tasks a-s

Task/N-queens-problem/Forth/n-queens-problem.fth

@@ -0,0 +1,29 @@
+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 recurse r> lowBit-
+    repeat
+  else 1 solutions +! then
+  drop 2drop ;
+
+: queens ( n -- )
+  0 solutions ! 0 nodes !
+  -1 -1 rot bits try
+  solutions @ . ." solutions, " nodes @ . ." nodes" ;
+
+8 queens  \ 92 solutions, 1965 nodes