adventofcode-2019/day04/Day04.rs

fn main() {
    part1(246515, 739105);
    part2(246515, 739105);
}

fn digit_n(digit: u32, n: usize) -> usize {
    let place = 10usize.pow(5 - digit);
    (n / place) % 10
}

fn is_increasing(n: usize) -> bool {
    for digit in 0 .. 5 {
        let this = digit_n(digit, n);
        let next = digit_n(digit + 1, n);
        if next < this {
            return false;
        }
    }
    true
}

fn has_double(n: usize) -> bool {
    for digit in 0 .. 5 {
        let this = digit_n(digit, n);
        let next = digit_n(digit + 1, n);
        if next == this {
            return true;
        }
    }
    false
}

fn has_strict_double(n: usize) -> bool {
    let mut doubles: [usize; 10] = Default::default();
    for digit in 0 .. 5 {
        let this = digit_n(digit, n);
        let next = digit_n(digit + 1, n);
        if next == this {
            doubles[this] += 1;
        }
    }
    doubles.contains(&1)
}

fn part1(lower: usize, upper: usize) {
    let mut count = 0;
    for n in lower ..= upper {
        if is_increasing(n) && has_double(n) {
            count += 1;
        }
    }
    println!("Part 1: {}", count);
}

fn part2(lower: usize, upper: usize) {
    let mut count = 0;
    for n in lower ..= upper {
        if is_increasing(n) && has_strict_double(n) {
            count += 1;
        }
    }
    println!("Part 2: {}", count);
}