forth/gcd.fs

\ gcd.fs -- Compute greatest common divisor (for Gforth manual
\ General Loops Tutorial)
\ David Meyer <papa@freeshell.org> 2010-03-17

: gcd ( n1 n2 -- n3 )
  assert( 2dup 0<> swap 0<> and ) \ Parameters are non-zero
  2dup < if swap then             \ Put parameters in descending order
  dup 1+                          \ Initialize candidate divisor
  begin 
	1-                        \ Decrement candidate divisor
	2dup mod 0=               \ n2 divisible by nc
	2over drop 2over nip mod 0= and \ n1 div. by nc
	over 1 = or               \ nc = 1
  until
  nip nip
\  dup 1 = if                     \ Result is 0 for mutual primes
\      drop 0
\  then       
;


: euclid ( n1 n2 -- n3 )           \ Calculate GCD with Euclid's
                                   \   method
  assert( 2dup 0<> swap 0<> and )  \ Parameters are non-zero
  2dup < if swap then              \ Put parameters in descending order
  begin
	dup 0<> 
  while
	tuck mod
  repeat
  drop                             \ Simple!
;