A snippet from the expansion/flood fill;

def flood_fill(used: [])
  new_dim = @dim * 3
  new_map = Array.new(new_dim.size, '.')

  puts "Expanding #{@dim} to #{new_dim}, with #{used.size} visited pipes." if $args.verbose

  # Mark all real points as inside on the expanded map
  (0..(@dim.y - 1)).each do |y|
    (0..(@dim.x - 1)).each do |x|
      expanded_point = Point.new x * 3 + 1, y * 3 + 1
      new_map[expanded_point.y * new_dim.x + expanded_point.x] = 'I'
    end
  end

  # Paint all used pipes onto the expanded map
  used.each do |used_p|
    expanded_point = Point.new used_p.x * 3 + 1, used_p.y * 3 + 1

    new_map[expanded_point.y * new_dim.x + expanded_point.x] = '#'
    offsets = @links[used_p].connections
    offsets.shift

    offsets.each do |offs|
      diff = offs - used_p
      new_map[(expanded_point.y + diff.y) * new_dim.x + (expanded_point.x + diff.x)] = '#'
    end
  end

  puts "Flooding expanded map..." if $args.verbose

  # Flood fill the expanded map from the top-left corner
  to_visit = [Point.new(0, 0)]
  until to_visit.empty?
    at = to_visit.shift
    new_map[at.y * new_dim.x + at.x] = ' '

    (-1..1).each do |off_y|
      (-1..1).each do |off_x|
        next if (off_x.zero? && off_y.zero?) || !(off_x.zero? || off_y.zero?)

        off_p = at + Point.new(off_x, off_y)
        next if off_p.x < 0 || off_p.y < 0 \
          || off_p.x >= new_dim.x || off_p.y >= new_dim.y \
          || to_visit.include?(off_p)

        val = new_map[off_p.y * new_dim.x + off_p.x]
        next unless %w[. I].include? val

        to_visit << off_p
      end
    end
  end

  return new_map, new_dim
end