stop the stopwatch
    let elapsed = Instant::now().duration_since(start).as_secs();
    format!(
        "{} is a prime number with {} digits.\nIt was computed in {} seconds.\n",
        prime_number, d, elapsed
    )
}

fn main() -> Result<(), std::io::Error> {
    use async_std::task;
    task::block_on(async {
        let mut app = tide::new();
        app.at("/prime/:digits").get(prime_number);
        app.listen("127.0.0.1:8080").await?;
        Ok(())
    })
}