Initial implementation of rfc2047_decode. Based on input from Kidong Lee <brian@ahmlhs.com>.

+/* GNU Mailutils -- a suite of utilities for electronic mail
+   Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.
+
+   GNU Mailutils is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Library Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   GNU Mailutils is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Library General Public License for more details.
+
+   You should have received a copy of the GNU Library General Public License
+   along with GNU Mailutils; if not, write to the Free Software
+   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.  */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+#include <string.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <mailutils/stream.h>
+#include <mailutils/filter.h>
+#include <mailutils/errno.h>
+
+int
+rfc2047_decode (const char *tocode, const char *input, char **ptostr)
+{
+  int status = 0;
+  char *tmpcopy, *fromstr;
+  char *start_position = NULL;
+  char *buffer;
+  size_t bufsize;
+  size_t bufpos;
+  
+  if (!tocode || !input || !ptostr)
+    return EINVAL;
+
+  /* Prepare a temporary copy of the input string (strtok_r is
+     going to modify it. */
+  tmpcopy = strdup (input);
+  if (!tmpcopy)
+    return ENOMEM;
+  fromstr = tmpcopy;
+
+  /* Allocate the buffer. It is assumed that encoded string is always
+     longer than it's decoded variant, so it's safe to use its length
+     as the first estimate */
+  bufsize = strlen (fromstr) + 1;
+  buffer = malloc (bufsize);
+  if (buffer == NULL)
+    {
+      free (tmpcopy);
+      return ENOMEM;
+    }
+  bufpos = 0;
+  
+  while (*fromstr)
+    {
+      char *fromcode = NULL;
+      char *encoding_type = NULL;
+      char *encoded_text = NULL;
+      stream_t filter = NULL;
+      stream_t in_stream = NULL;
+      char *pbuffer = NULL;
+      const char *filter_type = NULL;
+      size_t nbytes = 0, size;
+      char *sp = NULL;
+      char *end_position = NULL;
+
+      start_position = strstr (fromstr, "=?");
+      if (!start_position)
+	break;
+
+      /* Copy the unencoded part */
+      nbytes = start_position - fromstr;
+      if (bufpos + nbytes > bufsize) /* just in case */
+	{
+	  status = MU_ERR_BAD_2047_INPUT;
+	  break;
+	}
+	  
+      memcpy (buffer + bufpos, fromstr, nbytes);
+      bufpos += nbytes;
+      
+      fromcode = strtok_r (start_position + 2, "?", &sp);
+      encoding_type = strtok_r (NULL, "?", &sp);
+      encoded_text = strtok_r (NULL, "?", &sp);
+      if (sp[0] != '=')
+	{
+	  status = MU_ERR_BAD_2047_INPUT;
+	  break;
+	}
+      
+      if (fromcode == NULL || encoding_type == NULL || encoded_text == NULL)
+	{
+	  status = MU_ERR_BAD_2047_INPUT;
+	  break;
+	}
+
+      size = strlen (encoded_text);
+
+      switch (toupper (encoding_type[0]))
+	{
+	case 'B':
+	  filter_type = "base64";
+	  break;
+
+	case 'Q':
+	  filter_type = "quoted-printable";
+	  break;
+
+	default:
+	  status = MU_ERR_BAD_2047_INPUT;
+	  break;
+	}
+
+      if (status != 0)
+	break;
+
+      memory_stream_create (&in_stream, 0, 0);
+      stream_write (in_stream, encoded_text, size, 0, NULL);
+      filter_create (&filter, in_stream, filter_type, MU_FILTER_DECODE,
+                     MU_STREAM_READ);
+
+      while (stream_read (filter, buffer + bufpos, bufsize - bufpos, 0,
+			  &nbytes) == 0 && nbytes)
+        {
+          /* FIXME: Need to convert character set */
+	  bufpos += nbytes;
+        }
+
+      stream_close (filter);
+      stream_destroy (&filter, stream_get_owner (filter));
+      
+      fromstr = sp + 1;
+    }
+
+  if (*fromstr)
+    {
+      size_t len = strlen (fromstr);
+      if (bufpos + len + 1 > bufsize) /* just in case */
+	status = MU_ERR_BAD_2047_INPUT;
+      else
+	{
+	  memcpy (buffer + bufpos, fromstr, strlen (fromstr));
+	  bufpos += strlen (fromstr);
+	}
+    }
+
+  buffer[bufpos++] = 0;
+  
+  free (tmpcopy);
+  *ptostr = realloc (buffer, bufpos);
+  return status;
+}
+