IPAddr#mask!

# File tmp/rubies/ruby-3.3.0/lib/ipaddr.rb, line 529
def mask!(mask)
  case mask
  when String
    case mask
    when /\A(0|[1-9]+\d*)\z/
      prefixlen = mask.to_i
    when /\A\d+\z/
      raise InvalidPrefixError, "leading zeros in prefix"
    else
      m = IPAddr.new(mask)
      if m.family != @family
        raise InvalidPrefixError, "address family is not same: #{@addr}"
      end
      @mask_addr = m.to_i
      n = @mask_addr ^ m.instance_variable_get(:@mask_addr)
      unless ((n + 1) & n).zero?
        raise InvalidPrefixError, "invalid mask #{mask}: #{@addr}"
      end
      @addr &= @mask_addr
      return self
    end
  else
    prefixlen = mask
  end
  case @family
  when Socket::AF_INET
    if prefixlen < 0 || prefixlen > 32
      raise InvalidPrefixError, "invalid length: #{@addr}"
    end
    masklen = 32 - prefixlen
    @mask_addr = ((IN4MASK >> masklen) << masklen)
  when Socket::AF_INET6
    if prefixlen < 0 || prefixlen > 128
      raise InvalidPrefixError, "invalid length: #{@addr}"
    end
    masklen = 128 - prefixlen
    @mask_addr = ((IN6MASK >> masklen) << masklen)
  else
    raise AddressFamilyError, "unsupported address family"
  end
  @addr = ((@addr >> masklen) << masklen)
  return self
end