llvm-project/llvm/lib/Support/ConvertUTF.c

/*===--- ConvertUTF.c - Universal Character Names conversions ---------------===
 *
 *                     The LLVM Compiler Infrastructure
 *
 * This file is distributed under the University of Illinois Open Source
 * License. See LICENSE.TXT for details.
 *
 *===------------------------------------------------------------------------=*/
/*
 * Copyright 2001-2004 Unicode, Inc.
 * 
 * Disclaimer
 * 
 * This source code is provided as is by Unicode, Inc. No claims are
 * made as to fitness for any particular purpose. No warranties of any
 * kind are expressed or implied. The recipient agrees to determine
 * applicability of information provided. If this file has been
 * purchased on magnetic or optical media from Unicode, Inc., the
 * sole remedy for any claim will be exchange of defective media
 * within 90 days of receipt.
 * 
 * Limitations on Rights to Redistribute This Code
 * 
 * Unicode, Inc. hereby grants the right to freely use the information
 * supplied in this file in the creation of products supporting the
 * Unicode Standard, and to make copies of this file in any form
 * for internal or external distribution as long as this notice
 * remains attached.
 */

/* ---------------------------------------------------------------------

    Conversions between UTF32, UTF-16, and UTF-8. Source code file.
    Author: Mark E. Davis, 1994.
    Rev History: Rick McGowan, fixes & updates May 2001.
    Sept 2001: fixed const & error conditions per
        mods suggested by S. Parent & A. Lillich.
    June 2002: Tim Dodd added detection and handling of incomplete
        source sequences, enhanced error detection, added casts
        to eliminate compiler warnings.
    July 2003: slight mods to back out aggressive FFFE detection.
    Jan 2004: updated switches in from-UTF8 conversions.
    Oct 2004: updated to use UNI_MAX_LEGAL_UTF32 in UTF-32 conversions.

    See the header file "ConvertUTF.h" for complete documentation.

------------------------------------------------------------------------ */


#include "llvm/Support/ConvertUTF.h"
#ifdef CVTUTF_DEBUG
#include <stdio.h>
#endif
#include <assert.h>

static const int halfShift  = 10; /* used for shifting by 10 bits */

static const UTF32 halfBase = 0x0010000UL;
static const UTF32 halfMask = 0x3FFUL;

#define UNI_SUR_HIGH_START  (UTF32)0xD800
#define UNI_SUR_HIGH_END    (UTF32)0xDBFF
#define UNI_SUR_LOW_START   (UTF32)0xDC00
#define UNI_SUR_LOW_END     (UTF32)0xDFFF
#define false      0
#define true        1

/* --------------------------------------------------------------------- */

/*
 * Index into the table below with the first byte of a UTF-8 sequence to
 * get the number of trailing bytes that are supposed to follow it.
 * Note that *legal* UTF-8 values can't have 4 or 5-bytes. The table is
 * left as-is for anyone who may want to do such conversion, which was
 * allowed in earlier algorithms.
 */
static const char trailingBytesForUTF8[256] = {
    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,
    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,
    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,
    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,
    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,
    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,
    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,
    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
};

/*
 * Magic values subtracted from a buffer value during UTF8 conversion.
 * This table contains as many values as there might be trailing bytes
 * in a UTF-8 sequence.
 */
static const UTF32 offsetsFromUTF8[6] = { 0x00000000UL, 0x00003080UL, 0x000E2080UL, 
                     0x03C82080UL, 0xFA082080UL, 0x82082080UL };

/*
 * Once the bits are split out into bytes of UTF-8, this is a mask OR-ed
 * into the first byte, depending on how many bytes follow.  There are
 * as many entries in this table as there are UTF-8 sequence types.
 * (I.e., one byte sequence, two byte... etc.). Remember that sequencs
 * for *legal* UTF-8 will be 4 or fewer bytes total.
 */
static const UTF8 firstByteMark[7] = { 0x00, 0x00, 0xC0, 0xE0, 0xF0, 0xF8, 0xFC };

/* --------------------------------------------------------------------- */

/* The interface converts a whole buffer to avoid function-call overhead.
 * Constants have been gathered. Loops & conditionals have been removed as
 * much as possible for efficiency, in favor of drop-through switches.
 * (See "Note A" at the bottom of the file for equivalent code.)
 * If your compiler supports it, the "isLegalUTF8" call can be turned
 * into an inline function.
 */


/* --------------------------------------------------------------------- */

ConversionResult ConvertUTF32toUTF16 (
        const UTF32** sourceStart, const UTF32* sourceEnd, 
        UTF16** targetStart, UTF16* targetEnd, ConversionFlags flags) {
    ConversionResult result = conversionOK;
    const UTF32* source = *sourceStart;
    UTF16* target = *targetStart;
    while (source < sourceEnd) {
        UTF32 ch;
        if (target >= targetEnd) {
            result = targetExhausted; break;
        }
        ch = *source++;
        if (ch <= UNI_MAX_BMP) { /* Target is a character <= 0xFFFF */
            /* UTF-16 surrogate values are illegal in UTF-32; 0xffff or 0xfffe are both reserved values */
            if (ch >= UNI_SUR_HIGH_START && ch <= UNI_SUR_LOW_END) {
                if (flags == strictConversion) {
                    --source; /* return to the illegal value itself */
                    result = sourceIllegal;
                    break;
                } else {
                    *target++ = UNI_REPLACEMENT_CHAR;
                }
            } else {
                *target++ = (UTF16)ch; /* normal case */
            }
        } else if (ch > UNI_MAX_LEGAL_UTF32) {
            if (flags == strictConversion) {
                result = sourceIllegal;
            } else {
                *target++ = UNI_REPLACEMENT_CHAR;
            }
        } else {
            /* target is a character in range 0xFFFF - 0x10FFFF. */
            if (target + 1 >= targetEnd) {
                --source; /* Back up source pointer! */
                result = targetExhausted; break;
            }
            ch -= halfBase;
            *target++ = (UTF16)((ch >> halfShift) + UNI_SUR_HIGH_START);
            *target++ = (UTF16)((ch & halfMask) + UNI_SUR_LOW_START);
        }
    }
    *sourceStart = source;
    *targetStart = target;
    return result;
}

/* --------------------------------------------------------------------- */

ConversionResult ConvertUTF16toUTF32 (
        const UTF16** sourceStart, const UTF16* sourceEnd, 
        UTF32** targetStart, UTF32* targetEnd, ConversionFlags flags) {
    ConversionResult result = conversionOK;
    const UTF16* source = *sourceStart;
    UTF32* target = *targetStart;
    UTF32 ch, ch2;
    while (source < sourceEnd) {
        const UTF16* oldSource = source; /*  In case we have to back up because of target overflow. */
        ch = *source++;
        /* If we have a surrogate pair, convert to UTF32 first. */
        if (ch >= UNI_SUR_HIGH_START && ch <= UNI_SUR_HIGH_END) {
            /* If the 16 bits following the high surrogate are in the source buffer... */
            if (source < sourceEnd) {
                ch2 = *source;
                /* If it's a low surrogate, convert to UTF32. */
                if (ch2 >= UNI_SUR_LOW_START && ch2 <= UNI_SUR_LOW_END) {
                    ch = ((ch - UNI_SUR_HIGH_START) << halfShift)
                        + (ch2 - UNI_SUR_LOW_START) + halfBase;
                    ++source;
                } else if (flags == strictConversion) { /* it's an unpaired high surrogate */
                    --source; /* return to the illegal value itself */
                    result = sourceIllegal;
                    break;
                }
            } else { /* We don't have the 16 bits following the high surrogate. */
                --source; /* return to the high surrogate */
                result = sourceExhausted;
                break;
            }
        } else if (flags == strictConversion) {
            /* UTF-16 surrogate values are illegal in UTF-32 */
            if (ch >= UNI_SUR_LOW_START && ch <= UNI_SUR_LOW_END) {
                --source; /* return to the illegal value itself */
                result = sourceIllegal;
                break;
            }
        }
        if (target >= targetEnd) {
            source = oldSource; /* Back up source pointer! */
            result = targetExhausted; break;
        }
        *target++ = ch;
    }
    *sourceStart = source;
    *targetStart = target;
#ifdef CVTUTF_DEBUG
if (result == sourceIllegal) {
    fprintf(stderr, "ConvertUTF16toUTF32 illegal seq 0x%04x,%04x\n", ch, ch2);
    fflush(stderr);
}
#endif
    return result;
}
ConversionResult ConvertUTF16toUTF8 (
        const UTF16** sourceStart, const UTF16* sourceEnd, 
        UTF8** targetStart, UTF8* targetEnd, ConversionFlags flags) {
    ConversionResult result = conversionOK;
    const UTF16* source = *sourceStart;
    UTF8* target = *targetStart;
    while (source < sourceEnd) {
        UTF32 ch;
        unsigned short bytesToWrite = 0;
        const UTF32 byteMask = 0xBF;
        const UTF32 byteMark = 0x80; 
        const UTF16* oldSource = source; /* In case we have to back up because of target overflow. */
        ch = *source++;
        /* If we have a surrogate pair, convert to UTF32 first. */
        if (ch >= UNI_SUR_HIGH_START && ch <= UNI_SUR_HIGH_END) {
            /* If the 16 bits following the high surrogate are in the source buffer... */
            if (source < sourceEnd) {
                UTF32 ch2 = *source;
                /* If it's a low surrogate, convert to UTF32. */
                if (ch2 >= UNI_SUR_LOW_START && ch2 <= UNI_SUR_LOW_END) {
                    ch = ((ch - UNI_SUR_HIGH_START) << halfShift)
                        + (ch2 - UNI_SUR_LOW_START) + halfBase;
                    ++source;
                } else if (flags == strictConversion) { /* it's an unpaired high surrogate */
                    --source; /* return to the illegal value itself */
                    result = sourceIllegal;
                    break;
                }
            } else { /* We don't have the 16 bits following the high surrogate. */
                --source; /* return to the high surrogate */
                result = sourceExhausted;
                break;
            }
        } else if (flags == strictConversion) {
            /* UTF-16 surrogate values are illegal in UTF-32 */
            if (ch >= UNI_SUR_LOW_START && ch <= UNI_SUR_LOW_END) {
                --source; /* return to the illegal value itself */
                result = sourceIllegal;
                break;
            }
        }
        /* Figure out how many bytes the result will require */
        if (ch < (UTF32)0x80) {      bytesToWrite = 1;
        } else if (ch < (UTF32)0x800) {     bytesToWrite = 2;
        } else if (ch < (UTF32)0x10000) {   bytesToWrite = 3;
        } else if (ch < (UTF32)0x110000) {  bytesToWrite = 4;
        } else {                            bytesToWrite = 3;
                                            ch = UNI_REPLACEMENT_CHAR;
        }

        target += bytesToWrite;
        if (target > targetEnd) {
            source = oldSource; /* Back up source pointer! */
            target -= bytesToWrite; result = targetExhausted; break;
        }
        switch (bytesToWrite) { /* note: everything falls through. */
            case 4: *--target = (UTF8)((ch | byteMark) & byteMask); ch >>= 6;
            case 3: *--target = (UTF8)((ch | byteMark) & byteMask); ch >>= 6;
            case 2: *--target = (UTF8)((ch | byteMark) & byteMask); ch >>= 6;
            case 1: *--target =  (UTF8)(ch | firstByteMark[bytesToWrite]);
        }
        target += bytesToWrite;
    }
    *sourceStart = source;
    *targetStart = target;
    return result;
}

/* --------------------------------------------------------------------- */

ConversionResult ConvertUTF32toUTF8 (
        const UTF32** sourceStart, const UTF32* sourceEnd, 
        UTF8** targetStart, UTF8* targetEnd, ConversionFlags flags) {
    ConversionResult result = conversionOK;
    const UTF32* source = *sourceStart;
    UTF8* target = *targetStart;
    while (source < sourceEnd) {
        UTF32 ch;
        unsigned short bytesToWrite = 0;
        const UTF32 byteMask = 0xBF;
        const UTF32 byteMark = 0x80; 
        ch = *source++;
        if (flags == strictConversion ) {
            /* UTF-16 surrogate values are illegal in UTF-32 */
            if (ch >= UNI_SUR_HIGH_START && ch <= UNI_SUR_LOW_END) {
                --source; /* return to the illegal value itself */
                result = sourceIllegal;
                break;
            }
        }
        /*
         * Figure out how many bytes the result will require. Turn any
         * illegally large UTF32 things (> Plane 17) into replacement chars.
         */
        if (ch < (UTF32)0x80) {      bytesToWrite = 1;
        } else if (ch < (UTF32)0x800) {     bytesToWrite = 2;
        } else if (ch < (UTF32)0x10000) {   bytesToWrite = 3;
        } else if (ch <= UNI_MAX_LEGAL_UTF32) {  bytesToWrite = 4;
        } else {                            bytesToWrite = 3;
                                            ch = UNI_REPLACEMENT_CHAR;
                                            result = sourceIllegal;
        }
        
        target += bytesToWrite;
        if (target > targetEnd) {
            --source; /* Back up source pointer! */
            target -= bytesToWrite; result = targetExhausted; break;
        }
        switch (bytesToWrite) { /* note: everything falls through. */
            case 4: *--target = (UTF8)((ch | byteMark) & byteMask); ch >>= 6;
            case 3: *--target = (UTF8)((ch | byteMark) & byteMask); ch >>= 6;
            case 2: *--target = (UTF8)((ch | byteMark) & byteMask); ch >>= 6;
            case 1: *--target = (UTF8) (ch | firstByteMark[bytesToWrite]);
        }
        target += bytesToWrite;
    }
    *sourceStart = source;
    *targetStart = target;
    return result;
}

/* --------------------------------------------------------------------- */

/*
 * Utility routine to tell whether a sequence of bytes is legal UTF-8.
 * This must be called with the length pre-determined by the first byte.
 * If not calling this from ConvertUTF8to*, then the length can be set by:
 *  length = trailingBytesForUTF8[*source]+1;
 * and the sequence is illegal right away if there aren't that many bytes
 * available.
 * If presented with a length > 4, this returns false.  The Unicode
 * definition of UTF-8 goes up to 4-byte sequences.
 */

static Boolean isLegalUTF8(const UTF8 *source, int length) {
    UTF8 a;
    const UTF8 *srcptr = source+length;
    switch (length) {
    default: return false;
        /* Everything else falls through when "true"... */
    case 4: if ((a = (*--srcptr)) < 0x80 || a > 0xBF) return false;
    case 3: if ((a = (*--srcptr)) < 0x80 || a > 0xBF) return false;
    case 2: if ((a = (*--srcptr)) < 0x80 || a > 0xBF) return false;

        switch (*source) {
            /* no fall-through in this inner switch */
            case 0xE0: if (a < 0xA0) return false; break;
            case 0xED: if (a > 0x9F) return false; break;
            case 0xF0: if (a < 0x90) return false; break;
            case 0xF4: if (a > 0x8F) return false; break;
            default:   if (a < 0x80) return false;
        }

    case 1: if (*source >= 0x80 && *source < 0xC2) return false;
    }
    if (*source > 0xF4) return false;
    return true;
}

/* --------------------------------------------------------------------- */

/*
 * Exported function to return whether a UTF-8 sequence is legal or not.
 * This is not used here; it's just exported.
 */
Boolean isLegalUTF8Sequence(const UTF8 *source, const UTF8 *sourceEnd) {
    int length = trailingBytesForUTF8[*source]+1;
    if (length > sourceEnd - source) {
        return false;
    }
    return isLegalUTF8(source, length);
}

/* --------------------------------------------------------------------- */

static unsigned
findMaximalSubpartOfIllFormedUTF8Sequence(const UTF8 *source,
                                          const UTF8 *sourceEnd) {
  assert(!isLegalUTF8Sequence(source, sourceEnd));

  /*
   * Unicode 6.3.0, D93b:
   *
   *   Maximal subpart of an ill-formed subsequence: The longest code unit
   *   subsequence starting at an unconvertible offset that is either:
   *   a. the initial subsequence of a well-formed code unit sequence, or
   *   b. a subsequence of length one.
   */

  if (source == sourceEnd)
    return 0;

  /*
   * Perform case analysis.  See Unicode 6.3.0, Table 3-7. Well-Formed UTF-8
   * Byte Sequences.
   */

  UTF8 b1 = *source;
  ++source;
  if (b1 >= 0xC2 && b1 <= 0xDF) {
    /*
     * First byte is valid, but we know that this code unit sequence is
     * invalid, so the maximal subpart has to end after the first byte.
     */
    return 1;
  }

  if (source == sourceEnd)
    return 1;

  UTF8 b2 = *source;
  ++source;

  if (b1 == 0xE0) {
    return (b2 >= 0xA0 && b2 <= 0xBF) ? 2 : 1;
  }
  if (b1 >= 0xE1 && b1 <= 0xEC) {
    return (b2 >= 0x80 && b2 <= 0xBF) ? 2 : 1;
  }
  if (b1 == 0xED) {
    return (b2 >= 0x80 && b2 <= 0x9F) ? 2 : 1;
  }
  if (b1 >= 0xEE && b1 <= 0xEF) {
    return (b2 >= 0x80 && b2 <= 0xBF) ? 2 : 1;
  }
  if (b1 == 0xF0) {
    if (b2 >= 0x90 && b2 <= 0xBF) {
      if (source == sourceEnd)
        return 2;

      UTF8 b3 = *source;
      return (b3 >= 0x80 && b3 <= 0xBF) ? 3 : 2;
    }
    return 1;
  }
  if (b1 >= 0xF1 && b1 <= 0xF3) {
    if (b2 >= 0x80 && b2 <= 0xBF) {
      if (source == sourceEnd)
        return 2;

      UTF8 b3 = *source;
      return (b3 >= 0x80 && b3 <= 0xBF) ? 3 : 2;
    }
    return 1;
  }
  if (b1 == 0xF4) {
    if (b2 >= 0x80 && b2 <= 0x8F) {
      if (source == sourceEnd)
        return 2;

      UTF8 b3 = *source;
      return (b3 >= 0x80 && b3 <= 0xBF) ? 3 : 2;
    }
    return 1;
  }

  assert((b1 >= 0x80 && b1 <= 0xC1) || b1 >= 0xF5);
  /*
   * There are no valid sequences that start with these bytes.  Maximal subpart
   * is defined to have length 1 in these cases.
   */
  return 1;
}

/* --------------------------------------------------------------------- */

/*
 * Exported function to return the total number of bytes in a codepoint
 * represented in UTF-8, given the value of the first byte.
 */
unsigned getNumBytesForUTF8(UTF8 first) {
  return trailingBytesForUTF8[first] + 1;
}

/* --------------------------------------------------------------------- */

/*
 * Exported function to return whether a UTF-8 string is legal or not.
 * This is not used here; it's just exported.
 */
Boolean isLegalUTF8String(const UTF8 **source, const UTF8 *sourceEnd) {
    while (*source != sourceEnd) {
        int length = trailingBytesForUTF8[**source] + 1;
        if (length > sourceEnd - *source || !isLegalUTF8(*source, length))
            return false;
        *source += length;
    }
    return true;
}

/* --------------------------------------------------------------------- */

ConversionResult ConvertUTF8toUTF16 (
        const UTF8** sourceStart, const UTF8* sourceEnd, 
        UTF16** targetStart, UTF16* targetEnd, ConversionFlags flags) {
    ConversionResult result = conversionOK;
    const UTF8* source = *sourceStart;
    UTF16* target = *targetStart;
    while (source < sourceEnd) {
        UTF32 ch = 0;
        unsigned short extraBytesToRead = trailingBytesForUTF8[*source];
        if (extraBytesToRead >= sourceEnd - source) {
            result = sourceExhausted; break;
        }
        /* Do this check whether lenient or strict */
        if (!isLegalUTF8(source, extraBytesToRead+1)) {
            result = sourceIllegal;
            break;
        }
        /*
         * The cases all fall through. See "Note A" below.
         */
        switch (extraBytesToRead) {
            case 5: ch += *source++; ch <<= 6; /* remember, illegal UTF-8 */
            case 4: ch += *source++; ch <<= 6; /* remember, illegal UTF-8 */
            case 3: ch += *source++; ch <<= 6;
            case 2: ch += *source++; ch <<= 6;
            case 1: ch += *source++; ch <<= 6;
            case 0: ch += *source++;
        }
        ch -= offsetsFromUTF8[extraBytesToRead];

        if (target >= targetEnd) {
            source -= (extraBytesToRead+1); /* Back up source pointer! */
            result = targetExhausted; break;
        }
        if (ch <= UNI_MAX_BMP) { /* Target is a character <= 0xFFFF */
            /* UTF-16 surrogate values are illegal in UTF-32 */
            if (ch >= UNI_SUR_HIGH_START && ch <= UNI_SUR_LOW_END) {
                if (flags == strictConversion) {
                    source -= (extraBytesToRead+1); /* return to the illegal value itself */
                    result = sourceIllegal;
                    break;
                } else {
                    *target++ = UNI_REPLACEMENT_CHAR;
                }
            } else {
                *target++ = (UTF16)ch; /* normal case */
            }
        } else if (ch > UNI_MAX_UTF16) {
            if (flags == strictConversion) {
                result = sourceIllegal;
                source -= (extraBytesToRead+1); /* return to the start */
                break; /* Bail out; shouldn't continue */
            } else {
                *target++ = UNI_REPLACEMENT_CHAR;
            }
        } else {
            /* target is a character in range 0xFFFF - 0x10FFFF. */
            if (target + 1 >= targetEnd) {
                source -= (extraBytesToRead+1); /* Back up source pointer! */
                result = targetExhausted; break;
            }
            ch -= halfBase;
            *target++ = (UTF16)((ch >> halfShift) + UNI_SUR_HIGH_START);
            *target++ = (UTF16)((ch & halfMask) + UNI_SUR_LOW_START);
        }
    }
    *sourceStart = source;
    *targetStart = target;
    return result;
}

/* --------------------------------------------------------------------- */

static ConversionResult ConvertUTF8toUTF32Impl(
        const UTF8** sourceStart, const UTF8* sourceEnd, 
        UTF32** targetStart, UTF32* targetEnd, ConversionFlags flags,
        Boolean InputIsPartial) {
    ConversionResult result = conversionOK;
    const UTF8* source = *sourceStart;
    UTF32* target = *targetStart;
    while (source < sourceEnd) {
        UTF32 ch = 0;
        unsigned short extraBytesToRead = trailingBytesForUTF8[*source];
        if (extraBytesToRead >= sourceEnd - source) {
            if (flags == strictConversion || InputIsPartial) {
                result = sourceExhausted;
                break;
            } else {
                result = sourceIllegal;

                /*
                 * Replace the maximal subpart of ill-formed sequence with
                 * replacement character.
                 */
                source += findMaximalSubpartOfIllFormedUTF8Sequence(source,
                                                                    sourceEnd);
                *target++ = UNI_REPLACEMENT_CHAR;
                continue;
            }
        }
        if (target >= targetEnd) {
            result = targetExhausted; break;
        }

        /* Do this check whether lenient or strict */
        if (!isLegalUTF8(source, extraBytesToRead+1)) {
            result = sourceIllegal;
            if (flags == strictConversion) {
                /* Abort conversion. */
                break;
            } else {
                /*
                 * Replace the maximal subpart of ill-formed sequence with
                 * replacement character.
                 */
                source += findMaximalSubpartOfIllFormedUTF8Sequence(source,
                                                                    sourceEnd);
                *target++ = UNI_REPLACEMENT_CHAR;
                continue;
            }
        }
        /*
         * The cases all fall through. See "Note A" below.
         */
        switch (extraBytesToRead) {
            case 5: ch += *source++; ch <<= 6;
            case 4: ch += *source++; ch <<= 6;
            case 3: ch += *source++; ch <<= 6;
            case 2: ch += *source++; ch <<= 6;
            case 1: ch += *source++; ch <<= 6;
            case 0: ch += *source++;
        }
        ch -= offsetsFromUTF8[extraBytesToRead];

        if (ch <= UNI_MAX_LEGAL_UTF32) {
            /*
             * UTF-16 surrogate values are illegal in UTF-32, and anything
             * over Plane 17 (> 0x10FFFF) is illegal.
             */
            if (ch >= UNI_SUR_HIGH_START && ch <= UNI_SUR_LOW_END) {
                if (flags == strictConversion) {
                    source -= (extraBytesToRead+1); /* return to the illegal value itself */
                    result = sourceIllegal;
                    break;
                } else {
                    *target++ = UNI_REPLACEMENT_CHAR;
                }
            } else {
                *target++ = ch;
            }
        } else { /* i.e., ch > UNI_MAX_LEGAL_UTF32 */
            result = sourceIllegal;
            *target++ = UNI_REPLACEMENT_CHAR;
        }
    }
    *sourceStart = source;
    *targetStart = target;
    return result;
}

ConversionResult ConvertUTF8toUTF32Partial(const UTF8 **sourceStart,
                                           const UTF8 *sourceEnd,
                                           UTF32 **targetStart,
                                           UTF32 *targetEnd,
                                           ConversionFlags flags) {
  return ConvertUTF8toUTF32Impl(sourceStart, sourceEnd, targetStart, targetEnd,
                                flags, /*InputIsPartial=*/true);
}

ConversionResult ConvertUTF8toUTF32(const UTF8 **sourceStart,
                                    const UTF8 *sourceEnd, UTF32 **targetStart,
                                    UTF32 *targetEnd, ConversionFlags flags) {
  return ConvertUTF8toUTF32Impl(sourceStart, sourceEnd, targetStart, targetEnd,
                                flags, /*InputIsPartial=*/false);
}

/* ---------------------------------------------------------------------

    Note A.
    The fall-through switches in UTF-8 reading code save a
    temp variable, some decrements & conditionals.  The switches
    are equivalent to the following loop:
        {
            int tmpBytesToRead = extraBytesToRead+1;
            do {
                ch += *source++;
                --tmpBytesToRead;
                if (tmpBytesToRead) ch <<= 6;
            } while (tmpBytesToRead > 0);
        }
    In UTF-8 writing code, the switches on "bytesToWrite" are
    similarly unrolled loops.

   --------------------------------------------------------------------- */