[Unicode]  Technical Reports
 

Unicode Technical Standard #6

A Standard Compression Scheme for Unicode

Version 3.6
Authors Misha Wolf, Ken Whistler, Charles Wicksteed, Mark Davis, Asmus Freytag, and Markus Scherer
Date 2005-05-06
This Version http://www.unicode.org/reports/tr6/tr6-4.html
Previous Version http://www.unicode.org/reports/tr6/tr6-3.5.html
Latest Version http://www.unicode.org/reports/tr6/
Revision 4

Summary

This report presents the specifications of a compression scheme for Unicode and sample implementation [SampleCode].

Status

This document has been reviewed by Unicode members and other interested parties, and has been approved for publication by the Unicode Consortium. This is a stable document and may be used as reference material or cited as a normative reference by other specifications.

A Unicode Technical Standard (UTS) is an independent specification. Conformance to the Unicode Standard does not imply conformance to any UTS.

Please submit corrigenda and other comments with the online reporting form [Feedback]. Related information that is useful in understanding this document is found in the References. For the latest version of the Unicode Standard see [Unicode]. For a list of current Unicode Technical Reports see [Reports]. For more information about versions of the Unicode Standard, see [Versions].

Contents

1 Scope

The Standard Compression Scheme for Unicode will: It does not attempt to avoid the use of control bytes (including NUL) in the compressed stream, and does not attempt to preserve binary ordering of strings. 

The compression scheme is mainly intended for use with short to medium length Unicode strings. The resulting compressed format is intended for storage or transmission in bandwidth limited environments. It can be used stand-alone or as input to traditional general purpose data compression schemes. It is not intended as processing format or as general purpose interchange format.

2 Description

The following description is stated as an encoding of a sequence of Unicode characters as a compressed stream of bytes. It is therefore independent, for example, of whether the uncompressed data is encoded as UTF-8, UTF-16 or  UTF-32 (also known as UCS-4 in ISO 10646). If the compressed data consists of the same sequence of bytes, it represents the same sequence of characters. The reverse is not true — there are multiple ways of compressing any character sequence.

While the description uses the term character throughout, no limitation to assigned characters is implied; in other words, SCSU is defined in terms of code points.

2.1 Compression Scheme for Unicode

Compressing Unicode text for transmission or storage is often useful. The traditional general purpose data compression schemes such as Huffman or LZW are effective, but require considerable context for best results. In the course of implementing Unicode, it became apparent that there is a need for a compression scheme that is efficient even for short strings. The compression scheme described here compresses Unicode text into a sequence of bytes by taking advantage of the characteristics of Unicode text. The resulting compressed sequence can be used on its own or as further input to a general purpose compression scheme. The latter achieves even better compression than either method alone.

Some languages use a small repertoire of characters. Strings in such languages often contain runs of characters encoded close together in [Unicode]. These runs are typically interrupted only by punctuation characters, which are encoded in proximity to each other in Unicode, usually in the Basic Latin range.

The compression scheme sets up a so-called dynamically positioned window, which is a region of 128 consecutive characters in Unicode. This window can be positioned to contain the alphabetic characters in question. Each character that fits this window is represented as a byte between 0x80 and 0xFF in the compressed data stream, while any character from the Basic Latin range (as well as CR, LF, and TAB) is represented by a byte in the range 0x20 to 0x7F (as well as 0x0D, 0x0A or 0x09).

Runs of characters from a selected window which are intermixed only with characters from the range U+0020..U+007F can be compressed without requiring tag bytes beyond the initial setup of the window.

Tag bytes are bytes in the range 0x00 to 0x1F (except CR, LF, TAB) that are used as commands to select, define and position windows, or to escape to an uncompressed stream of Unicode text. Strings from languages using large alphabets use this uncompressed mode.

There are scripts for which the characters ordinarily show larger fluctuation in code values than can be contained in a dynamically positioned window. For these areas of the Unicode code space, windows cannot be set. Instead, an escape to uncompressed UTF-16 can be used.

2.2 Encoders and Decoders

There is more than one possible encoding for a given Unicode string, and it is possible to trade off speed of encoding against the compression achieved.

It is possible to write a simple encoder for this scheme which uses a subset of the allowed tags. For example, it could use only SCU, SD0, UQU and UC0 and still achieve respectable compression with typical text. See Section 8.4, Minimal Encoder for further discussion and sample code.

Encoders should follow the recommendations in Section 8.3, XML Suitability so that they can be used to encode XML, HTML and similar document formats.

2.3 Limitations

SCSU does not attempt to avoid the use of control bytes (including NUL) in the compressed stream. It is sometimes possible to escape control characters in the manner of Section 10.1, Avoiding Control Byte Values but this requires an additional agreement between sender and receiver. 

SCSU also does not attempt to preserve the binary ordering of strings, and is not MIME compatible, which limits its attractiveness as a processing format, particularly in databases, or as general purpose interchange format. If these features are required, a different compression scheme, such as [BOCU] could be employed. 

3 Definitions

All terms not defined here shall be as defined in the Unicode Standard [Unicode] or in the online [Glossary].
 
CD1. Single-Byte Mode
A mode where each character is represented in compressed form as a single byte.
 
CD2. Unicode Mode
A mode where each character is represented by big-endian UTF-16.
 
CD3. Window
A range of 128 consecutive Unicode character values.
 
CD4. Locking Shift
A permanent shift to a new active window.
 
CD5. Non-Locking Shift
A non-locking shift selects a window only for the immediately following character, before returning to the active window.
 
CD6. Dynamically Positioned Window
A window with a position that can be selected starting at a multiple of 128 or at one of several predefined locations. Dynamically positioned windows can be accessed by locking or non-locking shifts, and are only used in single-byte mode with bytes in the range 0x80 to 0xFF.
 
CD7. Static Window
A window with fixed position which can be accessed by non-locking shift only. They are used in single-byte mode with bytes in the range 0x00 to 0x7F.
 
CD8. Tag Byte
Any of the predefined single byte values that select compression functions in this scheme.
 
CD9. Index Byte
A byte that is used as an index into the offset table (for example, to select a window offset).
 
CD10. Supplementary Codespace
The codespace accessed by surrogate pairs in UTF-16.
 

4 Conformance

C1 Decoders are required to accept and interpret the full range of tags and arguments defined here. The action of a conformant decoder on illegal or reserved input is undefined.
C2 Conformant encoders must not emit illegal or reserved combinations of bytes. Encoders are not required to utilize (or be able to utilize) all the features of this compression scheme. Encoders must be able to encode strings containing any valid sequence of Unicode characters. The action of a conformant encoder on malformed input is undefined.
C3 Encoders and decoders must always start in the initial state defined below. Encoders must remain in Single-Byte Mode at least until the first code point is encountered that is not U+0000 (NUL), U+0009 (HT), U+000A (LF), U+000D (CR), or U+0020..U+00FF (Latin-1), or an initial U+FEFF. See Section 8.1, Signature Byte Sequence for SCSU and Section 8.3, XML Suitability.
C4 Conformance to SCSU requires conformance to Unicode 2.0.0 or later.

Conformance to SCSU excludes the options in Section 10, Possible Private Extensions. A higher-level protocol could define an extended form of SCSU that implements these or other extensions to SCSU. Such a higher-level protocol requires a separate agreement between sender and receiver.

5 Compression

The Unicode Compression Scheme compresses text by defining a set of windows into the [Unicode] codespace and interpreting byte values relative to the position of the window currently in force. Thus characters from languages that use a small alphabet can be encoded with one byte per character. By switching to Unicode mode, non-alphabetic scripts can be encoded with two bytes per character on the BMP or four bytes per supplementary character.

The compression scheme is capable of compressing strings containing any Unicode character. Some control character and private use character values overlap with the tag byte values. They can still be encoded, though at a cost of an additional byte per character.

There are two compression modes:

In the following text all byte values are given in hex.

5.1 Single-Byte Mode

Compressed text in single-byte mode consists of a tag byte followed by zero, one, or two argument bytes followed by one or more text bytes. Single-byte mode is in effect from initialization until the end of input or until an SCU tag. An SCU tag indicates that all following bytes are interpreted in Unicode mode as big-endian UTF-16. An SQU tag indicates that the following two bytes are interpreted as a sixteen bit Unicode BMP character, most significant byte first.

In single-byte mode, bytes between 00 and 1F are used as tags. The tags used in single-byte mode are shown in Table 1, their corresponding byte values are shown in Table 6.

Table 1. Tags for Use in Single-Byte Mode
Name  Meaning  Arguments  Function 
SQU  Quote Unicode hbyte, lbyte  Quote Unicode character = (hbyte << 8) + lbyte.
Used for isolated characters from the BMP that do not fit in any of the current windows.
SCU  Change to Unicode   Change to UTF-16 mode (locking shift).
Used for runs of characters not part of a small alphabet
SQn  Quote from Window n . byte  Non-locking shift to window n.
If the byte is in the range 00 to 7F, use static window n.
If the byte is in the range 80 to FF, use dynamically positioned window n.
SCn  Change to Window n   Change to window n (locking shift).
Use static window 0 for all following bytes that are in the range 20 to 7F, or CR, LF, HT.
Use dynamically positioned window n for all following bytes that are in the range 80 to FF.
SDn  Define Window n byte  Define window position n as OffsetTable[byte], and change to window n. 
SDX  Define Extended hbyte, lbyte Define window n in the supplementary codespace and change to it.
n = top 3 bits of hbyte.
Window base = 10000 + (80 * remaining 13 bits of hbyte and lbyte).

5.2 Unicode Mode

In Unicode mode, each character is encoded by two or four bytes as big-endian UTF-16, i.e. with the most significant byte first. This mode has its own set of reserved byte values which are used as tags, as shown in Table 2. Their corresponding byte values are shown in Table 6. Once selected by SCU, Unicode mode is in effect until the end of input, or until any tag that selects an active window.

5.2.1 Quoting in Unicode Mode

Note that in Unicode mode all tags are single bytes. Therefore all bytes which are not tag bytes are the most significant bytes (MSB) of a Unicode character. Each reserved tag value collides with 256 Unicode characters. A quoting mechanism is defined for Unicode mode to enable a character to be encoded whose first byte would collide with a tag value. The two bytes following a UQU tag are taken as a Unicode character on the BMP. The tags values used in Unicode mode are chosen so that they correspond to the most significant bytes of Unicode character values from the private use area, since private use characters are not in frequent use.
Table 2. Tags for Use in Unicode Mode
Name  Meaning  Arguments  Function 
UQU  Quote Unicode hbyte, lbyte  Quote a Unicode BMP character.
Used to quote tag bytes. 
UCn  Change to Window n   Change to single-byte mode, window n (locking shift).
Use static window 0 for all following bytes that are in the range 20 to 7F, or CR, LF, HT.
Use dynamically positioned window n for all following bytes that are in the range 80 to FF.
UDn  Define Window n byte  Define window position n as OffsetTable[byte], and change to window n. 
UDX Define Extended hbyte, lbyte Define window n in the supplementary codespace and change to it.
n = top 3 bits of hbyte
Window base = 10000 + (80 * remaining 13 bits of hbyte and lbyte)

6 Windows

Windows are always 128 code positions in length. There are two kinds of windows, static (or fixed position) windows and dynamically positioned windows.

6.1 Dynamically Positioned Windows

There are eight dynamically positioned windows used when compressing alphabetic text. Locking shift tags in the byte stream are used to select an active window, and other tags are used to redefine the position of any window. At initialization, the dynamically positioned windows are in their default positions shown in Table 5.

6.1.1 Locking Shifts (Dynamically Positioned Windows Only)

An SCn tag (or UCn tag in Unicode mode) is used for a locking shift to dynamically positioned window n. Following such a tag, bytes in the range 80 to FF represent characters in the active dynamically positioned window. Therefore any byte xx between 80 and FF encodes the Unicode character as follows:

Unicode character = DynamicOffset[n] + (xx - 80)

The values for the starting offsets of dynamically positioned windows can change. Their initial values are specified in Table 5. Bytes in the range 20 to 7F always represent the corresponding character from the Basic Latin block (U+0020 to U+007F). In addition, LF, CR and HT represent U+000A, U+000D and U+0009 respectively.

6.1.2 Window Positioning

An SDn tag (or UDn tag) followed by an index byte repositions window n and makes it the active window. To keep the encoding compact, the positions of the dynamically positioned windows are defined via a lookup table. Each window definition tag in the byte stream is followed by one byte that is used as an index into this table. The set of legal positions is defined by the Window Offset Table shown in Table 3.

The first part of the Window Offset Table defines half blocks covering the alphabetic scripts, symbols and the private use area. The individual entries from F9 onwards cover the scripts that cross a half-block boundary, plus one useful segment of European characters. Some collections of miscellaneous symbols and punctuation also cross half-block boundaries, but these characters are likely to occur rarely, or in isolation. Therefore no special offsets for them are included here.

Table 3. Window Offset Table
Byte x  OffsetTable[x]  Comment 
00  reserved  reserved for internal use 
01..67  x*80  half-blocks from U+0080 to U+3380 
68..A7  x*80+AC00  half-blocks from U+E000 to U+FF80 
A8..F8 reserved  reserved for future use 
F9  00C0  Latin-1 letters + half of Latin Extended-A 
FA  0250  IPA Extensions
FB  0370  Greek 
FC  0530  Armenian 
FD 3040  Hiragana 
FE 30A0 Katakana
FF  FF60  Halfwidth Katakana 

6.1.3 Extended Windows

An SDX tag (or UDX tag in Unicode mode) followed by two argument bytes (hbyte and lbyte) defines window n in the supplementary codespace and makes it the active window. The window index n is given by the top 3 bits of hbyte. The window offset is calculated from the remaining thirteen bits of hbyte and lbyte as follows:

offset = 10000 + (80 * ((hbyte & 1F) * 100 + lbyte))

where & is the bitwise AND operator and all values are in hexadecimal notation. After an extended window is defined each subsequent byte in the range 80 to FF represents a character from the supplementary codespace.

For example, when decoding SCSU into UTF-16, the bits in the two argument bytes following the SDX (or UDX) and a subsequent data byte map onto the bits in the resulting surrogate pair as shown in the following table:

 
Table 3a. Parameter Format Following SDX
High Surrogate Low Surrogate
110110wwwwwzzzzz 110111yyyxxxxxxx
nnnwwwww zzzzzyyy 1xxxxxxx
High Byte Low Byte Data Byte

6.2 Non-Locking Shifts and Static Windows

An SQn tag switches temporarily to a different window for just one character. The byte following the tag is interpreted relative to the window n, and then the window reverts to the previous value. This is called a non-locking shift. If the byte following the SQn is in the range 80 to FF, dynamically positioned window n is used.

6.2.1 Static Windows

There are eight static windows, seven of which are used only in conjunction with non-locking shifts. If any data byte following an SQn tag is in the range 00 to 7F, static window n is used. Therefore byte xx between 00 and 7F encodes the Unicode character as follows:

Unicode character = StartingOffset[n] + xx

The positions of static windows are as shown in Table 4 and cannot be changed. The static windows cover character ranges which contain characters that tend to occur in isolation and therefore are suitable for access via non-locking shifts. Static window 0 is also used when bytes following an SCn or UCn are in the range 20 to 7F.

Table 4. Static Window Positions
Window  Starting Offset  Major Area Covered 
0  0000  (for quoting of tags used in single-byte mode)
1  0080  Latin-1 Supplement 
2  0100  Latin Extended-A
3  0300  Combining Diacritical Marks
4  2000  General Punctuation 
5  2080 Currency Symbols
6  2100 Letterlike Symbols and Number Forms
7  3000 CJK Symbols & Punctuation 

6.2.2 Use of SQ0

SQ0 is used to quote characters that would otherwise collide with tag bytes. It may not be used with bytes in the range 20 to 7F. These values shall not be used by encoders. Decoders are not required to detect them as errors. Note that this restriction applies only to SQ0, which maps to ASCII. SQ1 to SQ7 may be followed by any byte value.

As in the general case of SCn, a following byte value in the range 80 to FF indicates use of dynamically positioned window 0.

7 Special Issues

7.1 Initial State

The initial state of encoder and decoder is as follows: Note: For APIs or data streams that mix text and data, it is expected that the encoder and decoder will be reinitialized at the beginning of each string or compressible chunk of text data.

7.2 Initial Window Settings

Encoder and Decoder are initialized with certain default settings for the windows. These allow use of the windows without predefining them, generally saving a few bytes. Encoder and Decoder always start with dynamically positioned window 0 active, so a string of characters that consists entirely of characters from the range U+0020..U+00FF plus CR, LF, TAB is effectively converted to ISO 8859-1.

Default positions are assigned based on the following criteria:

The choice of offsets makes it possible to handle most languages by requiring no more than the definition of one extra window, at the cost of a single byte. The default settings of the dynamically positioned windows are shown in Table 5. The static window positions are fixed and are shown in Table 4.

Table 5. Default Positions for Dynamically Positioned Windows
Window  Starting Offset  Major Area Covered 
0  0080  Latin-1 Supplement 
1  00C0  (combined partial Latin-1 Supplement/Latin Extended-A)
2 0400  Cyrillic
3 0600 Arabic
4  0900  Devanagari 
5 3040  Hiragana
6 30A0  Katakana 
7 FF00  Fullwidth ASCII 

7.3 Surrogate Pairs

A supplementary character, that is, a character corresponding to a surrogate pair in UTF-16, can be encoded in any of the following ways: It is not possible to set a window to the surrogate range, such that one byte would represent one half of a surrogate pair. However, the encoding for both halves of a surrogate pair is not required to use the same method.

Note: All conformant decoders that output UTF-8 or UTF-32 must be prepared to convert surrogate pairs to characters, even for the case SQU hbyte1 lbyte1 SQU hbyte2 lbyte2.

7.4 Private Use Area

A character in the Private Use Area on the BMP can be encoded in any of the following ways:

7.5 Tag Allocation

The tag byte values used in single-byte mode are shown in Table 6. In this table, "pass" means that the byte value (XX) represents the Unicode code point U+00XX.
Table 6. Single-Byte Mode Tag Values
Name  Value  Comment 
pass 00  NUL
SQ0 - SQ7 01 - 08   
pass  09 HT
pass  0A  LF
SDX 0B   
reserved 0C  reserved for future use
pass 0D CR
SQU 0E  
SCU 0F  
SC0 - SC7 10 - 17  
SD0 - SD7 18 - 1F  
pass 20 - 7F  

The tag byte values used in Unicode mode are shown in Table 7. In this table MSB means that the byte value is used as the most significant byte of a two byte sequence representing a Unicode code point on the BMP. There are no restrictions on the values of the byte immediately following an MSB.

Table 7. Unicode Mode Tag Values
Name  Value  Comment 
MSB 00 - DF Start of a Unicode character
UC0 - UC7 E0 - E7  
UD0 - UD7 E8 - EF  
UQU F0   
UDX F1   
reserved F2  reserved for future use
MSB F3 - FF Start of a Unicode character

8 Notes (Informative)

8.1 Signature Byte Sequence for SCSU

Where data streams are not tagged externally, it is useful to provide a signature at the beginning of the stream. For UTF-16, UTF-32 and UTF-8, this is done by using U+FEFF to allow identification of the text as Unicode and to distinguish little-endian from big-endian forms of UTF-16 and UTF-32.

Unlike the standard character encoding forms defined in [Unicode], SCSU does not have a single representation for U+FEFF. Depending on the implementation of an SCSU encoder, and depending on the following text, a leading U+FEFF character could be encoded as one of these initial byte sequences:

Table 8. Possible Encodings of Initial U+FEFF
Bytes  Commands  Comment 

Preferred

0E FE FF

SQU FE FF

Single-byte mode Quote Unicode

Not Recommended

0F FE FF

SCU FE FF

Single-byte mode Change to Unicode 

18 A5 FF

SD0 A5 FF

Single-byte mode Define dynamic window 0 to 0xFE80 

19 A5 FF

SD1 A5 FF

Single-byte mode Define dynamic window 1 to 0xFE80 

1A A5 FF

SD2 A5 FF

Single-byte mode Define dynamic window 2 to 0xFE80 

1B A5 FF

SD3 A5 FF

Single-byte mode Define dynamic window 3 to 0xFE80 

1C A5 FF

SD4 A5 FF

Single-byte mode Define dynamic window 4 to 0xFE80 

1D A5 FF

SD5 A5 FF

Single-byte mode Define dynamic window 5 to 0xFE80 

1E A5 FF

SD6 A5 FF

Single-byte mode Define dynamic window 6 to 0xFE80 

1F A5 FF

SD7 A5 FF

Single-byte mode Define dynamic window 7 to 0xFE80

It is recommended to use only the byte sequence <0E FE FF> for an initial U+FEFF character (0E is the "SQU" tag). This convention will assist receiving processes that use initial byte sequences to identify a data file or stream as being encoded in SCSU. Every SCSU encoder should write this particular initial byte sequence if a U+FEFF is encountered as the first character in the stream. Any further occurrences of this character may be encoded in the most compact way possible with SCSU. 

Note: The recommended sequence is the only one that does not affect the state of the encoder or decoder, and may be safely stripped by a receiver even before initiating a decoder.

A process reading text from a file or stream could interpret the initial bytes <0E FE FF> as a signature for SCSU and assume that the file or stream is encoded in SCSU. The process or SCSU decoder may or may not strip the initial U+FEFF character from the resulting text. Any other encoding of an initial U+FEFF character, and any encoding of a U+FEFF after the initial character are normally interpreted as a ZWNBSP.

If the input text starts with a U+FEFF that is to be interpreted as a ZWNBSP, then an encoder or sending process may prepend the text with another U+FEFF which may be safely recognized as an SCSU signature and stripped by a receiving process. Otherwise, the initial ZWNBSP could be misinterpreted as a signature and stripped by a receiving process. This is equivalent to sending and receiving text in UTF-16 or UTF-32. A signature should not be used where a protocol specification, database design, or out-of-band information or similar specifies the encoding.

8.2 Worst Case Behavior

By using SCU plus an input string in UTF-16, almost all Unicode strings can be represented with the same number of bytes as their UTF-16 encoding plus 1 byte. Strings containing private use characters in which the MSB collides with the tag byte values are the exception. These characters must be quoted with SQU or UQU, requiring three bytes instead of two bytes per character. Therefore, an absolute upper limit of required SCSU length is three bytes per UTF-16 code unit. (See also Section 5.2.1, Quoting in Unicode Mode). This upper limit is reached only for strings of n characters containing at least n-1 private use characters, subject to the quoting requirement.

Because the characters requiring SQU or UQU are in the BMP, an SCSU encoded string is never required to be longer than four bytes per character. In other words, it is never longer than its UTF-32 encoding. For supplementary characters there is no need for a one byte overhead, because any supplementary character can be represented using four bytes in SCSU by using SDX. (See also Section 6.1.3, Extended Windows).

A Unicode string consisting entirely of certain control characters will take up twice as much space in SCSU than in UTF-8, since each control character must be individually quoted with SQ0. (See also Section 5.1, Single-Byte Mode).

All of these upper limits can be exceeded, if an encoder deliberately chooses a particularly inefficient representation, such as using SQU or UQU to quote each surrogate separately for characters in the supplementary codespace (see also Section 7.3, Surrogate Pairs), or inserting redundant tags.

Typical compression of average text is markedly better than the worst case behavior, and normal text is encoded with fewer bytes in SCSU than in either UTF-8 or UTF-16.

8.3 XML Suitability

SCSU can be used for XML or HTML or similar documents if attention is paid to the in-document encoding declaration. The process emitting the document should place the encoding declaration at the earliest possible location, in front of any non-Latin-1 characters. Such documents can be parsed properly up to and including the encoding declaration, because many document parsers initially assume ASCII-compatible encodings. (See also Section F, Autodetection of Character Encodings of [XML 1.0].)

An SCSU encoder is XML-Suitable if it encodes all initial Latin-1 text (code points U+0000, U+0009, U+000A, U+000D, U+0020..U+00FF) in the shortest possible form. That is, it uses Single-Byte Mode without SQ0, SC0 or any other commands. This encodes initial Latin-1 text with the same bytes as with ISO 8859-1. It would be unusual for an SCSU encoder to not encode initial Latin-1 text in the shortest form, so most existing SCSU encoders are XML-Suitable.

If there were an initial U+FEFF indicating a Unicode encoding signature, it would be encoded with SQU (see Section 8.1, Signature Byte Sequence for SCSU). However, many HTML and XML parsers do not recognize Unicode encoding signatures other than for UTF-16, so such a signature should not be used with XML and HTML documents.

8.4 Minimal Encoder

While it is straightforward to write an SCSU decoder, writing an encoder may seem complicated because there are many ways to encode the same text. The choices that are made for an implementation affect the achievable compression ratio.

However, it is quite simple to write a minimal SCSU encoder that still produces valid and reasonable, even XML-suitable, output. The scsumini.c sample C code [SampleMini] demonstrates this; its encoder function consists of about 75 lines of C code and uses only a very small amount of state: a boolean flag for single-byte versus Unicode mode and an integer for the current window. It uses most SCSU commands, including quoting from and switching to all pre-defined windows, but does not define dynamic windows and does not use any look-ahead.

This kind of encoder is generally sufficient for text with mostly Latin/Cyrillic/Arabic/Devanagari/Japanese characters and CJK ideographs.

8.5 Encoder Strategies

Even an encoder with good compression performance is relatively easy to write. The following are tactics used:

For optimal compression, an encoder would have to look ahead several characters and probably compare multiple alternatives for sections of the text. The compression of normal text may improve only by a relatively small percentage compared to the strategy outlined in the previous paragraph.

9 Examples (Informative)

9.1 German

German can be written using only Basic Latin and the Latin-1 supplement, so all characters above 0x0080 use the default position of dynamically positioned window 0.

Sample text (9 characters)

Öl fließt

Unicode code points (9 code points):

00D6 006C 0020 0066 006C 0069 0065 00DF 0074

Compressed (9 bytes):

D6 6C 20 66 6C 69 65 DF 74

9.2 Russian

Russian can use the default position of window 2. The first byte of the compressed data is the tag SC2.

Sample text (6 characters)

Москва

Unicode code points (6 code points):

041C 043E 0441 043A 0432 0430

Compressed (7 bytes):

12 9C BE C1 BA B2 B0

9.3 Japanese

Japanese text almost always profits from the multiple predefined windows in SCSU. For more details on this sample see below.

Sample text (116 characters)

　♪リンゴ可愛いや可愛いやリンゴ。半世紀も前に流行した「リンゴの歌」がぴったりするかもしれない。米アップルコンピュータ社のパソコン「マック（マッキントッシュ）」を、こよなく愛する人たちのことだ。「アップル信者」なんて言い方まである。

Unicode code points (116 code points)

3000 266A 30EA 30F3 30B4 53EF 611B
3044 3084 53EF 611B 3044 3084 30EA 30F3
30B4 3002 534A 4E16 7D00 3082 524D 306B
6D41 884C 3057 305F 300C 30EA 30F3 30B4
306E 6B4C 300D 304C 3074 3063 305F 308A
3059 308B 304B 3082 3057 308C 306A 3044
3002 7C73 30A2 30C3 30D7 30EB 30B3 30F3
30D4 30E5 30FC 30BF 793E 306E 30D1 30BD
30B3 30F3 300C 30DE 30C3 30AF FF08 30DE
30C3 30AD 30F3 30C8 30C3 30B7 30E5 FF09
300D 3092 3001 3053 3088 306A 304F 611B
3059 308B 4EBA 305F 3061 306E 3053 3068
3060 3002 300C 30A2 30C3 30D7 30EB 4FE1
8005 300D 306A 3093 3066 8A00 3044 65B9
307E 3067 3042 308B 3002

Compressed (178 bytes)

08 00 1B 4C EA 16 CA D3 94 0F 53 EF 61 1B E5 84
C4 0F 53 EF 61 1B E5 84 C4 16 CA D3 94 08 02 0F
53 4A 4E 16 7D 00 30 82 52 4D 30 6B 6D 41 88 4C
E5 97 9F 08 0C 16 CA D3 94 15 AE 0E 6B 4C 08 0D
8C B4 A3 9F CA 99 CB 8B C2 97 CC AA 84 08 02 0E
7C 73 E2 16 A3 B7 CB 93 D3 B4 C5 DC 9F 0E 79 3E
06 AE B1 9D 93 D3 08 0C BE A3 8F 08 88 BE A3 8D
D3 A8 A3 97 C5 17 89 08 0D 15 D2 08 01 93 C8 AA
8F 0E 61 1B 99 CB 0E 4E BA 9F A1 AE 93 A8 A0 08
02 08 0C E2 16 A3 B7 CB 0F 4F E1 80 05 EC 60 8D
EA 06 D3 E6 0F 8A 00 30 44 65 B9 E4 FE E7 C2 06
CB 82

Details about the Japanese Text Example

Japanese example

The example above consists of a short piece of text found in a Japanese news story. Each character is color coded to indicate which characters can be encoded using the same window. The table lists the number of occurrences of characters for a given window divided by the number of runs, yielding the average run length.

The reference encoder will encode the 116 characters of this example into 178 bytes. This is approximately 3/4 of the size required to store the text in UTF-16, or any of the double byte character sets. A single window implementation, like the original Reuters' RCSU version of the Compression scheme would have required about a dozen window resets, plus would have had to resort to quoting Unicode a few more times. A complex example like this demonstrates the advantage of the multiple window implementation quite nicely.

9.4 All Features

The following sample compressed string contains all the features of the compression scheme, but limited to only representative instances of the eight SQn and the seventeen SCn/UCn, SDn/UDn, and SDX/UDX pairs. The text is repeated to demonstrate how the same substring can yield different compressed strings.

Unicode code points (18 code points):

0041 00DF 0401 015F 00DF 01DF F000 10FFFF 000D 000A 0041 00DF 0401 015F 00DF 01DF F000 10FFFF

UTF-16 code units (20 code units)

0041 00DF 0401 015F 00DF 01DF F000 DBFF DFFF 000D 000A 0041 00DF 0401 015F 00DF 01DF F000 DBFF DFFF

Compressed (35 bytes)

41 DF 12 81 03 5F 10 DF 1B 03 DF 1C 88 80 0B BF FF FF 0D 0A 41 10 DF 12 81 03 5F 10 DF 13 DF 14 80 15 FF

10 Possible Private Extensions (Informative)

During the design and review phase of the compression scheme, the extensions described in this section were suggested. Although these extensions were not accepted as part of the compression scheme itself, they are documented here as examples of how certain problems can be solved by adding higher-level protocols, for use by consenting parties.

10.1 Avoiding Control Byte Values

With a simple re-mapping, the SCSU encoded data stream can be made free of most control byte values so that it can be passed where ASCII text is expected. This re-mapping is not as costly as more general schemes for converting binary data to text and leaves the text parts of compressed Latin-1 text fully readable.

After encoding, replace any control byte by DLE (0x10) followed by the original byte plus 0x40. NUL becomes DLE followed by '@' (0x40). DLE is replaced by DLE followed by U+0050. Before decoding, the opposite transformation must be performed.

10.2 Handling Runs of the Same Character

Longer runs of the same character allow additional compression. Because this scenario is unusual, it was omitted from the standard algorithm. In situations where sender and receiver can agree on the additional specification and where runs are common, the following method is suggested:

Before encoding, replace any run of four or more Unicode characters by '@' (U+0040), followed by the character to repeat, followed by a 16-bit count (packed into one Unicode character). The sequence of 33 hyphens --------------------------------- becomes '@' '-' '!' (0x40, 0x2D, 0x21). Any occurrence of @ sign by itself is replaced by @@U+0001. After decoding, the reverse operation must be performed.

References

[BOCU]

BOCU-1: MIME-Compatible Unicode Compression
http://www.unicode.org/notes/tn6/
Binary Ordered Compression for Unicode (BOCU)
[FAQ] Unicode Frequently Asked Questions
http://www.unicode.org/faq/
For answers to common questions on technical issues; see in particular http://www.unicode.org/faq/compression.html
[Feedback] Reporting Errors and Requesting Information Online
http://www.unicode.org/reporting.html
[Glossary] Unicode Glossary
http://www.unicode.org/glossary/
For explanations of terminology used in this and other documents.
[Reports] Unicode Technical Reports
http://www.unicode.org/reports/
For information on the status and development process for technical reports, and for a list of technical reports.
[SampleCode] Sample Java code with a full implementation of SCSU
http://www.unicode.org/Public/PROGRAMS/SCSU/ or
ftp://ftp.unicode.org/Public/PROGRAMS/SCSU/
[SampleMini] Sample C code with a minimal implementation of an SCSU encoder; see Section 8.4, Minimal Encoder
http://www.unicode.org/Public/PROGRAMS/SCSUMini/ or
ftp://ftp.unicode.org/Public/PROGRAMS/SCSUMini/
[Unicode] The Unicode Consortium. The Unicode Standard, Version 4.0. Reading, MA, Addison-Wesley, 2003. 0-321-18578-1.
[Versions] Versions of the Unicode Standard
http://www.unicode.org/standard/versions/
For details on the precise contents of each version of the Unicode Standard, and how to cite them.
[XML 1.0] Extensible Markup Language (XML) 1.0 (Third Edition)
W3C Recommendation 04 February 2004
http://www.w3.org/TR/REC-xml/
In particular, see Section F, Autodetection of Character Encodings
 http://www.w3.org/TR/REC-xml/#sec-guessing

Acknowledgements

The authors would like to thank Dr. Laura Wideburg for assistance in copy editing. Thanks to David Pope, Doug Ewell and Roman Czyborra for bug reports. Markus Scherer proposed the signature sequence for SCSU. David Starner suggested a section on worst-case behavior.

Authors

The original concept of a standard compression scheme for Unicode was implemented at Reuters and proposed by Misha Wolf and Charles Wicksteed. Extensions and refinements were proposed by Mark Davis, Ken Whistler and Martin Duerst. The final text for the Technical Report and the original sample implementations were created by Asmus Freytag. The Technical Report is now maintained by Markus Scherer, who also contributed the scsumini sample.

Revisions

Note: none of the fixes imply a change to the specification.

Modifications

The following summarizes modifications from the previous version of this document.

4

Added 8.4 Minimal Encoder and 8.5 Encoder Strategies and the [SampleMini] sample code for a minimal encoder. Many editorial changes, including a move of sections 8.1..8.3 to 7.2..7.5. Included the formerly linked details page for the Japanese Text Example (9.3) into this text directly.

Adopted the common style of separate version number from document revision numbering.
3.5 Added recommendation to remain in Single-Byte Mode for initial Latin-1 text, and an informative section about the resulting XML suitability.
1.0 - 3.4 1. Russian uses SC2 instead of SC7 as claimed in the examples.

2. The 'All Features' example has been corrected.

3. A new Japanese example has been added.

4. Changed Table 3 from
 
68..A7  x*80+AE00  half-blocks from U+E000 to U+FF80 

to
 
68..A7  x*80+AC00  half-blocks from U+E000 to U+FF80 

to match the correct value used in the sample code.

5. Corrected 1FFF to 1F in the offset calculation equation for defining extended windows.

6. Corrected a few minor typographical errors [6/5/99].

7. Corrected dynamic offset in for Window 1 in sample code to 0x00C0 to match Table 5 of specification (updated internal version number of SCSU.java to 005 and commented changed source line).

8. Changed methods in the expander from private to protected to support a minor update of the driver program. (Updated internal version number to 005 in Expand.java and added a comment).

9. Minor improvements to the driver program. (Updated internal version number to 005 in CompressMain.java)

10. Editorial reformatting. [11/12/99]

11. Added the section on use of signature and changed version to 3.1 (The sample programs have not been updated to implement this recommendation).

12. Fixed HTML validation error. [3/11/00]

13. Added an informative section on worst-case behavior [10/31/01].

14. Changed references to 'expansion space' to 'supplementary coding space', to be more in line with terminology introduced in Unicode 3.1.

15. Clarified that the "Unicode" data in Unicode Mode is UTF-16BE. This clarification is necessary since later versions of the Unicode Standard add UTF-8 and UTF-32 on an equal basis.

16. Clarified that SCSU is an encoding of a sequence of code points, independent of the encoding form. This makes no change to the specification, since nothing in the original wording required the uncompressed data to be in UTF-16.

17. Clarified that SQU and UQU may only be applied to characters on the BMP, which are represented by two bytes in SCSU.

18. In 6.2.1, corrected

Static window 0 is also used when bytes following an SCn or UCn are in the range 80 to FF.

to

Static window 0 is also used when bytes following an SCn or UCn are in the range 20 to 7F.

19. Corrected the example in section 10.2.

20. Changed styles and template.

21. Added section 2.3 to discuss limitations of SCSU.  Added references. [05/08/02]

22. Changed "Unicode Values" to "code points" and made similar clarifications throughout.

Added restriction to remain in Single-Byte Mode for initial Latin-1 text, and an informative section about the resulting XML suitability.

 

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