\ fast queens program by Ian Osgood from
\ <7ec890f8.0405041426.44374d73@posting.google.com>

variable solutions  variable nodes
: lowBit  ( mask -- bit ) dup negate and ;
: lowBit- ( mask -- bits ) dup 1- and ;

: third ( a b c -- a b c a ) 2 pick ;    \ >r over r> swap ;
: poss ( a b c -- a b c a&b&c ) dup 2over and and ;
: next3 ( dl dr f Qfilebit -- dl dr f dl' dr' f' )
  invert >r third r@ and 2* 1+  third r@ and 2/  third r> and ;
: try ( dl dr f -- )
  dup if 1 nodes +!  poss
    begin ?dup while
      dup >r lowBit next3 recurse r> lowBit-
    repeat
  else 1 solutions +! then drop 2drop ;
: queens ( n -- )  0 solutions ! 0 nodes !  cr
  1 over lshift 1-  -1 dup rot try
  . ." queens: " solutions @ . ." solutions, " nodes @ . ." nodes" ;