examples.fifo

\ -*- forth -*-
\ Various example programs

: fibonacci
	dup 0= if
	else
		dup 1 = if
		else
			1 - dup fibonacci
			swap 1 - fibonacci +
		then
	then ;

: factorial ( n -- n! )
    dup 1 >  if
        dup 1-  factorial  *
    then ;

: gcd ( a b -- n )
  begin dup while tuck mod repeat drop ;

\ adapted from https://rosettacode.org/wiki/N-queens_problem#Forth

variable solutions
variable nodes

: bits ( n -- mask ) 1 swap lshift 1- ;
: lowBit  ( mask -- bit ) dup negate and ;
: lowBit- ( mask -- bits ) dup 1- and ;

: next3 ( dl dr f files -- dl dr f dl' dr' f' )
  invert >r
  2 pick r@ and 2* 1+
  2 pick r@ and 2/
  2 pick r> and ;

: try ( dl dr f -- )
  dup if
    1 nodes +!
    dup 2over and and
    begin ?dup while
      dup >r lowBit next3 try r> lowBit-
    repeat
  else 1 solutions +! then
  drop 2drop ;

: queens ( n -- )
  0 solutions ! 0 nodes !
  -1 -1 rot bits try
  solutions @ . nodes @ .  ;