#![allow(dead_code)]

use rand::prelude::*; // includes    `use rand::{Rng, SeedableRng, rngs::StdRng};`
//use rand::rngs::mock::StepRng;

fn main() {
    print_samples();
}

/// return the maximum of `k` draws from the interval `[0,n)`,
/// using `seed` to seed a rng.
///
/// pre-condition: k > 0, n > 0.
fn max_of_rands(n: u64, k: u64, seed: u64) -> u64 {
    let mut my_rng = rand::rngs::StdRng::seed_from_u64(seed);
    let mut max_so_far = 0_u64;
    assert!(k > 0);
    assert!(n > 0);
    for _ in 0..k {
        let next_roll = my_rng.random::<u64>() % n;
        max_so_far = if max_so_far > next_roll {
            max_so_far
        } else {
            next_roll
        }
    }
    max_so_far
}

// not a #[test], because it prints rather than asserts.
fn print_samples() -> () {
    println!("max of 2 d20s: {}", max_of_rands(20, 2, 2025));
    println!("max of 2 d20s: {}", max_of_rands(20, 2, 2026));
    println!("max of 2 d20s: {}", max_of_rands(20, 2, 2027));
    println!("max of 2 d20s: {}", max_of_rands(20, 2, 2028));

    println!("max of 3 d20s: {}", max_of_rands(20, 3, 2025));
    println!("max of 3 d20s: {}", max_of_rands(20, 3, 2026));
    println!("max of 3 d20s: {}", max_of_rands(20, 3, 2027));
    println!("max of 3 d20s: {}", max_of_rands(20, 3, 2028));
}

#[test]
fn test_max_of_rands() -> () {
    for seed in 30..200 {
        // Try many different seeds; we'll just assert that the result is in 0..20
        // (not a great test, but it's  all we can say for sure).
        let max_of_2_samples = max_of_rands(20, 2, seed);
        assert!(0 <= max_of_2_samples     );
        assert!(     max_of_2_samples < 20);
        // We can actually use a range's `contains`, to do the above 2 asserts at once:
        assert!((0..20).contains(&max_of_2_samples));
    }
}