2. The Unicode module — XML/Ada: the XML Library for Ada 2014 documentation

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# 2. The Unicode module

Unicode provides a unique number for every character, no matter what the platform, no matter what the program, no matter what the language.

Fundamentally, computers just deal with numbers. They store letters and other characters by assigning a number for each one. Before Unicode was invented, there were hundreds of different encoding systems for assigning these numbers. No single encoding could contain enough characters: for example, the European Union alone requires several different encodings to cover all its languages. Even for a single language like English no single encoding was adequate for all the letters, punctuation, and technical symbols in common use.

These encoding systems also conflict with one another. That is, two encodings can use the same number for two different characters, or use different numbers for the same character. Any given computer (especially servers) needs to support many different encodings; yet whenever data is passed between different encodings or platforms, that data always runs the risk of corruption.

Unicode provides a unique number for every character, no matter what the platform, no matter what the program, no matter what the language. The Unicode Standard has been adopted by such industry leaders as Apple, HP, IBM, JustSystem, Microsoft, Oracle, SAP, Sun, Sybase, Unisys and many others. Unicode is required by modern standards such as XML, Java, ECMAScript (JavaScript), LDAP, CORBA 3.0, WML, etc., and is the official way to implement ISO/IEC 10646. It is supported in many operating systems, all modern browsers, and many other products. The emergence of the Unicode Standard, and the availability of tools supporting it, are among the most significant recent global software technology trends.

The following sections explain the basic vocabulary and concepts associated with Unicode and encodings.

Most of the information comes from the official Unicode Web site, at http://www.unicode.org/unicode/reports/tr17.

Part of this documentation comes from http://www.unicode.org, the official web site for Unicode.

## 2.1. Glyphs

A glyph is a particular representation of a character or part of a character.

Several representations are possible, mostly depending on the exact font used at that time. A single glyph can correspond to a sequence of characters, or a single character to a sequence of glyphs.

The Unicode standard doesn’t deal with glyphs, although a suggested representation is given for each character in the standard. Likewise, this module doesn’t provide any graphical support for Unicode, and will just deal with textual memory representation and encodings.

Take a look at the GtkAda library that provides the graphical interface for unicode in the upcoming 2.0 version.

## 2.2. Repertoires and subsets

A repertoire is a set of abstract characters to be encoded, normally a familiar alphabet or symbol set. For instance, the alphabet used to spell English words, or the one used for the Russian alphabet are two such repertoires.

There exist two types of repertoires, close and open ones. The former is the most common one, and the two examples above are such repertoires. No character is ever added to them.

Unicode is also a repertoire, but an open one. New entries are added to it. However, it is guaranteed that none will ever be deleted from it. Unicode intends to be a universal repertoire, with all possible characters currently used in the world. It currently contains all the alphabets, including a number of alphabets associated with dead languages like hieroglyphs. It also contains a number of often used symbols, like mathematical signs.

The goal of this Unicode module is to convert all characters to entries in the Unicode repertoire, so that any applications can communicate with each other in a portable manner.

Given its size, most applications will only support a subset of Unicode. Some of the scripts, most notably Arabic and Asian languages, require a special support in the application (right-to-left writing,...), and thus will not be supported by some applications.

The Unicode standard includes a set of internal catalogs, called collections. Each character in these collections is given a special name, in addition to its code, to improve readability.

Several child packages (Unicode.Names.*) define those names. For instance:

Unicode.Names.Basic_Latin
  This contains the basic characters used in most western European languages, including the standard ASCII subset.
Unicode.Names.Cyrillic
  This contains the Russian alphabet.
Unicode.Names.Mathematical_Operators
  This contains several mathematical symbols

More than 80 such packages exist.

## 2.3. Character sets

A character set is a mapping from a set of abstract characters to some non-negative integers. The integer associated with a character is called its code point, and the character itself is called the encoded character.

There exist a number of standard character sets, unfortunately not compatible with each other. For instance, ASCII is one of these character sets, and contains 128 characters. A super-set of it is the ISO/8859-1 character set. Another character set is the JIS X 0208, used to encode Japanese characters.

Note that a character set is different from a repertoire. For instance, the same character C with cedilla doesn’t have the same integer value in the ISO/8859-1 character set and the ISO/8859-2 character set.

Unicode is also such a character set, that contains all the possible characters and associate a standard integer with them. A similar and fully compatible character set is ISO/10646. The only addition that Unicode does to ISO/10646 is that it also specifies algorithms for rendering presentation forms of some scripts (say Arabic), handling of bi-directional texts that mix for instance Latin and Hebrew, algorithms for sorting and string comparison, and much more.

Currently, our Unicode package doesn’t include any support for these algorithms.

Unicode and ISO 10646 define formally a 31-bit character set. However, of this huge code space, so far characters have been assigned only to the first 65534 positions (0x0000 to 0xFFFD). The characters that are expected to be encoded outside the 16-bit range belong all to rather exotic scripts (e.g., Hieroglyphics) that are only used by specialists for historic and scientific purposes

The Unicode module contains a set of packages to provide conversion from some of the most common character sets to and from Unicode. These are the Unicode.CCS.* packages.

All these packages have a common structure:

- They define a global variable of type Character_Set with two fields, ie the two conversion functions between the given character set and Unicode.

  These functions convert one character (actually its code point) at a time.

- They also define a number of standard names associated with this character set. For instance, the ISO/8859-1 set is also known as Latin1.

  The function Unicode.CCS.Get_Character_Set can be used to find a character set by its standard name.

Currently, the following sets are supported:

ISO/8859-1 aka Latin1

> This is the standard character set used to represent most Western European languages including: Albanian, Catalan, Danish, Dutch, English, Faroese, Finnish, French, Galician, German, Irish, Icelandic, Italian, Norwegian, Portuguese, Spanish and Swedish.

ISO/8859-2 aka Latin2

> This character set supports the Slavic languages of Central Europe which use the Latin alphabet. The ISO-8859-2 set is used for the following languages: Czech, Croat, German, Hungarian, Polish, Romanian, Slovak and Slovenian.

ISO/8859-3

> This character set is used for Esperanto, Galician, Maltese and Turkish

ISO/8859-4

> Some letters were added to the ISO-8859-4 to support languages such as Estonian, Latvian and Lithuanian. It is an incomplete precursor of the Latin 6 set.

## 2.4. Character encoding schemes

We now know how each encoded character can be represented by an integer value (code point) depending on the character set.

Character encoding schemes deal with the representation of a sequence of integers to a sequence of code units. A code unit is a sequence of bytes on a computer architecture.

There exists a number of possible encoding schemes. Some of them encode all integers on the same number of bytes. They are called fixed-width encoding forms, and include the standard encoding for Internet emails (7bits, but it can’t encode all characters), as well as the simple 8bits scheme, or the EBCDIC scheme. Among them is also the UTF-32 scheme which is defined in the Unicode standard.

Another set of encoding schemes encode integers on a variable number of bytes. These include two schemes that are also defined in the Unicode standard, namely Utf-8 and Utf-16.

Unicode doesn’t impose any specific encoding. However, it is most often associated with one of the Utf encodings. They each have their own properties and advantages:

Utf32

> This is the simplest of all these encodings. It simply encodes all the characters on 32 bits (4 bytes). This encodes all the possible characters in Unicode, and is obviously straightforward to manipulate. However, given that the first 65535 characters in Unicode are enough to encode all known languages currently in use, Utf32 is also a waste of space in most cases.

Utf16

> For the above reason, Utf16 was defined. Most characters are only encoded on two bytes (which is enough for the first 65535 and most current characters). In addition, a number of special code points have been defined, known as surrogate pairs, that make the encoding of integers greater than 65535 possible. The integers are then encoded on four bytes. As a result, Utf16 is thus much more memory-efficient and requires less space than Utf32 to encode sequences of characters. However, it is also more complex to decode.

Utf8

> This is an even more space-efficient encoding, but is also more complex to decode. More important, it is compatible with the most currently used simple 8bit encoding.

> Utf8 has the following properties:

> - Characters 0 to 127 (ASCII) are encoded simply as a single byte. This means that files and strings which contain only 7-bit ASCII characters have the same encoding under both ASCII and UTF-8.
> - Characters greater than 127 are encoded as a sequence of several bytes, each of which has the most significant bit set. Therefore, no ASCII byte can appear as part of any other character.
> - The first byte of a multibyte sequence that represents a non-ASCII character is always in the range 0xC0 to 0xFD and it indicates how many bytes follow for this character. All further bytes in a multibyte sequence are in the range 0x80 to 0xBF. This allows easy resynchronization and makes the encoding stateless and robust against missing bytes.
> - UTF-8 encoded characters may theoretically be up to six bytes long, however the first 16-bit characters are only up to three bytes long.

Note that the encodings above, except for Utf8, have two versions, depending on the chosen byte order on the machine.

The Ada95 Unicode module provides a set of packages that provide an easy conversion between all the encoding schemes, as well as basic manipulations of these byte sequences. These are the Unicode.CES.* packages. Currently, four encoding schemes are supported, the three Utf schemes and the basic 8bit encoding which corresponds to the standard Ada strings.

It also supports some routines to convert from one byte-order to another.

The following examples show a possible use of these packages:

    Converting a latin1 string coded on 8 bits to a Utf8 latin2 file
    involves the following steps:

       Latin1 string  (bytes associated with code points in Latin1)
         |    "use Unicode.CES.Basic_8bit.To_Utf32"
         v
       Utf32 latin1 string (contains code points in Latin1)
         |    "Convert argument to To_Utf32 should be
         v         Unicode.CCS.Iso_8859_1.Convert"
       Utf32 Unicode string (contains code points in Unicode)
         |    "use Unicode.CES.Utf8.From_Utf32"
         v
       Utf8 Unicode string (contains code points in Unicode)
         |    "Convert argument to From_Utf32 should be
         v         Unicode.CCS.Iso_8859_2.Convert"
       Utf8 Latin2 string (contains code points in Latin2)

## 2.5. Unicode_Encoding

XML/Ada groups the two notions of character sets and encoding schemes into a single type, Unicode.Encodings.Unicode_Encoding.

This package provides additional functions to manipulate these encodings, for instance to retrieve them by the common name that is associated with them (for instance “utf-8”, “iso-8859-15”,...), since very often the encoding scheme is implicit. If you are speaking of utf-8 string, most people always assume you also use the unicode character set. Likewise, if you are speaking of “iso-8859-1”, most people will assume you string is encoded as 8 byte characters.

The goal of the Unicode.Encodings package is to make these implicit associations more obvious.

It also provides one additional function Convert, which can be used to convert a sequence of bytes from one encoding to another. This is a convenience function that you can use when for instance creating DOM trees directly through Ada calls, since XML/Ada excepts all its strings to be in utf-8 by default.

## 2.6. Misc. functions

The package Unicode contains a series of Is_* functions, matching the Unicode standard.

Is_White_Space

> Return True if the character argument is a space character, ie a space, horizontal tab, line feed or carriage return.

Is_Letter

> Return True if the character argument is a letter. This includes the standard English letters, as well as some less current cases defined in the standard.

Is_Base_Char
  Return True if the character is a base character, ie a character whose meaning can be modified with a combining character.
Is_Digit
  Return True if the character is a digit (numeric character)
Is_Combining_Char

  Return True if the character is a combining character. Combining characters are accents or other diacritical marks that are added to the previous character.

  The most important accented characters, like those used in the orthographies of common languages, have codes of their own in Unicode to ensure backwards compatibility with older character sets. Accented characters that have their own code position, but could also be represented as a pair of another character followed by a combining character, are known as precomposed characters. Precomposed characters are available in Unicode for backwards compatibility with older encodings such as ISO 8859 that had no combining characters. The combining character mechanism allows to add accents and other diacritical marks to any character

  Note however that your application must provide specific support for combining characters, at least if you want to represent them visually.

Is_Extender
  True if Char is an extender character.
Is_Ideographic
  True if Char is an ideographic character. This is defined only for Asian languages.

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