#
# = net/imap.rb
#
# Copyright (C) 2000  Shugo Maeda <shugo@ruby-lang.org>
#
# 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. 
#


require "socket"
require "monitor"
require "digest/md5"
begin
  require "openssl"
rescue LoadError
end

module Net # :nodoc:

  #
  # 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 a 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 mail box 
  # from 1 up to the number of items in the mail box.  If new
  # message arrives during a session, it receives a sequence
  # number equal to the new size of the mail box.  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 cannot thus
  # 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-SAFENESS
  #
  # 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 exists; 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 our 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
    include MonitorMixin
    if defined?(OpenSSL)
      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

    # 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

    # Adds an authenticator for Net::IMAP#authenticate.  +auth_type+
    # is the type of authentication this authenticator supports
    # (for instance, "LOGIN").  The +authenticator+ is an object
    # which defines a process() method to handle authentication with
    # the server.  See Net::IMAP::LoginAuthenticator and 
    # Net::IMAP::CramMD5Authenticator for examples.
    #
    # If +auth_type+ refers to an existing authenticator, it will be
    # replaced by the new one.
    def self.add_authenticator(auth_type, authenticator)
      @@authenticators[auth_type] = authenticator
    end

    # Disconnects from the server.
    def disconnect
      @sock.shutdown unless @usessl
      @receiver_thread.join
      @sock.close
    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 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 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 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 these mechanisms, there should be two +args+: username
    # and (cleartext) password.  A server may not support one or 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 @@authenticators 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)
      auth_type = auth_type.upcase
      unless @@authenticators.has_key?(auth_type)
	raise ArgumentError,
	  format('unknown auth type - "%s"', auth_type)
      end
      authenticator = @@authenticators[auth_type].new(*args)
      send_command("AUTHENTICATE", auth_type) do |resp|
	if resp.instance_of?(ContinuationRequest)
	  data = authenticator.process(resp.data.text.unpack("m")[0])
	  send_data([data].pack("m").gsub(/\n/, ""))
	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 wilcards) 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 the GETQUOTAROOT command along with specified +mailbox+.
    # This command is generally available to both admin and user.
    # If mailbox exists, returns an array containing objects of
    # 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 generally is 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 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 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 that
    # we are request the status of.  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 to initially assing 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: shugo@ruby-lang.org
    #   To: shugo@ruby-lang.org
    #   
    #   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

    # As for #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 an array of
    # numbers or a Range object. The number is a message sequence
    # number.  +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. 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)
      return fetch_internal("FETCH", set, attr)
    end

    # As for #fetch(), but +set+ contains unique identifiers.
    def uid_fetch(set, attr)
      return fetch_internal("UID FETCH", set, attr)
    end

    # Sends a STORE command to alter data associated with messages
    # in the mailbox, in particular their flags. The +set+ parameter 
    # is a number or 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' means to replace the message's flag list
    # with the provided one; '+FLAGS' means to add the provided flags;
    # and '-FLAGS' means to 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

    # As for #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 or 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

    # As for #copy(), but +set+ contains unique identifiers.
    def uid_copy(set, mailbox)
      copy_internal("UID COPY", 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

    # As for #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 us a new EXISTS response (which normally
    # indicates new messages being added to the mail box), 
    # you could 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 = Proc.new)
      @response_handlers.push(handler)
    end

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

    # As for #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

    # As for #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

    # 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 utf7.
    def self.decode_utf7(s)
      return s.gsub(/&(.*?)-/n) {
	if $1.empty?
	  "&"
	else
	  base64 = $1.tr(",", "/")
	  x = base64.length % 4
	  if x > 0
	    base64.concat("=" * (4 - x))
	  end
	  u16tou8(base64.unpack("m")[0])
	end
      }
    end

    # Encode a string from UTF-8 format to modified UTF-7.
    def self.encode_utf7(s)
      return s.gsub(/(&)|([^\x20-\x25\x27-\x7e]+)/n) { |x|
	if $1
	  "&-"
	else
	  base64 = [u8tou16(x)].pack("m")
	  "&" + base64.delete("=\n").tr("/", ",") + "-"
	end
      }
    end

    private

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

    @@debug = false
    @@authenticators = {}

    # Creates a new Net::IMAP object and connects it to the specified
    # +port+ (143 by default) on the named +host+.  If +usessl+ 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.  The +certs+ parameter indicates
    # the path or file containing the CA cert of the server, and the
    # +verify+ parameter is for the OpenSSL verification callback.
    #
    # 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::NETUNREACH:: there is no route to that network.
    # SocketError:: hostname not known or other socket error.
    # Net::IMAP::ByeResponseError:: we connected to the host, but they 
    #                               immediately said goodbye to us.
    def initialize(host, port = PORT, usessl = false, certs = nil, verify = false)
      super()
      @host = host
      @port = port
      @tag_prefix = "RUBY"
      @tagno = 0
      @parser = ResponseParser.new
      @sock = TCPSocket.open(host, port)
      if usessl
        unless defined?(OpenSSL)
          raise "SSL extension not installed"
        end
        @usessl = true
        @sock = SSLSocket.new(@sock)

        # verify the server.
        @sock.ca_file = certs if certs && FileTest::file?(certs)
        @sock.ca_path = certs if certs && FileTest::directory?(certs)
        @sock.verify_mode = VERIFY_PEER if verify
        @sock.verify_callback = VerifyCallbackProc if defined?(VerifyCallbackProc)

        @sock.connect   # start ssl session.
      else
        @usessl = false
      end
      @responses = Hash.new([].freeze)
      @tagged_responses = {}
      @response_handlers = []
      @tag_arrival = new_cond

      @greeting = get_response
      if /\ABYE\z/ni =~ @greeting.name
	@sock.close
	raise ByeResponseError, resp[0]
      end

      @client_thread = Thread.current
      @receiver_thread = Thread.start {
	receive_responses
      }
    end

    def receive_responses
      while true
        begin
          resp = get_response
        rescue Exception
          @sock.close
          @client_thread.raise($!)
          break
        end
        break unless resp
        begin
          synchronize do
            case resp
            when TaggedResponse
              @tagged_responses[resp.tag] = resp
              @tag_arrival.broadcast
            when UntaggedResponse
              record_response(resp.name, resp.data)
              if resp.data.instance_of?(ResponseText) &&
                  (code = resp.data.code)
                record_response(code.name, code.data)
              end
            end
            @response_handlers.each do |handler|
              handler.call(resp)
            end
          end
        rescue Exception
          @client_thread.raise($!)
        end
      end
    end

    def get_tagged_response(tag, cmd)
      until @tagged_responses.key?(tag)
	@tag_arrival.wait
      end
      resp = @tagged_responses.delete(tag)
      case resp.name
      when /\A(?:NO)\z/ni
	raise NoResponseError, resp.data.text
      when /\A(?:BAD)\z/ni
	raise BadResponseError, resp.data.text
      else
	return resp
      end
    end

    def get_response
      buff = ""
      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
	tag = generate_tag
	data = args.collect {|i| format_data(i)}.join(" ")
	if data.length > 0
	  put_line(tag + " " + cmd + " " + data)
	else
	  put_line(tag + " " + cmd)
	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 send_data(*args)
      data = args.collect {|i| format_data(i)}.join(" ")
      put_line(data)
    end

    def put_line(line)
      line = line + CRLF
      @sock.print(line)
      if @@debug
        $stderr.print(line.gsub(/^/n, "C: "))
      end
    end

    def format_data(data)
      case data
      when nil
	return "NIL"
      when String
	return format_string(data)
      when Integer
	return format_number(data)
      when Array
	return format_list(data)
      when Time
	return format_time(data)
      when Symbol
	return format_symbol(data)
      else
	return data.format_data
      end
    end

    def format_string(str)
      case str
      when ""
	return '""'
      when /[\x80-\xff\r\n]/n
	# literal
	return "{" + str.length.to_s + "}" + CRLF + str
      when /[(){ \x00-\x1f\x7f%*"\\]/n
	# quoted string
	return '"' + str.gsub(/["\\]/n, "\\\\\\&") + '"'
      else
	# atom
	return str
      end
    end

    def format_number(num)
      if num < 0 || num >= 4294967296
	raise DataFormatError, num.to_s
      end
      return num.to_s
    end

    def format_list(list)
      contents = list.collect {|i| format_data(i)}.join(" ")
      return "(" + contents + ")"
    end

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

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

    def format_symbol(symbol)
      return "\\" + 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)
      if attr.instance_of?(String)
	attr = RawData.new(attr)
      end
      synchronize do
	@responses.delete("FETCH")
	send_command(cmd, MessageSet.new(set), attr)
	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 self.u16tou8(s)
      len = s.length
      if len < 2
	return ""
      end
      buf = ""
      i = 0
      while i < len
	c = s[i] << 8 | s[i + 1]
	i += 2
	if c == 0xfeff
	  next
	elsif c < 0x0080
	  buf.concat(c)
	elsif c < 0x0800
	  b2 = c & 0x003f
	  b1 = c >> 6
	  buf.concat(b1 | 0xc0)
	  buf.concat(b2 | 0x80)
	elsif c >= 0xdc00 && c < 0xe000
	  raise DataFormatError, "invalid surrogate detected"
	elsif c >= 0xd800 && c < 0xdc00
	  if i + 2 > len
	    raise DataFormatError, "invalid surrogate detected"
	  end
	  low = s[i] << 8 | s[i + 1]
	  i += 2
	  if low < 0xdc00 || low > 0xdfff
	    raise DataFormatError, "invalid surrogate detected"
	  end
	  c = (((c & 0x03ff)) << 10 | (low & 0x03ff)) + 0x10000
	  b4 = c & 0x003f
	  b3 = (c >> 6) & 0x003f
	  b2 = (c >> 12) & 0x003f
	  b1 = c >> 18;
	  buf.concat(b1 | 0xf0)
	  buf.concat(b2 | 0x80)
	  buf.concat(b3 | 0x80)
	  buf.concat(b4 | 0x80)
	else # 0x0800-0xffff
	  b3 = c & 0x003f
	  b2 = (c >> 6) & 0x003f
	  b1 = c >> 12
	  buf.concat(b1 | 0xe0)
	  buf.concat(b2 | 0x80)
	  buf.concat(b3 | 0x80)
	end
      end
      return buf
    end
    private_class_method :u16tou8

    def self.u8tou16(s)
      len = s.length
      buf = ""
      i = 0
      while i < len
	c = s[i]
	if (c & 0x80) == 0
	  buf.concat(0x00)
	  buf.concat(c)
	  i += 1
	elsif (c & 0xe0) == 0xc0 &&
	    inlen >= 2 &&
	    (s[i + 1] & 0xc0) == 0x80
	  if c == 0xc0 || c == 0xc1
	    raise DataFormatError, format("non-shortest UTF-8 sequence (%02x)", c)
	  end
	  u = ((c & 0x1f) << 6) | (s[i + 1] & 0x3f)
	  buf.concat(u >> 8)
	  buf.concat(u & 0x00ff)
	  i += 2
	elsif (c & 0xf0) == 0xe0 &&
	    i + 2 < len &&
	    (s[i + 1] & 0xc0) == 0x80 &&
	    (s[i + 2] & 0xc0) == 0x80
	  if c == 0xe0 && s[i + 1] < 0xa0
	    raise DataFormatError, format("non-shortest UTF-8 sequence (%02x)", c)
	  end
	  u = ((c & 0x0f) << 12) | ((s[i + 1] & 0x3f) << 6) | (s[i + 2] & 0x3f)
	  # surrogate chars
	  if u >= 0xd800 && u <= 0xdfff
	    raise DataFormatError, format("none-UTF-16 char detected (%04x)", u)
	  end
	  buf.concat(u >> 8)
	  buf.concat(u & 0x00ff)
	  i += 3
	elsif (c & 0xf8) == 0xf0 &&
	    i + 3 < len &&
	    (s[i + 1] & 0xc0) == 0x80 &&
	    (s[i + 2] & 0xc0) == 0x80 &&
	    (s[i + 3] & 0xc0) == 0x80
	  if c == 0xf0 && s[i + 1] < 0x90
	    raise DataFormatError, format("non-shortest UTF-8 sequence (%02x)", c)
	  end
	  u = ((c & 0x07) << 18) | ((s[i + 1] & 0x3f) << 12) |
	    ((s[i + 2] & 0x3f) << 6) | (s[i + 3] & 0x3f)
	  if u < 0x10000
	    buf.concat(u >> 8)
	    buf.concat(u & 0x00ff)
	  elsif u < 0x110000
	    high = ((u - 0x10000) >> 10) | 0xd800
	    low = (u & 0x03ff) | 0xdc00
	    buf.concat(high >> 8)
	    buf.concat(high & 0x00ff)
	    buf.concat(low >> 8)
	    buf.concat(low & 0x00ff)
	  else
	    raise DataFormatError, format("none-UTF-16 char detected (%04x)", u)
	  end
	  i += 4
	else
	  raise DataFormatError, format("illegal UTF-8 sequence (%02x)", c)
	end
      end
      return buf
    end
    private_class_method :u8tou16

    class RawData # :nodoc:
      def format_data
	return @data
      end

      private

      def initialize(data)
	@data = data
      end
    end

    class Atom # :nodoc:
      def format_data
	return @data
      end

      private

      def initialize(data)
	@data = data
      end
    end

    class QuotedString # :nodoc:
      def format_data
	return '"' + @data.gsub(/["\\]/n, "\\\\\\&") + '"'
      end

      private

      def initialize(data)
	@data = data
      end
    end

    class Literal # :nodoc:
      def format_data
	return "{" + @data.length.to_s + "}" + CRLF + @data
      end

      private

      def initialize(data)
	@data = data
      end
    end

    class MessageSet # :nodoc:
      def format_data
	return format_internal(@data)
      end

      private

      def initialize(data)
	@data = data
      end

      def format_internal(data)
	case data
	when "*"
	  return data
	when Integer
	  ensure_nz_number(data)
	  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(",")}
	else
	  raise DataFormatError, data.inspect
	end
      end

      def ensure_nz_number(num)
	if num < -1 || num == 0 || num >= 4294967296
	  raise DataFormatError, num.inspect
	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", "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::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. the name is one of "OK", "NO", "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 "]" SPACE] (text_mime2 / text)
    #                       ;; text SHOULD NOT begin with "[" or "="
    # 
    # ==== 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" / "PARSE" /
    #                       "PERMANENTFLAGS" SPACE "(" #(flag / "\*") ")" /
    #                       "READ-ONLY" / "READ-WRITE" / "TRYCREATE" /
    #                       "UIDVALIDITY" SPACE nz_number /
    #                       "UNSEEN" SPACE nz_number /
    #                       atom [SPACE 1*<any TEXT_CHAR except "]">]
    # 
    # ==== Fields:
    # 
    # name:: Returns the name such as "ALERT", "PERMANENTFLAGS", "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 mailbox.
    # 
    # quota:: Quota limit imposed on 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 effect the quota on the
    #              specified mailbox.
    #
    MailboxQuotaRoot = Struct.new(:mailbox, :quotaroots)

    # Net::IMAP::MailboxACLItem represents 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)

    # Net::IMAP::StatusData represents 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 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
	$stderr.printf("warning: media_subtype is obsolete.\n")
	$stderr.printf("         use subtype instead.\n")
	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
	$stderr.printf("warning: media_subtype is obsolete.\n")
	$stderr.printf("         use subtype instead.\n")
	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
	$stderr.printf("warning: media_subtype is obsolete.\n")
	$stderr.printf("         use subtype instead.\n")
	return subtype
      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
	$stderr.printf("warning: media_subtype is obsolete.\n")
	$stderr.printf("         use subtype instead.\n")
	return subtype
      end
    end

    class ResponseParser # :nodoc:
      def parse(str)
	@str = str
	@pos = 0
	@lex_state = EXPR_BEG
	@token = nil
	return response
      end

      private

      EXPR_BEG		= :EXPR_BEG
      EXPR_DATA		= :EXPR_DATA
      EXPR_TEXT		= :EXPR_TEXT
      EXPR_RTEXT	= :EXPR_RTEXT
      EXPR_CTEXT	= :EXPR_CTEXT

      T_SPACE	= :SPACE
      T_NIL	= :NIL
      T_NUMBER	= :NUMBER
      T_ATOM	= :ATOM
      T_QUOTED	= :QUOTED
      T_LPAR	= :LPAR
      T_RPAR	= :RPAR
      T_BSLASH	= :BSLASH
      T_STAR	= :STAR
      T_LBRA	= :LBRA
      T_RBRA	= :RBRA
      T_LITERAL	= :LITERAL
      T_PLUS	= :PLUS
      T_PERCENT	= :PERCENT
      T_CRLF	= :CRLF
      T_EOF	= :EOF
      T_TEXT	= :TEXT

      BEG_REGEXP = /\G(?:\
(?# 1:	SPACE	)( )|\
(?# 2:	NIL	)(NIL)(?=[\x80-\xff(){ \x00-\x1f\x7f%*"\\\[\]+])|\
(?# 3:	NUMBER	)(\d+)(?=[\x80-\xff(){ \x00-\x1f\x7f%*"\\\[\]+])|\
(?# 4:	ATOM	)([^\x80-\xff(){ \x00-\x1f\x7f%*"\\\[\]+]+)|\
(?# 5:	QUOTED	)"((?:[^\x80-\xff\x00\r\n"\\]|\\["\\])*)"|\
(?# 6:	LPAR	)(\()|\
(?# 7:	RPAR	)(\))|\
(?# 8:	BSLASH	)(\\)|\
(?# 9:	STAR	)(\*)|\
(?# 10:	LBRA	)(\[)|\
(?# 11:	RBRA	)(\])|\
(?# 12:	LITERAL	)\{(\d+)\}\r\n|\
(?# 13:	PLUS	)(\+)|\
(?# 14:	PERCENT	)(%)|\
(?# 15:	CRLF	)(\r\n)|\
(?# 16:	EOF	)(\z))/ni

      DATA_REGEXP = /\G(?:\
(?# 1:	SPACE	)( )|\
(?# 2:	NIL	)(NIL)|\
(?# 3:	NUMBER	)(\d+)|\
(?# 4:	QUOTED	)"((?:[^\x80-\xff\x00\r\n"\\]|\\["\\])*)"|\
(?# 5:	LITERAL	)\{(\d+)\}\r\n|\
(?# 6:	LPAR	)(\()|\
(?# 7:	RPAR	)(\)))/ni

      TEXT_REGEXP = /\G(?:\
(?# 1:	TEXT	)([^\x00\x80-\xff\r\n]*))/ni

      RTEXT_REGEXP = /\G(?:\
(?# 1:	LBRA	)(\[)|\
(?# 2:	TEXT	)([^\x00\x80-\xff\r\n]*))/ni

      CTEXT_REGEXP = /\G(?:\
(?# 1:	TEXT	)([^\x00\x80-\xff\r\n\]]*))/ni

      Token = Struct.new(:symbol, :value)

      def response
	token = lookahead
	case token.symbol
	when T_PLUS
	  result = continue_req
	when T_STAR
	  result = response_untagged
	else
	  result = response_tagged
	end
	match(T_CRLF)
	match(T_EOF)
	return result
      end

      def continue_req
	match(T_PLUS)
	match(T_SPACE)
	return ContinuationRequest.new(resp_text, @str)
      end

      def response_untagged
	match(T_STAR)
	match(T_SPACE)
	token = lookahead
	if token.symbol == T_NUMBER
	  return numeric_response
	elsif token.symbol == T_ATOM
	  case token.value
	  when /\A(?:OK|NO|BAD|BYE|PREAUTH)\z/ni
	    return response_cond
	  when /\A(?:FLAGS)\z/ni
	    return flags_response
	  when /\A(?:LIST|LSUB)\z/ni
	    return list_response
	  when /\A(?:QUOTA)\z/ni
	    return getquota_response
	  when /\A(?:QUOTAROOT)\z/ni
	    return getquotaroot_response
	  when /\A(?:ACL)\z/ni
	    return getacl_response
	  when /\A(?:SEARCH|SORT)\z/ni
	    return search_response
	  when /\A(?:THREAD)\z/ni
	    return thread_response
	  when /\A(?:STATUS)\z/ni
	    return status_response
	  when /\A(?:CAPABILITY)\z/ni
	    return capability_response
	  else
	    return text_response
	  end
	else
	  parse_error("unexpected token %s", token.symbol)
	end
      end

      def response_tagged
	tag = atom
	match(T_SPACE)
	token = match(T_ATOM)
	name = token.value.upcase
	match(T_SPACE)
	return TaggedResponse.new(tag, name, resp_text, @str)
      end

      def response_cond
	token = match(T_ATOM)
	name = token.value.upcase
	match(T_SPACE)
	return UntaggedResponse.new(name, resp_text, @str)
      end

      def numeric_response
	n = number
	match(T_SPACE)
	token = match(T_ATOM)
	name = token.value.upcase
	case name
	when "EXISTS", "RECENT", "EXPUNGE"
	  return UntaggedResponse.new(name, n, @str)
	when "FETCH"
	  shift_token
	  match(T_SPACE)
	  data = FetchData.new(n, msg_att)
	  return UntaggedResponse.new(name, data, @str)
	end
      end

      def msg_att
	match(T_LPAR)
	attr = {}
	while true
	  token = lookahead
	  case token.symbol
	  when T_RPAR
	    shift_token
	    break
	  when T_SPACE
	    shift_token
	    token = lookahead
	  end
	  case token.value
	  when /\A(?:ENVELOPE)\z/ni
	    name, val = envelope_data
	  when /\A(?:FLAGS)\z/ni
	    name, val = flags_data
	  when /\A(?:INTERNALDATE)\z/ni
	    name, val = internaldate_data
	  when /\A(?:RFC822(?:\.HEADER|\.TEXT)?)\z/ni
	    name, val = rfc822_text
	  when /\A(?:RFC822\.SIZE)\z/ni
	    name, val = rfc822_size
	  when /\A(?:BODY(?:STRUCTURE)?)\z/ni
	    name, val = body_data
	  when /\A(?:UID)\z/ni
	    name, val = uid_data
	  else
	    parse_error("unknown attribute `%s'", token.value)
	  end
	  attr[name] = val
	end
	return attr
      end

      def envelope_data
	token = match(T_ATOM)
	name = token.value.upcase
	match(T_SPACE)
	return name, envelope
      end

      def envelope
	@lex_state = EXPR_DATA
	match(T_LPAR)
	date = nstring
	match(T_SPACE)
	subject = nstring
	match(T_SPACE)
	from = address_list
	match(T_SPACE)
	sender = address_list
	match(T_SPACE)
	reply_to = address_list
	match(T_SPACE)
	to = address_list
	match(T_SPACE)
	cc = address_list
	match(T_SPACE)
	bcc = address_list
	match(T_SPACE)
	in_reply_to = nstring
	match(T_SPACE)
	message_id = nstring
	match(T_RPAR)
	@lex_state = EXPR_BEG
	return Envelope.new(date, subject, from, sender, reply_to,
			    to, cc, bcc, in_reply_to, message_id)
      end

      def flags_data
	token = match(T_ATOM)
	name = token.value.upcase
	match(T_SPACE)
	return name, flag_list
      end

      def internaldate_data
	token = match(T_ATOM)
	name = token.value.upcase
	match(T_SPACE)
	token = match(T_QUOTED)
	return name, token.value
      end

      def rfc822_text
	token = match(T_ATOM)
	name = token.value.upcase
	match(T_SPACE)
	return name, nstring
      end

      def rfc822_size
	token = match(T_ATOM)
	name = token.value.upcase
	match(T_SPACE)
	return name, number
      end

      def body_data
	token = match(T_ATOM)
	name = token.value.upcase
	token = lookahead
	if token.symbol == T_SPACE
	  shift_token
	  return name, body
	end
	name.concat(section)
	token = lookahead
	if token.symbol == T_ATOM
	  name.concat(token.value)
	  shift_token
	end
	match(T_SPACE)
	data = nstring
	return name, data
      end

      def body
	@lex_state = EXPR_DATA
	match(T_LPAR)
	token = lookahead
	if token.symbol == T_LPAR
	  result = body_type_mpart
	else
	  result = body_type_1part
	end
	match(T_RPAR)
	@lex_state = EXPR_BEG
	return result
      end

      def body_type_1part
	token = lookahead
	case token.value
	when /\A(?:TEXT)\z/ni
	  return body_type_text
	when /\A(?:MESSAGE)\z/ni
	  return body_type_msg
	else
	  return body_type_basic
	end
      end

      def body_type_basic
	mtype, msubtype = media_type
	match(T_SPACE)
	param, content_id, desc, enc, size = body_fields
	md5, disposition, language, extension = body_ext_1part
	return BodyTypeBasic.new(mtype, msubtype,
				 param, content_id,
				 desc, enc, size,
				 md5, disposition, language, extension)
      end

      def body_type_text
	mtype, msubtype = media_type
	match(T_SPACE)
	param, content_id, desc, enc, size = body_fields
	match(T_SPACE)
	lines = number
	md5, disposition, language, extension = body_ext_1part
	return BodyTypeText.new(mtype, msubtype,
				param, content_id,
				desc, enc, size,
				lines,
				md5, disposition, language, extension)
      end

      def body_type_msg
	mtype, msubtype = media_type
	match(T_SPACE)
	param, content_id, desc, enc, size = body_fields
	match(T_SPACE)
	env = envelope
	match(T_SPACE)
	b = body
	match(T_SPACE)
	lines = number
	md5, disposition, language, extension = body_ext_1part
	return BodyTypeMessage.new(mtype, msubtype,
				   param, content_id,
				   desc, enc, size,
				   env, b, lines,
				   md5, disposition, language, extension)
      end

      def body_type_mpart
	parts = []
	while true
	  token = lookahead
	  if token.symbol == T_SPACE
	    shift_token
	    break
	  end
	  parts.push(body)
	end
	mtype = "MULTIPART"
	msubtype = string.upcase
	param, disposition, language, extension = body_ext_mpart
	return BodyTypeMultipart.new(mtype, msubtype, parts,
				     param, disposition, language,
				     extension)
      end

      def media_type
	mtype = string.upcase
	match(T_SPACE)
	msubtype = string.upcase
	return mtype, msubtype
      end

      def body_fields
	param = body_fld_param
	match(T_SPACE)
	content_id = nstring
	match(T_SPACE)
	desc = nstring
	match(T_SPACE)
	enc = string.upcase
	match(T_SPACE)
	size = number
	return param, content_id, desc, enc, size
      end

      def body_fld_param
	token = lookahead
	if token.symbol == T_NIL
	  shift_token
	  return nil
	end
	match(T_LPAR)
	param = {}
	while true
	  token = lookahead
	  case token.symbol
	  when T_RPAR
	    shift_token
	    break
	  when T_SPACE
	    shift_token
	  end
	  name = string.upcase
	  match(T_SPACE)
	  val = string
	  param[name] = val
	end
	return param
      end

      def body_ext_1part
	token = lookahead
	if token.symbol == T_SPACE
	  shift_token
	else
	  return nil
	end
	md5 = nstring

	token = lookahead
	if token.symbol == T_SPACE
	  shift_token
	else
	  return md5
	end
	disposition = body_fld_dsp

	token = lookahead
	if token.symbol == T_SPACE
	  shift_token
	else
	  return md5, disposition
	end
	language = body_fld_lang

	token = lookahead
	if token.symbol == T_SPACE
	  shift_token
	else
	  return md5, disposition, language
	end

	extension = body_extensions
	return md5, disposition, language, extension
      end

      def body_ext_mpart
	token = lookahead
	if token.symbol == T_SPACE
	  shift_token
	else
	  return nil
	end
	param = body_fld_param

	token = lookahead
	if token.symbol == T_SPACE
	  shift_token
	else
	  return param
	end
	disposition = body_fld_dsp
	match(T_SPACE)
	language = body_fld_lang

	token = lookahead
	if token.symbol == T_SPACE
	  shift_token
	else
	  return param, disposition, language
	end

	extension = body_extensions
	return param, disposition, language, extension
      end

      def body_fld_dsp
	token = lookahead
	if token.symbol == T_NIL
	  shift_token
	  return nil
	end
	match(T_LPAR)
	dsp_type = string.upcase
	match(T_SPACE)
	param = body_fld_param
	match(T_RPAR)
	return ContentDisposition.new(dsp_type, param)
      end

      def body_fld_lang
	token = lookahead
	if token.symbol == T_LPAR
	  shift_token
	  result = []
	  while true
	    token = lookahead
	    case token.symbol
	    when T_RPAR
	      shift_token
	      return result
	    when T_SPACE
	      shift_token
	    end
	    result.push(string.upcase)
	  end
	else
	  lang = nstring
	  if lang
	    return lang.upcase
	  else
	    return lang
	  end
	end
      end

      def body_extensions
	result = []
	while true
	  token = lookahead
	  case token.symbol
	  when T_RPAR
	    return result
	  when T_SPACE
	    shift_token
	  end
	  result.push(body_extension)
	end
      end

      def body_extension
	token = lookahead
	case token.symbol
	when T_LPAR
	  shift_token
	  result = body_extensions
	  match(T_RPAR)
	  return result
	when T_NUMBER
	  return number
	else
	  return nstring
	end
      end

      def section
	str = ""
	token = match(T_LBRA)
	str.concat(token.value)
	token = match(T_ATOM, T_NUMBER, T_RBRA)
	if token.symbol == T_RBRA
	  str.concat(token.value)
	  return str
	end
	str.concat(token.value)
	token = lookahead
	if token.symbol == T_SPACE
	  shift_token
	  str.concat(token.value)
	  token = match(T_LPAR)
	  str.concat(token.value)
	  while true
	    token = lookahead
	    case token.symbol
	    when T_RPAR
	      str.concat(token.value)
	      shift_token
	      break
	    when T_SPACE
	      shift_token
	      str.concat(token.value)
	    end
	    str.concat(format_string(astring))
	  end
	end
	token = match(T_RBRA)
	str.concat(token.value)
	return str
      end

      def format_string(str)
	case str
	when ""
	  return '""'
	when /[\x80-\xff\r\n]/n
	  # literal
	  return "{" + str.length.to_s + "}" + CRLF + str
	when /[(){ \x00-\x1f\x7f%*"\\]/n
	  # quoted string
	  return '"' + str.gsub(/["\\]/n, "\\\\\\&") + '"'
	else
	  # atom
	  return str
	end
      end

      def uid_data
	token = match(T_ATOM)
	name = token.value.upcase
	match(T_SPACE)
	return name, number
      end

      def text_response
	token = match(T_ATOM)
	name = token.value.upcase
	match(T_SPACE)
	@lex_state = EXPR_TEXT
	token = match(T_TEXT)
	@lex_state = EXPR_BEG
	return UntaggedResponse.new(name, token.value)
      end

      def flags_response
	token = match(T_ATOM)
	name = token.value.upcase
	match(T_SPACE)
	return UntaggedResponse.new(name, flag_list, @str)
      end

      def list_response
	token = match(T_ATOM)
	name = token.value.upcase
	match(T_SPACE)
	return UntaggedResponse.new(name, mailbox_list, @str)
      end

      def mailbox_list
	attr = flag_list
	match(T_SPACE)
	token = match(T_QUOTED, T_NIL)
	if token.symbol == T_NIL
	  delim = nil
	else
	  delim = token.value
	end
	match(T_SPACE)
	name = astring
	return MailboxList.new(attr, delim, name)
      end

      def getquota_response
        # If quota never established, get back
        # `NO Quota root does not exist'.
        # If quota removed, get `()' after the
        # folder spec with no mention of `STORAGE'.
        token = match(T_ATOM)
        name = token.value.upcase
        match(T_SPACE)
        mailbox = astring
        match(T_SPACE)
        match(T_LPAR)
        token = lookahead
        case token.symbol
        when T_RPAR
          shift_token
          data = MailboxQuota.new(mailbox, nil, nil)
          return UntaggedResponse.new(name, data, @str)
        when T_ATOM
          shift_token
          match(T_SPACE)
          token = match(T_NUMBER)
          usage = token.value
          match(T_SPACE)
          token = match(T_NUMBER)
          quota = token.value
          match(T_RPAR)
          data = MailboxQuota.new(mailbox, usage, quota)
          return UntaggedResponse.new(name, data, @str)
        else
          parse_error("unexpected token %s", token.symbol)
        end
      end

      def getquotaroot_response
        # Similar to getquota, but only admin can use getquota.
        token = match(T_ATOM)
        name = token.value.upcase
        match(T_SPACE)
        mailbox = astring
        quotaroots = []
        while true
          token = lookahead
          break unless token.symbol == T_SPACE
          shift_token
          quotaroots.push(astring)
        end
        data = MailboxQuotaRoot.new(mailbox, quotaroots)
        return UntaggedResponse.new(name, data, @str)
      end

      def getacl_response
        token = match(T_ATOM)
        name = token.value.upcase
        match(T_SPACE)
        mailbox = astring
        data = []
        token = lookahead
        if token.symbol == T_SPACE
          shift_token
          while true
            token = lookahead
            case token.symbol
            when T_CRLF
              break
            when T_SPACE
              shift_token
            end
            user = astring
            match(T_SPACE)
            rights = astring
            ##XXX data.push([user, rights])
            data.push(MailboxACLItem.new(user, rights))
          end
        end
	return UntaggedResponse.new(name, data, @str)
      end

      def search_response
	token = match(T_ATOM)
	name = token.value.upcase
	token = lookahead
	if token.symbol == T_SPACE
	  shift_token
	  data = []
	  while true
	    token = lookahead
	    case token.symbol
	    when T_CRLF
	      break
	    when T_SPACE
	      shift_token
	    end
	    data.push(number)
	  end
	else
	  data = []
	end
	return UntaggedResponse.new(name, data, @str)
      end

      def thread_response
	token = match(T_ATOM)
	name = token.value.upcase
	token = lookahead

	if token.symbol == T_SPACE
	  threads = []

	  while true
	    shift_token
	    token = lookahead

	    case token.symbol
	    when T_LPAR
	      threads << thread_branch(token)
	    when T_CRLF
	      break
	    end
	  end
	else
	  # no member
	  threads = []
	end

	return UntaggedResponse.new(name, threads, @str)
      end

      def thread_branch(token)
	rootmember = nil
	lastmember = nil
	
	while true
	  shift_token    # ignore first T_LPAR
	  token = lookahead
	  
	  case token.symbol
	  when T_NUMBER
	    # new member
	    newmember = ThreadMember.new(number, [])
	    if rootmember.nil?
	      rootmember = newmember
	    else    
	      lastmember.children << newmember
	    end     
	    lastmember = newmember
	  when T_SPACE 
	    # do nothing 
	  when T_LPAR
	    if rootmember.nil?
	      # dummy member
	      lastmember = rootmember = ThreadMember.new(nil, [])
	    end     
	    
	    lastmember.children << thread_branch(token)
	  when T_RPAR
	    break   
	  end     
	end
	
	return rootmember
      end

      def status_response
	token = match(T_ATOM)
	name = token.value.upcase
	match(T_SPACE)
	mailbox = astring
	match(T_SPACE)
	match(T_LPAR)
	attr = {}
	while true
	  token = lookahead
	  case token.symbol
	  when T_RPAR
	    shift_token
	    break
	  when T_SPACE
	    shift_token
	  end
	  token = match(T_ATOM)
	  key = token.value.upcase
	  match(T_SPACE)
	  val = number
	  attr[key] = val
	end
	data = StatusData.new(mailbox, attr)
	return UntaggedResponse.new(name, data, @str)
      end

      def capability_response
	token = match(T_ATOM)
	name = token.value.upcase
	match(T_SPACE)
	data = []
	while true
	  token = lookahead
	  case token.symbol
	  when T_CRLF
	    break
	  when T_SPACE
	    shift_token
	  end
	  data.push(atom.upcase)
	end
	return UntaggedResponse.new(name, data, @str)
      end

      def resp_text
	@lex_state = EXPR_RTEXT
	token = lookahead
	if token.symbol == T_LBRA
	  code = resp_text_code
	else
	  code = nil
	end
	token = match(T_TEXT)
	@lex_state = EXPR_BEG
	return ResponseText.new(code, token.value)
      end

      def resp_text_code
	@lex_state = EXPR_BEG
	match(T_LBRA)
	token = match(T_ATOM)
	name = token.value.upcase
	case name
	when /\A(?:ALERT|PARSE|READ-ONLY|READ-WRITE|TRYCREATE)\z/n
	  result = ResponseCode.new(name, nil)
	when /\A(?:PERMANENTFLAGS)\z/n
	  match(T_SPACE)
	  result = ResponseCode.new(name, flag_list)
	when /\A(?:UIDVALIDITY|UIDNEXT|UNSEEN)\z/n
	  match(T_SPACE)
	  result = ResponseCode.new(name, number)
	else
	  match(T_SPACE)
	  @lex_state = EXPR_CTEXT
	  token = match(T_TEXT)
	  @lex_state = EXPR_BEG
	  result = ResponseCode.new(name, token.value)
	end
	match(T_RBRA)
	@lex_state = EXPR_RTEXT
	return result
      end

      def address_list
	token = lookahead
	if token.symbol == T_NIL
	  shift_token
	  return nil
	else
	  result = []
	  match(T_LPAR)
	  while true
	    token = lookahead
	    case token.symbol
	    when T_RPAR
	      shift_token
	      break
	    when T_SPACE
	      shift_token
	    end
	    result.push(address)
	  end
	  return result
	end
      end

      ADDRESS_REGEXP = /\G\
(?# 1: NAME	)(?:NIL|"((?:[^\x80-\xff\x00\r\n"\\]|\\["\\])*)") \
(?# 2: ROUTE	)(?:NIL|"((?:[^\x80-\xff\x00\r\n"\\]|\\["\\])*)") \
(?# 3: MAILBOX	)(?:NIL|"((?:[^\x80-\xff\x00\r\n"\\]|\\["\\])*)") \
(?# 4: HOST	)(?:NIL|"((?:[^\x80-\xff\x00\r\n"\\]|\\["\\])*)")\
\)/ni

      def address
	match(T_LPAR)
	if @str.index(ADDRESS_REGEXP, @pos)
	  # address does not include literal.
	  @pos = $~.end(0)
	  name = $1
	  route = $2
	  mailbox = $3
	  host = $4
	  for s in [name, route, mailbox, host]
	    if s
	      s.gsub!(/\\(["\\])/n, "\\1")
	    end
	  end
	else
	  name = nstring
	  match(T_SPACE)
	  route = nstring
	  match(T_SPACE)
	  mailbox = nstring
	  match(T_SPACE)
	  host = nstring
	  match(T_RPAR)
	end
	return Address.new(name, route, mailbox, host)
      end

#        def flag_list
#  	result = []
#  	match(T_LPAR)
#  	while true
#  	  token = lookahead
#  	  case token.symbol
#  	  when T_RPAR
#  	    shift_token
#  	    break
#  	  when T_SPACE
#  	    shift_token
#  	  end
#  	  result.push(flag)
#  	end
#  	return result
#        end

#        def flag
#  	token = lookahead
#  	if token.symbol == T_BSLASH
#  	  shift_token
#  	  token = lookahead
#  	  if token.symbol == T_STAR
#  	    shift_token
#  	    return token.value.intern
#  	  else
#  	    return atom.intern
#  	  end
#  	else
#  	  return atom
#  	end
#        end

      FLAG_REGEXP = /\
(?# FLAG	)\\([^\x80-\xff(){ \x00-\x1f\x7f%"\\]+)|\
(?# ATOM	)([^\x80-\xff(){ \x00-\x1f\x7f%*"\\]+)/n

      def flag_list
	if @str.index(/\(([^)]*)\)/ni, @pos)
	  @pos = $~.end(0)
	  return $1.scan(FLAG_REGEXP).collect { |flag, atom|
	    atom || flag.capitalize.intern
	  }
	else
	  parse_error("invalid flag list")
	end
      end

      def nstring
	token = lookahead
	if token.symbol == T_NIL
	  shift_token
	  return nil
	else
	  return string
	end
      end

      def astring
	token = lookahead
	if string_token?(token)
	  return string
	else
	  return atom
	end
      end

      def string
	token = match(T_QUOTED, T_LITERAL)
	return token.value
      end

      STRING_TOKENS = [T_QUOTED, T_LITERAL]

      def string_token?(token)
	return STRING_TOKENS.include?(token.symbol)
      end

      def atom
  	result = ""
  	while true
  	  token = lookahead
  	  if atom_token?(token)
  	    result.concat(token.value)
  	    shift_token
  	  else
  	    if result.empty?
  	      parse_error("unexpected token %s", token.symbol)
  	    else
  	      return result
  	    end
  	  end
  	end
      end

      ATOM_TOKENS = [
	T_ATOM,
	T_NUMBER,
	T_NIL,
	T_LBRA,
	T_RBRA,
	T_PLUS
      ]

      def atom_token?(token)
	return ATOM_TOKENS.include?(token.symbol)
      end

      def number
	token = match(T_NUMBER)
	return token.value.to_i
      end

      def nil_atom
	match(T_NIL)
	return nil
      end

      def match(*args)
	token = lookahead
	unless args.include?(token.symbol)
	  parse_error('unexpected token %s (expected %s)',
		      token.symbol.id2name,
		      args.collect {|i| i.id2name}.join(" or "))
	end
	shift_token
	return token
      end

      def lookahead
	unless @token
	  @token = next_token
	end
	return @token
      end

      def shift_token
	@token = nil
      end

      def next_token
	case @lex_state
	when EXPR_BEG
	  if @str.index(BEG_REGEXP, @pos)
	    @pos = $~.end(0)
	    if $1
	      return Token.new(T_SPACE, $+)
	    elsif $2
	      return Token.new(T_NIL, $+)
	    elsif $3
	      return Token.new(T_NUMBER, $+)
	    elsif $4
	      return Token.new(T_ATOM, $+)
	    elsif $5
	      return Token.new(T_QUOTED,
			       $+.gsub(/\\(["\\])/n, "\\1"))
	    elsif $6
	      return Token.new(T_LPAR, $+)
	    elsif $7
	      return Token.new(T_RPAR, $+)
	    elsif $8
	      return Token.new(T_BSLASH, $+)
	    elsif $9
	      return Token.new(T_STAR, $+)
	    elsif $10
	      return Token.new(T_LBRA, $+)
	    elsif $11
	      return Token.new(T_RBRA, $+)
	    elsif $12
	      len = $+.to_i
	      val = @str[@pos, len]
	      @pos += len
	      return Token.new(T_LITERAL, val)
	    elsif $13
	      return Token.new(T_PLUS, $+)
	    elsif $14
	      return Token.new(T_PERCENT, $+)
	    elsif $15
	      return Token.new(T_CRLF, $+)
	    elsif $16
	      return Token.new(T_EOF, $+)
	    else
	      parse_error("[Net::IMAP BUG] BEG_REGEXP is invalid")
	    end
	  else
	    @str.index(/\S*/n, @pos)
	    parse_error("unknown token - %s", $&.dump)
	  end
	when EXPR_DATA
	  if @str.index(DATA_REGEXP, @pos)
	    @pos = $~.end(0)
	    if $1
	      return Token.new(T_SPACE, $+)
	    elsif $2
	      return Token.new(T_NIL, $+)
	    elsif $3
	      return Token.new(T_NUMBER, $+)
	    elsif $4
	      return Token.new(T_QUOTED,
			       $+.gsub(/\\(["\\])/n, "\\1"))
	    elsif $5
	      len = $+.to_i
	      val = @str[@pos, len]
	      @pos += len
	      return Token.new(T_LITERAL, val)
	    elsif $6
	      return Token.new(T_LPAR, $+)
	    elsif $7
	      return Token.new(T_RPAR, $+)
	    else
	      parse_error("[Net::IMAP BUG] BEG_REGEXP is invalid")
	    end
	  else
	    @str.index(/\S*/n, @pos)
	    parse_error("unknown token - %s", $&.dump)
	  end
	when EXPR_TEXT
	  if @str.index(TEXT_REGEXP, @pos)
	    @pos = $~.end(0)
	    if $1
	      return Token.new(T_TEXT, $+)
	    else
	      parse_error("[Net::IMAP BUG] TEXT_REGEXP is invalid")
	    end
	  else
	    @str.index(/\S*/n, @pos)
	    parse_error("unknown token - %s", $&.dump)
	  end
	when EXPR_RTEXT
	  if @str.index(RTEXT_REGEXP, @pos)
	    @pos = $~.end(0)
	    if $1
	      return Token.new(T_LBRA, $+)
	    elsif $2
	      return Token.new(T_TEXT, $+)
	    else
	      parse_error("[Net::IMAP BUG] RTEXT_REGEXP is invalid")
	    end
	  else
	    @str.index(/\S*/n, @pos)
	    parse_error("unknown token - %s", $&.dump)
	  end
	when EXPR_CTEXT
	  if @str.index(CTEXT_REGEXP, @pos)
	    @pos = $~.end(0)
	    if $1
	      return Token.new(T_TEXT, $+)
	    else
	      parse_error("[Net::IMAP BUG] CTEXT_REGEXP is invalid")
	    end
	  else
	    @str.index(/\S*/n, @pos) #/
	    parse_error("unknown token - %s", $&.dump)
	  end
	else
	  parse_error("illegal @lex_state - %s", @lex_state.inspect)
	end
      end

      def parse_error(fmt, *args)
	if IMAP.debug
	  $stderr.printf("@str: %s\n", @str.dump)
	  $stderr.printf("@pos: %d\n", @pos)
	  $stderr.printf("@lex_state: %s\n", @lex_state)
	  if @token.symbol
	    $stderr.printf("@token.symbol: %s\n", @token.symbol)
	    $stderr.printf("@token.value: %s\n", @token.value.inspect)
	  end
	end
	raise ResponseParseError, format(fmt, *args)
      end
    end

    # Authenticator for the "LOGIN" authentication type.  See
    # #authenticate().
    class LoginAuthenticator
      def process(data)
	case @state
	when STATE_USER
	  @state = STATE_PASSWORD
	  return @user
	when STATE_PASSWORD
	  return @password
	end
      end

      private

      STATE_USER = :USER
      STATE_PASSWORD = :PASSWORD

      def initialize(user, password)
	@user = user
	@password = password
	@state = STATE_USER
      end
    end
    add_authenticator "LOGIN", LoginAuthenticator

    # Authenticator for the "CRAM-MD5" authentication type.  See
    # #authenticate().
    class CramMD5Authenticator
      def process(challenge)
	digest = hmac_md5(challenge, @password)
	return @user + " " + digest
      end

      private

      def initialize(user, password)
	@user = user
	@password = password
      end

      def hmac_md5(text, key)
	if key.length > 64
	  key = Digest::MD5.digest(key)
	end

	k_ipad = key + "\0" * (64 - key.length)
	k_opad = key + "\0" * (64 - key.length)
	for i in 0..63
	  k_ipad[i] ^= 0x36
	  k_opad[i] ^= 0x5c
	end

	digest = Digest::MD5.digest(k_ipad + text)

	return Digest::MD5.hexdigest(k_opad + digest)
      end
    end
    add_authenticator "CRAM-MD5", CramMD5Authenticator

    # 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
    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
  end
end

if __FILE__ == $0
  # :enddoc:
  require "getoptlong"

  $stdout.sync = true
  $port = "imap2"
  $user = ENV["USER"] || ENV["LOGNAME"]
  $auth = "cram-md5"

  def usage
    $stderr.print <<EOF
usage: #{$0} [options] <host>

  --help                        print this message
  --port=PORT                   specifies port
  --user=USER                   specifies user
  --auth=AUTH                   specifies auth type
EOF
  end

  def get_password
    print "password: "
    system("stty", "-echo")
    begin
      return gets.chop
    ensure
      system("stty", "echo")
      print "\n"
    end
  end

  def get_command
    printf("%s@%s> ", $user, $host)
    if line = gets
      return line.strip.split(/\s+/)
    else
      return nil
    end
  end

  parser = GetoptLong.new
  parser.set_options(['--help', GetoptLong::NO_ARGUMENT],
		     ['--port', GetoptLong::REQUIRED_ARGUMENT],
		     ['--user', GetoptLong::REQUIRED_ARGUMENT],
		     ['--auth', GetoptLong::REQUIRED_ARGUMENT])
  begin
    parser.each_option do |name, arg|
      case name
      when "--port"
	$port = arg
      when "--user"
	$user = arg
      when "--auth"
	$auth = arg
      when "--help"
	usage
	exit(1)
      end
    end
  rescue
    usage
    exit(1)
  end

  $host = ARGV.shift
  unless $host
    usage
    exit(1)
  end
    
  imap = Net::IMAP.new($host, $port)
  begin
    password = get_password
    imap.authenticate($auth, $user, password)
    while true
      cmd, *args = get_command
      break unless cmd
      begin
	case cmd
	when "list"
	  for mbox in imap.list("", args[0] || "*")
	    if mbox.attr.include?(Net::IMAP::NOSELECT)
	      prefix = "!"
	    elsif mbox.attr.include?(Net::IMAP::MARKED)
	      prefix = "*"
	    else
	      prefix = " "
	    end
	    print prefix, mbox.name, "\n"
	  end
	when "select"
	  imap.select(args[0] || "inbox")
	  print "ok\n"
	when "close"
	  imap.close
	  print "ok\n"
	when "summary"
	  unless messages = imap.responses["EXISTS"][-1]
	    puts "not selected"
	    next
	  end
	  if messages > 0
	    for data in imap.fetch(1..-1, ["ENVELOPE"])
	      print data.seqno, ": ", data.attr["ENVELOPE"].subject, "\n"
	    end
	  else
	    puts "no message"
	  end
	when "fetch"
	  if args[0]
	    data = imap.fetch(args[0].to_i, ["RFC822.HEADER", "RFC822.TEXT"])[0]
	    puts data.attr["RFC822.HEADER"]
	    puts data.attr["RFC822.TEXT"]
	  else
	    puts "missing argument"
	  end
	when "logout", "exit", "quit"
	  break
	when "help", "?"
	  print <<EOF
list [pattern]			list mailboxes
select [mailbox]		select mailbox
close				close mailbox
summary				display summary
fetch [msgno]			display message
logout				logout
help, ?				display help message
EOF
	else
	  print "unknown command: ", cmd, "\n"
	end
      rescue Net::IMAP::Error
	puts $!
      end
    end
  ensure
    imap.logout
    imap.disconnect
  end
end