Permutations.fs

[<RequireQualifiedAccess>]
module Array =

    let private swap (arr: _[]) i j =
        let buf = arr.[i]
        arr.[i] <- arr.[j]
        arr.[j] <- buf

    let permutations arr =
        match arr with
        | null | [||] -> [||]
        | arr ->
            let last = arr.Length - 1
            let arr = Array.copy arr
            let rec perm arr k =
                let arr = Array.copy arr
                [|
                    if k = last then
                        yield arr
                    else
                        for i in k .. last do
                            swap arr k i
                            yield! perm arr (k + 1)
                |]
            perm arr 0