'
' Compute nth Fibonacci number
'
' open a window for display
OPENW 1
CLEARW 1
' Display some fibonacci numbers
' Fib(46) is the largest number GFA Basic can reach
' (long integers are 4 bytes)
FOR i%=0 TO 46
  PRINT "fib(";i%;")=";@fib(i%)
NEXT i%
' wait for a key press and tidy up
~INP(2)
CLOSEW 1
'
' Function to compute nth fibonacci number
' n must be in range 0 to 46, inclusive
'
FUNCTION fib(n%)
  LOCAL n0%,n1%,nn%,i%
  n0%=0
  n1%=1
  SELECT n%
  CASE 0
    RETURN n0%
  CASE 1
    RETURN n1%
  DEFAULT
    FOR i%=2 TO n%
      nn%=n0%+n1%
      n0%=n1%
      n1%=nn%
    NEXT i%
    RETURN nn%
  ENDSELECT
ENDFUNC