advent-of-code-2025/src/bin/04.rs

advent_of_code::solution!(4);

pub fn part_one(input: &str) -> Option<u64> {
    let char_grid: Vec<Vec<char>> = input
        .split('\n')
        .filter(|el| !el.is_empty())
        .map(|line| line.chars().collect())
        .collect();
    let mut num_rolls = 0;
    for (i, row) in char_grid.iter().enumerate() {
        for (j, c) in row.iter().enumerate() {
            let mut num = 0;
            if *c == '@' {
                if (i + 1) < char_grid.len() && char_grid[i + 1][j] == '@' {
                    num += 1;
                }

                if (i + 1) < char_grid.len()
                    && (j + 1) < row.len()
                    && char_grid[i + 1][j + 1] == '@'
                {
                    num += 1;
                }

                if (j + 1) < row.len() && char_grid[i][j + 1] == '@' {
                    num += 1;
                }

                if (i as i32 - 1) >= 0 && (j + 1) < row.len() && char_grid[i - 1][j + 1] == '@' {
                    num += 1;
                }

                if (i as i32 - 1) as i32 >= 0 && char_grid[i - 1][j] == '@' {
                    num += 1;
                }

                if (i as i32 - 1) as i32 >= 0
                    && (j as i32 - 1) >= 0
                    && char_grid[i - 1][j - 1] == '@'
                {
                    num += 1;
                }

                if (j as i32 - 1) >= 0 && char_grid[i][j - 1] == '@' {
                    num += 1;
                }

                if i + 1 < char_grid.len() && (j as i32 - 1) >= 0 && char_grid[i + 1][j - 1] == '@'
                {
                    num += 1;
                }
                if num < 4 {
                    num_rolls += 1;
                }
            }
        }
    }
    Some(num_rolls)
}

pub fn part_two(input: &str) -> Option<u64> {
    let mut char_grid: Vec<Vec<char>> = input
        .split('\n')
        .filter(|el| !el.is_empty())
        .map(|line| line.chars().collect())
        .collect();
    let mut num_rolls = 0;
    let mut done = false;
    while !done {
        done = true;
        for (i, row) in char_grid.clone().iter().enumerate() {
            for (j, c) in row.clone().iter().enumerate() {
                let mut num = 0;
                if *c == '@' {
                    if (i + 1) < char_grid.len() && char_grid[i + 1][j] == '@' {
                        num += 1;
                    }

                    if (i + 1) < char_grid.len()
                        && (j + 1) < row.len()
                        && char_grid[i + 1][j + 1] == '@'
                    {
                        num += 1;
                    }

                    if (j + 1) < row.len() && char_grid[i][j + 1] == '@' {
                        num += 1;
                    }

                    if (i as i32 - 1) >= 0 && (j + 1) < row.len() && char_grid[i - 1][j + 1] == '@'
                    {
                        num += 1;
                    }

                    if (i as i32 - 1) as i32 >= 0 && char_grid[i - 1][j] == '@' {
                        num += 1;
                    }

                    if (i as i32 - 1) as i32 >= 0
                        && (j as i32 - 1) >= 0
                        && char_grid[i - 1][j - 1] == '@'
                    {
                        num += 1;
                    }

                    if (j as i32 - 1) >= 0 && char_grid[i][j - 1] == '@' {
                        num += 1;
                    }

                    if i + 1 < char_grid.len()
                        && (j as i32 - 1) >= 0
                        && char_grid[i + 1][j - 1] == '@'
                    {
                        num += 1;
                    }
                    if num < 4 {
                        num_rolls += 1;
                        char_grid[i][j] = '.';
                        done = false;
                    }
                }
            }
        }
    }
    Some(num_rolls)
}

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

    #[test]
    fn test_part_one() {
        let result = part_one(&advent_of_code::template::read_file("examples", DAY));
        assert_eq!(result, Some(13));
    }

    #[test]
    fn test_part_two() {
        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
        assert_eq!(result, Some(43));
    }
}