static OFFSETS: &'static[(i32,i32)] = &[
    (-1,-1), (-1, 0), (-1, 1),
    ( 0,-1),          ( 0, 1),
    ( 1,-1), ( 1, 0), ( 1, 1)
];

pub fn annotate(minefield: &[&str]) -> Vec<String> {
    let height = minefield.len() as i32;
    (0..height).map(|i| {
        let width = minefield[i as usize].len() as i32;
        (0..width).map(|j| {
            if minefield[i as usize].as_bytes()[j as usize] == b' ' {
                let count = OFFSETS
                    .iter()
                    .map(|(oi,oj)| (i+oi,j+oj))
                    .filter(|&(i,j)| i >= 0 && i < height && j >= 0 && j < width)
                    .filter(|&(i,j)| minefield[i as usize].as_bytes()[j as usize] == b'*')
                    .count();
                if count > 0 { char::from_digit(count as u32,10).unwrap() } else { ' ' }
            } else {
                '*'
            }
        }).collect()
    }).collect()
}