' version 04-07-2018
' compile with: fbc -s console

Function Damm(digit_str As String) As UInteger

    Dim As UInteger table(10,10) => { { 0, 3, 1, 7, 5, 9, 8, 6, 4, 2 } , _
   { 7, 0, 9, 2, 1, 5, 4, 8, 6, 3 } , { 4, 2, 0, 6, 8, 7, 1, 3, 5, 9 } , _
   { 1, 7, 5, 0, 9, 8, 3, 4, 2, 6 } , { 6, 1, 2, 3, 0, 4, 5, 9, 7, 8 } , _
   { 3, 6, 7, 4, 2, 0, 9, 5, 8, 1 } , { 5, 8, 6, 9, 7, 2, 0, 1, 3, 4 } , _
   { 8, 9, 4, 5, 3, 6, 2, 0, 1, 7 } , { 9, 4, 3, 8, 6, 1, 7, 2, 0, 5 } , _
   { 2, 5, 8, 1, 4, 3, 6, 7, 9, 0 } }

    Dim As UInteger i, col_i, old_row_i, new_row_i

    For i = 0 To Len(digit_str) -1
        col_i = digit_str[i] - Asc("0")
        new_row_i = table(old_row_i, col_i)
        old_row_i = new_row_i
    Next

    Return new_row_i

End Function

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

Data "5724", "5727", "112946", ""

Dim As UInteger checksum, t
Dim As String test_string

Do

    Read test_string
    If test_string = "" Then Exit Do
    Print "Checksum test: ";test_string;

    checksum = Damm(test_string)
    If checksum = 0 Then
        Print " is valid"
    Else
        Print " is invalid"
    End If

Loop

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