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hz encoded files can be viewed with hztty, anon-ftp ifcss.org:software/unix


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


Fung F. Lee ~{@n7c7e~} (lee@umunhum.stanford.edu)
Stanford Univeristy

September 4, 1989


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' (), 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.