June 2018 Update

Task/N-queens-problem/Wart/n-queens-problem.wart

@@ -0,0 +1,24 @@
+def (nqueens n queens)
+  prn "step: " queens  # show progress
+  if (len.queens = n)
+    prn "solution! " queens
+    # else
+    let row (if queens (queens.zero.zero + 1) 0)
+      for col 0 (col < n) ++col
+        let new_queens (cons (list row col) queens)
+          if (no conflicts.new_queens)
+            (nqueens n new_queens)
+
+# check if the first queen in 'queens' lies on the same column or diagonal as
+# any of the others
+def (conflicts queens)
+  let (curr ... rest) queens
+    or (let curr_column curr.1
+         (some (fn(_) (= _ curr_column))
+               (map cadr rest)))  # columns
+       (some (fn(_) (diagonal_match curr _))
+             rest)
+
+def (diagonal_match curr other)
+  (= (abs (curr.0 - other.0))
+     (abs (curr.1 - other.1)))