'Fibonacci extended
'Freebasic version 24  Windows
Dim Shared ADDQmod(0 To 19) As Ubyte
Dim Shared ADDbool(0 To 19) As Ubyte

For z As Integer=0 To 19
    ADDQmod(z)=(z Mod 10+48)
    ADDbool(z)=(-(10<=z))
Next z

Function plusINT(NUM1 As String,NUM2 As String) As String
    Dim As Byte flag
    #macro finish()
    three=Ltrim(three,"0")
    If three="" Then Return "0"
    If flag=1 Then Swap NUM2,NUM1
    Return three
    Exit Function
    #endmacro
    var lenf=Len(NUM1)
    var lens=Len(NUM2)
    If lens>lenf Then
        Swap NUM2,NUM1
        Swap lens,lenf
        flag=1
    End If

    var diff=lenf-lens-Sgn(lenf-lens)
    var three="0"+NUM1
    var two=String(lenf-lens,"0")+NUM2
    Dim As Integer n2
    Dim As Ubyte addup,addcarry

    addcarry=0

    For n2=lenf-1 To diff Step -1
        addup=two[n2]+NUM1[n2]-96
        three[n2+1]=addQmod(addup+addcarry)
        addcarry=addbool(addup+addcarry)
    Next n2
    If addcarry=0 Then
        finish()
    End If
    If n2=-1 Then
        three[0]=addcarry+48
        finish()
    End If

    For n2=n2 To 0 Step -1
        addup=two[n2]+NUM1[n2]-96
        three[n2+1]=addQmod(addup+addcarry)
        addcarry=addbool(addup+addcarry)
    Next n2
    three[0]=addcarry+48
    finish()
End Function

Function  fibonacci(n As Integer) As String
    Dim As String sl,l,term
    sl="0": l="1"
    If n=1 Then Return "0"
    If n=2 Then Return "1"
    n=n-2
    For x As Integer= 1 To n
        term=plusINT(l,sl)
        sl=l
        l=term
    Next x
    Function =term
End Function

'==============  EXAMPLE ===============
print "THE SEQUENCE TO 10:"
print
For n As Integer=1 To 10
    Print "term";n;": "; fibonacci(n)
Next n
print
print "Selected Fibonacci number"
print "Fibonacci 500"
print
print fibonacci(500)
Sleep