summaryrefslogtreecommitdiff
path: root/gcc/ada/g-decstr.adb
blob: e7c8a2612f4048d03cd3c65f226f050f9b6f2d5b (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
------------------------------------------------------------------------------
--                                                                          --
--                         GNAT RUN-TIME COMPONENTS                         --
--                                                                          --
--                    G N A T . D E C O D E _ S T R I N G                   --
--                                                                          --
--                                 S p e c                                  --
--                                                                          --
--                     Copyright (C) 2007-2008, AdaCore                     --
--                                                                          --
-- GNAT is free software;  you can  redistribute it  and/or modify it under --
-- terms of the  GNU General Public License as published  by the Free Soft- --
-- ware  Foundation;  either version 2,  or (at your option) any later ver- --
-- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
-- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
-- or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License --
-- for  more details.  You should have  received  a copy of the GNU General --
-- Public License  distributed with GNAT;  see file COPYING.  If not, write --
-- to  the  Free Software Foundation,  51  Franklin  Street,  Fifth  Floor, --
-- Boston, MA 02110-1301, USA.                                              --
--                                                                          --
-- As a special exception,  if other files  instantiate  generics from this --
-- unit, or you link  this unit with other files  to produce an executable, --
-- this  unit  does not  by itself cause  the resulting  executable  to  be --
-- covered  by the  GNU  General  Public  License.  This exception does not --
-- however invalidate  any other reasons why  the executable file  might be --
-- covered by the  GNU Public License.                                      --
--                                                                          --
-- GNAT was originally developed  by the GNAT team at  New York University. --
-- Extensive contributions were provided by Ada Core Technologies Inc.      --
--                                                                          --
------------------------------------------------------------------------------

--  This package provides a utility routine for converting from an encoded
--  string to a corresponding Wide_String or Wide_Wide_String value.

with Interfaces; use Interfaces;

with System.WCh_Cnv; use System.WCh_Cnv;
with System.WCh_Con; use System.WCh_Con;

package body GNAT.Decode_String is

   -----------------------
   -- Local Subprograms --
   -----------------------

   procedure Bad;
   pragma No_Return (Bad);
   --  Raise error for bad encoding

   procedure Past_End;
   pragma No_Return (Past_End);
   --  Raise error for off end of string

   ---------
   -- Bad --
   ---------

   procedure Bad is
   begin
      raise Constraint_Error with
        "bad encoding or character out of range";
   end Bad;

   ---------------------------
   -- Decode_Wide_Character --
   ---------------------------

   procedure Decode_Wide_Character
     (Input  : String;
      Ptr    : in out Natural;
      Result : out Wide_Character)
   is
      Char : Wide_Wide_Character;
   begin
      Decode_Wide_Wide_Character (Input, Ptr, Char);

      if Wide_Wide_Character'Pos (Char) > 16#FFFF# then
         Bad;
      else
         Result := Wide_Character'Val (Wide_Wide_Character'Pos (Char));
      end if;
   end Decode_Wide_Character;

   ------------------------
   -- Decode_Wide_String --
   ------------------------

   function Decode_Wide_String (S : String) return Wide_String is
      Result : Wide_String (1 .. S'Length);
      Length : Natural;
   begin
      Decode_Wide_String (S, Result, Length);
      return Result (1 .. Length);
   end Decode_Wide_String;

   procedure Decode_Wide_String
     (S      : String;
      Result : out Wide_String;
      Length : out Natural)
   is
      Ptr : Natural;

   begin
      Ptr := S'First;
      Length := 0;
      while Ptr <= S'Last loop
         if Length >= Result'Last then
            Past_End;
         end if;

         Length := Length + 1;
         Decode_Wide_Character (S, Ptr, Result (Length));
      end loop;
   end Decode_Wide_String;

   --------------------------------
   -- Decode_Wide_Wide_Character --
   --------------------------------

   procedure Decode_Wide_Wide_Character
     (Input  : String;
      Ptr    : in out Natural;
      Result : out Wide_Wide_Character)
   is
      C : Character;

      function In_Char return Character;
      pragma Inline (In_Char);
      --  Function to get one input character

      -------------
      -- In_Char --
      -------------

      function In_Char return Character is
      begin
         if Ptr <= Input'Last then
            Ptr := Ptr + 1;
            return Input (Ptr - 1);
         else
            Past_End;
         end if;
      end In_Char;

   --  Start of processing for Decode_Wide_Wide_Character

   begin
      C := In_Char;

      --  Special fast processing for UTF-8 case

      if Encoding_Method = WCEM_UTF8 then
         UTF8 : declare
            U : Unsigned_32;
            W : Unsigned_32;

            procedure Get_UTF_Byte;
            pragma Inline (Get_UTF_Byte);
            --  Used to interpret 2#10xxxxxx# continuation byte in UTF-8 mode.
            --  Reads a byte, and raises CE if the first two bits are not 10.
            --  Otherwise shifts W 6 bits left and or's in the 6 xxxxxx bits.

            ------------------
            -- Get_UTF_Byte --
            ------------------

            procedure Get_UTF_Byte is
            begin
               U := Unsigned_32 (Character'Pos (In_Char));

               if (U and 2#11000000#) /= 2#10_000000# then
                  Bad;
               end if;

               W := Shift_Left (W, 6) or (U and 2#00111111#);
            end Get_UTF_Byte;

         --  Start of processing for UTF8 case

         begin
            --  Note: for details of UTF8 encoding see RFC 3629

            U := Unsigned_32 (Character'Pos (C));

            --  16#00_0000#-16#00_007F#: 0xxxxxxx

            if (U and 2#10000000#) = 2#00000000# then
               Result := Wide_Wide_Character'Val (Character'Pos (C));

            --  16#00_0080#-16#00_07FF#: 110xxxxx 10xxxxxx

            elsif (U and 2#11100000#) = 2#110_00000# then
               W := U and 2#00011111#;
               Get_UTF_Byte;
               Result := Wide_Wide_Character'Val (W);

            --  16#00_0800#-16#00_ffff#: 1110xxxx 10xxxxxx 10xxxxxx

            elsif (U and 2#11110000#) = 2#1110_0000# then
               W := U and 2#00001111#;
               Get_UTF_Byte;
               Get_UTF_Byte;
               Result := Wide_Wide_Character'Val (W);

            --  16#01_0000#-16#10_FFFF#: 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx

            elsif (U and 2#11111000#) = 2#11110_000# then
               W := U and 2#00000111#;

               for K in 1 .. 3 loop
                  Get_UTF_Byte;
               end loop;

               Result := Wide_Wide_Character'Val (W);

            --  16#0020_0000#-16#03FF_FFFF#: 111110xx 10xxxxxx 10xxxxxx
            --                               10xxxxxx 10xxxxxx

            elsif (U and 2#11111100#) = 2#111110_00# then
               W := U and 2#00000011#;

               for K in 1 .. 4 loop
                  Get_UTF_Byte;
               end loop;

               Result := Wide_Wide_Character'Val (W);

            --  All other cases are invalid, note that this includes:

            --  16#0400_0000#-16#7FFF_FFFF#: 1111110x 10xxxxxx 10xxxxxx
            --                               10xxxxxx 10xxxxxx 10xxxxxx

            --  since Wide_Wide_Character does not include code values
            --  greater than 16#03FF_FFFF#.

            else
               Bad;
            end if;
         end UTF8;

      --  All encoding functions other than UTF-8

      else
         Non_UTF8 : declare
            function Char_Sequence_To_UTF is
              new Char_Sequence_To_UTF_32 (In_Char);

         begin
            --  For brackets, must test for specific case of [ not followed by
            --  quotation, where we must not call Char_Sequence_To_UTF, but
            --  instead just return the bracket unchanged.

            if Encoding_Method = WCEM_Brackets
              and then C = '['
              and then (Ptr > Input'Last or else Input (Ptr) /= '"')
            then
               Result := '[';

            --  All other cases including [" with Brackets

            else
               Result :=
                 Wide_Wide_Character'Val
                   (Char_Sequence_To_UTF (C, Encoding_Method));
            end if;
         end Non_UTF8;
      end if;
   end Decode_Wide_Wide_Character;

   -----------------------------
   -- Decode_Wide_Wide_String --
   -----------------------------

   function Decode_Wide_Wide_String (S : String) return Wide_Wide_String is
      Result : Wide_Wide_String (1 .. S'Length);
      Length : Natural;
   begin
      Decode_Wide_Wide_String (S, Result, Length);
      return Result (1 .. Length);
   end Decode_Wide_Wide_String;

   procedure Decode_Wide_Wide_String
     (S      : String;
      Result : out Wide_Wide_String;
      Length : out Natural)
   is
      Ptr : Natural;

   begin
      Ptr := S'First;
      Length := 0;
      while Ptr <= S'Last loop
         if Length >= Result'Last then
            Past_End;
         end if;

         Length := Length + 1;
         Decode_Wide_Wide_Character (S, Ptr, Result (Length));
      end loop;
   end Decode_Wide_Wide_String;

   -------------------------
   -- Next_Wide_Character --
   -------------------------

   procedure Next_Wide_Character (Input : String; Ptr : in out Natural) is
   begin
      if Ptr < Input'First then
         Past_End;
      end if;

      --  Special efficient encoding for UTF-8 case

      if Encoding_Method = WCEM_UTF8 then
         UTF8 : declare
            U : Unsigned_32;

            procedure Getc;
            pragma Inline (Getc);
            --  Gets the character at Input (Ptr) and returns code in U as
            --  Unsigned_32 value. On return Ptr is bumped past the character.

            procedure Skip_UTF_Byte;
            pragma Inline (Skip_UTF_Byte);
            --  Skips past one encoded byte which must be 2#10xxxxxx#

            ----------
            -- Getc --
            ----------

            procedure Getc is
            begin
               if Ptr > Input'Last then
                  Past_End;
               else
                  U := Unsigned_32 (Character'Pos (Input (Ptr)));
                  Ptr := Ptr + 1;
               end if;
            end Getc;

            -------------------
            -- Skip_UTF_Byte --
            -------------------

            procedure Skip_UTF_Byte is
            begin
               Getc;

               if (U and 2#11000000#) /= 2#10_000000# then
                  Bad;
               end if;
            end Skip_UTF_Byte;

         --  Start of processing for UTF-8 case

         begin
            --  16#00_0000#-16#00_007F#: 0xxxxxxx

            Getc;

            if (U and 2#10000000#) = 2#00000000# then
               return;

            --  16#00_0080#-16#00_07FF#: 110xxxxx 10xxxxxx

            elsif (U and 2#11100000#) = 2#110_00000# then
               Skip_UTF_Byte;

            --  16#00_0800#-16#00_ffff#: 1110xxxx 10xxxxxx 10xxxxxx

            elsif (U and 2#11110000#) = 2#1110_0000# then
               Skip_UTF_Byte;
               Skip_UTF_Byte;

            --  Any other code is invalid, note that this includes:

            --  16#01_0000#-16#10_FFFF#: 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx

            --  16#0020_0000#-16#03FF_FFFF#: 111110xx 10xxxxxx 10xxxxxx
            --                               10xxxxxx 10xxxxxx

            --  16#0400_0000#-16#7FFF_FFFF#: 1111110x 10xxxxxx 10xxxxxx
            --                               10xxxxxx 10xxxxxx 10xxxxxx

            --  since Wide_Character does not allow codes > 16#FFFF#

            else
               Bad;
            end if;
         end UTF8;

      --  Non-UTF-8 case

      else
         declare
            Discard : Wide_Character;
         begin
            Decode_Wide_Character (Input, Ptr, Discard);
         end;
      end if;
   end Next_Wide_Character;

   ------------------------------
   -- Next_Wide_Wide_Character --
   ------------------------------

   procedure Next_Wide_Wide_Character (Input : String; Ptr : in out Natural) is
   begin
      --  Special efficient encoding for UTF-8 case

      if Encoding_Method = WCEM_UTF8 then
         UTF8 : declare
            U : Unsigned_32;

            procedure Getc;
            pragma Inline (Getc);
            --  Gets the character at Input (Ptr) and returns code in U as
            --  Unsigned_32 value. On return Ptr is bumped past the character.

            procedure Skip_UTF_Byte;
            pragma Inline (Skip_UTF_Byte);
            --  Skips past one encoded byte which must be 2#10xxxxxx#

            ----------
            -- Getc --
            ----------

            procedure Getc is
            begin
               if Ptr > Input'Last then
                  Past_End;
               else
                  U := Unsigned_32 (Character'Pos (Input (Ptr)));
                  Ptr := Ptr + 1;
               end if;
            end Getc;

            -------------------
            -- Skip_UTF_Byte --
            -------------------

            procedure Skip_UTF_Byte is
            begin
               Getc;

               if (U and 2#11000000#) /= 2#10_000000# then
                  Bad;
               end if;
            end Skip_UTF_Byte;

         --  Start of processing for UTF-8 case

         begin
            if Ptr < Input'First then
               Past_End;
            end if;

            --  16#00_0000#-16#00_007F#: 0xxxxxxx

            Getc;

            if (U and 2#10000000#) = 2#00000000# then
               null;

            --  16#00_0080#-16#00_07FF#: 110xxxxx 10xxxxxx

            elsif (U and 2#11100000#) = 2#110_00000# then
               Skip_UTF_Byte;

            --  16#00_0800#-16#00_ffff#: 1110xxxx 10xxxxxx 10xxxxxx

            elsif (U and 2#11110000#) = 2#1110_0000# then
               Skip_UTF_Byte;
               Skip_UTF_Byte;

            --  16#01_0000#-16#10_FFFF#: 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx

            elsif (U and 2#11111000#) = 2#11110_000# then
               for K in 1 .. 3 loop
                  Skip_UTF_Byte;
               end loop;

            --  16#0020_0000#-16#03FF_FFFF#: 111110xx 10xxxxxx 10xxxxxx
            --                               10xxxxxx 10xxxxxx

            elsif (U and 2#11111100#) = 2#111110_00# then
               for K in 1 .. 4 loop
                  Skip_UTF_Byte;
               end loop;

            --  Any other code is invalid, note that this includes:

            --  16#0400_0000#-16#7FFF_FFFF#: 1111110x 10xxxxxx 10xxxxxx
            --                               10xxxxxx 10xxxxxx 10xxxxxx

            --  since Wide_Wide_Character does not allow codes > 16#03FF_FFFF#

            else
               Bad;
            end if;
         end UTF8;

      --  Non-UTF-8 case

      else
         declare
            Discard : Wide_Wide_Character;
         begin
            Decode_Wide_Wide_Character (Input, Ptr, Discard);
         end;
      end if;
   end Next_Wide_Wide_Character;

   --------------
   -- Past_End --
   --------------

   procedure Past_End is
   begin
      raise Constraint_Error with "past end of string";
   end Past_End;

   -------------------------
   -- Prev_Wide_Character --
   -------------------------

   procedure Prev_Wide_Character (Input : String; Ptr : in out Natural) is
   begin
      if Ptr > Input'Last + 1 then
         Past_End;
      end if;

      --  Special efficient encoding for UTF-8 case

      if Encoding_Method = WCEM_UTF8 then
         UTF8 : declare
            U : Unsigned_32;

            procedure Getc;
            pragma Inline (Getc);
            --  Gets the character at Input (Ptr - 1) and returns code in U as
            --  Unsigned_32 value. On return Ptr is decremented by one.

            procedure Skip_UTF_Byte;
            pragma Inline (Skip_UTF_Byte);
            --  Checks that U is 2#10xxxxxx# and then calls Get

            ----------
            -- Getc --
            ----------

            procedure Getc is
            begin
               if Ptr <= Input'First then
                  Past_End;
               else
                  Ptr := Ptr - 1;
                  U := Unsigned_32 (Character'Pos (Input (Ptr)));
               end if;
            end Getc;

            -------------------
            -- Skip_UTF_Byte --
            -------------------

            procedure Skip_UTF_Byte is
            begin
               if (U and 2#11000000#) = 2#10_000000# then
                  Getc;
               else
                  Bad;
               end if;
            end Skip_UTF_Byte;

         --  Start of processing for UTF-8 case

         begin
            --  16#00_0000#-16#00_007F#: 0xxxxxxx

            Getc;

            if (U and 2#10000000#) = 2#00000000# then
               return;

            --  16#00_0080#-16#00_07FF#: 110xxxxx 10xxxxxx

            else
               Skip_UTF_Byte;

               if (U and 2#11100000#) = 2#110_00000# then
                  return;

               --  16#00_0800#-16#00_ffff#: 1110xxxx 10xxxxxx 10xxxxxx

               else
                  Skip_UTF_Byte;

                  if (U and 2#11110000#) = 2#1110_0000# then
                     return;

                     --  Any other code is invalid, note that this includes:

                     --  16#01_0000#-16#10_FFFF#: 11110xxx 10xxxxxx 10xxxxxx
                     --                           10xxxxxx

                     --  16#0020_0000#-16#03FF_FFFF#: 111110xx 10xxxxxx
                     --                               10xxxxxx 10xxxxxx
                     --                               10xxxxxx

                     --  16#0400_0000#-16#7FFF_FFFF#: 1111110x 10xxxxxx
                     --                               10xxxxxx 10xxxxxx
                     --                               10xxxxxx 10xxxxxx

                     --  since Wide_Character does not allow codes > 16#FFFF#

                  else
                     Bad;
                  end if;
               end if;
            end if;
         end UTF8;

      --  Special efficient encoding for brackets case

      elsif Encoding_Method = WCEM_Brackets then
         Brackets : declare
            P : Natural;
            S : Natural;

         begin
            --  See if we have "] at end positions

            if Ptr > Input'First + 1
              and then Input (Ptr - 1) = ']'
              and then Input (Ptr - 2) = '"'
            then
               P := Ptr - 2;

               --  Loop back looking for [" at start

               while P >= Ptr - 10 loop
                  if P <= Input'First + 1 then
                     Bad;

                  elsif Input (P - 1) = '"'
                    and then Input (P - 2) = '['
                  then
                     --  Found ["..."], scan forward to check it

                     S := P - 2;
                     P := S;
                     Next_Wide_Character (Input, P);

                     --  OK if at original pointer, else error

                     if P = Ptr then
                        Ptr := S;
                        return;
                     else
                        Bad;
                     end if;
                  end if;

                  P := P - 1;
               end loop;

               --  Falling through loop means more than 8 chars between the
               --  enclosing brackets (or simply a missing left bracket)

               Bad;

            --  Here if no bracket sequence present

            else
               if Ptr = Input'First then
                  Past_End;
               else
                  Ptr := Ptr - 1;
               end if;
            end if;
         end Brackets;

      --  Non-UTF-8/Brackets. These are the inefficient cases where we have to
      --  go to the start of the string and skip forwards till Ptr matches.

      else
         Non_UTF_Brackets : declare
            Discard : Wide_Character;
            PtrS    : Natural;
            PtrP    : Natural;

         begin
            PtrS := Input'First;

            if Ptr <= PtrS then
               Past_End;
            end if;

            loop
               PtrP := PtrS;
               Decode_Wide_Character (Input, PtrS, Discard);

               if PtrS = Ptr then
                  Ptr := PtrP;
                  return;

               elsif PtrS > Ptr then
                  Bad;
               end if;
            end loop;

         exception
            when Constraint_Error =>
               Bad;
         end Non_UTF_Brackets;
      end if;
   end Prev_Wide_Character;

   ------------------------------
   -- Prev_Wide_Wide_Character --
   ------------------------------

   procedure Prev_Wide_Wide_Character (Input : String; Ptr : in out Natural) is
   begin
      if Ptr > Input'Last + 1 then
         Past_End;
      end if;

      --  Special efficient encoding for UTF-8 case

      if Encoding_Method = WCEM_UTF8 then
         UTF8 : declare
            U : Unsigned_32;

            procedure Getc;
            pragma Inline (Getc);
            --  Gets the character at Input (Ptr - 1) and returns code in U as
            --  Unsigned_32 value. On return Ptr is decremented by one.

            procedure Skip_UTF_Byte;
            pragma Inline (Skip_UTF_Byte);
            --  Checks that U is 2#10xxxxxx# and then calls Get

            ----------
            -- Getc --
            ----------

            procedure Getc is
            begin
               if Ptr <= Input'First then
                  Past_End;
               else
                  Ptr := Ptr - 1;
                  U := Unsigned_32 (Character'Pos (Input (Ptr)));
               end if;
            end Getc;

            -------------------
            -- Skip_UTF_Byte --
            -------------------

            procedure Skip_UTF_Byte is
            begin
               if (U and 2#11000000#) = 2#10_000000# then
                  Getc;
               else
                  Bad;
               end if;
            end Skip_UTF_Byte;

         --  Start of processing for UTF-8 case

         begin
            --  16#00_0000#-16#00_007F#: 0xxxxxxx

            Getc;

            if (U and 2#10000000#) = 2#00000000# then
               return;

            --  16#00_0080#-16#00_07FF#: 110xxxxx 10xxxxxx

            else
               Skip_UTF_Byte;

               if (U and 2#11100000#) = 2#110_00000# then
                  return;

               --  16#00_0800#-16#00_ffff#: 1110xxxx 10xxxxxx 10xxxxxx

               else
                  Skip_UTF_Byte;

                  if (U and 2#11110000#) = 2#1110_0000# then
                     return;

                  --  16#01_0000#-16#10_FFFF#: 11110xxx 10xxxxxx 10xxxxxx
                  --                           10xxxxxx

                  else
                     Skip_UTF_Byte;

                     if (U and 2#11111000#) = 2#11110_000# then
                        return;

                     --  16#0020_0000#-16#03FF_FFFF#: 111110xx 10xxxxxx
                     --                               10xxxxxx 10xxxxxx
                     --                               10xxxxxx

                     else
                        Skip_UTF_Byte;

                        if (U and 2#11111100#) = 2#111110_00# then
                           return;

                        --  Any other code is invalid, note that this includes:

                        --  16#0400_0000#-16#7FFF_FFFF#: 1111110x 10xxxxxx
                        --                               10xxxxxx 10xxxxxx
                        --                               10xxxxxx 10xxxxxx

                        --  since Wide_Wide_Character does not allow codes
                        --  greater than 16#03FF_FFFF#

                        else
                           Bad;
                        end if;
                     end if;
                  end if;
               end if;
            end if;
         end UTF8;

      --  Special efficient encoding for brackets case

      elsif Encoding_Method = WCEM_Brackets then
         Brackets : declare
            P : Natural;
            S : Natural;

         begin
            --  See if we have "] at end positions

            if Ptr > Input'First + 1
              and then Input (Ptr - 1) = ']'
              and then Input (Ptr - 2) = '"'
            then
               P := Ptr - 2;

               --  Loop back looking for [" at start

               while P >= Ptr - 10 loop
                  if P <= Input'First + 1 then
                     Bad;

                  elsif Input (P - 1) = '"'
                    and then Input (P - 2) = '['
                  then
                     --  Found ["..."], scan forward to check it

                     S := P - 2;
                     P := S;
                     Next_Wide_Wide_Character (Input, P);

                     --  OK if at original pointer, else error

                     if P = Ptr then
                        Ptr := S;
                        return;
                     else
                        Bad;
                     end if;
                  end if;

                  P := P - 1;
               end loop;

               --  Falling through loop means more than 8 chars between the
               --  enclosing brackets (or simply a missing left bracket)

               Bad;

            --  Here if no bracket sequence present

            else
               if Ptr = Input'First then
                  Past_End;
               else
                  Ptr := Ptr - 1;
               end if;
            end if;
         end Brackets;

      --  Non-UTF-8/Brackets. These are the inefficient cases where we have to
      --  go to the start of the string and skip forwards till Ptr matches.

      else
         Non_UTF8_Brackets : declare
            Discard : Wide_Wide_Character;
            PtrS    : Natural;
            PtrP    : Natural;

         begin
            PtrS := Input'First;

            if Ptr <= PtrS then
               Past_End;
            end if;

            loop
               PtrP := PtrS;
               Decode_Wide_Wide_Character (Input, PtrS, Discard);

               if PtrS = Ptr then
                  Ptr := PtrP;
                  return;

               elsif PtrS > Ptr then
                  Bad;
               end if;
            end loop;

         exception
            when Constraint_Error =>
               Bad;
         end Non_UTF8_Brackets;
      end if;
   end Prev_Wide_Wide_Character;

   --------------------------
   -- Validate_Wide_String --
   --------------------------

   function Validate_Wide_String (S : String) return Boolean is
      Ptr : Natural;

   begin
      Ptr := S'First;
      while Ptr <= S'Last loop
         Next_Wide_Character (S, Ptr);
      end loop;

      return True;

   exception
      when Constraint_Error =>
         return False;
   end Validate_Wide_String;

   -------------------------------
   -- Validate_Wide_Wide_String --
   -------------------------------

   function Validate_Wide_Wide_String (S : String) return Boolean is
      Ptr : Natural;

   begin
      Ptr := S'First;
      while Ptr <= S'Last loop
         Next_Wide_Wide_Character (S, Ptr);
      end loop;

      return True;

   exception
      when Constraint_Error =>
         return False;
   end Validate_Wide_Wide_String;

end GNAT.Decode_String;