Solution: Eight Queens Puzzle

def print_chessboard
    @chessboard.length.times do |i|
        puts " #{@chessboard[i].join(" | ")}"
        puts "--- " * @chessboard[i].size
    end
end
# calculating diagonal values
def get_diagonal_values pos
    diagonals = []
    len = @chessboard.length
    @chessboard.length.times do |i|
        neg_x = pos[0] - i
        neg_y = pos[1] - i
        pos_x = pos[0] + i
        pos_y = pos[1] + i
        
        diagonals << @chessboard[neg_x][neg_y] if neg_x >= 0 && neg_y >= 0
        diagonals << @chessboard[neg_x][pos_y] if neg_x >= 0 && pos_y < len
        diagonals << @chessboard[pos_x][neg_y] if neg_y >= 0 && pos_x < len
    end
    return diagonals
end
def check_constraints pos
    diagonals = get_diagonal_values(pos)
    @chessboard[pos[0]].all? { | x | x == "." } &&     # no queen in same row
    @chessboard.collect { | sqr | sqr[pos[1]] }.all? { | x | x == "." } &&  # no queen in same column
    diagonals.all? { | x | x == "." } &&    # no queen in all diagonals
    @chessboard[pos[0]][pos[1]] == "."  # unvisited location
end
@queens = 0
@max_queens = 8
@chessboard  = Array.new(8){ Array.new(8, '.')}
def place_queens row
    @chessboard[row][@queens] = "Q"   # placing queen in chessboard
    @queens += 1
    # end cause
    if @queens == @max_queens
        print_chessboard
        exit
    end
    # trying all possible locations in a row
    @chessboard.length.times do |row_item|
        place_queens(row_item) if check_constraints([row_item, @queens])    # only place queen when constraints are met
    end
    # backtracking
    @queens -= 1
    @chessboard[row][@queens] = "."
end
@chessboard.length.times do |x|
    place_queens(x)
end
# loop ended without finding a solution
puts "Solution not found!"