mod minesweeper {
    use std;

    pub fn is_mine(string: &str, x_pos: usize) -> bool {
        string.chars().nth(x_pos).unwrap() == '*'
    }

    pub fn fill(empty: Vec<&str>) -> Vec<std::string::String> {
        let mut result = vec![];

        for (y_pos, row) in empty.iter().enumerate() {
            let mut new_row = String::from(empty[y_pos]);

            for (x_pos, c) in row.chars().enumerate() {
                let b_up: bool = y_pos > 0;
                let b_down: bool = empty.len() > y_pos + 1;
                let b_left: bool = x_pos > 0;
                let b_right: bool = row.len() > x_pos + 1;

                let mine_count = [
                    // looking up
                    b_up && is_mine(empty[y_pos - 1], x_pos),
                    // looking down
                    b_down && is_mine(empty[y_pos + 1], x_pos),
                    // looking left
                    b_left && is_mine(empty[y_pos], x_pos - 1),
                    // looking right
                    b_right && is_mine(empty[y_pos], x_pos + 1),
                    // looking up left
                    b_up && b_left && is_mine(empty[y_pos - 1], x_pos - 1),
                    // looking up right
                    b_up && b_right && is_mine(empty[y_pos - 1], x_pos + 1),
                    // looking down left
                    b_down && b_left && is_mine(empty[y_pos + 1], x_pos - 1),
                    // looking down right
                    b_down && b_right && is_mine(empty[y_pos + 1], x_pos + 1)
                ].iter().filter(|x| **x ).count();

                if c == ' ' && mine_count > 0 {
                    new_row.remove(x_pos);
                    new_row.insert(x_pos, mine_count.to_string()
                                                    .chars()
                                                    .next()
                                                    .unwrap());
                }
            }
            result.push(new_row);
        };

        result
    }
}

#[test]
fn add_numbers_to_empty_sweeper_board() {
    assert_eq!(
        minesweeper::fill(vec![
            "   ",
            "   ",
            "   "
        ]),
        vec![
            "   ",
            "   ",
            "   "
        ]
    )
}

#[test]
fn add_numbers_to_filled_sweeper_board() {
    assert_eq!(
        minesweeper::fill(vec![
            "** ",
            "   ",
            "   "
        ]),
        vec![
            "**1",
            "221",
            "   "
        ]
    )
}

#[test]
fn add_numbers_to_all_mines() {
    assert_eq!(
        minesweeper::fill(vec![
            "***",
            "* *",
            "***"
        ]),
        vec![
            "***",
            "*8*",
            "***"
        ]
    )
}

#[test]
fn add_numbers_to_bigger_filled_sweeper_board() {
    assert_eq!(
        minesweeper::fill(vec![
            "**  ",
            "    ",
            "  * ",
            "    "
        ]),
        vec![
            "**1 ",
            "2321",
            " 1*1",
            " 111"
        ]
    )
}