# ETOOBUSY ðŸš€ minimal blogging for the impatient

# Conway's Game of Life - Sweeping counting

**TL;DR**

Letâ€™s add a little improvement to the implementation of Conwayâ€™s Game of Life.

In the Game of Life, the rules are pretty simple. The next state of a cell depends in the previous state like this:

- if the cell is alive, then it will survive if and only if it is
*surrounded*by 2 or 3 alive cells; - if the cell is empty, it will spawn new life if it has exactly 3 alive cells around, otherwise it will stay empty.

Soâ€¦ counting the number of cells around the target one is pretty important.

In the first implementation, itâ€™s only about counting all values around the target cells:

```
1 sub alive_around ($field, $y, $x) {
2 my $n = 0;
3 for my $dy (-1 .. 1) {
4 for my $dx (-1 .. 1) {
5 ++$n if $field->[$y + $dy][$x + $dx] eq '#';
6 }
7 }
8 --$n if $field->[$y][$x] eq '#';
9 return $n;
10 } ## end sub alive_around
```

Each cellâ€™s contents will be counted overâ€¦ and overâ€¦ and over. About least 9 times, as a matter of fact. Can we do better?

The new implementation keeps track of the surrounding cells horizontally, on three lines. This allows calculating the value at a target cell by simply adding up these values, possibly removing one for the specific target cell (line 19, where it only makes sense).

```
1 sub life_tick ($field) {
2 my @retval;
3 my $nx = $field->[0]->@*;
4 my @previous = my @current = (0) x $nx;
5 for my $y (0 .. $#$field - 1) {
6 my ($irow, $nrow) = $field->@[$y, $y + 1];
7 my @next = (0) x $nx;
8 $next[-1] = $nrow->[-2] eq '#' ? 1 : 0;
9 my @row = (' ') x $nx;
10 push @retval, \@row;
11 for my $x (0 .. $nx - 1) {
12 $next[$x] = $next[$x - 1] - ($nrow->[$x - 2] eq '#' ? 1 : 0)
13 + ($nrow->[($x + 1) % $nx] eq '#' ? 1 : 0);
14 my $around = $previous[$x] + $current[$x] + $next[$x];
15 if ($irow->[$x] eq ' ') {
16 $row[$x] = '#' if $around == 3;
17 }
18 elsif ($irow->[$x] eq '#') {
19 $around--; # the item itself must not be counted
20 $row[$x] = '#' if $around == 2 || $around == 3;
21 }
22 else {
23 $row[$x] = $irow->[$x];
24 }
25 } ## end for my $x (0 .. $nx - 1)
26 @previous = @current;
27 @current = @next;
28 } ## end for my $y (1 .. $#$field...)
29 push @retval, $field->[-1];
30 return \@retval;
31 } ## end sub life_tick ($field)
```

The three arrays `@previous`

, `@current`

and `@next`

keep track of these
horizontal values. When a row is complete (i.e. after line 25), these
array are *shifted* to prepare for the next loop.

After doing this, I realize that I donâ€™t want to benchmark the improvements, if anyâ€¦ Iâ€™m too scared!!!

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