1------------------------------------------------------------------------------
2--                                                                          --
3--                         GNAT COMPILER COMPONENTS                         --
4--                                                                          --
5--                       S Y S T E M . W C H _ C O N                        --
6--                                                                          --
7--                                 S p e c                                  --
8--                                                                          --
9--          Copyright (C) 1992-2013, Free Software Foundation, Inc.         --
10--                                                                          --
11-- GNAT is free software;  you can  redistribute it  and/or modify it under --
12-- terms of the  GNU General Public License as published  by the Free Soft- --
13-- ware  Foundation;  either version 3,  or (at your option) any later ver- --
14-- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
15-- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
16-- or FITNESS FOR A PARTICULAR PURPOSE.                                     --
17--                                                                          --
18-- As a special exception under Section 7 of GPL version 3, you are granted --
19-- additional permissions described in the GCC Runtime Library Exception,   --
20-- version 3.1, as published by the Free Software Foundation.               --
21--                                                                          --
22-- You should have received a copy of the GNU General Public License and    --
23-- a copy of the GCC Runtime Library Exception along with this program;     --
24-- see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see    --
25-- <http://www.gnu.org/licenses/>.                                          --
26--                                                                          --
27-- GNAT was originally developed  by the GNAT team at  New York University. --
28-- Extensive contributions were provided by Ada Core Technologies Inc.      --
29--                                                                          --
30------------------------------------------------------------------------------
31
32--  This package defines the codes used to identify the encoding method for
33--  wide characters in string and character constants. This is needed both
34--  at compile time and at runtime (for the wide character runtime routines)
35
36--  This unit may be used directly from an application program by providing
37--  an appropriate WITH, and the interface can be expected to remain stable.
38
39pragma Compiler_Unit_Warning;
40
41package System.WCh_Con is
42   pragma Pure;
43
44   -------------------------------------
45   -- Wide_Character Encoding Methods --
46   -------------------------------------
47
48   --  A wide character encoding method is a method for uniquely representing
49   --  a Wide_Character or Wide_Wide_Character value using a one or more
50   --  Character values. Three types of encoding method are supported by GNAT:
51
52   --    An escape encoding method uses ESC as the first character of the
53   --    sequence, and subsequent characters determine the wide character
54   --    value that is represented. Any character other than ESC stands
55   --    for itself as a single byte (i.e. any character in Latin-1, other
56   --    than ESC itself, is represented as a single character: itself).
57
58   --    An upper half encoding method uses a character in the upper half
59   --    range (i.e. in the range 16#80# .. 16#FF#) as the first byte of
60   --    a wide character encoding sequence. Subsequent characters are
61   --    used to determine the wide character value that is represented.
62   --    Any character in the lower half (16#00# .. 16#7F#) represents
63   --    itself as a single character.
64
65   --    The brackets notation, where a wide character is represented by the
66   --    sequence ["xx"] or ["xxxx"] or ["xxxxxx"] where xx are hexadecimal
67   --    characters. Note that currently this is the only encoding that
68   --    supports the full UTF-32 range.
69
70   --  Note that GNAT does not currently support escape-in, escape-out
71   --  encoding methods, where an escape sequence is used to set a mode
72   --  used to recognize subsequent characters. All encoding methods use
73   --  individual character-by-character encodings, so that a sequence of
74   --  wide characters is represented by a sequence of encodings.
75
76   --  To add new encoding methods, the following steps are required:
77
78   --     1.  Define a code for a new value of type WC_Encoding_Method
79   --     2.  Adjust the definition of WC_Encoding_Method accordingly
80   --     3.  Provide appropriate conversion routines in System.Wch_Cnv
81   --     4.  Adjust definition of WC_Longest_Sequence if necessary
82   --     5.  Add an entry in WC_Encoding_Letters for the new method
83   --     6.  Add proper code to s-wchstw.adb, s-wchwts.adb, s-widwch.adb
84   --     7.  Update documentation (remember section on form strings)
85
86   --  Note that the WC_Encoding_Method values must be kept ordered so that
87   --  the definitions of the subtypes WC_Upper_Half_Encoding_Method and
88   --  WC_ESC_Encoding_Method are still correct.
89
90   ---------------------------------
91   -- Encoding Method Definitions --
92   ---------------------------------
93
94   type WC_Encoding_Method is range 1 .. 6;
95   --  Type covering the range of values used to represent wide character
96   --  encoding methods. An enumeration type might be a little neater, but
97   --  more trouble than it's worth, given the need to pass these values
98   --  from the compiler to the backend, and to record them in the ALI file.
99
100   WCEM_Hex : constant WC_Encoding_Method := 1;
101   --  The wide character with code 16#abcd# is represented by the escape
102   --  sequence ESC a b c d (five characters, where abcd are ASCII hex
103   --  characters, using upper case for letters). This method is easy
104   --  to deal with in external environments that do not support wide
105   --  characters, and covers the whole 16-bit BMP. Codes larger than
106   --  16#FFFF# are not representable using this encoding method.
107
108   WCEM_Upper : constant WC_Encoding_Method := 2;
109   --  The wide character with encoding 16#abcd#, where the upper bit is on
110   --  (i.e. a is in the range 8-F) is represented as two bytes 16#ab# and
111   --  16#cd#. The second byte may never be a format control character, but
112   --  is not required to be in the upper half. This method can be also used
113   --  for shift-JIS or EUC where the internal coding matches the external
114   --  coding. Codes larger than 16#FFFF# are not representable using this
115   --  encoding method.
116
117   WCEM_Shift_JIS : constant WC_Encoding_Method := 3;
118   --  A wide character is represented by a two character sequence 16#ab#
119   --  and 16#cd#, with the restrictions described for upper half encoding
120   --  as described above. The internal character code is the corresponding
121   --  JIS character according to the standard algorithm for Shift-JIS
122   --  conversion. See the body of package System.JIS_Conversions for
123   --  further details. Codes larger than 16#FFFF are not representable
124   --  using this encoding method.
125
126   WCEM_EUC : constant WC_Encoding_Method := 4;
127   --  A wide character is represented by a two character sequence 16#ab# and
128   --  16#cd#, with both characters being in the upper half set. The internal
129   --  character code is the corresponding JIS character according to the EUC
130   --  encoding algorithm. See the body of package System.JIS_Conversions for
131   --  further details. Codes larger than 16#FFFF# are not representable using
132   --  this encoding method.
133
134   WCEM_UTF8 : constant WC_Encoding_Method := 5;
135   --  An ISO 10646-1 BMP/Unicode wide character is represented in UCS
136   --  Transformation Format 8 (UTF-8), as defined in Annex R of ISO
137   --  10646-1/Am.2. Depending on the character value, a Unicode character
138   --  is represented as the one to six byte sequence.
139   --
140   --    16#0000_0000#-16#0000_007f#: 2#0xxxxxxx#
141   --    16#0000_0080#-16#0000_07ff#: 2#110xxxxx# 2#10xxxxxx#
142   --    16#0000_0800#-16#0000_ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#
143   --    16#0001_0000#-16#001F_FFFF#: 2#11110xxx# 2#10xxxxxx# 2#10xxxxxx#
144   --                                 2#10xxxxxx#
145   --    16#0020_0000#-16#03FF_FFFF#: 2#111110xx# 2#10xxxxxx# 2#10xxxxxx#
146   --                                 2#10xxxxxx# 2#10xxxxxx#
147   --    16#0400_0000#-16#7FFF_FFFF#: 2#1111110x# 2#10xxxxxx# 2#10xxxxxx#
148   --                                 2#10xxxxxx# 2#10xxxxxx# 2#10xxxxxx#
149   --
150   --  where the xxx bits correspond to the left-padded bits of the
151   --  16-bit character value. Note that all lower half ASCII characters
152   --  are represented as ASCII bytes and all upper half characters and
153   --  other wide characters are represented as sequences of upper-half. This
154   --  encoding method can represent the entire range of Wide_Wide_Character.
155
156   WCEM_Brackets : constant WC_Encoding_Method := 6;
157   --  A wide character is represented using one of the following sequences:
158   --
159   --    ["xx"]
160   --    ["xxxx"]
161   --    ["xxxxxx"]
162   --    ["xxxxxxxx"]
163   --
164   --  where xx are hexadecimal digits representing the character code. This
165   --  encoding method can represent the entire range of Wide_Wide_Character
166   --  but in the general case results in ambiguous representations (there is
167   --  no ambiguity in Ada sources, since the above sequences are illegal Ada).
168
169   WC_Encoding_Letters : constant array (WC_Encoding_Method) of Character :=
170     (WCEM_Hex       => 'h',
171      WCEM_Upper     => 'u',
172      WCEM_Shift_JIS => 's',
173      WCEM_EUC       => 'e',
174      WCEM_UTF8      => '8',
175      WCEM_Brackets  => 'b');
176   --  Letters used for selection of wide character encoding method in the
177   --  compiler options (-gnatW? switch) and for Wide_Text_IO (WCEM parameter
178   --  in the form string).
179
180   subtype WC_ESC_Encoding_Method is
181     WC_Encoding_Method range WCEM_Hex .. WCEM_Hex;
182   --  Encoding methods using an ESC character at the start of the sequence
183
184   subtype WC_Upper_Half_Encoding_Method is
185     WC_Encoding_Method range WCEM_Upper .. WCEM_UTF8;
186   --  Encoding methods using an upper half character (16#80#..16#FF) at
187   --  the start of the sequence.
188
189   WC_Longest_Sequence : constant := 12;
190   --  The longest number of characters that can be used for a wide character
191   --  or wide wide character sequence for any of the active encoding methods.
192
193   WC_Longest_Sequences : constant array (WC_Encoding_Method) of Natural :=
194     (WCEM_Hex       => 5,
195      WCEM_Upper     => 2,
196      WCEM_Shift_JIS => 2,
197      WCEM_EUC       => 2,
198      WCEM_UTF8      => 6,
199      WCEM_Brackets  => 12);
200   --  The longest number of characters that can be used for a wide character
201   --  or wide wide character sequence using the given encoding method.
202
203   function Get_WC_Encoding_Method (C : Character) return WC_Encoding_Method;
204   --  Given a character C, returns corresponding encoding method (see array
205   --  WC_Encoding_Letters above). Raises Constraint_Error if not in list.
206
207   function Get_WC_Encoding_Method (S : String) return WC_Encoding_Method;
208   --  Given a lower case string that is one of hex, upper, shift_jis, euc,
209   --  utf8, brackets, return the corresponding encoding method. Raises
210   --  Constraint_Error if not in list.
211
212   function Is_Start_Of_Encoding
213     (C  : Character;
214      EM : WC_Encoding_Method) return Boolean;
215   pragma Inline (Is_Start_Of_Encoding);
216   --  Returns True if the Character C is the start of a multi-character
217   --  encoding sequence for the given encoding method EM. If EM is set to
218   --  WCEM_Brackets, this function always returns False.
219
220end System.WCh_Con;
221