release/src/router/transmission/libtransmission/ConvertUTF.c

   1 /*
   2  * Copyright 2001-2004 Unicode, Inc.
   3  *
   4  * Disclaimer
   5  *
   6  * This source code is provided as is by Unicode, Inc. No claims are
   7  * made as to fitness for any particular purpose. No warranties of any
   8  * kind are expressed or implied. The recipient agrees to determine
   9  * applicability of information provided. If this file has been
  10  * purchased on magnetic or optical media from Unicode, Inc., the
  11  * sole remedy for any claim will be exchange of defective media
  12  * within 90 days of receipt.
  13  *
  14  * Limitations on Rights to Redistribute This Code
  15  *
  16  * Unicode, Inc. hereby grants the right to freely use the information
  17  * supplied in this file in the creation of products supporting the
  18  * Unicode Standard, and to make copies of this file in any form
  19  * for internal or external distribution as long as this notice
  20  * remains attached.
  21  */
  22
  23 /* ---------------------------------------------------------------------
  24
  25     Conversions between UTF32, UTF-16, and UTF-8. Source code file.
  26     Author: Mark E. Davis, 1994.
  27     Rev History: Rick McGowan, fixes & updates May 2001.
  28     Sept 2001: fixed const & error conditions per
  29         mods suggested by S. Parent & A. Lillich.
  30     June 2002: Tim Dodd added detection and handling of incomplete
  31         source sequences, enhanced error detection, added casts
  32         to eliminate compiler warnings.
  33     July 2003: slight mods to back out aggressive FFFE detection.
  34     Jan 2004: updated switches in from-UTF8 conversions.
  35     Oct 2004: updated to use UNI_MAX_LEGAL_UTF32 in UTF-32 conversions.
  36     May 2006: updated isLegalUTF8Sequence.
  37
  38     See the header file "ConvertUTF.h" for complete documentation.
  39
  40 ------------------------------------------------------------------------ */
  41
  42 #ifdef CVTUTF_DEBUG
  43  #include <stdio.h>
  44 #endif
  45 #include <string.h> /* strlen() */
  46 #include "ConvertUTF.h"
  47
  48 static const int halfShift  = 10; /* used for shifting by 10 bits */
  49
  50 static const UTF32 halfBase = 0x0010000UL;
  51 static const UTF32 halfMask = 0x3FFUL;
  52
  53 #define UNI_SUR_HIGH_START  (UTF32)0xD800
  54 #define UNI_SUR_HIGH_END    (UTF32)0xDBFF
  55 #define UNI_SUR_LOW_START   (UTF32)0xDC00
  56 #define UNI_SUR_LOW_END     (UTF32)0xDFFF
  57 #define false      0
  58 #define true        1
  59
  60 /* --------------------------------------------------------------------- */
  61
  62 ConversionResult ConvertUTF32toUTF16 (
  63         const UTF32** sourceStart, const UTF32* sourceEnd,
  64         UTF16** targetStart, UTF16* targetEnd, ConversionFlags flags) {
  65     ConversionResult result = conversionOK;
  66     const UTF32* source = *sourceStart;
  67     UTF16* target = *targetStart;
  68     while (source < sourceEnd) {
  69         UTF32 ch;
  70         if (target >= targetEnd) {
  71             result = targetExhausted; break;
  72         }
  73         ch = *source++;
  74         if (ch <= UNI_MAX_BMP) { /* Target is a character <= 0xFFFF */
  75             /* UTF-16 surrogate values are illegal in UTF-32; 0xffff or 0xfffe are both reserved values */
  76             if (ch >= UNI_SUR_HIGH_START && ch <= UNI_SUR_LOW_END) {
  77                 if (flags == strictConversion) {
  78                     --source; /* return to the illegal value itself */
  79                     result = sourceIllegal;
  80                     break;
  81                 } else {
  82                     *target++ = UNI_REPLACEMENT_CHAR;
  83                 }
  84             } else {
  85                 *target++ = (UTF16)ch; /* normal case */
  86             }
  87         } else if (ch > UNI_MAX_LEGAL_UTF32) {
  88             if (flags == strictConversion) {
  89                 result = sourceIllegal;
  90             } else {
  91                 *target++ = UNI_REPLACEMENT_CHAR;
  92             }
  93         } else {
  94             /* target is a character in range 0xFFFF - 0x10FFFF. */
  95             if (target + 1 >= targetEnd) {
  96                 --source; /* Back up source pointer! */
  97                 result = targetExhausted; break;
  98             }
  99             ch -= halfBase;
 100             *target++ = (UTF16)((ch >> halfShift) + UNI_SUR_HIGH_START);
 101             *target++ = (UTF16)((ch & halfMask) + UNI_SUR_LOW_START);
 102         }
 103     }
 104     *sourceStart = source;
 105     *targetStart = target;
 106     return result;
 107 }
 108
 109 /* --------------------------------------------------------------------- */
 110
 111 ConversionResult ConvertUTF16toUTF32 (
 112         const UTF16** sourceStart, const UTF16* sourceEnd,
 113         UTF32** targetStart, UTF32* targetEnd, ConversionFlags flags) {
 114     ConversionResult result = conversionOK;
 115     const UTF16* source = *sourceStart;
 116     UTF32* target = *targetStart;
 117     UTF32 ch, ch2;
 118     while (source < sourceEnd) {
 119         const UTF16* oldSource = source; /*  In case we have to back up because of target overflow. */
 120         ch = *source++;
 121         /* If we have a surrogate pair, convert to UTF32 first. */
 122         if (ch >= UNI_SUR_HIGH_START && ch <= UNI_SUR_HIGH_END) {
 123             /* If the 16 bits following the high surrogate are in the source buffer... */
 124             if (source < sourceEnd) {
 125                 ch2 = *source;
 126                 /* If it's a low surrogate, convert to UTF32. */
 127                 if (ch2 >= UNI_SUR_LOW_START && ch2 <= UNI_SUR_LOW_END) {
 128                     ch = ((ch - UNI_SUR_HIGH_START) << halfShift)
 129                         + (ch2 - UNI_SUR_LOW_START) + halfBase;
 130                     ++source;
 131                 } else if (flags == strictConversion) { /* it's an unpaired high surrogate */
 132                     --source; /* return to the illegal value itself */
 133                     result = sourceIllegal;
 134                     break;
 135                 }
 136             } else { /* We don't have the 16 bits following the high surrogate. */
 137                 --source; /* return to the high surrogate */
 138                 result = sourceExhausted;
 139                 break;
 140             }
 141         } else if (flags == strictConversion) {
 142             /* UTF-16 surrogate values are illegal in UTF-32 */
 143             if (ch >= UNI_SUR_LOW_START && ch <= UNI_SUR_LOW_END) {
 144                 --source; /* return to the illegal value itself */
 145                 result = sourceIllegal;
 146                 break;
 147             }
 148         }
 149         if (target >= targetEnd) {
 150             source = oldSource; /* Back up source pointer! */
 151             result = targetExhausted; break;
 152         }
 153         *target++ = ch;
 154     }
 155     *sourceStart = source;
 156     *targetStart = target;
 157 #ifdef CVTUTF_DEBUG
 158 if (result == sourceIllegal) {
 159     fprintf(stderr, "ConvertUTF16toUTF32 illegal seq 0x%04x,%04x\n", ch, ch2);
 160     fflush(stderr);
 161 }
 162 #endif
 163     return result;
 164 }
 165
 166 /* --------------------------------------------------------------------- */
 167
 168 /*
 169  * Index into the table below with the first byte of a UTF-8 sequence to
 170  * get the number of trailing bytes that are supposed to follow it.
 171  * Note that *legal* UTF-8 values can't have 4 or 5-bytes. The table is
 172  * left as-is for anyone who may want to do such conversion, which was
 173  * allowed in earlier algorithms.
 174  */
 175 static const char trailingBytesForUTF8[256] = {
 176     0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
 177     0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
 178     0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
 179     0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
 180     0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
 181     0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
 182     1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
 183     2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2, 3,3,3,3,3,3,3,3,4,4,4,4,5,5,5,5
 184 };
 185
 186 /*
 187  * Magic values subtracted from a buffer value during UTF8 conversion.
 188  * This table contains as many values as there might be trailing bytes
 189  * in a UTF-8 sequence.
 190  */
 191 static const UTF32 offsetsFromUTF8[6] = { 0x00000000UL, 0x00003080UL, 0x000E2080UL,
 192                      0x03C82080UL, 0xFA082080UL, 0x82082080UL };
 193
 194 /*
 195  * Once the bits are split out into bytes of UTF-8, this is a mask OR-ed
 196  * into the first byte, depending on how many bytes follow. There are
 197  * as many entries in this table as there are UTF-8 sequence types.
 198  * (I.e., one byte sequence, two byte... etc.). Remember that sequencs
 199  * for *legal* UTF-8 will be 4 or fewer bytes total.
 200  */
 201 static const UTF8 firstByteMark[7] = { 0x00, 0x00, 0xC0, 0xE0, 0xF0, 0xF8, 0xFC };
 202
 203 /* --------------------------------------------------------------------- */
 204
 205 /* The interface converts a whole buffer to avoid function-call overhead.
 206  * Constants have been gathered. Loops & conditionals have been removed as
 207  * much as possible for efficiency, in favor of drop-through switches.
 208  * (See "Note A" at the bottom of the file for equivalent code.)
 209  * If your compiler supports it, the "isLegalUTF8" call can be turned
 210  * into an inline function.
 211  */
 212
 213 /* --------------------------------------------------------------------- */
 214
 215 ConversionResult ConvertUTF16toUTF8 (
 216         const UTF16** sourceStart, const UTF16* sourceEnd,
 217         UTF8** targetStart, UTF8* targetEnd, ConversionFlags flags) {
 218     ConversionResult result = conversionOK;
 219     const UTF16* source = *sourceStart;
 220     UTF8* target = *targetStart;
 221     while (source < sourceEnd) {
 222         UTF32 ch;
 223         unsigned short bytesToWrite = 0;
 224         const UTF32 byteMask = 0xBF;
 225         const UTF32 byteMark = 0x80;
 226         const UTF16* oldSource = source; /* In case we have to back up because of target overflow. */
 227         ch = *source++;
 228         /* If we have a surrogate pair, convert to UTF32 first. */
 229         if (ch >= UNI_SUR_HIGH_START && ch <= UNI_SUR_HIGH_END) {
 230             /* If the 16 bits following the high surrogate are in the source buffer... */
 231             if (source < sourceEnd) {
 232                 UTF32 ch2 = *source;
 233                 /* If it's a low surrogate, convert to UTF32. */
 234                 if (ch2 >= UNI_SUR_LOW_START && ch2 <= UNI_SUR_LOW_END) {
 235                     ch = ((ch - UNI_SUR_HIGH_START) << halfShift)
 236                         + (ch2 - UNI_SUR_LOW_START) + halfBase;
 237                     ++source;
 238                 } else if (flags == strictConversion) { /* it's an unpaired high surrogate */
 239                     --source; /* return to the illegal value itself */
 240                     result = sourceIllegal;
 241                     break;
 242                 }
 243             } else { /* We don't have the 16 bits following the high surrogate. */
 244                 --source; /* return to the high surrogate */
 245                 result = sourceExhausted;
 246                 break;
 247             }
 248         } else if (flags == strictConversion) {
 249             /* UTF-16 surrogate values are illegal in UTF-32 */
 250             if (ch >= UNI_SUR_LOW_START && ch <= UNI_SUR_LOW_END) {
 251                 --source; /* return to the illegal value itself */
 252                 result = sourceIllegal;
 253                 break;
 254             }
 255         }
 256         /* Figure out how many bytes the result will require */
 257         if (ch < (UTF32)0x80) {      bytesToWrite = 1;
 258         } else if (ch < (UTF32)0x800) {     bytesToWrite = 2;
 259         } else if (ch < (UTF32)0x10000) {   bytesToWrite = 3;
 260         } else if (ch < (UTF32)0x110000) {  bytesToWrite = 4;
 261         } else {                            bytesToWrite = 3;
 262                                             ch = UNI_REPLACEMENT_CHAR;
 263         }
 264
 265         target += bytesToWrite;
 266         if (target > targetEnd) {
 267             source = oldSource; /* Back up source pointer! */
 268             target -= bytesToWrite; result = targetExhausted; break;
 269         }
 270         switch (bytesToWrite) { /* note: everything falls through. */
 271             case 4: *--target = (UTF8)((ch | byteMark) & byteMask); ch >>= 6;
 272             case 3: *--target = (UTF8)((ch | byteMark) & byteMask); ch >>= 6;
 273             case 2: *--target = (UTF8)((ch | byteMark) & byteMask); ch >>= 6;
 274             case 1: *--target =  (UTF8)(ch | firstByteMark[bytesToWrite]);
 275         }
 276         target += bytesToWrite;
 277     }
 278     *sourceStart = source;
 279     *targetStart = target;
 280     return result;
 281 }
 282
 283 /* --------------------------------------------------------------------- */
 284
 285 /*
 286  * Utility routine to tell whether a sequence of bytes is legal UTF-8.
 287  * This must be called with the length pre-determined by the first byte.
 288  * If not calling this from ConvertUTF8to*, then the length can be set by:
 289  *  length = trailingBytesForUTF8[*source]+1;
 290  * and the sequence is illegal right away if there aren't that many bytes
 291  * available.
 292  * If presented with a length > 4, this returns false. The Unicode
 293  * definition of UTF-8 goes up to 4-byte sequences.
 294  */
 295
 296 static Boolean isLegalUTF8(const UTF8 *source, int length) {
 297     UTF8 a;
 298     const UTF8 *srcptr = source+length;
 299     switch (length) {
 300     default: return false;
 301         /* Everything else falls through when "true"... */
 302     case 4: if ((a = (*--srcptr)) < 0x80 || a > 0xBF) return false;
 303     case 3: if ((a = (*--srcptr)) < 0x80 || a > 0xBF) return false;
 304     case 2: if ((a = (*--srcptr)) > 0xBF) return false;
 305
 306         switch (*source) {
 307             /* no fall-through in this inner switch */
 308             case 0xE0: if (a < 0xA0) return false; break;
 309             case 0xED: if ((a < 0x80) || (a > 0x9F)) return false; break;
 310             case 0xF0: if (a < 0x90) return false; break;
 311             case 0xF4: if (a > 0x8F) return false; break;
 312             default:   if (a < 0x80) return false;
 313         }
 314
 315     case 1: if (*source >= 0x80 && *source < 0xC2) return false;
 316     }
 317     if (*source > 0xF4) return false;
 318     return true;
 319 }
 320
 321 /* --------------------------------------------------------------------- */
 322
 323 /*
 324  * Exported function to return whether a UTF-8 sequence is legal or not.
 325  * This is not used here; it's just exported.
 326  */
 327
 328 Boolean isLegalUTF8Sequence(const UTF8 *source, const UTF8 *sourceEnd) {
 329     int length;
 330     if (source == sourceEnd) {
 331         return true;
 332     }
 333     while (true) {
 334         length = trailingBytesForUTF8[*source]+1;
 335         if (source+length > sourceEnd) {
 336             return false;
 337         }
 338         if (!isLegalUTF8(source, length)) {
 339             return false;
 340         }
 341         source += length;
 342         if (source >= sourceEnd) {
 343             return true;
 344         }
 345     }
 346 }
 347
 348 /**
 349  * This is a variation of isLegalUTF8Sequence() that behaves like g_utf8_validate().
 350  * In addition to knowing if the sequence is legal, it also tells you the last good character.
 351  */
 352 Boolean
 353 tr_utf8_validate( const char * str, int max_len, const char ** end )
 354 {
 355     const UTF8* source = (const UTF8*) str;
 356     const UTF8* sourceEnd;
 357
 358     if( max_len == 0 )
 359         return true;
 360
 361     if( str == NULL )
 362         return false;
 363
 364     sourceEnd = source + ((max_len < 0) ? strlen(str) : (size_t)max_len);
 365
 366     if( source == sourceEnd )
 367     {
 368         if( end != NULL )
 369             *end = (const char*) source;
 370         return true;
 371     }
 372
 373     for( ;; )
 374     {
 375         const int length = trailingBytesForUTF8[*source] + 1;
 376         if (source + length > sourceEnd) {
 377             if( end != NULL )
 378                 *end = (const char*) source;
 379             return false;
 380         }
 381         if (!isLegalUTF8(source, length)) {
 382             if( end != NULL )
 383                 *end = (const char*) source;
 384             return false;
 385         }
 386         source += length;
 387         if (source >= sourceEnd) {
 388             if( end != NULL )
 389                 *end = (const char*) source;
 390             return true;
 391         }
 392     }
 393
 394
 395 }
 396
 397
 398 /* --------------------------------------------------------------------- */
 399
 400 ConversionResult ConvertUTF8toUTF16 (
 401         const UTF8** sourceStart, const UTF8* sourceEnd,
 402         UTF16** targetStart, UTF16* targetEnd, ConversionFlags flags) {
 403     ConversionResult result = conversionOK;
 404     const UTF8* source = *sourceStart;
 405     UTF16* target = *targetStart;
 406     while (source < sourceEnd) {
 407         UTF32 ch = 0;
 408         unsigned short extraBytesToRead = trailingBytesForUTF8[*source];
 409         if (source + extraBytesToRead >= sourceEnd) {
 410             result = sourceExhausted; break;
 411         }
 412         /* Do this check whether lenient or strict */
 413         if (! isLegalUTF8(source, extraBytesToRead+1)) {
 414             result = sourceIllegal;
 415             break;
 416         }
 417         /*
 418          * The cases all fall through. See "Note A" below.
 419          */
 420         switch (extraBytesToRead) {
 421             case 5: ch += *source++; ch <<= 6; /* remember, illegal UTF-8 */
 422             case 4: ch += *source++; ch <<= 6; /* remember, illegal UTF-8 */
 423             case 3: ch += *source++; ch <<= 6;
 424             case 2: ch += *source++; ch <<= 6;
 425             case 1: ch += *source++; ch <<= 6;
 426             case 0: ch += *source++;
 427         }
 428         ch -= offsetsFromUTF8[extraBytesToRead];
 429
 430         if (target >= targetEnd) {
 431             source -= (extraBytesToRead+1); /* Back up source pointer! */
 432             result = targetExhausted; break;
 433         }
 434         if (ch <= UNI_MAX_BMP) { /* Target is a character <= 0xFFFF */
 435             /* UTF-16 surrogate values are illegal in UTF-32 */
 436             if (ch >= UNI_SUR_HIGH_START && ch <= UNI_SUR_LOW_END) {
 437                 if (flags == strictConversion) {
 438                     source -= (extraBytesToRead+1); /* return to the illegal value itself */
 439                     result = sourceIllegal;
 440                     break;
 441                 } else {
 442                     *target++ = UNI_REPLACEMENT_CHAR;
 443                 }
 444             } else {
 445                 *target++ = (UTF16)ch; /* normal case */
 446             }
 447         } else if (ch > UNI_MAX_UTF16) {
 448             if (flags == strictConversion) {
 449                 result = sourceIllegal;
 450                 source -= (extraBytesToRead+1); /* return to the start */
 451                 break; /* Bail out; shouldn't continue */
 452             } else {
 453                 *target++ = UNI_REPLACEMENT_CHAR;
 454             }
 455         } else {
 456             /* target is a character in range 0xFFFF - 0x10FFFF. */
 457             if (target + 1 >= targetEnd) {
 458                 source -= (extraBytesToRead+1); /* Back up source pointer! */
 459                 result = targetExhausted; break;
 460             }
 461             ch -= halfBase;
 462             *target++ = (UTF16)((ch >> halfShift) + UNI_SUR_HIGH_START);
 463             *target++ = (UTF16)((ch & halfMask) + UNI_SUR_LOW_START);
 464         }
 465     }
 466     *sourceStart = source;
 467     *targetStart = target;
 468     return result;
 469 }
 470
 471 /* --------------------------------------------------------------------- */
 472
 473 ConversionResult ConvertUTF32toUTF8 (
 474         const UTF32** sourceStart, const UTF32* sourceEnd,
 475         UTF8** targetStart, UTF8* targetEnd, ConversionFlags flags) {
 476     ConversionResult result = conversionOK;
 477     const UTF32* source = *sourceStart;
 478     UTF8* target = *targetStart;
 479     while (source < sourceEnd) {
 480         UTF32 ch;
 481         unsigned short bytesToWrite = 0;
 482         const UTF32 byteMask = 0xBF;
 483         const UTF32 byteMark = 0x80;
 484         ch = *source++;
 485         if (flags == strictConversion ) {
 486             /* UTF-16 surrogate values are illegal in UTF-32 */
 487             if (ch >= UNI_SUR_HIGH_START && ch <= UNI_SUR_LOW_END) {
 488                 --source; /* return to the illegal value itself */
 489                 result = sourceIllegal;
 490                 break;
 491             }
 492         }
 493         /*
 494          * Figure out how many bytes the result will require. Turn any
 495          * illegally large UTF32 things (> Plane 17) into replacement chars.
 496          */
 497         if (ch < (UTF32)0x80) {      bytesToWrite = 1;
 498         } else if (ch < (UTF32)0x800) {     bytesToWrite = 2;
 499         } else if (ch < (UTF32)0x10000) {   bytesToWrite = 3;
 500         } else if (ch <= UNI_MAX_LEGAL_UTF32) {  bytesToWrite = 4;
 501         } else {                            bytesToWrite = 3;
 502                                             ch = UNI_REPLACEMENT_CHAR;
 503                                             result = sourceIllegal;
 504         }
 505
 506         target += bytesToWrite;
 507         if (target > targetEnd) {
 508             --source; /* Back up source pointer! */
 509             target -= bytesToWrite; result = targetExhausted; break;
 510         }
 511         switch (bytesToWrite) { /* note: everything falls through. */
 512             case 4: *--target = (UTF8)((ch | byteMark) & byteMask); ch >>= 6;
 513             case 3: *--target = (UTF8)((ch | byteMark) & byteMask); ch >>= 6;
 514             case 2: *--target = (UTF8)((ch | byteMark) & byteMask); ch >>= 6;
 515             case 1: *--target = (UTF8) (ch | firstByteMark[bytesToWrite]);
 516         }
 517         target += bytesToWrite;
 518     }
 519     *sourceStart = source;
 520     *targetStart = target;
 521     return result;
 522 }
 523
 524 /* --------------------------------------------------------------------- */
 525
 526 ConversionResult ConvertUTF8toUTF32 (
 527         const UTF8** sourceStart, const UTF8* sourceEnd,
 528         UTF32** targetStart, UTF32* targetEnd, ConversionFlags flags) {
 529     ConversionResult result = conversionOK;
 530     const UTF8* source = *sourceStart;
 531     UTF32* target = *targetStart;
 532     while (source < sourceEnd) {
 533         UTF32 ch = 0;
 534         unsigned short extraBytesToRead = trailingBytesForUTF8[*source];
 535         if (source + extraBytesToRead >= sourceEnd) {
 536             result = sourceExhausted; break;
 537         }
 538         /* Do this check whether lenient or strict */
 539         if (! isLegalUTF8(source, extraBytesToRead+1)) {
 540             result = sourceIllegal;
 541             break;
 542         }
 543         /*
 544          * The cases all fall through. See "Note A" below.
 545          */
 546         switch (extraBytesToRead) {
 547             case 5: ch += *source++; ch <<= 6;
 548             case 4: ch += *source++; ch <<= 6;
 549             case 3: ch += *source++; ch <<= 6;
 550             case 2: ch += *source++; ch <<= 6;
 551             case 1: ch += *source++; ch <<= 6;
 552             case 0: ch += *source++;
 553         }
 554         ch -= offsetsFromUTF8[extraBytesToRead];
 555
 556         if (target >= targetEnd) {
 557             source -= (extraBytesToRead+1); /* Back up the source pointer! */
 558             result = targetExhausted; break;
 559         }
 560         if (ch <= UNI_MAX_LEGAL_UTF32) {
 561             /*
 562              * UTF-16 surrogate values are illegal in UTF-32, and anything
 563              * over Plane 17 (> 0x10FFFF) is illegal.
 564              */
 565             if (ch >= UNI_SUR_HIGH_START && ch <= UNI_SUR_LOW_END) {
 566                 if (flags == strictConversion) {
 567                     source -= (extraBytesToRead+1); /* return to the illegal value itself */
 568                     result = sourceIllegal;
 569                     break;
 570                 } else {
 571                     *target++ = UNI_REPLACEMENT_CHAR;
 572                 }
 573             } else {
 574                 *target++ = ch;
 575             }
 576         } else { /* i.e., ch > UNI_MAX_LEGAL_UTF32 */
 577             result = sourceIllegal;
 578             *target++ = UNI_REPLACEMENT_CHAR;
 579         }
 580     }
 581     *sourceStart = source;
 582     *targetStart = target;
 583     return result;
 584 }
 585
 586 /* ---------------------------------------------------------------------
 587
 588     Note A.
 589     The fall-through switches in UTF-8 reading code save a
 590     temp variable, some decrements & conditionals. The switches
 591     are equivalent to the following loop:
 592         {
 593             int tmpBytesToRead = extraBytesToRead+1;
 594             do {
 595                 ch += *source++;
 596                 --tmpBytesToRead;
 597                 if (tmpBytesToRead) ch <<= 6;
 598             } while (tmpBytesToRead > 0);
 599         }
 600     In UTF-8 writing code, the switches on "bytesToWrite" are
 601     similarly unrolled loops.
 602
 603    --------------------------------------------------------------------- */