Minesweeper

Instructions

Add the mine counts to a completed Minesweeper board.

Minesweeper is a popular game where the user has to find the mines using numeric hints that indicate how many mines are directly adjacent (horizontally, vertically, diagonally) to a square.

In this exercise you have to create some code that counts the number of mines adjacent to a given empty square and replaces that square with the count.

The board is a rectangle composed of blank space (' ') characters. A mine is represented by an asterisk ('*') character.

If a given space has no adjacent mines at all, leave that square blank.

Examples

For example you may receive a 5 x 4 board like this (empty spaces are represented here with the '·' character for display on screen):

·*·*·
··*··
··*··
·····

And your code will transform it into this:

1*3*1
13*31
·2*2·
·111·

Performance Hint

All the inputs and outputs are in Ascii. Rust Strings and &str are utf8, so while one might expect "Hello".chars() to be simple, it actually has to check each char to see if it's 1, 2, 3 or 4 u8s long. If we know a &str is ascii then we can call .as_bytes() and refer to the underlying data via a &[u8] slice. Iterating over a u8 slice of ascii is much quicker as there are no codepoints involved - every ascii char is one u8 long.

Can you complete the challenge without cloning the input?