use std::fs;

fn solve_puzzle(first: &str, num_rows: usize) -> usize {
    let mut total = first.matches('.').count();

    let mut tiles: Vec<char> = first.chars().collect();
    for _i in 1..num_rows {
        let mut next = Vec::new();

        for j in 0..tiles.len() {
            let is_trap;

            // At left wall
            if j == 0 {
                is_trap = tiles[j + 1] == '^'; // rule 2 + 4
            // At right wall
            } else if j == tiles.len() - 1 {
                is_trap = tiles[j - 1] == '^'; // rule 1 + 3
            // Away from the walls
            } else {
                is_trap = tiles[j - 1] == '^' && tiles[j + 1] == '.'  // rule 1 + 3
                       || tiles[j - 1] == '.' && tiles[j + 1] == '^'; // rule 2 + 4
            }

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

        tiles = next;
        total += tiles.iter().filter(|&c| *c == '.').count();
    }

    total
}

fn main() {
    // Read my personal puzzle input
    let input = fs::read_to_string("input.txt").unwrap();
    let first = input.trim_end();

    // Part 1 gives 1939 for me
    println!("Part 1 solution: {}", solve_puzzle(first, 40));

    // Part 2 gives 19999535 for me
    println!("Part 1 solution: {}", solve_puzzle(first, 400000));
}

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

    #[test]
    fn part1_example1() {
        let input = "..^^.";
        assert_eq!(solve_puzzle(input, 3), 6);
    }

    #[test]
    fn part1_example2() {
        let input = ".^^.^.^^^^";
        assert_eq!(solve_puzzle(input, 10), 38);
    }
}