Jarkko Hietaniemi > perl-5.7.3 > EncodeFormat

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NAME ^

EncodeFormat - the format of encoding tables of the Encode extension

DESCRIPTION ^

The format used in the encoding tables of the Encode extension has been borrowed from Tcl, as has the following documentation been borrowed from the same. The documentation has been reformatted as Perl pod.

Space would prohibit precompiling into Tcl every possible encoding algorithm, so many encodings are stored on disk as dynamically-loadable encoding files. This behavior also allows the user to create additional encoding files that can be loaded using the same mechanism. These encoding files contain information about the tables and/or escape sequences used to map between an external encoding and Unicode. The external encoding may consist of single-byte, multi-byte, or double-byte characters.

Each dynamically-loadable encoding is represented as a text file. The initial line of the file, beginning with a ``#'' symbol, is a comment that provides a human-readable description of the file. The next line identifies the type of encoding file. It can be one of the following letters:

[1] S

A single-byte encoding, where one character is always one byte long in the encoding. An example is iso8859-1, used by many European languages.

[2] D

A double-byte encoding, where one character is always two bytes long in the encoding. An example is big5, used for Chinese text.

[3] M

A multi-byte encoding, where one character may be either one or two bytes long. Certain bytes are a lead bytes, indicating that another byte must follow and that together the two bytes represent one character. Other bytes are not lead bytes and represent themselves. An example is shiftjis, used by many Japanese computers.

[4] E

An escape-sequence encoding, specifying that certain sequences of bytes do not represent characters, but commands that describe how following bytes should be interpreted.

The rest of the lines in the file depend on the type.

Cases [1], [2], and [3] are collectively referred to as table-based encoding files. The lines in a table-based encoding file are in the same format as this example taken from the shiftjis encoding (this is not the complete file):

 # Encoding file: shiftjis, multi-byte
 M
 003F 0 40
 00
 0000000100020003000400050006000700080009000A000B000C000D000E000F
 0010001100120013001400150016001700180019001A001B001C001D001E001F
 0020002100220023002400250026002700280029002A002B002C002D002E002F
 0030003100320033003400350036003700380039003A003B003C003D003E003F
 0040004100420043004400450046004700480049004A004B004C004D004E004F
 0050005100520053005400550056005700580059005A005B005C005D005E005F
 0060006100620063006400650066006700680069006A006B006C006D006E006F
 0070007100720073007400750076007700780079007A007B007C007D203E007F
 0080000000000000000000000000000000000000000000000000000000000000
 0000000000000000000000000000000000000000000000000000000000000000
 0000FF61FF62FF63FF64FF65FF66FF67FF68FF69FF6AFF6BFF6CFF6DFF6EFF6F
 FF70FF71FF72FF73FF74FF75FF76FF77FF78FF79FF7AFF7BFF7CFF7DFF7EFF7F
 FF80FF81FF82FF83FF84FF85FF86FF87FF88FF89FF8AFF8BFF8CFF8DFF8EFF8F
 FF90FF91FF92FF93FF94FF95FF96FF97FF98FF99FF9AFF9BFF9CFF9DFF9EFF9F
 0000000000000000000000000000000000000000000000000000000000000000
 0000000000000000000000000000000000000000000000000000000000000000
 81
 0000000000000000000000000000000000000000000000000000000000000000
 0000000000000000000000000000000000000000000000000000000000000000
 0000000000000000000000000000000000000000000000000000000000000000
 0000000000000000000000000000000000000000000000000000000000000000
 300030013002FF0CFF0E30FBFF1AFF1BFF1FFF01309B309C00B4FF4000A8FF3E
 FFE3FF3F30FD30FE309D309E30034EDD30053006300730FC20152010FF0F005C
 301C2016FF5C2026202520182019201C201DFF08FF0930143015FF3BFF3DFF5B
 FF5D30083009300A300B300C300D300E300F30103011FF0B221200B100D70000
 00F7FF1D2260FF1CFF1E22662267221E22342642264000B0203220332103FFE5
 FF0400A200A3FF05FF03FF06FF0AFF2000A72606260525CB25CF25CE25C725C6
 25A125A025B325B225BD25BC203B301221922190219121933013000000000000
 000000000000000000000000000000002208220B2286228722822283222A2229
 000000000000000000000000000000002227222800AC21D221D4220022030000
 0000000000000000000000000000000000000000222022A52312220222072261
 2252226A226B221A223D221D2235222B222C0000000000000000000000000000
 212B2030266F266D266A2020202100B6000000000000000025EF000000000000

The third line of the file is three numbers. The first number is the fallback character (in base 16) to use when converting from UTF-8 to this encoding. The second number is a 1 if this file represents the encoding for a symbol font, or 0 otherwise. The last number (in base 10) is how many pages of data follow.

Subsequent lines in the example above are pages that describe how to map from the encoding into 2-byte Unicode. The first line in a page identifies the page number. Following it are 256 double-byte numbers, arranged as 16 rows of 16 numbers. Given a character in the encoding, the high byte of that character is used to select which page, and the low byte of that character is used as an index to select one of the double-byte numbers in that page - the value obtained being the corresponding Unicode character. By examination of the example above, one can see that the characters 0x7E and 0x8163 in shiftjis map to 203E and 2026 in Unicode, respectively.

Following the first page will be all the other pages, each in the same format as the first: one number identifying the page followed by 256 double-byte Unicode characters. If a character in the encoding maps to the Unicode character 0000, it means that the character doesn't actually exist. If all characters on a page would map to 0000, that page can be omitted.

Case [4] is the escape-sequence encoding file. The lines in an this type of file are in the same format as this example taken from the iso2022-jp encoding:

 # Encoding file: iso2022-jp, escape-driven
 E
 init           {}
 final          {}
 iso8859-1      \\x1b(B
 jis0201                \\x1b(J
 jis0208                \\x1b$@
 jis0208                \\x1b$B
 jis0212                \\x1b$(D
 gb2312         \\x1b$A
 ksc5601                \\x1b$(C

In the file, the first column represents an option and the second column is the associated value. init is a string to emit or expect before the first character is converted, while final is a string to emit or expect after the last character. All other options are names of table-based encodings; the associated value is the escape-sequence that marks that encoding. Tcl syntax is used for the values; in the above example, for instance, ``{}'' represents the empty string and ``\\x1b'' represents character 27.

Completely Tcl-specific paragraph, ignore in the context of Perl When Tcl_GetEncoding encounters an encoding name that has not been loaded, it attempts to load an encoding file called name.enc from the encoding subdirectory of each directory specified in the library path $tcl_libPath. If the encoding file exists, but is malformed, an error message will be left in interp.

KEYWORDS ^

utf, encoding, convert

COPYRIGHT ^

  #  Copyright (c) 1997-1998 Sun Microsystems, Inc.
  #  See the file "license.terms" for information on usage and redistribution
  #  of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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