September 2017 Update

Task/N-queens-problem/Phix/n-queens-problem.phix

@@ -0,0 +1,54 @@
+--
+-- demo\rosetta\n_queens.exw
+-- =========================
+--
+sequence co,    -- columns occupied
+                -- (ro is implicit)
+         fd,    -- forward diagonals
+         bd,    -- backward diagonals
+         board
+
+atom count
+
+procedure solve(integer row, integer N, integer show)
+    for col=1 to N do
+        if not co[col] then
+            integer fdi = col+row-1,
+                    bdi = row-col+N
+            if not fd[fdi]
+            and not bd[bdi] then
+                board[row][col] = 'Q'
+                co[col] = 1
+                fd[fdi] = 1
+                bd[bdi] = 1
+                if row=N then
+                    if show then
+                        puts(1,join(board,"\n")&"\n")
+                        puts(1,repeat('=',N)&"\n")
+                    end if
+                    count += 1
+                else
+                    solve(row+1,N,show)
+                end if
+                board[row][col] = '.'
+                co[col] = 0
+                fd[fdi] = 0
+                bd[bdi] = 0
+            end if
+        end if
+    end for
+end procedure
+
+procedure n_queens(integer N=8, integer show=1)
+    co = repeat(0,N)
+    fd = repeat(0,N*2-1)
+    bd = repeat(0,N*2-1)
+    board  = repeat(repeat('.',N),N)
+    count = 0
+    solve(1,N,show)
+    printf(1,"%d queens: %d solutions\n",{N,count})
+end procedure
+
+for N=1 to 14 do
+    n_queens(N,N<5)
+end for