Code Golf: Lights out

Answer 1

F#, 23 lines

Uses brute force and a liberal amount of bitmasking to find a solution:

open System.Collections
let solve(r:string) =
    let s = r.Replace("\n", "")
    let size = s.Length|>float|>sqrt|>int

    let buttons =
        [| for i in 0 .. (size*size)-1 do
            let x = new BitArray(size*size)
            { 0 .. (size*size)-1 } |> Seq.iter (fun j ->
                let toXY n = n / size, n % size
                let (ir, ic), (jr, jc) = toXY i, toXY j
                x.[j] <- ir=jr&&abs(ic-jc)<2||ic=jc&&abs(ir-jr)<2)
            yield x |]

    let testPerm permutation =
        let b = new BitArray(s.Length)
        s |> Seq.iteri (fun i x -> if x = '*' then b.[i] <- true)
        permutation |> Seq.iteri (fun i x -> if x = '1' then b.Xor(buttons.[i]);() )
        b |> Seq.cast |> Seq.forall (fun x -> not x)

    {for a in 0 .. (1 <<< (size * size)) - 1 -> System.Convert.ToString(a, 2).PadLeft(size * size, '0') }
    |> Seq.pick (fun p -> if testPerm p then Some p else None)
    |> Seq.iteri (fun i s -> printf "%s%s" (if s = '1' then "X" else ".") (if (i + 1) % size = 0 then "\n" else "") )

Usage:

> solve ".*.
***
.*.";;

...
.X.
...
val it : unit = ()

> solve "**.**
*.*.*
.***.
*.*.*
**.**";;

..X..
X.X.X
..X..
X.X.X
..X..
val it : unit = ()

> solve "*...*
**.**
..*..
*.*..
*.**.";;

.....
X...X
.....
X.X.X
....X