utf8: Add imap4_utf_7 codec

Add code to reencode IMAP folder names to regular utf-8.
This starts an implementation that will add a new config option
`utf8foldernames` on account level which will fix #299 and on the
long run replace the current `decodefoldernames` option.

This commit introduces code to register an `imap4_utf_7` codec
on which two-way conversion methods will later be built.

Original code by
(https://www.blogger.com/profile/16648963337079496096),
taken from
http://piao-tech.blogspot.no/2010/03/get-offlineimap-working-with-non-ascii.html

In the comment
http://piao-tech.blogspot.com/2010/03/get-offlineimap-working-with-non-ascii.html?showComment=1316041409339#c669880170006851138
indicates that this code is expected to be incorporated into offlineIMAP and therefore the author implicitly agrees to put it under this license.

Signed-off-by: Urs Liska <git@ursliska.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/imaputil.py

@@ -17,6 +17,8 @@
 
 import re
 import string
+import binascii
+import codecs
 from offlineimap.ui import getglobalui
 
 
@@ -370,3 +372,73 @@ def decode_mailbox_name(name):
         return ret.decode('utf-7').encode('utf-8')
     except (UnicodeDecodeError, UnicodeEncodeError):
         return name
+
+# Functionality to convert folder names encoded in IMAP_utf_7 to utf_8.
+# This is achieved by defining 'imap4_utf_7' as a proper encoding scheme.
+
+def modified_base64(s):
+    s = s.encode('utf-16be')
+    return binascii.b2a_base64(s).rstrip('\n=').replace('/', ',')
+
+def doB64(_in, r):
+    if _in:
+        r.append('&%s-' % modified_base64(''.join(_in)))
+        del _in[:]
+
+def encoder(s):
+    r = []
+    _in = []
+    for c in s:
+        ordC = ord(c)
+        if 0x20 <= ordC <= 0x25 or 0x27 <= ordC <= 0x7e:
+            doB64(_in, r)
+            r.append(c)
+        elif c == '&':
+            doB64(_in, r)
+            r.append('&-')
+        else:
+            _in.append(c)
+    doB64(_in, r)
+    return (str(''.join(r)), len(s))
+
+# decoding
+def modified_unbase64(s):
+    b = binascii.a2b_base64(s.replace(',', '/') + '===')
+    return unicode(b, 'utf-16be')
+
+def decoder(s):
+    r = []
+    decode = []
+    for c in s:
+        if c == '&' and not decode:
+            decode.append('&')
+        elif c == '-' and decode:
+            if len(decode) == 1:
+                r.append('&')
+            else:
+                r.append(modified_unbase64(''.join(decode[1:])))
+            decode = []
+        elif decode:
+            decode.append(c)
+        else:
+            r.append(c)
+
+    if decode:
+        r.append(modified_unbase64(''.join(decode[1:])))
+    bin_str = ''.join(r)
+    return (bin_str, len(s))
+
+class StreamReader(codecs.StreamReader):
+    def decode(self, s, errors='strict'):
+        return decoder(s)
+
+class StreamWriter(codecs.StreamWriter):
+    def decode(self, s, errors='strict'):
+        return encoder(s)
+
+def imap4_utf_7(name):
+    if name == 'imap4-utf-7':
+        return (encoder, decoder, StreamReader, StreamWriter)
+
+
+codecs.register(imap4_utf_7)