#	QUEENS
#
#	Solutions to n nonattacking queens
#
#	Stephen B. Wampler
#
#	Last modified 8/14/84
#

global n, solution

procedure main(args)
   local i
   n := args[1] | 6		# 6 queens by default
   if not(0 < integer(n)) then stop("usage [ n ]")
   solution := list(n)		# ... and a list of column solutions
   write(n,"-Queens:")
   every q(1)			# start by placing queen in first col.
end

# q(c) - place a queen in column c.
#
procedure q(c)
   local r
   static up, down, rows
   initial {
      up := list(2*n-1,0)
      down := list(2*n-1,0)
      rows := list(n,0)
      }
   every 0 = rows[r := 1 to n] = up[n+r-c] = down[r+c-1] &
      rows[r] <- up[n+r-c] <- down[r+c-1] <- 1        do {
         solution[c] := r	# record placement.
         if c = n then show()
         else q(c + 1)		# try to place next queen.
         }
end

# show the solution on a chess board.
#
procedure show()
   static count, line, border
   initial {
      count := 0
      line := repl("|   ",n) || "|"
      border := repl("----",n) || "-"
      }
   write("solution: ", count+:=1)
   write("  ", border)
   every line[4*(!solution - 1) + 3] <- "Q" do {
      write("  ", line)
      write("  ", border)
      }
   write()
end