day18.rs

use std::fs;

pub(crate) fn day18() {
    println!("{}", solve(fs::read_to_string("input/2016/day18/input.txt").unwrap(), 40));
    println!("{}", solve(fs::read_to_string("input/2016/day18/input.txt").unwrap(), 400000));
}

fn solve(input: String, total_rows: usize) -> usize {
    let mut ans = 0;
    let mut prev_row: Vec<char> = input.chars().collect();
    ans += prev_row.iter().filter(|c| **c == '.').count();

    for _ in 1..total_rows {
        let mut new_row = vec!();
        for i in 0..prev_row.len() {
            let left = if i > 0 { prev_row[i - 1] } else { '.' };
            let center = prev_row[i];
            let right = if i < prev_row.len() - 1 { prev_row[i + 1] } else { '.' };

            let is_trap = (left == '^' && center == '^' && right == '.')
                || (left == '.' && center == '^' && right == '^')
                || (left == '^' && center == '.' && right == '.')
                || (left == '.' && center == '.' && right == '^');

            new_row.push(if is_trap { '^' } else { '.' });
        }

        ans += new_row.iter().filter(|c| **c == '.').count();
        prev_row = new_row
    }

    ans
}

#[cfg(test)]
mod day18_tests {
    use std::fs;

    use crate::y2016::day18::{solve};

    #[test]
    fn test_works() {
        assert_eq!(38, solve(fs::read_to_string("input/2016/day18/test.txt").unwrap(), 10));
    }

    #[test]
    fn input_works() {
        assert_eq!(2005, solve(fs::read_to_string("input/2016/day18/input.txt").unwrap(), 40));
        assert_eq!(20008491, solve(fs::read_to_string("input/2016/day18/input.txt").unwrap(), 400000));
    }
}