### The Unicode Character Set

Because TADS 3 uses Unicode internally, TADS programs can work on practically any computer, in any national language, using any character set. Unicode is a "universal" character set that encodes almost every character from almost every written language in the world.

### Character Mapping Files

Sadly, most computers today do not use Unicode as their native character sets. In order for TADS to work on non-Unicode computers, TADS must translate characters between Unicode and native encodings. To accomplish this without tying itself to any one type of computer, TADS uses character mapping files. A character mapping file describes the association between a computer's native character set and Unicode.

The Unicode consortium, which is the organization that defines the Unicode standard, publishes files that describe the correspondence between Unicode and most native character sets that are in use today. TADS uses these files as a starting point.

### Creating a TADS Character Mapping File

To create a character mapping for use with TADS, follow these steps.

- Identify the character set you're using. This is a function of the type of computer you are using and your computer's localization. For example, Microsoft Windows systems localized for Eastern or Central Europe use Windows Code Page 1250.
- Obtain the official Unicode character mapping for your character set, if it's available. The Unicode consortium does not permit third-party distribution of its character mapping database files, so those files are not included with TADS. However, you can download these files directly from the Unicode consortium's ftp site. See the download locations table for a list of common mappings and where to find them.
- The character mapping files as provided on the Unicode ftp site are almost ready to use, but you must make one small addition. At the end of the file, add a line that looks like this:

    ·

    ·                       default_display 0x81

For the "0x81", however, you should substitute the character that you want to use as the default display character. This defines the character that TADS will display when a Unicode character has no mapping in your native character set. Most native character sets include an "invalid" character for just this purpose; this character looks like an empty rectangle in many fonts, but each character set varies. You should find an appropriate character in your native character set, and define it for the "default_display" character.

- Use the `mkchrtab` ("make character table") utility, included with the TADS distribution, to "compile" the mapping file into the format that TADS uses:

    ·

    ·                      mkchrtab cp1250.txt cp1250.tcm

The first argument is the source file that you downloaded from the Unicode site. The second argument is the name of the "tcm" file (TADS Character Map) that TADS will use.

### Download Locations for Common Mappings

ISO 8859 (ISO Latin-n) code pages | ftp://ftp.unicode.org/Public/MAPPINGS/ISO8859/
MS-DOS code pages | ftp://ftp.unicode.org/Public/MAPPINGS/VENDORS/MICSFT/PC/
Microsoft Windows code pages | ftp://ftp.unicode.org/Public/MAPPINGS/VENDORS/MICSFT/WINDOWS/
Apple Macintosh character sets | ftp://ftp.unicode.org/Public/MAPPINGS/VENDORS/APPLE/
NeXT character sets | ftp://ftp.unicode.org/Public/MAPPINGS/VENDORS/NEXT/

### Approximation Mappings

Since the Unicode character set includes character assignments for nearly every character used in a written language anywhere in the world, and most local character sets on existing operating systems and terminals include character assignments for only a few languages, most of the characters in the Unicode character set will not be displayable on most computers.

As described earlier, the `default_display` setting in the character mapping file allows you to specify the glyph to display when a character cannot be mapped to the local character set. So, if you're running a game that uses kanji characters on a machine localized for the United States, the kanji characters will be unmappable and so will be displayed using the `default_display` setting.

In many cases, however, you might want to provide an "approximate" mapping for some characters. An approximate mapping lets you specify a local character that is similar enough to a particular Unicode character that you'd rather display that local character as an approximation than display the `default_display` glyph. For example, if your computer can display only plain ASCII characters (so it doesn't have any accented letters in its character set), you might want to use a regular "a" as an approximation for the various accented "a" characters ("&aacute;", "à", "â", and so on). While displaying a regular "a" for an accented "a" loses some information, it might give the reader a better chance of understanding the original text than displaying the `default_display` character, which doesn't tell the reader anything other than that the original character can't be displayed on this system.

In addition, some glyphs can be approximated visually by combining two or more glyphs from a more limited character set. For example, left and right arrows can be approximated by "<-" and "->".

The TADS 3 character mapping system provides an optional mechanism for making these types of approximations. If you wish to provide approximations, you must place them after the local-to-Unicode mappings described earlier (the local-to-Unicode mappings are usually the ones you obtain from mapping files found on the Unicode web site). After all of the local-to-Unicode mappings, add this line to your mapping source file:

       display_mappings

After this line, provide your approximation mappings. These mappings can take two forms, as shown in these examples:

       &mdash  '--'

       0x0141      'L'

In the first form, you can specify an HTML entity name, written with a leading ampersand ("&") as it is in HTML source code. In the second form, you use a number (the "0x" prefix indicates that the number is in hexadecimal; you can specify a decimal number by omitting the "0x" prefix, but the Unicode listings specify everything in hex so you would probably want to use hex most of the time).

In both forms, the first item on the line specifies the Unicode character to be mapped to a local approximation, and the rest of the line specifies a string of Unicode characters to substitute for the original Unicode character. The simplest way to specify the substitution string is to enclose a string in single quote marks; this works if all of the substituted characters are plain ASCII characters (in the Unicode range 0 to 255). If you want to substitute any Unicode characters outside of the plain ASCII range, use numbers to specify the Unicode code points of the substitution:

       &mdash  0x2d 0x2d

The numeric form eliminates any character set translation issues with respect to the character mapping source file itself (the last thing we need to worry about at this point is how mapping files are mapped!).

Important: the substitution string is a set of Unicode characters, not local characters. These Unicode characters are themselves further translated into local characters. Note that the Unicode characters in the substitution string must appear in the local-to-Unicode mapping section; you cannot substitute a character with no mapping, and you cannot substitute a character that itself has an approximation mapping. The character mapper automatically translates the substitution string to the local character set for display.

The reason that the substitution string is specified with Unicode characters rather than local characters is that this allows programs to perform the approximation substitutions entirely in the Unicode domain, and then know for certain that each Unicode character in the substituted string will translate to exactly one local character. This is important because it allows programs using the character mappings to process the results of any multiple-character substitutions in the Unicode domain; for example, the TADS Interpreter's output formatter uses this information to perform word wrapping on the text that results from substituting the approximation mappings, which must be done in the Unicode domain.

You might wonder why the character mapping file has two separate sections - one for mappings from local to Unicode, and the other for approximation mappings. The reason for this dichotomy is that the character mapper treats the local-to-Unicode mappings as reversible - each mapping establishes that the local character will be translated on input to the given Unicode character, and also that the Unicode character will be translated to the local character on output; in order to be reversible, the local-to-Unicode entries must be unique, one-to-one mappings. The Unicode-to-local section, on the other hand, does not need to provide a unique mapping for each character, because the character mapper does not assume that these mappings are reversible - this is a one-way mapping. This is all designed as a time-saver for people creating character mappings - if one or the other of these sections weren't reversible, you'd always have to specify a full set for both directions, because neither set could be reversed to infer the other; the logical choice for the reversible mappings is the local-to-Unicode set, because the local character set will almost always be a small subset of the Unicode set, and hence it is frequently desirable to map several Unicode characters to a single local character, but not vice versa.

For some approximation mapping examples, refer to the default 7-bit ASCII mapping file "us-ascii.txt" (in the "charmap" subdirectory of the TADS 3 distribution).