File : unicode-ces-utf16.ads


-----------------------------------------------------------------------

--                XML/Ada - An XML suite for Ada95                   --

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--                       Copyright (C) 2001                          --

--                            ACT-Europe                             --

--                       Author: Emmanuel Briot                      --

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-----------------------------------------------------------------------


--  This package provides support for Utf16 encoding of characters.

--

--  Although utf32 is simpler, it requires too much space to encode typical

--  strings. Indeed, only two bytes are currently needed to encode every

--  unicode character. Thus, utf16 was created in that purpose. Most characters

--  are encoded only on two bytes, although there is provision for a four-byte

--  encoding in some cases, using what is known as surrogate pairs.

--

--  As a result, utf16 is a variable-width encoding, and traversing a string is

--  a little bit less efficient that with Utf32.

--

--  Another inconvenient is that you can not easily search an ASCII character

--  in a string with the standard string manipulation functions, since you

--  might find the high-byte of a two-byte sequence.


with Unicode.CES.Utf32;
with Unicode.CCS;
with Unchecked_Deallocation;

package Unicode.CES.Utf16 is

   -----------

   -- Types --

   -----------


   subtype Utf16_String is String;
   type Utf16_String_Access is access Utf16_String;
   --  A UTF16-encoded string, undefined byte-order


   subtype Utf16_LE_String is Utf16_String;
   type Utf16_LE_String_Access is access Utf16_LE_String;
   --  Similar, but with little-endian byte-order


   subtype Utf16_BE_String is Utf16_String;
   --  Similar, but with big-endian byte-order


   -------------------------------------------

   -- Conversion to and from byte sequences --

   -------------------------------------------


   function Encode (Char : Unicode_Char) return Utf16_LE_String;
   --  Return the byte sequence representing Char in the Utf16 character

   --  encoding form (little-endian)


   function Read (Str : Utf16_LE_String; Index : Positive) return Unicode_Char;
   --  Return the character starting at location Index in Str


   function Read_BE (Str : Utf16_BE_String; Index : Positive)
      return Unicode_Char;
   --  Return the character starting at location Index in Str (big-endian)


   function Encode_BE (Char : Unicode_Char) return Utf16_BE_String;
   --  Return the byte sequence representing Char in the Utf16 character

   --  encoding form (big-endian)


   function Width (Char : Unicode_Char) return Natural;
   --  Return the number of bytes occupied by the Utf16 representation of Char


   function Length (Str : Utf16_String) return Natural;
   --  Return the number of characters in Str


   -------------------------------------------

   -- Conversion to and from Utf32-encoding --

   -------------------------------------------


   function From_Utf32
     (Str : Unicode.CES.Utf32.Utf32_LE_String)
      return Utf16_LE_String;
   --  Return a new utf16-encoded string, from a Utf32-encoded string.


   function To_Utf32
     (Str : Utf16_LE_String)
      return Unicode.CES.Utf32.Utf32_LE_String;
   --  Return a new utf32-encoded string, from a Utf16-encoded string.


   ---------------------------

   -- Byte order conversion --

   ---------------------------


   function To_Unicode_LE
     (Str   : Utf16_String;
      Cs    : Unicode.CCS.Character_Set := Unicode.CCS.Unicode_Character_Set;
      Order : Byte_Order := Default_Byte_Order) return Utf16_LE_String;
   --  Convert Str to a Unicode, little-endian utf16-encoding byte sequence.

   --  Order is the order in which bytes are coded in Str. This is silently

   --  overriden in case Str as a BOM (byte-order-marker) at the beginning

   --  that specifies an explicit order.

   --  The BOM is removed from the resulting string

   --  Invalid_Encoding is raised if Str contains a BOM that indicates an

   --  encoding other than Utf16.


   function To_CS
     (Str   : Utf16_LE_String;
      Cs    : Unicode.CCS.Character_Set := Unicode.CCS.Unicode_Character_Set;
      Order : Byte_Order := Default_Byte_Order) return Utf16_String;
   --  Convert Str from Unicode to another character set, and possibly

   --  another ordering.

   --  No BOM is checked


   ---------------------

   -- Encoding Scheme --

   ---------------------


   Utf16_LE_Encoding : constant Encoding_Scheme :=
     (Read   => Read'Access,
      Width  => Width'Access,
      Encode => Encode_Function' (Encode'Access),
      Length => Length'Access);

   Utf16_BE_Encoding : constant Encoding_Scheme :=
     (Read   => Read_BE'Access,
      Width  => Width'Access,
      Encode => Encode_Function' (Encode_BE'Access),
      Length => Length'Access);

   ------------------

   -- Deallocation --

   ------------------


   procedure Free is new Unchecked_Deallocation
     (Utf16_String, Utf16_String_Access);
   procedure Free is new Unchecked_Deallocation
     (Utf16_LE_String, Utf16_LE_String_Access);
   --  Free the memory occupied by a utf16-encoded string


private
   pragma Inline (Width);
end Unicode.CES.Utf16;