Unicode Extended Strings (UXStrings)

Motivation

Ada GUI library Gnoga internal character strings implementation is based on both Ada types String and Unbounded_String. The native Ada String encoding is Latin-1 whereas transactions with the Javascript part are in UTF-8 encoding.

Some drawbacks come up, for instance, with internationalization of programs (see Localize Gnoga demo):

• several conversions between String and Unbounded_String objects
• it isn't usable out of Latin-1 character set, characters out of Latin-1 set are blanked
• continuous conversions between Latin-1 and UTF-8, each sent and received transaction between Ada and Javascript parts

Two ways of possible improvement for native Ada String: dynamic length handling and Unicode support.

Workarounds

First possibility is using UTF-8 as internal implementation in Unbounded_String objects. The simplest way but Gnoga uses many times character indexes to parse Javascript messages that is not easy to achieved with UTF-8 which may have several lengths to represent one character. String parsing will be time consuming. Some combinations may lead to incorrect UTF-8 representation.

Second possibility is to use Unbounded_Wide_String or Unbounded_Wide_Wide_String. Using Unbounded_Wide_String is quite being in the middle of the river might as well use Unbounded_Wide_Wide_String. In this latter case the memory penalty is heavy for only few accentuated character occurrences. So back to Unbounded_Wide_String but you'll miss the so essential emojis ;-)

Third possibility is to make no choice between Latin-1, Wide and Wide_Wide characters. The object shall adapt its inner implementation to the actual content. For instance with English language the most often use case will be Latin-1 inner implementation, for French language the most often will be Latin-1 with some exceptions with BMP (Unicode Basic Multilingual Plane) implementation such as in "cœur", for Greek language the most often will be BMP implementation. The programmer won't make any representation choice when for example receiving UTF-8 messages:

   S2 : UXString;
   ...
   S2 := "Received: " & From_UTF_8 (Message);

Automatically S2 will adapt its inner representation to the received characters.

UXStrings packages

Package named UXStrings (Unicode Extended String) and its Text_IO child package are proposed to bring String enhancements using some Ada 202x features.

The first part of UXString package contains renaming statements of current Ada types. Ada current String type is structurally an array of Latin-1 characters thus is renamed as Latin_1_Character_Array. And so on.

The second part defines the USXString type as a tagged private type which has got aspects such as Constant_Indexing, Variable_Indexing, Iterable and String_Literal, so we can write:

   S1, S2, S3 : UXString;
   C          : Character;
   WC         : Wide_Character;
   WWC        : Wide_Wide_Character;
   ...
   S1 := "était blah blah";
   C   := S1 (3);
   WC  := S1 (2);
   WWC := S1 (1);
   S1 (3) := WWC;
   S1 (2) := WC;
   S1 (1) := C;
   S3  := "une soirée passée à étudier les mathématiques ℕ⊂𝕂...";
   for I in S3 loop
      C   := S3 (I);
      WC  := S3 (I);
      WWC := S3 (I);
      Put_Line (Character'pos (C)'img & Wide_Character'pos (WC)'img & Wide_Wide_Character'pos (WWC)'img);
   end loop;

The third part defines conversion functions between UXString and various encoding such as Latin-1, BMP (USC-2), Unicode (USC-4), UTF-8 or UTF-16, so we can write:

   S1  := From_Latin_1 ("blah blah");
   S2  := From_BMP ("une soirée passée à étudier la physique ω=Δθ/Δt...");
   S3  := From_Unicode ("une soirée passée à étudier les mathématiques ℕ⊂𝕂...");
   Send (To_UTF_8 (S1) & To_UTF_8 (S3));

The fourth part defines various API coming from Unbounded_String such as Append, "&", Slice, "=", Index and so on.

Note: Iterable is a GNAT specific aspect.

UXStrings implementations

UXStrings 1

A first proof of concept implementation is provided. The source code files are ending with the number 1 as for instance "uxstrings1.ads". A GNAT project file "uxstrings1.gpr" is provided with some naming conventions for both packages UXStrings and UXStrings.Text_IO.

Implementation choices

UTF-8 encoding is chosen for internal representation. The Strings_Edit library is used for UTF-8 encoding management. GNAT.OS_Lib is chosen for input / output management.

This implementation which is only for demonstrate the possible usages of UXString has many limitations.

Limitations:

• not thread safe
• single character assignment is not implemented
• only few API are implemented

UXStrings 2

A second proof of concept implementation is provided. The source code files are ending with the number 2 as for instance "uxstrings2.ads". A GNAT project file "uxstrings2.gpr" is provided with some naming conventions for both packages UXStrings and UXStrings.Text_IO.

Implementation choices

In addition to implementation UXStrings 1, some API have been added to support ASCII 7 bits encoding. ASCII is a subset of UTF-8 thus no change with the internal representation. However, the API are now aware if content is full ASCII. On one hand, this permits to access directly to the position of one character without iterating on UTF-8 characters. Thus this is a time improvement when content is full ASCII. On the other hand, when content is changing the API check if the content is full ASCII. Thus this is a time penalty when changes are not full ASCII.

This implementation which is only for demonstrate the possible usages of UXString has many limitations.

Limitations:

• not thread safe
• single character assignment is not implemented
• only few API are implemented

Future implementations

Here are some ideas:

• Use memory management as implemented in XStrings from GNATColl.
• Use Unbounded_Strings with UTF-8 encoding.
• Use Unbounded_Wide_Wide_Strings
• Adapt the inner implementation to the actual content with 8 bits character encodings, 16 bits or 32 bits.

Tests

One test program test_uxstrings.adb is provided for UXStrings tests and an other test program test_uxstrings_text_io.adb is provided for UXStrings.Text_IO tests.

Dependencies

UXStrings library depends on Strings Edit library. This latter is also part of Simple Components framework available on Alire. Get one of these and add the path of strings_edit.gpr in your GPR_PROJECT_PATH before building with UXStrings.

Using Alire

In your own Alire project, add UXStrings dependency:

% alr with uxstrings

Then you can import the Ada UXStrings packages in your programs.

Note: Alire will take care of dependencies.

Licence

The provided UXStrings specifications are intend to be public. Constructive criticism and altenative implementations of these specifications are expected. The actual proposed implementation is under CeCILL licence.

Feedbacks

Feel free to send feedback about UXStrings specification source code on Github.

Pascal Pignard, April 2021, August 2022.