Network Working Group                                             F. Lee
Request for Comments: 1843                           Stanford University
Category: Informational                                      August 1995

HZ - A Data Format for Exchanging Files of
Arbitrarily Mixed Chinese and ASCII characters

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   The content of this memo is identical to an article of the same title
   written by the author on September 4, 1989.  In this memo, GB stands
   for GB2312-80.  Note that the title is kept only for historical
   reasons.  HZ has been widely used for purposes other than "file
   exchange".

1. Introduction

   Most existing computer systems which can handle a text file of
   arbitrarily mixed Chinese and ASCII characters use 8-bit codes.  To
   exchange such text files through electronic mail on ASCII computer
   systems, it is necessary to encode them in a 7-bit format.  A generic
   binary to ASCII encoder is not sufficient, because there is currently
   no universal standard for such 8-bit codes. For example, CCDOS and
   Macintosh's Chinese OS use different internal codes.  Fortunately,
   there is a PRC national standard, GuoBiao (GB), for the encoding of
   Chinese characters, and Chinese characters encoded in the above
   systems can be easily converted to GB by a simple formula. (* The ROC
   standard BIG-5 is outside the scope of this article.)

   HZ is a 7-bit data format proposed for arbitrarily mixed GB and ASCII
   text file exchange.  HZ is also intended for the design of terminal
   emulators that display and edit mixed Chinese and ASCII text files in
   real time.

2. Specification

   The format of HZ is described in the following.

   Without loss of generality, we assume that all Chinese characters
   (HanZi) have already been encoded in GB.  A GB (GB1 and GB2) code is
   a two byte code, where the first byte is in the range $21-$77
   (hexadecimal), and the second byte is in the range $21-$7E.

   A graphical ASCII character is a byte in the range $21-$7E. A non-
   graphical ASCII character is a byte in the range $0-$20 or of the
   value $7F.

   Since the range of a graphical ASCII character overlaps that of a GB
   byte, a byte in the range $21-$7E is interpreted according to the
   mode it is in.  There are two modes, namely ASCII mode and GB mode.

   By convention, a non-graphical ASCII character should only appear in
   ASCII mode.

   The default mode is ASCII mode.

   In ASCII mode, a byte is interpreted as an ASCII character, unless a
   '~' is encountered. The character '~' is an escape character. By
   convention, it must be immediately followed ONLY by '~', '{' or '\n'
   (<LF>), with the following special meaning.

   o The escape sequence '~~' is interpreted as a '~'.
   o The escape-to-GB sequence '~{' switches the mode from ASCII to
     GB.
   o The escape sequence '~\n' is a line-continuation marker to be
     consumed with no output produced.

   In GB mode, characters are interpreted two bytes at a time as (pure)
   GB codes until the escape-from-GB code '~}' is read. This code
   switches the mode from GB back to ASCII.  (Note that the escape-
   from-GB code '~}' ($7E7D) is outside the defined GB range.)

   The decoding process is clear from the above description.

   The encoding process is straightforward. Note that an (ASCII) '~' is
   always encoded as '~~'. A sequence of GB codes is enclosed in '~{'
   and '~}'.

3. Remarks & Recommendations

   We choose to encode any ASCII character except '~' as it is, rather
   than as a two byte code, and we choose ASCII as the default mode for
   the following reasons. The computer systems we use is ASCII based.  A
   HZ file containing pure ASCII characters (i.e. no Chinese characters)
   except '~' is precisely a pure ASCII file. In general, the English
   (ASCII) portion of a HZ file is directly readable.

   The escape character '~' is chosen not only because it is commonly
   used in the ASCII world, but also because '~' ($7E) is outside the
   defined range ($21-$77) of the first byte of a GB code.

   In ASCII mode, other potential escape sequences, i.e., two byte
   sequences beginning with '~' (other than '~~', '~{', '~\n') are
   currently invalid HZ sequences. Hence, they can be used for future
   extension of HZ with total upward compatibility.

   The line-continuation marker '~\n' is useful if one wants to encode
   long lines in the original text into short lines in this data format
   without introducing extra newline characters in the decoding process.

   There is no limit on the length of a line. In fact, the whole file
   could be one long line or even contain no newline characters. Any
   DECODER of this HZ data format should not and has no need to operate
   on the concept of a line.

   It is easy to write encoders and decoders for HZ. An encoder or
   decoder needs to lookahead at most one character in the input data
   stream.

   Given the current mode, it is also possible and easy to decode a HZ
   data stream by scanning backward. One of the implication is that
   "backspaces" can be handled correctly by a terminal emulator.

   To facilitate the effective use of programs supporting line/page
   skips such as "more" on UNIX with a terminal emulator understanding
   the HZ format, it is RECOMMENDED that the ENCODER (which outputs in
   HZ) sets a maximum line size of less than 80 characters.  Since '\n'
   is an ASCII character, the syntax of HZ then automatically implies
   that GB codes appearing at the end of a line must be terminated with
   the escape-from-GB code '~}', and the line-continuation marker '~\n'
   should be inserted appropriately. The price to paid is that the
   encoded file size is slightly larger.

   It is important to understand the following distinction.  Note that
   the above recommendation does NOT change the HZ format.  It is simply
   an encoding "style" which follows the syntax of HZ. Note that this

   "style" is not built into HZ. It is an additional convention built
   "on top of" HZ.  Other applications may require different "styles",
   but the same basic HZ DECODER will always work. The essence of HZ is
   to provide such a flexible basic data format for files of arbitrarily
   mixed Chinese and ASCII characters.

4. Examples

   To illustrate the "stylistic" issue of HZ encoding, we give the
   following four examples of encoded text, which should produce the
   same decoded output. (The recommendation in the last section refers
   to Example 2.)

   Example 1:  (Suppose there is no line size limit.)
   This sentence is in ASCII.
   The next sentence is in GB.~{<:Ky2;S{#,NpJ)l6HK!#~}Bye.

   Example 2:  (Suppose the maximum line size is 42.)
   This sentence is in ASCII.
   The next sentence is in GB.~{<:Ky2;S{#,~}~
   ~{NpJ)l6HK!#~}Bye.

   Example 3: (Suppose a new line is started whenever there is a mode
              switch.)
   This sentence is in ASCII.
   The next sentence is in GB.~
   ~{<:Ky2;S{#,NpJ)l6HK!#~}~
   Bye.

Acknowledgement

   Edmund Lai was the first one who brought my attention to this topic.
   Discussions with Ed, Tin-Fook Ngai, Yagui Wei and Ricky Yeung were
   very helpful in shaping the ideas in this article. Thanks to Tin-Fook
   for his careful review of the draft and numerous interesting
   suggestions.

References

   [1] Fung Fung Lee, "HZ - A Data Format for Exchanging Files of
       Arbitrarily Mixed Chinese and ASCII Characters," September 4,
       1989.
       As part of //ftp.ifcss.org/software/unix/convert/HZ-2.0.tar.gz

Security Considerations

   Security issues are not addressed in this memo.

Author's Address

   Fung Fung Lee
   Computer Systems Laboratory
   Stanford University
   Stanford, CA 94309

   Phone: +1 415 723 1450
   EMail: lee@csl.stanford.edu