path: root/lib/net/imap.rb

# frozen_string_literal: true
#
# = net/imap.rb
#
# Copyright (C) 2000  Shugo Maeda <[email protected]>
#
# This library is distributed under the terms of the Ruby license.
# You can freely distribute/modify this library.
#
# Documentation: Shugo Maeda, with RDoc conversion and overview by William
# Webber.
#
# See Net::IMAP for documentation.
#


require "socket"
require "monitor"
require 'net/protocol'
begin
  require "openssl"
rescue LoadError
end

require_relative "imap/response_parser"

module Net

  #
  # Net::IMAP implements Internet Message Access Protocol (IMAP) client
  # functionality.  The protocol is described in [IMAP].
  #
  # == IMAP Overview
  #
  # An IMAP client connects to a server, and then authenticates
  # itself using either #authenticate() or #login().  Having
  # authenticated itself, there is a range of commands
  # available to it.  Most work with mailboxes, which may be
  # arranged in an hierarchical namespace, and each of which
  # contains zero or more messages.  How this is implemented on
  # the server is implementation-dependent; on a UNIX server, it
  # will frequently be implemented as files in mailbox format
  # within a hierarchy of directories.
  #
  # To work on the messages within a mailbox, the client must
  # first select that mailbox, using either #select() or (for
  # read-only access) #examine().  Once the client has successfully
  # selected a mailbox, they enter _selected_ state, and that
  # mailbox becomes the _current_ mailbox, on which mail-item
  # related commands implicitly operate.
  #
  # Messages have two sorts of identifiers: message sequence
  # numbers and UIDs.
  #
  # Message sequence numbers number messages within a mailbox
  # from 1 up to the number of items in the mailbox.  If a new
  # message arrives during a session, it receives a sequence
  # number equal to the new size of the mailbox.  If messages
  # are expunged from the mailbox, remaining messages have their
  # sequence numbers "shuffled down" to fill the gaps.
  #
  # UIDs, on the other hand, are permanently guaranteed not to
  # identify another message within the same mailbox, even if
  # the existing message is deleted.  UIDs are required to
  # be assigned in ascending (but not necessarily sequential)
  # order within a mailbox; this means that if a non-IMAP client
  # rearranges the order of mailitems within a mailbox, the
  # UIDs have to be reassigned.  An IMAP client thus cannot
  # rearrange message orders.
  #
  # == Examples of Usage
  #
  # === List sender and subject of all recent messages in the default mailbox
  #
  #   imap = Net::IMAP.new('mail.example.com')
  #   imap.authenticate('LOGIN', 'joe_user', 'joes_password')
  #   imap.examine('INBOX')
  #   imap.search(["RECENT"]).each do |message_id|
  #     envelope = imap.fetch(message_id, "ENVELOPE")[0].attr["ENVELOPE"]
  #     puts "#{envelope.from[0].name}: \t#{envelope.subject}"
  #   end
  #
  # === Move all messages from April 2003 from "Mail/sent-mail" to "Mail/sent-apr03"
  #
  #   imap = Net::IMAP.new('mail.example.com')
  #   imap.authenticate('LOGIN', 'joe_user', 'joes_password')
  #   imap.select('Mail/sent-mail')
  #   if not imap.list('Mail/', 'sent-apr03')
  #     imap.create('Mail/sent-apr03')
  #   end
  #   imap.search(["BEFORE", "30-Apr-2003", "SINCE", "1-Apr-2003"]).each do |message_id|
  #     imap.copy(message_id, "Mail/sent-apr03")
  #     imap.store(message_id, "+FLAGS", [:Deleted])
  #   end
  #   imap.expunge
  #
  # == Thread Safety
  #
  # Net::IMAP supports concurrent threads. For example,
  #
  #   imap = Net::IMAP.new("imap.foo.net", "imap2")
  #   imap.authenticate("cram-md5", "bar", "password")
  #   imap.select("inbox")
  #   fetch_thread = Thread.start { imap.fetch(1..-1, "UID") }
  #   search_result = imap.search(["BODY", "hello"])
  #   fetch_result = fetch_thread.value
  #   imap.disconnect
  #
  # This script invokes the FETCH command and the SEARCH command concurrently.
  #
  # == Errors
  #
  # An IMAP server can send three different types of responses to indicate
  # failure:
  #
  # NO:: the attempted command could not be successfully completed.  For
  #      instance, the username/password used for logging in are incorrect;
  #      the selected mailbox does not exist; etc.
  #
  # BAD:: the request from the client does not follow the server's
  #       understanding of the IMAP protocol.  This includes attempting
  #       commands from the wrong client state; for instance, attempting
  #       to perform a SEARCH command without having SELECTed a current
  #       mailbox.  It can also signal an internal server
  #       failure (such as a disk crash) has occurred.
  #
  # BYE:: the server is saying goodbye.  This can be part of a normal
  #       logout sequence, and can be used as part of a login sequence
  #       to indicate that the server is (for some reason) unwilling
  #       to accept your connection.  As a response to any other command,
  #       it indicates either that the server is shutting down, or that
  #       the server is timing out the client connection due to inactivity.
  #
  # These three error response are represented by the errors
  # Net::IMAP::NoResponseError, Net::IMAP::BadResponseError, and
  # Net::IMAP::ByeResponseError, all of which are subclasses of
  # Net::IMAP::ResponseError.  Essentially, all methods that involve
  # sending a request to the server can generate one of these errors.
  # Only the most pertinent instances have been documented below.
  #
  # Because the IMAP class uses Sockets for communication, its methods
  # are also susceptible to the various errors that can occur when
  # working with sockets.  These are generally represented as
  # Errno errors.  For instance, any method that involves sending a
  # request to the server and/or receiving a response from it could
  # raise an Errno::EPIPE error if the network connection unexpectedly
  # goes down.  See the socket(7), ip(7), tcp(7), socket(2), connect(2),
  # and associated man pages.
  #
  # Finally, a Net::IMAP::DataFormatError is thrown if low-level data
  # is found to be in an incorrect format (for instance, when converting
  # between UTF-8 and UTF-16), and Net::IMAP::ResponseParseError is
  # thrown if a server response is non-parseable.
  #
  #
  # == References
  #
  # [[IMAP]]
  #    M. Crispin, "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1",
  #    RFC 2060, December 1996.  (Note: since obsoleted by RFC 3501)
  #
  # [[LANGUAGE-TAGS]]
  #    Alvestrand, H., "Tags for the Identification of
  #    Languages", RFC 1766, March 1995.
  #
  # [[MD5]]
  #    Myers, J., and M. Rose, "The Content-MD5 Header Field", RFC
  #    1864, October 1995.
  #
  # [[MIME-IMB]]
  #    Freed, N., and N. Borenstein, "MIME (Multipurpose Internet
  #    Mail Extensions) Part One: Format of Internet Message Bodies", RFC
  #    2045, November 1996.
  #
  # [[RFC-822]]
  #    Crocker, D., "Standard for the Format of ARPA Internet Text
  #    Messages", STD 11, RFC 822, University of Delaware, August 1982.
  #
  # [[RFC-2087]]
  #    Myers, J., "IMAP4 QUOTA extension", RFC 2087, January 1997.
  #
  # [[RFC-2086]]
  #    Myers, J., "IMAP4 ACL extension", RFC 2086, January 1997.
  #
  # [[RFC-2195]]
  #    Klensin, J., Catoe, R., and Krumviede, P., "IMAP/POP AUTHorize Extension
  #    for Simple Challenge/Response", RFC 2195, September 1997.
  #
  # [[SORT-THREAD-EXT]]
  #    Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - SORT and THREAD
  #    Extensions", draft-ietf-imapext-sort, May 2003.
  #
  # [[OSSL]]
  #    http://www.openssl.org
  #
  # [[RSSL]]
  #    http://savannah.gnu.org/projects/rubypki
  #
  # [[UTF7]]
  #    Goldsmith, D. and Davis, M., "UTF-7: A Mail-Safe Transformation Format of
  #    Unicode", RFC 2152, May 1997.
  #
  class IMAP < Protocol
    VERSION = "0.2.1"

    include MonitorMixin
    if defined?(OpenSSL::SSL)
      include OpenSSL
      include SSL
    end

    #  Returns an initial greeting response from the server.
    attr_reader :greeting

    # Returns recorded untagged responses.  For example:
    #
    #   imap.select("inbox")
    #   p imap.responses["EXISTS"][-1]
    #   #=> 2
    #   p imap.responses["UIDVALIDITY"][-1]
    #   #=> 968263756
    attr_reader :responses

    # Returns all response handlers.
    attr_reader :response_handlers

    # Seconds to wait until a connection is opened.
    # If the IMAP object cannot open a connection within this time,
    # it raises a Net::OpenTimeout exception. The default value is 30 seconds.
    attr_reader :open_timeout

    # Seconds to wait until an IDLE response is received.
    attr_reader :idle_response_timeout

    # The thread to receive exceptions.
    attr_accessor :client_thread

    # Flag indicating a message has been seen.
    SEEN = :Seen

    # Flag indicating a message has been answered.
    ANSWERED = :Answered

    # Flag indicating a message has been flagged for special or urgent
    # attention.
    FLAGGED = :Flagged

    # Flag indicating a message has been marked for deletion.  This
    # will occur when the mailbox is closed or expunged.
    DELETED = :Deleted

    # Flag indicating a message is only a draft or work-in-progress version.
    DRAFT = :Draft

    # Flag indicating that the message is "recent," meaning that this
    # session is the first session in which the client has been notified
    # of this message.
    RECENT = :Recent

    # Flag indicating that a mailbox context name cannot contain
    # children.
    NOINFERIORS = :Noinferiors

    # Flag indicating that a mailbox is not selected.
    NOSELECT = :Noselect

    # Flag indicating that a mailbox has been marked "interesting" by
    # the server; this commonly indicates that the mailbox contains
    # new messages.
    MARKED = :Marked

    # Flag indicating that the mailbox does not contains new messages.
    UNMARKED = :Unmarked

    # Returns the debug mode.
    def self.debug
      return @@debug
    end

    # Sets the debug mode.
    def self.debug=(val)
      return @@debug = val
    end

    # Returns the max number of flags interned to symbols.
    def self.max_flag_count
      return @@max_flag_count
    end

    # Sets the max number of flags interned to symbols.
    def self.max_flag_count=(count)
      @@max_flag_count = count
    end

    # The default port for IMAP connections, port 143
    def self.default_port
      return PORT
    end

    # The default port for IMAPS connections, port 993
    def self.default_tls_port
      return SSL_PORT
    end

    class << self
      alias default_imap_port default_port
      alias default_imaps_port default_tls_port
      alias default_ssl_port default_tls_port
    end

    # Disconnects from the server.
    def disconnect
      return if disconnected?
      begin
        begin
          # try to call SSL::SSLSocket#io.
          @sock.io.shutdown
        rescue NoMethodError
          # @sock is not an SSL::SSLSocket.
          @sock.shutdown
        end
      rescue Errno::ENOTCONN
        # ignore `Errno::ENOTCONN: Socket is not connected' on some platforms.
      rescue Exception => e
        @receiver_thread.raise(e)
      end
      @receiver_thread.join
      synchronize do
        @sock.close
      end
      raise e if e
    end

    # Returns true if disconnected from the server.
    def disconnected?
      return @sock.closed?
    end

    # Sends a CAPABILITY command, and returns an array of
    # capabilities that the server supports.  Each capability
    # is a string.  See [IMAP] for a list of possible
    # capabilities.
    #
    # Note that the Net::IMAP class does not modify its
    # behaviour according to the capabilities of the server;
    # it is up to the user of the class to ensure that
    # a certain capability is supported by a server before
    # using it.
    def capability
      synchronize do
        send_command("CAPABILITY")
        return @responses.delete("CAPABILITY")[-1]
      end
    end

    # Sends an ID command, and returns a hash of the server's
    # response, or nil if the server does not identify itself.
    #
    # Note that the user should first check if the server supports the ID
    # capability. For example:
    #
    #    capabilities = imap.capability
    #    if capabilities.include?("ID")
    #      id = imap.id(
    #        name: "my IMAP client (ruby)",
    #        version: MyIMAP::VERSION,
    #        "support-url": "mailto:[email protected]",
    #        os: RbConfig::CONFIG["host_os"],
    #      )
    #    end
    #
    # See RFC 2971, Section 3.3, for defined fields.
    def id(client_id=nil)
      synchronize do
        send_command("ID", ClientID.new(client_id))
        @responses.delete("ID")&.last
      end
    end

    # Sends a NOOP command to the server. It does nothing.
    def noop
      send_command("NOOP")
    end

    # Sends a LOGOUT command to inform the server that the client is
    # done with the connection.
    def logout
      send_command("LOGOUT")
    end

    # Sends a STARTTLS command to start TLS session.
    def starttls(options = {}, verify = true)
      send_command("STARTTLS") do |resp|
        if resp.kind_of?(TaggedResponse) && resp.name == "OK"
          begin
            # for backward compatibility
            certs = options.to_str
            options = create_ssl_params(certs, verify)
          rescue NoMethodError
          end
          start_tls_session(options)
        end
      end
    end

    # Sends an AUTHENTICATE command to authenticate the client.
    # The +auth_type+ parameter is a string that represents
    # the authentication mechanism to be used. Currently Net::IMAP
    # supports the authentication mechanisms:
    #
    #   LOGIN:: login using cleartext user and password.
    #   CRAM-MD5:: login with cleartext user and encrypted password
    #              (see [RFC-2195] for a full description).  This
    #              mechanism requires that the server have the user's
    #              password stored in clear-text password.
    #
    # For both of these mechanisms, there should be two +args+: username
    # and (cleartext) password.  A server may not support one or the other
    # of these mechanisms; check #capability() for a capability of
    # the form "AUTH=LOGIN" or "AUTH=CRAM-MD5".
    #
    # Authentication is done using the appropriate authenticator object:
    # see +add_authenticator+ for more information on plugging in your own
    # authenticator.
    #
    # For example:
    #
    #    imap.authenticate('LOGIN', user, password)
    #
    # A Net::IMAP::NoResponseError is raised if authentication fails.
    def authenticate(auth_type, *args)
      authenticator = self.class.authenticator(auth_type, *args)
      send_command("AUTHENTICATE", auth_type) do |resp|
        if resp.instance_of?(ContinuationRequest)
          data = authenticator.process(resp.data.text.unpack("m")[0])
          s = [data].pack("m0")
          send_string_data(s)
          put_string(CRLF)
        end
      end
    end

    # Sends a LOGIN command to identify the client and carries
    # the plaintext +password+ authenticating this +user+.  Note
    # that, unlike calling #authenticate() with an +auth_type+
    # of "LOGIN", #login() does *not* use the login authenticator.
    #
    # A Net::IMAP::NoResponseError is raised if authentication fails.
    def login(user, password)
      send_command("LOGIN", user, password)
    end

    # Sends a SELECT command to select a +mailbox+ so that messages
    # in the +mailbox+ can be accessed.
    #
    # After you have selected a mailbox, you may retrieve the
    # number of items in that mailbox from +@responses["EXISTS"][-1]+,
    # and the number of recent messages from +@responses["RECENT"][-1]+.
    # Note that these values can change if new messages arrive
    # during a session; see #add_response_handler() for a way of
    # detecting this event.
    #
    # A Net::IMAP::NoResponseError is raised if the mailbox does not
    # exist or is for some reason non-selectable.
    def select(mailbox)
      synchronize do
        @responses.clear
        send_command("SELECT", mailbox)
      end
    end

    # Sends a EXAMINE command to select a +mailbox+ so that messages
    # in the +mailbox+ can be accessed.  Behaves the same as #select(),
    # except that the selected +mailbox+ is identified as read-only.
    #
    # A Net::IMAP::NoResponseError is raised if the mailbox does not
    # exist or is for some reason non-examinable.
    def examine(mailbox)
      synchronize do
        @responses.clear
        send_command("EXAMINE", mailbox)
      end
    end

    # Sends a CREATE command to create a new +mailbox+.
    #
    # A Net::IMAP::NoResponseError is raised if a mailbox with that name
    # cannot be created.
    def create(mailbox)
      send_command("CREATE", mailbox)
    end

    # Sends a DELETE command to remove the +mailbox+.
    #
    # A Net::IMAP::NoResponseError is raised if a mailbox with that name
    # cannot be deleted, either because it does not exist or because the
    # client does not have permission to delete it.
    def delete(mailbox)
      send_command("DELETE", mailbox)
    end

    # Sends a RENAME command to change the name of the +mailbox+ to
    # +newname+.
    #
    # A Net::IMAP::NoResponseError is raised if a mailbox with the
    # name +mailbox+ cannot be renamed to +newname+ for whatever
    # reason; for instance, because +mailbox+ does not exist, or
    # because there is already a mailbox with the name +newname+.
    def rename(mailbox, newname)
      send_command("RENAME", mailbox, newname)
    end

    # Sends a SUBSCRIBE command to add the specified +mailbox+ name to
    # the server's set of "active" or "subscribed" mailboxes as returned
    # by #lsub().
    #
    # A Net::IMAP::NoResponseError is raised if +mailbox+ cannot be
    # subscribed to; for instance, because it does not exist.
    def subscribe(mailbox)
      send_command("SUBSCRIBE", mailbox)
    end

    # Sends a UNSUBSCRIBE command to remove the specified +mailbox+ name
    # from the server's set of "active" or "subscribed" mailboxes.
    #
    # A Net::IMAP::NoResponseError is raised if +mailbox+ cannot be
    # unsubscribed from; for instance, because the client is not currently
    # subscribed to it.
    def unsubscribe(mailbox)
      send_command("UNSUBSCRIBE", mailbox)
    end

    # Sends a LIST command, and returns a subset of names from
    # the complete set of all names available to the client.
    # +refname+ provides a context (for instance, a base directory
    # in a directory-based mailbox hierarchy).  +mailbox+ specifies
    # a mailbox or (via wildcards) mailboxes under that context.
    # Two wildcards may be used in +mailbox+: '*', which matches
    # all characters *including* the hierarchy delimiter (for instance,
    # '/' on a UNIX-hosted directory-based mailbox hierarchy); and '%',
    # which matches all characters *except* the hierarchy delimiter.
    #
    # If +refname+ is empty, +mailbox+ is used directly to determine
    # which mailboxes to match.  If +mailbox+ is empty, the root
    # name of +refname+ and the hierarchy delimiter are returned.
    #
    # The return value is an array of +Net::IMAP::MailboxList+. For example:
    #
    #   imap.create("foo/bar")
    #   imap.create("foo/baz")
    #   p imap.list("", "foo/%")
    #   #=> [#<Net::IMAP::MailboxList attr=[:Noselect], delim="/", name="foo/">, \\
    #        #<Net::IMAP::MailboxList attr=[:Noinferiors, :Marked], delim="/", name="foo/bar">, \\
    #        #<Net::IMAP::MailboxList attr=[:Noinferiors], delim="/", name="foo/baz">]
    def list(refname, mailbox)
      synchronize do
        send_command("LIST", refname, mailbox)
        return @responses.delete("LIST")
      end
    end

    # Sends a NAMESPACE command [RFC2342] and returns the namespaces that are
    # available. The NAMESPACE command allows a client to discover the prefixes
    # of namespaces used by a server for personal mailboxes, other users'
    # mailboxes, and shared mailboxes.
    #
    # This extension predates IMAP4rev1 (RFC3501), so most IMAP servers support
    # it. Many popular IMAP servers are configured with the default personal
    # namespaces as `("" "/")`: no prefix and "/" hierarchy delimiter. In that
    # common case, the naive client may not have any trouble naming mailboxes.
    #
    # But many servers are configured with the default personal namespace as
    # e.g. `("INBOX." ".")`, placing all personal folders under INBOX, with "."
    # as the hierarchy delimiter. If the client does not check for this, but
    # naively assumes it can use the same folder names for all servers, then
    # folder creation (and listing, moving, etc) can lead to errors.
    #
    # From RFC2342:
    #
    #    Although typically a server will support only a single Personal
    #    Namespace, and a single Other User's Namespace, circumstances exist
    #    where there MAY be multiples of these, and a client MUST be prepared
    #    for them. If a client is configured such that it is required to create
    #    a certain mailbox, there can be circumstances where it is unclear which
    #    Personal Namespaces it should create the mailbox in. In these
    #    situations a client SHOULD let the user select which namespaces to
    #    create the mailbox in.
    #
    # The user of this method should first check if the server supports the
    # NAMESPACE capability.  The return value is a +Net::IMAP::Namespaces+
    # object which has +personal+, +other+, and +shared+ fields, each an array
    # of +Net::IMAP::Namespace+ objects. These arrays will be empty when the
    # server responds with nil.
    #
    # For example:
    #
    #    capabilities = imap.capability
    #    if capabilities.include?("NAMESPACE")
    #      namespaces = imap.namespace
    #      if namespace = namespaces.personal.first
    #        prefix = namespace.prefix  # e.g. "" or "INBOX."
    #        delim  = namespace.delim   # e.g. "/" or "."
    #        # personal folders should use the prefix and delimiter
    #        imap.create(prefix + "foo")
    #        imap.create(prefix + "bar")
    #        imap.create(prefix + %w[path to my folder].join(delim))
    #      end
    #    end
    def namespace
      synchronize do
        send_command("NAMESPACE")
        return @responses.delete("NAMESPACE")[-1]
      end
    end

    # Sends a XLIST command, and returns a subset of names from
    # the complete set of all names available to the client.
    # +refname+ provides a context (for instance, a base directory
    # in a directory-based mailbox hierarchy).  +mailbox+ specifies
    # a mailbox or (via wildcards) mailboxes under that context.
    # Two wildcards may be used in +mailbox+: '*', which matches
    # all characters *including* the hierarchy delimiter (for instance,
    # '/' on a UNIX-hosted directory-based mailbox hierarchy); and '%',
    # which matches all characters *except* the hierarchy delimiter.
    #
    # If +refname+ is empty, +mailbox+ is used directly to determine
    # which mailboxes to match.  If +mailbox+ is empty, the root
    # name of +refname+ and the hierarchy delimiter are returned.
    #
    # The XLIST command is like the LIST command except that the flags
    # returned refer to the function of the folder/mailbox, e.g. :Sent
    #
    # The return value is an array of +Net::IMAP::MailboxList+. For example:
    #
    #   imap.create("foo/bar")
    #   imap.create("foo/baz")
    #   p imap.xlist("", "foo/%")
    #   #=> [#<Net::IMAP::MailboxList attr=[:Noselect], delim="/", name="foo/">, \\
    #        #<Net::IMAP::MailboxList attr=[:Noinferiors, :Marked], delim="/", name="foo/bar">, \\
    #        #<Net::IMAP::MailboxList attr=[:Noinferiors], delim="/", name="foo/baz">]
    def xlist(refname, mailbox)
      synchronize do
        send_command("XLIST", refname, mailbox)
        return @responses.delete("XLIST")
      end
    end

    # Sends the GETQUOTAROOT command along with the specified +mailbox+.
    # This command is generally available to both admin and user.
    # If this mailbox exists, it returns an array containing objects of type
    # Net::IMAP::MailboxQuotaRoot and Net::IMAP::MailboxQuota.
    def getquotaroot(mailbox)
      synchronize do
        send_command("GETQUOTAROOT", mailbox)
        result = []
        result.concat(@responses.delete("QUOTAROOT"))
        result.concat(@responses.delete("QUOTA"))
        return result
      end
    end

    # Sends the GETQUOTA command along with specified +mailbox+.
    # If this mailbox exists, then an array containing a
    # Net::IMAP::MailboxQuota object is returned.  This
    # command is generally only available to server admin.
    def getquota(mailbox)
      synchronize do
        send_command("GETQUOTA", mailbox)
        return @responses.delete("QUOTA")
      end
    end

    # Sends a SETQUOTA command along with the specified +mailbox+ and
    # +quota+.  If +quota+ is nil, then +quota+ will be unset for that
    # mailbox.  Typically one needs to be logged in as a server admin
    # for this to work.  The IMAP quota commands are described in
    # [RFC-2087].
    def setquota(mailbox, quota)
      if quota.nil?
        data = '()'
      else
        data = '(STORAGE ' + quota.to_s + ')'
      end
      send_command("SETQUOTA", mailbox, RawData.new(data))
    end

    # Sends the SETACL command along with +mailbox+, +user+ and the
    # +rights+ that user is to have on that mailbox.  If +rights+ is nil,
    # then that user will be stripped of any rights to that mailbox.
    # The IMAP ACL commands are described in [RFC-2086].
    def setacl(mailbox, user, rights)
      if rights.nil?
        send_command("SETACL", mailbox, user, "")
      else
        send_command("SETACL", mailbox, user, rights)
      end
    end

    # Send the GETACL command along with a specified +mailbox+.
    # If this mailbox exists, an array containing objects of
    # Net::IMAP::MailboxACLItem will be returned.
    def getacl(mailbox)
      synchronize do
        send_command("GETACL", mailbox)
        return @responses.delete("ACL")[-1]
      end
    end

    # Sends a LSUB command, and returns a subset of names from the set
    # of names that the user has declared as being "active" or
    # "subscribed."  +refname+ and +mailbox+ are interpreted as
    # for #list().
    # The return value is an array of +Net::IMAP::MailboxList+.
    def lsub(refname, mailbox)
      synchronize do
        send_command("LSUB", refname, mailbox)
        return @responses.delete("LSUB")
      end
    end

    # Sends a STATUS command, and returns the status of the indicated
    # +mailbox+. +attr+ is a list of one or more attributes whose
    # statuses are to be requested.  Supported attributes include:
    #
    #   MESSAGES:: the number of messages in the mailbox.
    #   RECENT:: the number of recent messages in the mailbox.
    #   UNSEEN:: the number of unseen messages in the mailbox.
    #
    # The return value is a hash of attributes. For example:
    #
    #   p imap.status("inbox", ["MESSAGES", "RECENT"])
    #   #=> {"RECENT"=>0, "MESSAGES"=>44}
    #
    # A Net::IMAP::NoResponseError is raised if status values
    # for +mailbox+ cannot be returned; for instance, because it
    # does not exist.
    def status(mailbox, attr)
      synchronize do
        send_command("STATUS", mailbox, attr)
        return @responses.delete("STATUS")[-1].attr
      end
    end

    # Sends a APPEND command to append the +message+ to the end of
    # the +mailbox+. The optional +flags+ argument is an array of
    # flags initially passed to the new message.  The optional
    # +date_time+ argument specifies the creation time to assign to the
    # new message; it defaults to the current time.
    # For example:
    #
    #   imap.append("inbox", <<EOF.gsub(/\n/, "\r\n"), [:Seen], Time.now)
    #   Subject: hello
    #   From: [email protected]
    #   To: [email protected]
    #
    #   hello world
    #   EOF
    #
    # A Net::IMAP::NoResponseError is raised if the mailbox does
    # not exist (it is not created automatically), or if the flags,
    # date_time, or message arguments contain errors.
    def append(mailbox, message, flags = nil, date_time = nil)
      args = []
      if flags
        args.push(flags)
      end
      args.push(date_time) if date_time
      args.push(Literal.new(message))
      send_command("APPEND", mailbox, *args)
    end

    # Sends a CHECK command to request a checkpoint of the currently
    # selected mailbox.  This performs implementation-specific
    # housekeeping; for instance, reconciling the mailbox's
    # in-memory and on-disk state.
    def check
      send_command("CHECK")
    end

    # Sends a CLOSE command to close the currently selected mailbox.
    # The CLOSE command permanently removes from the mailbox all
    # messages that have the \Deleted flag set.
    def close
      send_command("CLOSE")
    end

    # Sends a EXPUNGE command to permanently remove from the currently
    # selected mailbox all messages that have the \Deleted flag set.
    def expunge
      synchronize do
        send_command("EXPUNGE")
        return @responses.delete("EXPUNGE")
      end
    end

    # Sends a SEARCH command to search the mailbox for messages that
    # match the given searching criteria, and returns message sequence
    # numbers.  +keys+ can either be a string holding the entire
    # search string, or a single-dimension array of search keywords and
    # arguments.  The following are some common search criteria;
    # see [IMAP] section 6.4.4 for a full list.
    #
    # <message set>:: a set of message sequence numbers.  ',' indicates
    #                 an interval, ':' indicates a range.  For instance,
    #                 '2,10:12,15' means "2,10,11,12,15".
    #
    # BEFORE <date>:: messages with an internal date strictly before
    #                 <date>.  The date argument has a format similar
    #                 to 8-Aug-2002.
    #
    # BODY <string>:: messages that contain <string> within their body.
    #
    # CC <string>:: messages containing <string> in their CC field.
    #
    # FROM <string>:: messages that contain <string> in their FROM field.
    #
    # NEW:: messages with the \Recent, but not the \Seen, flag set.
    #
    # NOT <search-key>:: negate the following search key.
    #
    # OR <search-key> <search-key>:: "or" two search keys together.
    #
    # ON <date>:: messages with an internal date exactly equal to <date>,
    #             which has a format similar to 8-Aug-2002.
    #
    # SINCE <date>:: messages with an internal date on or after <date>.
    #
    # SUBJECT <string>:: messages with <string> in their subject.
    #
    # TO <string>:: messages with <string> in their TO field.
    #
    # For example:
    #
    #   p imap.search(["SUBJECT", "hello", "NOT", "NEW"])
    #   #=> [1, 6, 7, 8]
    def search(keys, charset = nil)
      return search_internal("SEARCH", keys, charset)
    end

    # Similar to #search(), but returns unique identifiers.
    def uid_search(keys, charset = nil)
      return search_internal("UID SEARCH", keys, charset)
    end

    # Sends a FETCH command to retrieve data associated with a message
    # in the mailbox.
    #
    # The +set+ parameter is a number or a range between two numbers,
    # or an array of those.  The number is a message sequence number,
    # where -1 represents a '*' for use in range notation like 100..-1
    # being interpreted as '100:*'.  Beware that the +exclude_end?+
    # property of a Range object is ignored, and the contents of a
    # range are independent of the order of the range endpoints as per
    # the protocol specification, so 1...5, 5..1 and 5...1 are all
    # equivalent to 1..5.
    #
    # +attr+ is a list of attributes to fetch; see the documentation
    # for Net::IMAP::FetchData for a list of valid attributes.
    #
    # The return value is an array of Net::IMAP::FetchData or nil
    # (instead of an empty array) if there is no matching message.
    #
    # For example:
    #
    #   p imap.fetch(6..8, "UID")
    #   #=> [#<Net::IMAP::FetchData seqno=6, attr={"UID"=>98}>, \\
    #        #<Net::IMAP::FetchData seqno=7, attr={"UID"=>99}>, \\
    #        #<Net::IMAP::FetchData seqno=8, attr={"UID"=>100}>]
    #   p imap.fetch(6, "BODY[HEADER.FIELDS (SUBJECT)]")
    #   #=> [#<Net::IMAP::FetchData seqno=6, attr={"BODY[HEADER.FIELDS (SUBJECT)]"=>"Subject: test\r\n\r\n"}>]
    #   data = imap.uid_fetch(98, ["RFC822.SIZE", "INTERNALDATE"])[0]
    #   p data.seqno
    #   #=> 6
    #   p data.attr["RFC822.SIZE"]
    #   #=> 611
    #   p data.attr["INTERNALDATE"]
    #   #=> "12-Oct-2000 22:40:59 +0900"
    #   p data.attr["UID"]
    #   #=> 98
    def fetch(set, attr, mod = nil)
      return fetch_internal("FETCH", set, attr, mod)
    end

    # Similar to #fetch(), but +set+ contains unique identifiers.
    def uid_fetch(set, attr, mod = nil)
      return fetch_internal("UID FETCH", set, attr, mod)
    end

    # Sends a STORE command to alter data associated with messages
    # in the mailbox, in particular their flags. The +set+ parameter
    # is a number, an array of numbers, or a Range object. Each number
    # is a message sequence number.  +attr+ is the name of a data item
    # to store: 'FLAGS' will replace the message's flag list
    # with the provided one, '+FLAGS' will add the provided flags,
    # and '-FLAGS' will remove them.  +flags+ is a list of flags.
    #
    # The return value is an array of Net::IMAP::FetchData. For example:
    #
    #   p imap.store(6..8, "+FLAGS", [:Deleted])
    #   #=> [#<Net::IMAP::FetchData seqno=6, attr={"FLAGS"=>[:Seen, :Deleted]}>, \\
    #        #<Net::IMAP::FetchData seqno=7, attr={"FLAGS"=>[:Seen, :Deleted]}>, \\
    #        #<Net::IMAP::FetchData seqno=8, attr={"FLAGS"=>[:Seen, :Deleted]}>]
    def store(set, attr, flags)
      return store_internal("STORE", set, attr, flags)
    end

    # Similar to #store(), but +set+ contains unique identifiers.
    def uid_store(set, attr, flags)
      return store_internal("UID STORE", set, attr, flags)
    end

    # Sends a COPY command to copy the specified message(s) to the end
    # of the specified destination +mailbox+. The +set+ parameter is
    # a number, an array of numbers, or a Range object. The number is
    # a message sequence number.
    def copy(set, mailbox)
      copy_internal("COPY", set, mailbox)
    end

    # Similar to #copy(), but +set+ contains unique identifiers.
    def uid_copy(set, mailbox)
      copy_internal("UID COPY", set, mailbox)
    end

    # Sends a MOVE command to move the specified message(s) to the end
    # of the specified destination +mailbox+. The +set+ parameter is
    # a number, an array of numbers, or a Range object. The number is
    # a message sequence number.
    # The IMAP MOVE extension is described in [RFC-6851].
    def move(set, mailbox)
      copy_internal("MOVE", set, mailbox)
    end

    # Similar to #move(), but +set+ contains unique identifiers.
    def uid_move(set, mailbox)
      copy_internal("UID MOVE", set, mailbox)
    end

    # Sends a SORT command to sort messages in the mailbox.
    # Returns an array of message sequence numbers. For example:
    #
    #   p imap.sort(["FROM"], ["ALL"], "US-ASCII")
    #   #=> [1, 2, 3, 5, 6, 7, 8, 4, 9]
    #   p imap.sort(["DATE"], ["SUBJECT", "hello"], "US-ASCII")
    #   #=> [6, 7, 8, 1]
    #
    # See [SORT-THREAD-EXT] for more details.
    def sort(sort_keys, search_keys, charset)
      return sort_internal("SORT", sort_keys, search_keys, charset)
    end

    # Similar to #sort(), but returns an array of unique identifiers.
    def uid_sort(sort_keys, search_keys, charset)
      return sort_internal("UID SORT", sort_keys, search_keys, charset)
    end

    # Adds a response handler. For example, to detect when
    # the server sends a new EXISTS response (which normally
    # indicates new messages being added to the mailbox),
    # add the following handler after selecting the
    # mailbox:
    #
    #   imap.add_response_handler { |resp|
    #     if resp.kind_of?(Net::IMAP::UntaggedResponse) and resp.name == "EXISTS"
    #       puts "Mailbox now has #{resp.data} messages"
    #     end
    #   }
    #
    def add_response_handler(handler = nil, &block)
      raise ArgumentError, "two Procs are passed" if handler && block
      @response_handlers.push(block || handler)
    end

    # Removes the response handler.
    def remove_response_handler(handler)
      @response_handlers.delete(handler)
    end

    # Similar to #search(), but returns message sequence numbers in threaded
    # format, as a Net::IMAP::ThreadMember tree.  The supported algorithms
    # are:
    #
    # ORDEREDSUBJECT:: split into single-level threads according to subject,
    #                  ordered by date.
    # REFERENCES:: split into threads by parent/child relationships determined
    #              by which message is a reply to which.
    #
    # Unlike #search(), +charset+ is a required argument.  US-ASCII
    # and UTF-8 are sample values.
    #
    # See [SORT-THREAD-EXT] for more details.
    def thread(algorithm, search_keys, charset)
      return thread_internal("THREAD", algorithm, search_keys, charset)
    end

    # Similar to #thread(), but returns unique identifiers instead of
    # message sequence numbers.
    def uid_thread(algorithm, search_keys, charset)
      return thread_internal("UID THREAD", algorithm, search_keys, charset)
    end

    # Sends an IDLE command that waits for notifications of new or expunged
    # messages.  Yields responses from the server during the IDLE.
    #
    # Use #idle_done() to leave IDLE.
    #
    # If +timeout+ is given, this method returns after +timeout+ seconds passed.
    # +timeout+ can be used for keep-alive.  For example, the following code
    # checks the connection for each 60 seconds.
    #
    #   loop do
    #     imap.idle(60) do |res|
    #       ...
    #     end
    #   end
    def idle(timeout = nil, &response_handler)
      raise LocalJumpError, "no block given" unless response_handler

      response = nil

      synchronize do
        tag = Thread.current[:net_imap_tag] = generate_tag
        put_string("#{tag} IDLE#{CRLF}")

        begin
          add_response_handler(&response_handler)
          @idle_done_cond = new_cond
          @idle_done_cond.wait(timeout)
          @idle_done_cond = nil
          if @receiver_thread_terminating
            raise @exception || Net::IMAP::Error.new("connection closed")
          end
        ensure
          unless @receiver_thread_terminating
            remove_response_handler(response_handler)
            put_string("DONE#{CRLF}")
            response = get_tagged_response(tag, "IDLE", @idle_response_timeout)
          end
        end
      end

      return response
    end

    # Leaves IDLE.
    def idle_done
      synchronize do
        if @idle_done_cond.nil?
          raise Net::IMAP::Error, "not during IDLE"
        end
        @idle_done_cond.signal
      end
    end

    # Decode a string from modified UTF-7 format to UTF-8.
    #
    # UTF-7 is a 7-bit encoding of Unicode [UTF7].  IMAP uses a
    # slightly modified version of this to encode mailbox names
    # containing non-ASCII characters; see [IMAP] section 5.1.3.
    #
    # Net::IMAP does _not_ automatically encode and decode
    # mailbox names to and from UTF-7.
    def self.decode_utf7(s)
      return s.gsub(/&([^-]+)?-/n) {
        if $1
          ($1.tr(",", "/") + "===").unpack1("m").encode(Encoding::UTF_8, Encoding::UTF_16BE)
        else
          "&"
        end
      }
    end

    # Encode a string from UTF-8 format to modified UTF-7.
    def self.encode_utf7(s)
      return s.gsub(/(&)|[^\x20-\x7e]+/) {
        if $1
          "&-"
        else
          base64 = [$&.encode(Encoding::UTF_16BE)].pack("m0")
          "&" + base64.delete("=").tr("/", ",") + "-"
        end
      }.force_encoding("ASCII-8BIT")
    end

    # Formats +time+ as an IMAP-style date.
    def self.format_date(time)
      return time.strftime('%d-%b-%Y')
    end

    # Formats +time+ as an IMAP-style date-time.
    def self.format_datetime(time)
      return time.strftime('%d-%b-%Y %H:%M %z')
    end

    private

    CRLF = "\r\n"      # :nodoc:
    PORT = 143         # :nodoc:
    SSL_PORT = 993   # :nodoc:

    @@debug = false
    @@max_flag_count = 10000

    # :call-seq:
    #    Net::IMAP.new(host, options = {})
    #
    # Creates a new Net::IMAP object and connects it to the specified
    # +host+.
    #
    # +options+ is an option hash, each key of which is a symbol.
    #
    # The available options are:
    #
    # port::  Port number (default value is 143 for imap, or 993 for imaps)
    # ssl::   If +options[:ssl]+ is true, then an attempt will be made
    #         to use SSL (now TLS) to connect to the server.  For this to work
    #         OpenSSL [OSSL] and the Ruby OpenSSL [RSSL] extensions need to
    #         be installed.
    #         If +options[:ssl]+ is a hash, it's passed to
    #         OpenSSL::SSL::SSLContext#set_params as parameters.
    # open_timeout:: Seconds to wait until a connection is opened
    # idle_response_timeout:: Seconds to wait until an IDLE response is received
    #
    # The most common errors are:
    #
    # Errno::ECONNREFUSED:: Connection refused by +host+ or an intervening
    #                       firewall.
    # Errno::ETIMEDOUT:: Connection timed out (possibly due to packets
    #                    being dropped by an intervening firewall).
    # Errno::ENETUNREACH:: There is no route to that network.
    # SocketError:: Hostname not known or other socket error.
    # Net::IMAP::ByeResponseError:: The connected to the host was successful, but
    #                               it immediately said goodbye.
    def initialize(host, port_or_options = {},
                   usessl = false, certs = nil, verify = true)
      super()
      @host = host
      begin
        options = port_or_options.to_hash
      rescue NoMethodError
        # for backward compatibility
        options = {}
        options[:port] = port_or_options
        if usessl
          options[:ssl] = create_ssl_params(certs, verify)
        end
      end
      @port = options[:port] || (options[:ssl] ? SSL_PORT : PORT)
      @tag_prefix = "RUBY"
      @tagno = 0
      @open_timeout = options[:open_timeout] || 30
      @idle_response_timeout = options[:idle_response_timeout] || 5
      @parser = ResponseParser.new
      @sock = tcp_socket(@host, @port)
      begin
        if options[:ssl]
          start_tls_session(options[:ssl])
          @usessl = true
        else
          @usessl = false
        end
        @responses = Hash.new([].freeze)
        @tagged_responses = {}
        @response_handlers = []
        @tagged_response_arrival = new_cond
        @continued_command_tag = nil
        @continuation_request_arrival = new_cond
        @continuation_request_exception = nil
        @idle_done_cond = nil
        @logout_command_tag = nil
        @debug_output_bol = true
        @exception = nil

        @greeting = get_response
        if @greeting.nil?
          raise Error, "connection closed"
        end
        if @greeting.name == "BYE"
          raise ByeResponseError, @greeting
        end

        @client_thread = Thread.current
        @receiver_thread = Thread.start {
          begin
            receive_responses
          rescue Exception
          end
        }
        @receiver_thread_terminating = false
      rescue Exception
        @sock.close
        raise
      end
    end

    def tcp_socket(host, port)
      s = Socket.tcp(host, port, :connect_timeout => @open_timeout)
      s.setsockopt(:SOL_SOCKET, :SO_KEEPALIVE, true)
      s
    rescue Errno::ETIMEDOUT
      raise Net::OpenTimeout, "Timeout to open TCP connection to " +
        "#{host}:#{port} (exceeds #{@open_timeout} seconds)"
    end

    def receive_responses
      connection_closed = false
      until connection_closed
        synchronize do
          @exception = nil
        end
        begin
          resp = get_response
        rescue Exception => e
          synchronize do
            @sock.close
            @exception = e
          end
          break
        end
        unless resp
          synchronize do
            @exception = EOFError.new("end of file reached")
          end
          break
        end
        begin
          synchronize do
            case resp
            when TaggedResponse
              @tagged_responses[resp.tag] = resp
              @tagged_response_arrival.broadcast
              case resp.tag
              when @logout_command_tag
                return
              when @continued_command_tag
                @continuation_request_exception =
                  RESPONSE_ERRORS[resp.name].new(resp)
                @continuation_request_arrival.signal
              end
            when UntaggedResponse
              record_response(resp.name, resp.data)
              if resp.data.instance_of?(ResponseText) &&
                  (code = resp.data.code)
                record_response(code.name, code.data)
              end
              if resp.name == "BYE" && @logout_command_tag.nil?
                @sock.close
                @exception = ByeResponseError.new(resp)
                connection_closed = true
              end
            when ContinuationRequest
              @continuation_request_arrival.signal
            end
            @response_handlers.each do |handler|
              handler.call(resp)
            end
          end
        rescue Exception => e
          @exception = e
          synchronize do
            @tagged_response_arrival.broadcast
            @continuation_request_arrival.broadcast
          end
        end
      end
      synchronize do
        @receiver_thread_terminating = true
        @tagged_response_arrival.broadcast
        @continuation_request_arrival.broadcast
        if @idle_done_cond
          @idle_done_cond.signal
        end
      end
    end

    def get_tagged_response(tag, cmd, timeout = nil)
      if timeout
        deadline = Time.now + timeout
      end
      until @tagged_responses.key?(tag)
        raise @exception if @exception
        if timeout
          timeout = deadline - Time.now
          if timeout <= 0
            return nil
          end
        end
        @tagged_response_arrival.wait(timeout)
      end
      resp = @tagged_responses.delete(tag)
      case resp.name
      when /\A(?:NO)\z/ni
        raise NoResponseError, resp
      when /\A(?:BAD)\z/ni
        raise BadResponseError, resp
      else
        return resp
      end
    end

    def get_response
      buff = String.new
      while true
        s = @sock.gets(CRLF)
        break unless s
        buff.concat(s)
        if /\{(\d+)\}\r\n/n =~ s
          s = @sock.read($1.to_i)
          buff.concat(s)
        else
          break
        end
      end
      return nil if buff.length == 0
      if @@debug
        $stderr.print(buff.gsub(/^/n, "S: "))
      end
      return @parser.parse(buff)
    end

    def record_response(name, data)
      unless @responses.has_key?(name)
        @responses[name] = []
      end
      @responses[name].push(data)
    end

    def send_command(cmd, *args, &block)
      synchronize do
        args.each do |i|
          validate_data(i)
        end
        tag = generate_tag
        put_string(tag + " " + cmd)
        args.each do |i|
          put_string(" ")
          send_data(i, tag)
        end
        put_string(CRLF)
        if cmd == "LOGOUT"
          @logout_command_tag = tag
        end
        if block
          add_response_handler(&block)
        end
        begin
          return get_tagged_response(tag, cmd)
        ensure
          if block
            remove_response_handler(block)
          end
        end
      end
    end

    def generate_tag
      @tagno += 1
      return format("%s%04d", @tag_prefix, @tagno)
    end

    def put_string(str)
      @sock.print(str)
      if @@debug
        if @debug_output_bol
          $stderr.print("C: ")
        end
        $stderr.print(str.gsub(/\n(?!\z)/n, "\nC: "))
        if /\r\n\z/n.match(str)
          @debug_output_bol = true
        else
          @debug_output_bol = false
        end
      end
    end

    def validate_data(data)
      case data
      when nil
      when String
      when Integer
        NumValidator.ensure_number(data)
      when Array
        if data[0] == 'CHANGEDSINCE'
          NumValidator.ensure_mod_sequence_value(data[1])
        else
          data.each do |i|
            validate_data(i)
          end
        end
      when Time
      when Symbol
      else
        data.validate
      end
    end

    def send_data(data, tag = nil)
      case data
      when nil
        put_string("NIL")
      when String
        send_string_data(data, tag)
      when Integer
        send_number_data(data)
      when Array
        send_list_data(data, tag)
      when Time
        send_time_data(data)
      when Symbol
        send_symbol_data(data)
      else
        data.send_data(self, tag)
      end
    end

    def send_string_data(str, tag = nil)
      case str
      when ""
        put_string('""')
      when /[\x80-\xff\r\n]/n
        # literal
        send_literal(str, tag)
      when /[(){ \x00-\x1f\x7f%*"\\]/n
        # quoted string
        send_quoted_string(str)
      else
        put_string(str)
      end
    end

    def send_quoted_string(str)
      put_string('"' + str.gsub(/["\\]/n, "\\\\\\&") + '"')
    end

    def send_literal(str, tag = nil)
      synchronize do
        put_string("{" + str.bytesize.to_s + "}" + CRLF)
        @continued_command_tag = tag
        @continuation_request_exception = nil
        begin
          @continuation_request_arrival.wait
          e = @continuation_request_exception || @exception
          raise e if e
          put_string(str)
        ensure
          @continued_command_tag = nil
          @continuation_request_exception = nil
        end
      end
    end

    def send_number_data(num)
      put_string(num.to_s)
    end

    def send_list_data(list, tag = nil)
      put_string("(")
      first = true
      list.each do |i|
        if first
          first = false
        else
          put_string(" ")
        end
        send_data(i, tag)
      end
      put_string(")")
    end

    DATE_MONTH = %w(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec)

    def send_time_data(time)
      t = time.dup.gmtime
      s = format('"%2d-%3s-%4d %02d:%02d:%02d +0000"',
                 t.day, DATE_MONTH[t.month - 1], t.year,
                 t.hour, t.min, t.sec)
      put_string(s)
    end

    def send_symbol_data(symbol)
      put_string("\\" + symbol.to_s)
    end

    def search_internal(cmd, keys, charset)
      if keys.instance_of?(String)
        keys = [RawData.new(keys)]
      else
        normalize_searching_criteria(keys)
      end
      synchronize do
        if charset
          send_command(cmd, "CHARSET", charset, *keys)
        else
          send_command(cmd, *keys)
        end
        return @responses.delete("SEARCH")[-1]
      end
    end

    def fetch_internal(cmd, set, attr, mod = nil)
      case attr
      when String then
        attr = RawData.new(attr)
      when Array then
        attr = attr.map { |arg|
          arg.is_a?(String) ? RawData.new(arg) : arg
        }
      end

      synchronize do
        @responses.delete("FETCH")
        if mod
          send_command(cmd, MessageSet.new(set), attr, mod)
        else
          send_command(cmd, MessageSet.new(set), attr)
        end
        return @responses.delete("FETCH")
      end
    end

    def store_internal(cmd, set, attr, flags)
      if attr.instance_of?(String)
        attr = RawData.new(attr)
      end
      synchronize do
        @responses.delete("FETCH")
        send_command(cmd, MessageSet.new(set), attr, flags)
        return @responses.delete("FETCH")
      end
    end

    def copy_internal(cmd, set, mailbox)
      send_command(cmd, MessageSet.new(set), mailbox)
    end

    def sort_internal(cmd, sort_keys, search_keys, charset)
      if search_keys.instance_of?(String)
        search_keys = [RawData.new(search_keys)]
      else
        normalize_searching_criteria(search_keys)
      end
      normalize_searching_criteria(search_keys)
      synchronize do
        send_command(cmd, sort_keys, charset, *search_keys)
        return @responses.delete("SORT")[-1]
      end
    end

    def thread_internal(cmd, algorithm, search_keys, charset)
      if search_keys.instance_of?(String)
        search_keys = [RawData.new(search_keys)]
      else
        normalize_searching_criteria(search_keys)
      end
      normalize_searching_criteria(search_keys)
      send_command(cmd, algorithm, charset, *search_keys)
      return @responses.delete("THREAD")[-1]
    end

    def normalize_searching_criteria(keys)
      keys.collect! do |i|
        case i
        when -1, Range, Array
          MessageSet.new(i)
        else
          i
        end
      end
    end

    def create_ssl_params(certs = nil, verify = true)
      params = {}
      if certs
        if File.file?(certs)
          params[:ca_file] = certs
        elsif File.directory?(certs)
          params[:ca_path] = certs
        end
      end
      if verify
        params[:verify_mode] = VERIFY_PEER
      else
        params[:verify_mode] = VERIFY_NONE
      end
      return params
    end

    def start_tls_session(params = {})
      unless defined?(OpenSSL::SSL)
        raise "SSL extension not installed"
      end
      if @sock.kind_of?(OpenSSL::SSL::SSLSocket)
        raise RuntimeError, "already using SSL"
      end
      begin
        params = params.to_hash
      rescue NoMethodError
        params = {}
      end
      context = SSLContext.new
      context.set_params(params)
      if defined?(VerifyCallbackProc)
        context.verify_callback = VerifyCallbackProc
      end
      @sock = SSLSocket.new(@sock, context)
      @sock.sync_close = true
      @sock.hostname = @host if @sock.respond_to? :hostname=
      ssl_socket_connect(@sock, @open_timeout)
      if context.verify_mode != VERIFY_NONE
        @sock.post_connection_check(@host)
      end
    end

    class RawData # :nodoc:
      def send_data(imap, tag)
        imap.__send__(:put_string, @data)
      end

      def validate
      end

      private

      def initialize(data)
        @data = data
      end
    end

    class Atom # :nodoc:
      def send_data(imap, tag)
        imap.__send__(:put_string, @data)
      end

      def validate
      end

      private

      def initialize(data)
        @data = data
      end
    end

    class QuotedString # :nodoc:
      def send_data(imap, tag)
        imap.__send__(:send_quoted_string, @data)
      end

      def validate
      end

      private

      def initialize(data)
        @data = data
      end
    end

    class Literal # :nodoc:
      def send_data(imap, tag)
        imap.__send__(:send_literal, @data, tag)
      end

      def validate
      end

      private

      def initialize(data)
        @data = data
      end
    end

    class MessageSet # :nodoc:
      def send_data(imap, tag)
        imap.__send__(:put_string, format_internal(@data))
      end

      def validate
        validate_internal(@data)
      end

      private

      def initialize(data)
        @data = data
      end

      def format_internal(data)
        case data
        when "*"
          return data
        when Integer
          if data == -1
            return "*"
          else
            return data.to_s
          end
        when Range
          return format_internal(data.first) +
            ":" + format_internal(data.last)
        when Array
          return data.collect {|i| format_internal(i)}.join(",")
        when ThreadMember
          return data.seqno.to_s +
            ":" + data.children.collect {|i| format_internal(i).join(",")}
        end
      end

      def validate_internal(data)
        case data
        when "*"
        when Integer
          NumValidator.ensure_nz_number(data)
        when Range
        when Array
          data.each do |i|
            validate_internal(i)
          end
        when ThreadMember
          data.children.each do |i|
            validate_internal(i)
          end
        else
          raise DataFormatError, data.inspect
        end
      end
    end

    class ClientID # :nodoc:

      def send_data(imap, tag)
        imap.__send__(:send_data, format_internal(@data), tag)
      end

      def validate
        validate_internal(@data)
      end

      private

      def initialize(data)
        @data = data
      end

      def validate_internal(client_id)
        client_id.to_h.each do |k,v|
          unless StringFormatter.valid_string?(k)
            raise DataFormatError, client_id.inspect
          end
        end
      rescue NoMethodError, TypeError # to_h failed
        raise DataFormatError, client_id.inspect
      end

      def format_internal(client_id)
        return nil if client_id.nil?
        client_id.to_h.flat_map {|k,v|
          [StringFormatter.string(k), StringFormatter.nstring(v)]
        }
      end

    end

    module StringFormatter

      LITERAL_REGEX = /[\x80-\xff\r\n]/n

      module_function

      # Allows symbols in addition to strings
      def valid_string?(str)
        str.is_a?(Symbol) || str.respond_to?(:to_str)
      end

      # Allows nil, symbols, and strings
      def valid_nstring?(str)
        str.nil? || valid_string?(str)
      end

      # coerces using +to_s+
      def string(str)
        str = str.to_s
        if str =~ LITERAL_REGEX
          Literal.new(str)
        else
          QuotedString.new(str)
        end
      end

      # coerces non-nil using +to_s+
      def nstring(str)
        str.nil? ? nil : string(str)
      end

    end

    # Common validators of number and nz_number types
    module NumValidator # :nodoc
      class << self
        # Check is passed argument valid 'number' in RFC 3501 terminology
        def valid_number?(num)
          # [RFC 3501]
          # number          = 1*DIGIT
          #                    ; Unsigned 32-bit integer
          #                    ; (0 <= n < 4,294,967,296)
          num >= 0 && num < 4294967296
        end

        # Check is passed argument valid 'nz_number' in RFC 3501 terminology
        def valid_nz_number?(num)
          # [RFC 3501]
          # nz-number       = digit-nz *DIGIT
          #                    ; Non-zero unsigned 32-bit integer
          #                    ; (0 < n < 4,294,967,296)
          num != 0 && valid_number?(num)
        end

        # Check is passed argument valid 'mod_sequence_value' in RFC 4551 terminology
        def valid_mod_sequence_value?(num)
          # mod-sequence-value  = 1*DIGIT
          #                        ; Positive unsigned 64-bit integer
          #                        ; (mod-sequence)
          #                        ; (1 <= n < 18,446,744,073,709,551,615)
          num >= 1 && num < 18446744073709551615
        end

        # Ensure argument is 'number' or raise DataFormatError
        def ensure_number(num)
          return if valid_number?(num)

          msg = "number must be unsigned 32-bit integer: #{num}"
          raise DataFormatError, msg
        end

        # Ensure argument is 'nz_number' or raise DataFormatError
        def ensure_nz_number(num)
          return if valid_nz_number?(num)

          msg = "nz_number must be non-zero unsigned 32-bit integer: #{num}"
          raise DataFormatError, msg
        end

        # Ensure argument is 'mod_sequence_value' or raise DataFormatError
        def ensure_mod_sequence_value(num)
          return if valid_mod_sequence_value?(num)

          msg = "mod_sequence_value must be unsigned 64-bit integer: #{num}"
          raise DataFormatError, msg
        end
      end
    end

    # Net::IMAP::ContinuationRequest represents command continuation requests.
    #
    # The command continuation request response is indicated by a "+" token
    # instead of a tag.  This form of response indicates that the server is
    # ready to accept the continuation of a command from the client.  The
    # remainder of this response is a line of text.
    #
    #   continue_req    ::= "+" SPACE (resp_text / base64)
    #
    # ==== Fields:
    #
    # data:: Returns the data (Net::IMAP::ResponseText).
    #
    # raw_data:: Returns the raw data string.
    ContinuationRequest = Struct.new(:data, :raw_data)

    # Net::IMAP::UntaggedResponse represents untagged responses.
    #
    # Data transmitted by the server to the client and status responses
    # that do not indicate command completion are prefixed with the token
    # "*", and are called untagged responses.
    #
    #   response_data   ::= "*" SPACE (resp_cond_state / resp_cond_bye /
    #                       mailbox_data / message_data / capability_data)
    #
    # ==== Fields:
    #
    # name:: Returns the name, such as "FLAGS", "LIST", or "FETCH".
    #
    # data:: Returns the data such as an array of flag symbols,
    #        a ((<Net::IMAP::MailboxList>)) object.
    #
    # raw_data:: Returns the raw data string.
    UntaggedResponse = Struct.new(:name, :data, :raw_data)

    # Net::IMAP::IgnoredResponse represents intentionally ignored responses.
    #
    # This includes untagged response "NOOP" sent by eg. Zimbra to avoid some
    # clients to close the connection.
    #
    # It matches no IMAP standard.
    #
    # ==== Fields:
    #
    # raw_data:: Returns the raw data string.
    IgnoredResponse = Struct.new(:raw_data)

    # Net::IMAP::TaggedResponse represents tagged responses.
    #
    # The server completion result response indicates the success or
    # failure of the operation.  It is tagged with the same tag as the
    # client command which began the operation.
    #
    #   response_tagged ::= tag SPACE resp_cond_state CRLF
    #
    #   tag             ::= 1*<any ATOM_CHAR except "+">
    #
    #   resp_cond_state ::= ("OK" / "NO" / "BAD") SPACE resp_text
    #
    # ==== Fields:
    #
    # tag:: Returns the tag.
    #
    # name:: Returns the name, one of "OK", "NO", or "BAD".
    #
    # data:: Returns the data. See ((<Net::IMAP::ResponseText>)).
    #
    # raw_data:: Returns the raw data string.
    #
    TaggedResponse = Struct.new(:tag, :name, :data, :raw_data)

    # Net::IMAP::ResponseText represents texts of responses.
    # The text may be prefixed by the response code.
    #
    #   resp_text       ::= ["[" resp-text-code "]" SP] text
    #
    # ==== Fields:
    #
    # code:: Returns the response code. See ((<Net::IMAP::ResponseCode>)).
    #
    # text:: Returns the text.
    #
    ResponseText = Struct.new(:code, :text)

    # Net::IMAP::ResponseCode represents response codes.
    #
    #   resp_text_code  ::= "ALERT" /
    #                       "BADCHARSET" [SP "(" astring *(SP astring) ")" ] /
    #                       capability_data / "PARSE" /
    #                       "PERMANENTFLAGS" SP "("
    #                       [flag_perm *(SP flag_perm)] ")" /
    #                       "READ-ONLY" / "READ-WRITE" / "TRYCREATE" /
    #                       "UIDNEXT" SP nz_number / "UIDVALIDITY" SP nz_number /
    #                       "UNSEEN" SP nz_number /
    #                       atom [SP 1*<any TEXT-CHAR except "]">]
    #
    # ==== Fields:
    #
    # name:: Returns the name, such as "ALERT", "PERMANENTFLAGS", or "UIDVALIDITY".
    #
    # data:: Returns the data, if it exists.
    #
    ResponseCode = Struct.new(:name, :data)

    # Net::IMAP::MailboxList represents contents of the LIST response.
    #
    #   mailbox_list    ::= "(" #("\Marked" / "\Noinferiors" /
    #                       "\Noselect" / "\Unmarked" / flag_extension) ")"
    #                       SPACE (<"> QUOTED_CHAR <"> / nil) SPACE mailbox
    #
    # ==== Fields:
    #
    # attr:: Returns the name attributes. Each name attribute is a symbol
    #        capitalized by String#capitalize, such as :Noselect (not :NoSelect).
    #
    # delim:: Returns the hierarchy delimiter.
    #
    # name:: Returns the mailbox name.
    #
    MailboxList = Struct.new(:attr, :delim, :name)

    # Net::IMAP::MailboxQuota represents contents of GETQUOTA response.
    # This object can also be a response to GETQUOTAROOT.  In the syntax
    # specification below, the delimiter used with the "#" construct is a
    # single space (SPACE).
    #
    #    quota_list      ::= "(" #quota_resource ")"
    #
    #    quota_resource  ::= atom SPACE number SPACE number
    #
    #    quota_response  ::= "QUOTA" SPACE astring SPACE quota_list
    #
    # ==== Fields:
    #
    # mailbox:: The mailbox with the associated quota.
    #
    # usage:: Current storage usage of the mailbox.
    #
    # quota:: Quota limit imposed on the mailbox.
    #
    MailboxQuota = Struct.new(:mailbox, :usage, :quota)

    # Net::IMAP::MailboxQuotaRoot represents part of the GETQUOTAROOT
    # response. (GETQUOTAROOT can also return Net::IMAP::MailboxQuota.)
    #
    #    quotaroot_response ::= "QUOTAROOT" SPACE astring *(SPACE astring)
    #
    # ==== Fields:
    #
    # mailbox:: The mailbox with the associated quota.
    #
    # quotaroots:: Zero or more quotaroots that affect the quota on the
    #              specified mailbox.
    #
    MailboxQuotaRoot = Struct.new(:mailbox, :quotaroots)

    # Net::IMAP::MailboxACLItem represents the response from GETACL.
    #
    #    acl_data        ::= "ACL" SPACE mailbox *(SPACE identifier SPACE rights)
    #
    #    identifier      ::= astring
    #
    #    rights          ::= astring
    #
    # ==== Fields:
    #
    # user:: Login name that has certain rights to the mailbox
    #        that was specified with the getacl command.
    #
    # rights:: The access rights the indicated user has to the
    #          mailbox.
    #
    MailboxACLItem = Struct.new(:user, :rights, :mailbox)

    # Net::IMAP::Namespace represents a single [RFC-2342] namespace.
    #
    #    Namespace = nil / "(" 1*( "(" string SP  (<"> QUOTED_CHAR <"> /
    #       nil) *(Namespace_Response_Extension) ")" ) ")"
    #
    #    Namespace_Response_Extension = SP string SP "(" string *(SP string)
    #       ")"
    #
    # ==== Fields:
    #
    # prefix:: Returns the namespace prefix string.
    # delim:: Returns nil or the hierarchy delimiter character.
    # extensions:: Returns a hash of extension names to extension flag arrays.
    #
    Namespace = Struct.new(:prefix, :delim, :extensions)

    # Net::IMAP::Namespaces represents the response from [RFC-2342] NAMESPACE.
    #
    #    Namespace_Response = "*" SP "NAMESPACE" SP Namespace SP Namespace SP
    #       Namespace
    #
    #       ; The first Namespace is the Personal Namespace(s)
    #       ; The second Namespace is the Other Users' Namespace(s)
    #       ; The third Namespace is the Shared Namespace(s)
    #
    # ==== Fields:
    #
    # personal:: Returns an array of Personal Net::IMAP::Namespace objects.
    # other:: Returns an array of Other Users' Net::IMAP::Namespace objects.
    # shared:: Returns an array of Shared Net::IMAP::Namespace objects.
    #
    Namespaces = Struct.new(:personal, :other, :shared)

    # Net::IMAP::StatusData represents the contents of the STATUS response.
    #
    # ==== Fields:
    #
    # mailbox:: Returns the mailbox name.
    #
    # attr:: Returns a hash. Each key is one of "MESSAGES", "RECENT", "UIDNEXT",
    #        "UIDVALIDITY", "UNSEEN". Each value is a number.
    #
    StatusData = Struct.new(:mailbox, :attr)

    # Net::IMAP::FetchData represents the contents of the FETCH response.
    #
    # ==== Fields:
    #
    # seqno:: Returns the message sequence number.
    #         (Note: not the unique identifier, even for the UID command response.)
    #
    # attr:: Returns a hash. Each key is a data item name, and each value is
    #        its value.
    #
    #        The current data items are:
    #
    #        [BODY]
    #           A form of BODYSTRUCTURE without extension data.
    #        [BODY[<section>]<<origin_octet>>]
    #           A string expressing the body contents of the specified section.
    #        [BODYSTRUCTURE]
    #           An object that describes the [MIME-IMB] body structure of a message.
    #           See Net::IMAP::BodyTypeBasic, Net::IMAP::BodyTypeText,
    #           Net::IMAP::BodyTypeMessage, Net::IMAP::BodyTypeMultipart.
    #        [ENVELOPE]
    #           A Net::IMAP::Envelope object that describes the envelope
    #           structure of a message.
    #        [FLAGS]
    #           A array of flag symbols that are set for this message. Flag symbols
    #           are capitalized by String#capitalize.
    #        [INTERNALDATE]
    #           A string representing the internal date of the message.
    #        [RFC822]
    #           Equivalent to +BODY[]+.
    #        [RFC822.HEADER]
    #           Equivalent to +BODY.PEEK[HEADER]+.
    #        [RFC822.SIZE]
    #           A number expressing the [RFC-822] size of the message.
    #        [RFC822.TEXT]
    #           Equivalent to +BODY[TEXT]+.
    #        [UID]
    #           A number expressing the unique identifier of the message.
    #
    FetchData = Struct.new(:seqno, :attr)

    # Net::IMAP::Envelope represents envelope structures of messages.
    #
    # ==== Fields:
    #
    # date:: Returns a string that represents the date.
    #
    # subject:: Returns a string that represents the subject.
    #
    # from:: Returns an array of Net::IMAP::Address that represents the from.
    #
    # sender:: Returns an array of Net::IMAP::Address that represents the sender.
    #
    # reply_to:: Returns an array of Net::IMAP::Address that represents the reply-to.
    #
    # to:: Returns an array of Net::IMAP::Address that represents the to.
    #
    # cc:: Returns an array of Net::IMAP::Address that represents the cc.
    #
    # bcc:: Returns an array of Net::IMAP::Address that represents the bcc.
    #
    # in_reply_to:: Returns a string that represents the in-reply-to.
    #
    # message_id:: Returns a string that represents the message-id.
    #
    Envelope = Struct.new(:date, :subject, :from, :sender, :reply_to,
                          :to, :cc, :bcc, :in_reply_to, :message_id)

    #
    # Net::IMAP::Address represents electronic mail addresses.
    #
    # ==== Fields:
    #
    # name:: Returns the phrase from [RFC-822] mailbox.
    #
    # route:: Returns the route from [RFC-822] route-addr.
    #
    # mailbox:: nil indicates end of [RFC-822] group.
    #           If non-nil and host is nil, returns [RFC-822] group name.
    #           Otherwise, returns [RFC-822] local-part.
    #
    # host:: nil indicates [RFC-822] group syntax.
    #        Otherwise, returns [RFC-822] domain name.
    #
    Address = Struct.new(:name, :route, :mailbox, :host)

    #
    # Net::IMAP::ContentDisposition represents Content-Disposition fields.
    #
    # ==== Fields:
    #
    # dsp_type:: Returns the disposition type.
    #
    # param:: Returns a hash that represents parameters of the Content-Disposition
    #         field.
    #
    ContentDisposition = Struct.new(:dsp_type, :param)

    # Net::IMAP::ThreadMember represents a thread-node returned
    # by Net::IMAP#thread.
    #
    # ==== Fields:
    #
    # seqno:: The sequence number of this message.
    #
    # children:: An array of Net::IMAP::ThreadMember objects for mail
    #            items that are children of this in the thread.
    #
    ThreadMember = Struct.new(:seqno, :children)

    # Net::IMAP::BodyTypeBasic represents basic body structures of messages.
    #
    # ==== Fields:
    #
    # media_type:: Returns the content media type name as defined in [MIME-IMB].
    #
    # subtype:: Returns the content subtype name as defined in [MIME-IMB].
    #
    # param:: Returns a hash that represents parameters as defined in [MIME-IMB].
    #
    # content_id:: Returns a string giving the content id as defined in [MIME-IMB].
    #
    # description:: Returns a string giving the content description as defined in
    #               [MIME-IMB].
    #
    # encoding:: Returns a string giving the content transfer encoding as defined in
    #            [MIME-IMB].
    #
    # size:: Returns a number giving the size of the body in octets.
    #
    # md5:: Returns a string giving the body MD5 value as defined in [MD5].
    #
    # disposition:: Returns a Net::IMAP::ContentDisposition object giving
    #               the content disposition.
    #
    # language:: Returns a string or an array of strings giving the body
    #            language value as defined in [LANGUAGE-TAGS].
    #
    # extension:: Returns extension data.
    #
    # multipart?:: Returns false.
    #
    class BodyTypeBasic < Struct.new(:media_type, :subtype,
                                     :param, :content_id,
                                     :description, :encoding, :size,
                                     :md5, :disposition, :language,
                                     :extension)
      def multipart?
        return false
      end

      # Obsolete: use +subtype+ instead.  Calling this will
      # generate a warning message to +stderr+, then return
      # the value of +subtype+.
      def media_subtype
        warn("media_subtype is obsolete, use subtype instead.\n", uplevel: 1)
        return subtype
      end
    end

    # Net::IMAP::BodyTypeText represents TEXT body structures of messages.
    #
    # ==== Fields:
    #
    # lines:: Returns the size of the body in text lines.
    #
    # And Net::IMAP::BodyTypeText has all fields of Net::IMAP::BodyTypeBasic.
    #
    class BodyTypeText < Struct.new(:media_type, :subtype,
                                    :param, :content_id,
                                    :description, :encoding, :size,
                                    :lines,
                                    :md5, :disposition, :language,
                                    :extension)
      def multipart?
        return false
      end

      # Obsolete: use +subtype+ instead.  Calling this will
      # generate a warning message to +stderr+, then return
      # the value of +subtype+.
      def media_subtype
        warn("media_subtype is obsolete, use subtype instead.\n", uplevel: 1)
        return subtype
      end
    end

    # Net::IMAP::BodyTypeMessage represents MESSAGE/RFC822 body structures of messages.
    #
    # ==== Fields:
    #
    # envelope:: Returns a Net::IMAP::Envelope giving the envelope structure.
    #
    # body:: Returns an object giving the body structure.
    #
    # And Net::IMAP::BodyTypeMessage has all methods of Net::IMAP::BodyTypeText.
    #
    class BodyTypeMessage < Struct.new(:media_type, :subtype,
                                       :param, :content_id,
                                       :description, :encoding, :size,
                                       :envelope, :body, :lines,
                                       :md5, :disposition, :language,
                                       :extension)
      def multipart?
        return false
      end

      # Obsolete: use +subtype+ instead.  Calling this will
      # generate a warning message to +stderr+, then return
      # the value of +subtype+.
      def media_subtype
        warn("media_subtype is obsolete, use subtype instead.\n", uplevel: 1)
        return subtype
      end
    end

    # Net::IMAP::BodyTypeAttachment represents attachment body structures
    # of messages.
    #
    # ==== Fields:
    #
    # media_type:: Returns the content media type name.
    #
    # subtype:: Returns +nil+.
    #
    # param:: Returns a hash that represents parameters.
    #
    # multipart?:: Returns false.
    #
    class BodyTypeAttachment < Struct.new(:media_type, :subtype,
                                          :param)
      def multipart?
        return false
      end
    end

    # Net::IMAP::BodyTypeMultipart represents multipart body structures
    # of messages.
    #
    # ==== Fields:
    #
    # media_type:: Returns the content media type name as defined in [MIME-IMB].
    #
    # subtype:: Returns the content subtype name as defined in [MIME-IMB].
    #
    # parts:: Returns multiple parts.
    #
    # param:: Returns a hash that represents parameters as defined in [MIME-IMB].
    #
    # disposition:: Returns a Net::IMAP::ContentDisposition object giving
    #               the content disposition.
    #
    # language:: Returns a string or an array of strings giving the body
    #            language value as defined in [LANGUAGE-TAGS].
    #
    # extension:: Returns extension data.
    #
    # multipart?:: Returns true.
    #
    class BodyTypeMultipart < Struct.new(:media_type, :subtype,
                                         :parts,
                                         :param, :disposition, :language,
                                         :extension)
      def multipart?
        return true
      end

      # Obsolete: use +subtype+ instead.  Calling this will
      # generate a warning message to +stderr+, then return
      # the value of +subtype+.
      def media_subtype
        warn("media_subtype is obsolete, use subtype instead.\n", uplevel: 1)
        return subtype
      end
    end

    class BodyTypeExtension < Struct.new(:media_type, :subtype,
                                         :params, :content_id,
                                         :description, :encoding, :size)
      def multipart?
        return false
      end
    end

    # Superclass of IMAP errors.
    class Error < StandardError
    end

    # Error raised when data is in the incorrect format.
    class DataFormatError < Error
    end

    # Error raised when a response from the server is non-parseable.
    class ResponseParseError < Error
    end

    # Superclass of all errors used to encapsulate "fail" responses
    # from the server.
    class ResponseError < Error

      # The response that caused this error
      attr_accessor :response

      def initialize(response)
        @response = response

        super @response.data.text
      end

    end

    # Error raised upon a "NO" response from the server, indicating
    # that the client command could not be completed successfully.
    class NoResponseError < ResponseError
    end

    # Error raised upon a "BAD" response from the server, indicating
    # that the client command violated the IMAP protocol, or an internal
    # server failure has occurred.
    class BadResponseError < ResponseError
    end

    # Error raised upon a "BYE" response from the server, indicating
    # that the client is not being allowed to login, or has been timed
    # out due to inactivity.
    class ByeResponseError < ResponseError
    end

    RESPONSE_ERRORS = Hash.new(ResponseError)
    RESPONSE_ERRORS["NO"] = NoResponseError
    RESPONSE_ERRORS["BAD"] = BadResponseError

    # Error raised when too many flags are interned to symbols.
    class FlagCountError < Error
    end
  end
end

require_relative "imap/authenticators"