day20.rs

//! [Day 20: Infinite Elves and Infinite Houses](https://adventofcode.com/2015/day/20)

struct Puzzle {
    house_present: usize,
}

impl Puzzle {
    fn new(data: &str) -> Self {
        Self {
            house_present: data.trim_ascii().parse::<usize>().unwrap(),
        }
    }

    fn part1(&self) -> usize {
        let mut house_number: usize = 0;
        let mut present_count: usize = 0;
        while present_count < self.house_present {
            house_number += 1;

            present_count = 10 * divisors::get_divisors(house_number).iter().sum::<usize>();

            // The crate `divisors` forgot to include 1 as a divisor. It also forgot to include
            // the number itself, except for 2. For instance:
            // - `divisors::get_divisors(1)` returns { }
            // - `divisors::get_divisors(2)` returns { 2 }
            // - `divisors::get_divisors(3)` returns { }
            // - `divisors::get_divisors(10)` returns { 2, 5 }
            if house_number == 1 || house_number == 2 {
                present_count += 10;
            } else {
                present_count += 10 * (1 + house_number);
            }
        }

        house_number
    }

    fn part2(&self) -> usize {
        let mut house_number: usize = 0;
        let mut present_count: usize = 0;
        while present_count < self.house_present {
            house_number += 1;

            present_count = 11
                * divisors::get_divisors(house_number)
                    .iter()
                    .filter(|x| 50 * **x >= house_number)
                    .sum::<usize>();

            // The crate `divisors` forgot to include 1 as a divisor. It also forgot to include
            // the number itself, except for 2. For instance:
            // - `divisors::get_divisors(1)` returns { }
            // - `divisors::get_divisors(2)` returns { 2 }
            // - `divisors::get_divisors(3)` returns { }
            // - `divisors::get_divisors(10)` returns { 2, 5 }
            if house_number == 1 || house_number == 2 {
                present_count += 11;
            } else {
                if house_number <= 50 {
                    present_count += 11;
                }
                present_count += 11 * house_number;
            }
        }

        house_number
    }
}

/// # Panics
#[must_use]
pub fn solve(data: &str) -> (usize, usize) {
    let puzzle = Puzzle::new(data);
    (puzzle.part1(), puzzle.part2())
}

pub fn main() {
    let args = aoc::parse_args();
    args.run(solve);
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn test01() {
        let puzzle = Puzzle::new("10");
        assert_eq!(puzzle.part1(), 1);
        assert_eq!(puzzle.part2(), 1);
    }

    #[test]
    fn test02() {
        let puzzle = Puzzle::new("30");
        assert_eq!(puzzle.part1(), 2);
        assert_eq!(puzzle.part2(), 2);
    }

    #[test]
    fn test03() {
        let puzzle = Puzzle::new("40");
        assert_eq!(puzzle.part1(), 3);
        assert_eq!(puzzle.part2(), 3);
    }

    #[test]
    fn test04() {
        let puzzle = Puzzle::new("60");
        assert_eq!(puzzle.part1(), 4);
        assert_eq!(puzzle.part2(), 4);
    }

    #[test]
    fn test05() {
        let puzzle = Puzzle::new("70");
        assert_eq!(puzzle.part1(), 4);
        assert_eq!(puzzle.part2(), 4);
    }

    #[test]
    fn test06() {
        let puzzle = Puzzle::new("80");
        assert_eq!(puzzle.part1(), 6);
        assert_eq!(puzzle.part2(), 6);
    }

    #[test]
    fn test07() {
        let puzzle = Puzzle::new("120");
        assert_eq!(puzzle.part1(), 6);
        assert_eq!(puzzle.part2(), 6);
    }

    #[test]
    fn test08() {
        let puzzle = Puzzle::new("130");
        assert_eq!(puzzle.part1(), 8);
        assert_eq!(puzzle.part2(), 6);
    }

    #[test]
    fn test09() {
        let puzzle = Puzzle::new("150");
        assert_eq!(puzzle.part1(), 8);
        assert_eq!(puzzle.part2(), 8);
    }
}