The shortest code by character count to solve the input lights out board.
The lights out board is a 2d square grid of varying size composed of two characters - . for a light that is off and * for a light that is on.
To solve the board, all "lights" have to be turned off. Toggling a light (i.e. turning off when it is on, turning on when it is off) is made 5 lights at a time - the light selected and the lights surround it in a + (plus) shape. "Selecting" the middle light will solve the board:
.*.
***
.*.
Since Lights Out! solution order does not matter, the output will be a new board with markings on what bulbs to select. The above board's solution is
...
.X.
...
Turning off a light in a corner where there are no side bulbs to turn off will not overflow:
...
..*
.**
Selecting the lower-right bulb will only turn off 3 bulbs in this case.
Input:
**.**
*.*.*
.***.
*.*.*
**.**
Output:
X...X
.....
..X..
.....
X...X
Input:
.*.*.
**.**
.*.*.
*.*.*
*.*.*
Output:
.....
.X.X.
.....
.....
X.X.X
Input:
*...*
**.**
..*..
*.*..
*.**.
Output:
X.X.X
..X..
.....
.....
X.X..
Code count includes input/output (i.e full program).
Perl, 333 251 203 197 190 172 characters. In this version, we randomly push buttons until all of the lights are out.
map{$N++;$E+=/\*/*1<<$t++for/./g}<>;
$C^=$b=1<<($%=rand$t),
$E^=$b|$b>>$N|($%<$t-$N)*$b<<$N|($%%$N&&$b/2)|(++$%%$N&&$b*2)while$E;
die map{('.',X)[1&$C>>$_-1],$_%$N?"":$/}1..$t