' version 02-03-2019
' compile with: fbc -s console

#Define max 2000

Dim Shared As UInteger stern(max +2)

Sub stern_brocot

    stern(1) = 1
    stern(2) = 1

    Dim As UInteger i = 2 , n = 2, ub = UBound(stern)

    Do While i < ub
        i += 1
        stern(i) = stern(n) + stern(n -1)
        i += 1
        stern(i) = stern(n)
        n += 1
    Loop

End Sub

Function gcd(x As UInteger, y As UInteger) As UInteger

    Dim As UInteger t

    While y
        t = y
        y = x Mod y
        x = t
    Wend

    Return x

End Function

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

Dim As UInteger i

stern_brocot

Print "The first 15 are: " ;
For i = 1 To 15
    Print stern(i); " ";
Next

Print : Print
Print "  Index   First nr."
Dim As UInteger d = 1
For i = 1 To max
    If stern(i) = d Then
        Print Using " ######"; i; stern(i)
        d += 1
        If d = 11 Then d = 100
        If d = 101 Then Exit For
        i = 0
    End If
Next

Print : Print
d = 0
For i = 1 To 1000
    If gcd(stern(i), stern(i +1)) <> 1 Then
        d = gcd(stern(i), stern(i +1))
        Exit For
    End If
Next

If d = 0 Then
    Print "GCD of two consecutive members of the series up to the 1000th member is 1"
Else
    Print "The GCD for index "; i; " and "; i +1; " = "; d
End If

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