(50g) Nth Fibonacci Number

[Gerald H]

Another way to do it for integer input:

::
CK1&Dispatch
# FF
::
::
DUP
Z2_
Z<
case
::
BINT4
NDUPN
DROP
;
FPTR2 ^Z>ZH
Z-2_
Z0Z1_
4PICK
FPTR2 ^ZBits
SWAPDROP#1-_
ZERO_DO
FPTR2 ^ZSQ_
SWAP
FPTR2 ^ZSQ_
2DUP
FPTR2 ^RSUBext
DUP
FPTR2 ^RADDext
3PICK
FPTR2 ^RADDext
3UNROLL
FPTR2 ^RADDext
SWAPROT
FPTR2 ^RADDext
3PICK
ISTOP-INDEX
#1-
FPTR2 ^ZBit?
SWAPDROP
ITE
::
DUPUNROT
FPTR2 ^QAdd
Z-2_
;
Z2_
3UNROLL
LOOP
;
4UNROLL3DROP
;
;