4a.rs

use std::error::Error;

fn check_some_adjacent_eq(pwd: &[u8; 6]) -> bool {
    (0..pwd.len()-1).any(|i| pwd[i] == pwd[i + 1])
}

fn check_non_decreasing(pwd: &[u8; 6]) -> bool {
    (0..pwd.len()-1).all(|i| pwd[i] <= pwd[i + 1])
}

fn check(pwd: &[u8; 6]) -> bool {
    check_some_adjacent_eq(pwd) && check_non_decreasing(pwd)
}

fn bump(pwd: &mut [u8; 6]) {
    for digit in pwd.iter_mut().rev() {
        if *digit < 9 {
            *digit += 1;
            return;
        }
        *digit = 0;
    }
}

fn main() -> Result<(), Box<dyn Error>> {
    assert!(check(&[1,1,1,1,1,1]));
    assert!(!check(&[2,2,3,4,5,0]));
    assert!(!check(&[1,2,3,7,8,9]));

    let first = [2,4,0,9,2,0];
    let last = [7,8,9,8,5,7];

    let mut cnt = 0;
    let mut pwd = first.clone();
    while pwd != last {
        if check(&pwd) {
            cnt += 1;
        }
        bump(&mut pwd);
    }
    println!("{}", cnt);

    Ok(())
}