' version 07-04-2017
' compile with: fbc -s console

' Heap's algorithm non-recursive
Sub perms(n As Long)

    Dim As ULong i, j, count = 1
    Dim As ULong a(0 To n -1), c(0 To n -1)

    For j = 0 To n -1
        a(j) = j +1
        Print a(j);
    Next
    Print " ";

    i = 0
    While i < n
        If c(i) < i Then
            If (i And 1) = 0 Then
                Swap a(0), a(i)
            Else
                Swap a(c(i)), a(i)
            End If
            For j = 0 To n -1
                Print a(j);
            Next
            count += 1
            If count = 12 Then
                Print
                count = 0
            Else
                Print " ";
            End If
            c(i) += 1
            i = 0
        Else
            c(i) = 0
            i += 1
        End If
    Wend

End Sub

' ------=< MAIN >=------

perms(4)

' empty keyboard buffer
While Inkey <> "" : Wend
Print : Print "hit any key to end program"
Sleep
End