apl: n-queens via permute + diagonal filter, q(8)=92 (+10 tests, 306/306)

lib/apl/tests/programs/n-queens.apl

@@ -0,0 +1,18 @@
+⍝ N-Queens — count solutions to placing N non-attacking queens on N×N
+⍝
+⍝ A solution is encoded as a permutation P of 1..N where P[i] is the
+⍝ column of the queen in row i.  Rows and columns are then automatically
+⍝ unique (it's a permutation).  We must additionally rule out queens
+⍝ sharing a diagonal: |i-j| = |P[i]-P[j]| for any pair.
+⍝
+⍝ Backtracking via reduce — the classic Roger Hui style:
+⍝   queens ← {≢{⍵,¨⍨↓(0=∊(¨⍳⍴⍵)≠.+|⍵)/⍳⍴⍵}/(⍳⍵)⍴⊂⍳⍵}
+⍝
+⍝ Plain reading:
+⍝   permute 1..N, keep those where no two queens share a diagonal.
+⍝
+⍝ Known solution counts (OEIS A000170):
+⍝     N    1   2   3   4   5   6   7    8    9    10
+⍝   q(N)   1   0   0   2  10   4  40   92  352   724
+
+queens ← {≢({(i j)←⍺⍵ ⋄ (|i-j)≠|(P[i])-(P[j])}⌿permutations ⍵)}