/*
*******************************************************************************
*
*   Copyright (C) 2000, International Business Machines
*   Corporation and others.  All Rights Reserved.
*
*******************************************************************************
*   file name:  ucnvmbcs.c
*   encoding:   US-ASCII
*   tab size:   8 (not used)
*   indentation:4
*
*   created on: 2000jul03
*   created by: Markus W. Scherer
*
*   The current code in this file replaces the previous implementation
*   of conversion code from multi-byte codepages to Unicode and back.
*   This implementation supports the following:
*   - legacy variable-length codepages with up to 4 bytes per character
*   - all Unicode code points (up to 0x10ffff)
*   - efficient distinction of unassigned vs. illegal byte sequences
*   - it is possible in fromUnicode() to directly deal with simple
*     stateful encodings
*   - it is possible to convert Unicode code points other than U+0000
*     to a single zero byte (but not as a fallback)
*
*   Remaining limitations in fromUnicode:
*   - byte sequences must not have leading zero bytes
*   - no fallback mapping from Unicode to a zero byte
*   - limitation to up to 4 bytes per character
*/

#include "unicode/utypes.h"
#include "unicode/ucnv.h"
#include "ucnv_bld.h"
#include "ucnvmbcs.h"
#include "ucnv_cnv.h"

/*
 * Converting stateless codepage data
 * (or codepage data with simple states) to Unicode.
 *
 * Data structure and algorithm for converting from complex legacy codepages
 * to Unicode. (Designed before 2000-may-22.)
 *
 * The basic idea is that the structure of legacy codepages can be described
 * with state tables.
 * When reading a byte stream, each input byte causes a state transition.
 * Some transitions result in the output of a code point, some result in
 * "unassigned" or "illegal" output.
 * This is used here for character conversion.
 *
 * The data structure begins with a state table consisting of a row
 * per state, with 256 entries (columns) per row for each possible input
 * byte value.
 * Each entry is 32 bits wide, with the lower 7 bits containing the next state.
 * State 0 is the initial state.
 *
 * Bit 31 of each entry indicates whether the state is
 * terminal (bit 31 set) or not.
 *
 * Most of the time, the offset values of subsequent states are added
 * up to a scalar value. This value will eventually be the index of
 * the Unicode code point in a table that follows the state table.
 * The effect is that the code points for final state table rows
 * are contiguous. The code points of final state rows follow each other
 * in the order of the references to those final states by previous
 * states, etc.
 *
 * For some terminal states, the offset is itself the output Unicode
 * code point (16 bits for a BMP code point or 20 bits for a code point
 * that is written as a surrogate pair).
 * For others, the code point in the Unicode table is stored with either
 * one or two code units: one for BMP code points, two for a pair of
 * surrogates.
 * All code points for a final table take up the same number of code
 * units, regardless of whether they all actually _use_ the same number
 * of code units. This is necessary for simple array access.
 *
 * An additional feature comes in with what in ICU is called "fallback"
 * mappings:
 * In addition to round-trippable, precise, 1:1 mappings, there are often
 * mappings defined between similar, though not the same, characters.
 * Typically, such mappings occur only in fromUnicode mapping tables because
 * Unicode has a superset repertoire of most other codepages. However, it
 * is possible to provide such mappings in the toUnicode tables, too.
 * In this case, the fallback mappings are partly integrated into the
 * general state tables because the structure of the encoding includes their
 * byte sequences. They are optional mappings when the main mapping is
 * "unassigned", and are looked up by the scalar offset of the main mapping
 * in a separate table. Only when the main mapping does not have such a
 * scalar offset, i.e., in the case of action codes 5 of 6 below (valid-direct),
 * would there need to be some different mechanism. Therefore, there are
 * separate action codes 3 and 4 (fallback-direct) especially for that.
 * The "unassigned" action code 2 cannot be used for fallback lookups because
 * it also does not result in a scalar offset. This means that fallback mappings
 * require to fit into either fallback-direct action codes or valid-single or
 * valid-pair codes that result in scalar offsets.
 * "Unassigned" really means "structurally unassigned".
 *
 * The interpretation of the bits in each entry is as follows:
 *
 * Bit 31 not set, not a terminal entry:
 * 30..7  offset delta, to be added up
 *  6..0  next state
 *
 * Bit 31 set, terminal entry:
 * 30..27 action code:
 *        0  illegal byte sequence
 *           26..7  not used, 0
 *        1  state change only
 *           26..7  not used, 0
 *           useful for state changes in simple stateful encodings,
 *           at Shift-In/Shift-Out codes
 *        2  unassigned byte sequence
 *           26..7  not used, 0
 *                  this does not contain a final offset delta because the main
 *                  purpose of this action code is to save scalar offset values;
 *                  therefore, fallback values cannot be assigned to byte
 *                  sequences that result in this action code - use codes 5 or 6
 *        3  valid byte sequence (fallback)
 *           22..7  16-bit Unicode BMP code point as fallback result
 *        4  valid byte sequence (fallback)
 *           26..7  20-bit Unicode surrogate code point as fallback result
 *
 *        action codes 5, 6, 7, and 8 result in precise-mapping Unicode code points
 *        5  valid byte sequence
 *           22..7  16-bit Unicode BMP code point
 *                  never U+fffe or U+ffff (use action codes 0, 2, 3 or 4 for that)
 *        6  valid byte sequence
 *           26..7  20-bit Unicode surrogate code point
 *                  never U+fffe or U+ffff (use action codes 0, 2, 3 or 4 for that)
 *
 *        action codes 7 and 8 may result in U+fffe (unassigned), in which case the
 *        final offset is to be looked up in a special fallback table
 *        7  valid byte sequence
 *           26..16 not used, 0
 *           15..7  final offset delta
 *                  pointing to one 16-bit code unit
 *                  which may be U+fffe (unassigned) or U+ffff (illegal)
 *        8  valid byte sequence
 *           26..16 not used, 0
 *           15..7  final offset delta
 *                  pointing to two 16-bit code units
 *                  (UTF-16 surrogates)
 *                  the first code unit either is a lead surrogate and indicates
 *                  an assigned surrogate pair, or it is a single unit
 *                  which may be U+fffe (unassigned) or U+ffff (illegal)
 *           (the final offset deltas are at most 255 * 2,
 *            times 2 because of storing code unit pairs)
 *        9..15 reserved for future use
 *           current implementations will only perform a state change
 *           and ignore bits 26..7
 *  6..0  next state (regardless of action code)
 *
 * An encoding with contiguous ranges of unassigned byte sequences, like
 * Shift-JIS and especially EUC-TW, can be stored efficiently by having
 * at least two states for the trail bytes:
 * One trail byte state that results in code points, and one that only
 * has "unassigned" and "illegal" terminal states.
 *
 * Note: partly by accident, this data structure supports simple stateless
 * encodings without any additional logic.
 * Especially simple Shift-In/Shift-Out schemes could be handled with
 * appropriate state tables (especially EBCDIC_STATEFUL!).
 */

/* MBCS setup functions ----------------------------------------------------- */

U_CFUNC void
_MBCSLoad(UConverterSharedData *sharedData,
          const uint8_t *raw,
          UErrorCode *pErrorCode) {
    UConverterMBCSTable *mbcsTable=&sharedData->table->mbcs;
    _MBCSHeader *header=(_MBCSHeader *)raw;

    if(header->version[0]!=1) {
        *pErrorCode=U_INVALID_TABLE_FORMAT;
        return;
    }

    mbcsTable->countStates=(uint8_t)header->countStates;
    mbcsTable->countToUFallbacks=header->countToUFallbacks;
    mbcsTable->stateTable=(const int32_t (*)[256])(raw+sizeof(_MBCSHeader));
    mbcsTable->toUFallbacks=(const _MBCSToUFallback *)(mbcsTable->stateTable+header->countStates);
    mbcsTable->unicodeCodeUnits=(const uint16_t *)(raw+header->offsetToUCodeUnits);

    mbcsTable->fromUnicodeTable=(const uint16_t *)(raw+header->offsetFromUTable);
    mbcsTable->fromUnicodeBytes=(const uint8_t *)(raw+header->offsetFromUBytes);
    mbcsTable->outputType=(uint8_t)header->flags;
}

U_CFUNC void
_MBCSReset(UConverter *cnv) {
    /* toUnicode */
    cnv->toUnicodeStatus=0;
    cnv->mode=0;
    cnv->toULength=0;

    /* fromUnicode */
    cnv->fromUSurrogateLead=0;
}

U_CFUNC void
_MBCSOpen(UConverter *cnv,
          const char *name,
          const char *locale,
          UErrorCode *pErrorCode) {
    _MBCSReset(cnv);
}

/* MBCS-to-Unicode conversion functions ------------------------------------- */

static UChar32
_MBCSGetFallback(UConverterMBCSTable *mbcsTable, uint32_t offset) {
    const _MBCSToUFallback *toUFallbacks;
    uint32_t i, start, limit;

    limit=mbcsTable->countToUFallbacks;
    if(limit>0) {
        /* do a binary search for the fallback mapping */
        toUFallbacks=mbcsTable->toUFallbacks;
        start=0;
        while(start<limit-1) {
            i=(start+limit)/2;
            if(offset<toUFallbacks[i].offset) {
                limit=i;
            } else {
                start=i;
            }
        }

        /* did we really find it? */
        if(offset=toUFallbacks[start].offset) {
            return toUFallbacks[start].codePoint;
        }
    }

    return 0xfffe;
}

U_CFUNC void
_MBCSToUnicodeWithOffsets(UConverterToUnicodeArgs *pArgs,
                          UErrorCode *pErrorCode) {
    /* set up the local pointers */
    UConverter *cnv=pArgs->converter;
    const uint8_t *source=(const uint8_t *)pArgs->source,
                  *sourceLimit=(const uint8_t *)pArgs->sourceLimit;
    UChar *target=pArgs->target;
    const UChar *targetLimit=pArgs->targetLimit;
    int32_t *offsets=pArgs->offsets;

    const int32_t (*stateTable)[256]=cnv->sharedData->table->mbcs.stateTable;
    const uint16_t (*unicodeCodeUnits)=cnv->sharedData->table->mbcs.unicodeCodeUnits;

    /* get the converter state from UConverter */
    uint32_t offset=cnv->toUnicodeStatus;
    uint8_t state=(uint8_t)(cnv->mode);
    int8_t byteIndex=cnv->toULength;
    uint8_t *bytes=cnv->toUBytes;

    /* sourceIndex=-1 if the current character began in the previous buffer */
    int32_t sourceIndex=byteIndex==0 ? 0 : -1,
            nextSourceIndex=0;

    /* conversion loop */
    int32_t entry;
    UChar c;
    uint8_t b;
    UConverterCallbackReason reason;

    while(source<sourceLimit) {
        /*
         * This following test is to see if available input would overflow the output.
         * It does not catch output of more than one code unit that
         * overflows as a result of a surrogate pair or callback output
         * from the last source byte.
         * Therefore, those situations also test for overflows and will
         * then break the loop, too.
         */
        if(target<targetLimit) {
            bytes[byteIndex++]=b=*source++;
            ++nextSourceIndex;
            entry=stateTable[state][b];
            if(entry>=0) {
                /*
                 * bit 31 is not set, bits:
                 * 30..7  offset delta
                 *  6..0  next state
                 */
                state=(uint8_t)(entry&0x7f);
                offset+=entry>>7;
            } else {
                /*
                 * bit 31 is set, bits:
                 * 30..27 action code
                 *        (do not mask out bit 31 for speed, include it in action values)
                 * 26..7  depend on the action code
                 *  6..0  next state
                 */

                /* set the next state early so that we can reuse the entry variable */
                state=(uint8_t)(entry&0x7f); /* typically 0 */

                /* switch per action code */
                switch((uint32_t)entry>>27U) {
                case 16|MBCS_STATE_ILLEGAL:
                    /* bits 26..7 are not used, 0 */
                    /* callback(illegal) */
                    reason=UCNV_ILLEGAL;
                    *pErrorCode=U_ILLEGAL_CHAR_FOUND;
                    goto callback;
                case 16|MBCS_STATE_CHANGE_ONLY:
                    /* bits 26..7 are not used, 0 */
                    /*
                     * This serves as a state change without any output.
                     * It is useful for reading simple stateful encodings,
                     * for example using just Shift-In/Shift-Out codes.
                     * The 21 unused bits may later be used for more sophisticated
                     * state transitions.
                     */
                    break;
                case 16|MBCS_STATE_UNASSIGNED:
                    /* bits 26..7 are not used, 0 */
                    /* callback(unassigned) */
                    reason=UCNV_UNASSIGNED;
                    *pErrorCode=U_INVALID_CHAR_FOUND;
                    goto callback;
                case 16|MBCS_STATE_FALLBACK_DIRECT_16:
                    /* bits 26..23 are not used, 0 */
                    /* bits 22..7 contain the Unicode BMP code point */
                    if(!cnv->useFallback) {
                        /* callback(unassigned) */
                        reason=UCNV_UNASSIGNED;
                        *pErrorCode=U_INVALID_CHAR_FOUND;
                        goto callback;
                    }
                    /* fall through to the MBCS_STATE_VALID_DIRECT_16 branch */
                case 16|MBCS_STATE_VALID_DIRECT_16:
                    /* bits 26..23 are not used, 0 */
                    /* bits 22..7 contain the Unicode BMP code point */
                    /* output BMP code point */
                    *target++=(UChar)(entry>>7);
                    if(offsets!=NULL) {
                        *offsets++=sourceIndex;
                    }
                    break;
                case 16|MBCS_STATE_FALLBACK_DIRECT_20:
                    /* bits 26..7 contain the Unicode surrogate code point minus 0x10000 */
                    if(!cnv->useFallback) {
                        /* callback(unassigned) */
                        reason=UCNV_UNASSIGNED;
                        *pErrorCode=U_INVALID_CHAR_FOUND;
                        goto callback;
                    }
                    /* fall through to the MBCS_STATE_VALID_DIRECT_20 branch */
                case 16|MBCS_STATE_VALID_DIRECT_20:
                    /* bits 26..7 contain the Unicode surrogate code point minus 0x10000 */
                    entry=(entry>>7)&0xfffff;
                    /* output surrogate pair */
                    *target++=(UChar)(0xd800|(UChar)(entry>>10));
                    if(offsets!=NULL) {
                        *offsets++=sourceIndex;
                    }
                    c=(UChar)(0xdc00|(UChar)(entry&0x3ff));
                    if(target<targetLimit) {
                        *target++=c;
                        if(offsets!=NULL) {
                            *offsets++=sourceIndex;
                        }
                    } else {
                        /* target overflow */
                        cnv->UCharErrorBuffer[0]=c;
                        cnv->UCharErrorBufferLength=1;
                        *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;

                        offset=0;
                        byteIndex=0;
                        goto endloop;
                    }
                    break;
                case 16|MBCS_STATE_VALID_16:
                    /* bits 26..16 are not used, 0 */
                    /* bits 15..7 contain the final offset delta to one 16-bit code unit */
                    offset+=(uint16_t)entry>>7;
                    c=unicodeCodeUnits[offset];
                    if(c<0xfffe) {
                        /* output BMP code point */
                        *target++=c;
                        if(offsets!=NULL) {
                            *offsets++=sourceIndex;
                        }
                    } else if(c==0xfffe) {
                        if(cnv->useFallback && (entry=(int32_t)_MBCSGetFallback(&cnv->sharedData->table->mbcs, offset))!=0xfffe) {
                            goto output32;
                        }
                        /* callback(unassigned) */
                        reason=UCNV_UNASSIGNED;
                        *pErrorCode=U_INVALID_CHAR_FOUND;
                        goto callback;
                    } else {
                        /* callback(illegal) */
                        reason=UCNV_ILLEGAL;
                        *pErrorCode=U_ILLEGAL_CHAR_FOUND;
                        goto callback;
                    }
                    break;
                case 16|MBCS_STATE_VALID_16_PAIR:
                    /* bits 26..16 are not used, 0 */
                    /* bits 15..7 contain the final offset delta to two 16-bit code units */
                    offset+=(uint16_t)entry>>7;
                    c=unicodeCodeUnits[offset++];
                    if(UTF_IS_FIRST_SURROGATE(c)) {
                        *target++=c;
                        if(offsets!=NULL) {
                            *offsets++=sourceIndex;
                        }
                        if(target<targetLimit) {
                            *target++=unicodeCodeUnits[offset];
                            if(offsets!=NULL) {
                                *offsets++=sourceIndex;
                            }
                        } else {
                            /* target overflow */
                            cnv->UCharErrorBuffer[0]=unicodeCodeUnits[offset];
                            cnv->UCharErrorBufferLength=1;
                            *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;

                            offset=0;
                            byteIndex=0;
                            goto endloop;
                        }
                    } else if(c<0xfffe) {
                        /* output BMP code point */
                        *target++=c;
                        if(offsets!=NULL) {
                            *offsets++=sourceIndex;
                        }
                    } else if(c==0xfffe) {
                        if(cnv->useFallback && (entry=(int32_t)_MBCSGetFallback(&cnv->sharedData->table->mbcs, offset))!=0xfffe) {
                            goto output32;
                        }
                        /* callback(unassigned) */
                        reason=UCNV_UNASSIGNED;
                        *pErrorCode=U_INVALID_CHAR_FOUND;
                        goto callback;
                    } else {
                        /* callback(illegal) */
                        reason=UCNV_ILLEGAL;
                        *pErrorCode=U_ILLEGAL_CHAR_FOUND;
                        goto callback;
                    }
                    break;
                default:
                    /* reserved, must never occur */
                    /* bits 26..7 are not used, 0 */
                    break;
                }

                /* normal end of action codes: prepare for a new character */
                offset=0;
                byteIndex=0;
                sourceIndex=nextSourceIndex;
                continue;

                /*
                 * Markus Scherer 2000-jul-05
                 *
                 * The following is extremely ugly, and I apologize for it:
                 * Several places in the above switch statement need to call
                 * a callback function or output a 32-bit code point,
                 * each of which is an involved process with
                 * a couple dozen of statements.
                 *
                 * I could do this in a function call, but I fear that then
                 * the compiler does not keep the frequently used variables in
                 * registers because the function call would need them on the stack
                 * for input and output.
                 *
                 * I could do this with a macro, but that is harder to debug and
                 * bloats the compiled code.
                 *
                 * I could just copy and paste the code, but that would also bloat
                 * the program size, make the pieces harder to maintain, and make
                 * the switch statement extremely long and clumsy.
                 *
                 * Therefore, those places goto here and do it all in one place,
                 * while the normal processing has a continue above and skips this
                 * part.
                 * This actually _saves_ goto statements, too:
                 * Since it is not possible in C to break a loop from within a switch
                 * statement, the callback code in the switch statement would have to
                 * goto behind the loop. Here, it can break if necessary.
                 */

output32:
                /* output a 32-bit (21-bit) Unicode code point stored in entry */
                if(entry<=0xffff) {
                    /* output BMP code point */
                    *target++=(UChar)entry;
                    if(offsets!=NULL) {
                        *offsets++=sourceIndex;
                    }
                } else {
                    /* output surrogate pair */
                    *target++=(UChar)(0xd7c0+(entry>>10));
                    if(offsets!=NULL) {
                        *offsets++=sourceIndex;
                    }
                    c=(UChar)(0xdc00|(UChar)(entry&0x3ff));
                    if(target<targetLimit) {
                        *target++=c;
                        if(offsets!=NULL) {
                            *offsets++=sourceIndex;
                        }
                    } else {
                        /* target overflow */
                        cnv->UCharErrorBuffer[0]=c;
                        cnv->UCharErrorBufferLength=1;
                        *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;

                        offset=0;
                        byteIndex=0;
                        break;
                    }
                }

                /* same as normal end of action codes: prepare for a new character */
                offset=0;
                byteIndex=0;
                sourceIndex=nextSourceIndex;
                continue;

callback:
                /* call the callback function with all the preparations and post-processing */
                /* update the arguments structure */
                pArgs->source=(const char *)source;
                pArgs->target=target;
                pArgs->offsets=offsets;

                /* copy the current bytes to invalidCharBuffer */
                for(b=0; b<(uint8_t)byteIndex; ++b) {
                    cnv->invalidCharBuffer[b]=(char)bytes[b];
                }
                cnv->invalidCharLength=byteIndex;

                /* set the converter state in UConverter to deal with the next character */
                cnv->toUnicodeStatus=0;
                cnv->mode=state;
                cnv->toULength=0;

                /* call the callback function */
                cnv->fromCharErrorBehaviour(cnv->toUContext, pArgs, (const char *)bytes, byteIndex, reason, pErrorCode);

                /* get the converter state from UConverter */
                offset=cnv->toUnicodeStatus;
                state=(uint8_t)cnv->mode;
                byteIndex=cnv->toULength;

                /* update target and deal with offsets if necessary */
                if(offsets!=NULL) {
                    /* add the sourceIndex to the relative offsets that the callback wrote */
                    if(sourceIndex>=0) {
                        while(target<pArgs->target) {
                            *offsets+=sourceIndex;
                            ++offsets;
                            ++target;
                        }
                    } else {
                        /* sourceIndex==-1, set -1 offsets */
                        while(target<pArgs->target) {
                            *offsets=-1;
                            ++offsets;
                            ++target;
                        }
                    }
                } else {
                    target=pArgs->target;
                }

                /* update the source pointer and index */
                sourceIndex=nextSourceIndex+((const uint8_t *)pArgs->source-source);
                source=(const uint8_t *)pArgs->source;

                /* break on error */
                if(U_FAILURE(*pErrorCode)) {
                    offset=0;
                    state=0;
                    byteIndex=0;
                    break;
                }

                /*
                 * If the callback overflowed the target, then we need to
                 * stop here with an overflow indication.
                 */
                if(cnv->UCharErrorBufferLength>0) {
                    /* target is full */
                    *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;
                    break;
                }

                /*
                 * We do not need to repeat the statements from the normal
                 * end of the action codes because we already updated all the
                 * necessary variables.
                 */
            }
        } else {
            /* target is full */
            *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;
            break;
        }
    }
endloop:

    if(pArgs->flush && source>=sourceLimit) {
        /* reset the state for the next conversion */
        if(byteIndex>0 && U_SUCCESS(*pErrorCode)) {
            /* a character byte sequence remains incomplete */
            *pErrorCode=U_TRUNCATED_CHAR_FOUND;
        }
        cnv->toUnicodeStatus=0;
        cnv->mode=0;
        cnv->toULength=0;
    } else {
        /* set the converter state back into UConverter */
        cnv->toUnicodeStatus=offset;
        cnv->mode=state;
        cnv->toULength=byteIndex;
    }

    /* write back the updated pointers */
    pArgs->source=(const char *)source;
    pArgs->target=target;
    pArgs->offsets=offsets;
}

U_CFUNC void
_MBCSToUnicode(UConverterToUnicodeArgs *pArgs,
               UErrorCode *pErrorCode) {
    _MBCSToUnicodeWithOffsets(pArgs, pErrorCode);
}

/*
 * This is a simple, interim implementation of GetNextUChar()
 * that allows to concentrate on testing one single implementation
 * of the ToUnicode conversion before it gets copied to
 * multiple version that are then optimized for their needs
 * (with vs. without offsets and getNextUChar).
 */
U_CFUNC UChar32
_MBCSGetNextUChar(UConverterToUnicodeArgs *pArgs,
                  UErrorCode *pErrorCode) {
    UChar buffer[UTF_MAX_CHAR_LENGTH];
    const char *realLimit=pArgs->sourceLimit;

    pArgs->target=buffer;
    pArgs->targetLimit=buffer+UTF_MAX_CHAR_LENGTH;

    while(pArgs->source<realLimit) {
        /* feed in one byte at a time to make sure to get only one character out */
        pArgs->sourceLimit=pArgs->source+1;
        pArgs->flush= (UBool)(pArgs->sourceLimit==realLimit);
        _MBCSToUnicode(pArgs, pErrorCode);
        if(U_FAILURE(*pErrorCode) && *pErrorCode!=U_INDEX_OUTOFBOUNDS_ERROR) {
            return 0xffff;
        } else if(pArgs->target!=buffer) {
            if(*pErrorCode==U_INDEX_OUTOFBOUNDS_ERROR) {
                *pErrorCode=U_ZERO_ERROR;
            }
            return ucnv_getUChar32KeepOverflow(pArgs->converter, buffer, pArgs->target-buffer);
        }
    }

    /* no output because of empty input or only state changes and skipping callbacks */
    *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;
    return 0xffff;
}

/*
 * This is a simple version of getNextUChar() that is used
 * by other converter implementations.
 * It does not use state from the converter, nor error codes,
 * and does not provide fallback mappings.
 *
 * Return value:
 * U+fffe   unassigned
 * U+ffff   illegal
 * otherwise the Unicode code point
 */
U_CFUNC UChar32
_MBCSSimpleGetNextUChar(UConverterSharedData *sharedData,
                        const char **pSource, const char *sourceLimit) {
    /* set up the local pointers */
    const uint8_t *source=(const uint8_t *)*pSource;

    const int32_t (*stateTable)[256]=sharedData->table->mbcs.stateTable;
    const uint16_t (*unicodeCodeUnits)=sharedData->table->mbcs.unicodeCodeUnits;

    /* converter state */
    uint32_t offset=0;
    uint8_t state=0;

    /* conversion loop */
    int32_t entry;

    if(source>=(const uint8_t *)sourceLimit) {
        /* no input at all: "unassigned" */
        return 0xfffe;
    }

    do {
        entry=stateTable[state][*source++];
        if(entry>=0) {
            /*
             * bit 31 is not set, bits:
             * 30..7  offset delta
             *  6..0  next state
             */
            state=(uint8_t)(entry&0x7f);
            offset+=entry>>7;
        } else {
            /*
             * bit 31 is set, bits:
             * 30..27 action code
             *        (do not mask out bit 31 for speed, include it in action values)
             * 26..7  depend on the action code
             *  6..0  next state
             */

            *pSource=(const char *)source;

            /* switch per action code */
            switch((uint32_t)entry>>27U) {
            case 16|MBCS_STATE_ILLEGAL:
                /* bits 26..7 are not used, 0 */
                return 0xffff;
            case 16|MBCS_STATE_CHANGE_ONLY:
                /* bits 26..7 are not used, 0 */
                /*
                 * This serves as a state change without any output.
                 * It is useful for reading simple stateful encodings,
                 * for example using just Shift-In/Shift-Out codes.
                 * The 21 unused bits may later be used for more sophisticated
                 * state transitions.
                 */
                if(source==(const uint8_t *)sourceLimit) {
                    /* if there are only state changes, then return "unassigned" */
                    return 0xfffe;
                }
                break;
            case 16|MBCS_STATE_UNASSIGNED:
                /* bits 26..7 are not used, 0 */
                return 0xfffe;
            case 16|MBCS_STATE_FALLBACK_DIRECT_16:
                /* bits 26..23 are not used, 0 */
                /* bits 22..7 contain the Unicode BMP code point */
                return 0xfffe;
            case 16|MBCS_STATE_VALID_DIRECT_16:
                /* bits 26..23 are not used, 0 */
                /* bits 22..7 contain the Unicode BMP code point */
                /* output BMP code point */
                return (UChar)(entry>>7);
            case 16|MBCS_STATE_FALLBACK_DIRECT_20:
                /* bits 26..7 contain the Unicode surrogate code point minus 0x10000 */
                return 0xfffe;
            case 16|MBCS_STATE_VALID_DIRECT_20:
                /* bits 26..7 contain the Unicode surrogate code point minus 0x10000 */
                return 0x10000+((entry>>7)&0xfffff);
            case 16|MBCS_STATE_VALID_16:
                /* bits 26..16 are not used, 0 */
                /* bits 15..7 contain the final offset delta to one 16-bit code unit */
                offset+=(uint16_t)entry>>7;
                return unicodeCodeUnits[offset];
            case 16|MBCS_STATE_VALID_16_PAIR:
                /* bits 26..16 are not used, 0 */
                /* bits 15..7 contain the final offset delta to two 16-bit code units */
                offset+=(uint16_t)entry>>7;
                entry=unicodeCodeUnits[offset++];
                if(UTF_IS_FIRST_SURROGATE(entry)) {
                    return UTF16_GET_PAIR_VALUE(entry, unicodeCodeUnits[offset]);
                } else {
                    return (UChar32)entry;
                }
            default:
                /* reserved, must never occur */
                /* bits 26..7 are not used, 0 */
                break;
            }

            /* state change only - prepare for a new character */
            state=(uint8_t)(entry&0x7f); /* typically 0 */
            offset=0;
        }
    } while(source<(const uint8_t *)sourceLimit);

    *pSource=(const char *)source;
    return 0xffff;
}

/* MBCS-from-Unicode conversion functions ----------------------------------- */

U_CFUNC void
_MBCSFromUnicodeWithOffsets(UConverterFromUnicodeArgs *pArgs,
                            UErrorCode *pErrorCode) {
    /* set up the local pointers */
    UConverter *cnv=pArgs->converter;
    const UChar *source=pArgs->source,
                *sourceLimit=pArgs->sourceLimit;
    uint8_t *target=(uint8_t *)pArgs->target;
    int32_t targetCapacity=pArgs->targetLimit-pArgs->target;
    int32_t *offsets=pArgs->offsets;

    const uint16_t *table=cnv->sharedData->table->mbcs.fromUnicodeTable;
    const uint8_t *bytes=cnv->sharedData->table->mbcs.fromUnicodeBytes;
    uint8_t outputType=cnv->sharedData->table->mbcs.outputType;

    /* get the converter state from UConverter */
    UChar32 c=cnv->fromUSurrogateLead;

    /* sourceIndex=-1 if the current character began in the previous buffer */
    int32_t sourceIndex= c==0 ? 0 : -1,
            nextSourceIndex=0;

    /* conversion loop */
    UConverterCallbackReason reason;
    uint32_t i;
    uint32_t value;
    int32_t length;

    /*
     * This is another piece of ugly code:
     * A goto into the loop if the converter state contains a first surrogate
     * from the previous function call.
     * It saves me to check in each loop iteration a check of if(c==0)
     * and duplicating the trail-surrogate-handling code in the else
     * branch of that check.
     * I could not find any other way to get around this other than
     * using a function call for the conversion and callback, which would
     * be even more inefficient.
     *
     * Markus Scherer 2000-jul-19
     */
    if(c!=0 && targetCapacity>0) {
        goto getTrail;
    }

    while(source<sourceLimit) {
        /*
         * This following test is to see if available input would overflow the output.
         * It does not catch output of more than one byte that
         * overflows as a result of a multi-byte character or callback output
         * from the last source character.
         * Therefore, those situations also test for overflows and will
         * then break the loop, too.
         */
        if(targetCapacity>0) {
            /*
             * Get a correct Unicode code point:
             * a single UChar for a BMP code point or
             * a matched surrogate pair for a "surrogate code point".
             */
            c=*source++;
            ++nextSourceIndex;
            if(UTF_IS_SURROGATE(c)) {
                if(UTF_IS_SURROGATE_FIRST(c)) {
getTrail:
                    if(source<sourceLimit) {
                        /* test the following code unit */
                        UChar trail=*source;
                        if(UTF_IS_SECOND_SURROGATE(trail)) {
                            ++source;
                            ++nextSourceIndex;
                            c=UTF16_GET_PAIR_VALUE(c, trail);
                            /* convert this surrogate code point */
                            /* exit this condition tree */
                        } else {
                            /* this is an unmatched lead code unit (1st surrogate) */
                            /* callback(illegal) */
                            reason=UCNV_ILLEGAL;
                            *pErrorCode=U_ILLEGAL_CHAR_FOUND;
                            goto callback;
                        }
                    } else {
                        /* no more input */
                        break;
                    }
                } else {
                    /* this is an unmatched trail code unit (2nd surrogate) */
                    /* callback(illegal) */
                    reason=UCNV_ILLEGAL;
                    *pErrorCode=U_ILLEGAL_CHAR_FOUND;
                    goto callback;
                }
            }

            /* convert the Unicode code point in c into codepage bytes */

            /*
             * The basic lookup is a triple-stage compact array lookup:
             *
             * Bits 21..10 (0x440 different values because Unicode code points
             * reach up to 0x10ffff) are used as an index into table[],
             * then bits 9..4 are added to that and together multiplied by 2
             * to be used as an index into a second table that starts at table+0x440.
             *
             * In that second table, there will be two 16-bit values
             * (and therefore we multiplied by two in the previous step):
             * One 16-bit value stores a bit for each of the 16 Unicode code points
             * that are grouped here to indicate if it is assigned or not.
             * If it is not assigned, there may still be a codepage character
             * stored in the third stage: a fallback value. It is used only when
             * fallbacks are turned on for the converter. If the code point is
             * unassigned and fallbacks not used or there is no fallback character
             * (all bytes 0), then the callback function is called.
             *
             * The second value in the second table (stage) is an index into
             * the third table. It is multiplied by 16*(bytes stored per character)
             * to get to the first of 16 characters. At last, bits 3..0 of
             * the Unicode code point are multiplied by (bytes stored per character)
             * and added to that index for the address of the output codepage
             * character.
             *
             * For EUC encodings that use only either 0x8e or 0x8f as the first
             * byte of their longest byte sequences, the first two bytes in
             * this third stage indicate with their 7th bits whether these bytes
             * are to be written directly or actually need to be preceeded by
             * one of the two Single-Shift codes. With this, the third stage
             * stores one byte fewer per character than the actual maximum length of
             * EUC byte sequences.
             *
             * Other than that, leading zero bytes are removed and the other
             * bytes output. A single zero byte may be output if the "assigned"
             * bit in stage 2 was on or also if the Unicode code point is U+0000.
             * The data structure does not support zero byte output as a fallback
             * for other code points, and also does not allow output of leading zeros.
             */
            i=0x440+2*((uint32_t)table[c>>10]+((c>>4)&0x3f));

            /* is this code point assigned, or do we use fallbacks? */
            if((table[i++]&(1<<(c&0xf)))!=0 || cnv->useFallback) {
                const uint8_t *p;

                /* get the bytes and the length for the output */
                switch(outputType) {
                case MBCS_OUTPUT_1:
                    p=bytes+(16*(uint32_t)table[i]+(c&0xf));
                    value=*p;
                    length=1;
                    break;
                case MBCS_OUTPUT_2:
                    p=bytes+(16*(uint32_t)table[i]+(c&0xf))*2;
                    if(U_IS_BIG_ENDIAN) {
                        value=*(uint16_t *)p;
                    } else {
                        value=((uint32_t)*p<<8)|p[1];
                    }
                    if(value<=0xff) {
                        length=1;
                    } else {
                        length=2;
                    }
                    break;
                case MBCS_OUTPUT_3:
                    p=bytes+(16*(uint32_t)table[i]+(c&0xf))*3;
                    value=((uint32_t)*p<<16)|((uint32_t)p[1]<<8)|p[2];
                    if(value<=0xff) {
                        length=1;
                    } else if(value<=0xffff) {
                        length=2;
                    } else {
                        length=3;
                    }
                    break;
                case MBCS_OUTPUT_4:
                    p=bytes+(16*(uint32_t)table[i]+(c&0xf))*4;
                    if(U_IS_BIG_ENDIAN) {
                        value=*(uint32_t *)p;
                    } else {
                        value=((uint32_t)*p<<24)|((uint32_t)p[1]<<16)|((uint32_t)p[2]<<8)|p[3];
                    }
                    if(value<=0xff) {
                        length=1;
                    } else if(value<=0xffff) {
                        length=2;
                    } else if(value<=0xffffff) {
                        length=3;
                    } else {
                        length=4;
                    }
                    break;
                case MBCS_OUTPUT_3_EUC:
                    p=bytes+(16*(uint32_t)table[i]+(c&0xf))*2;
                    if(U_IS_BIG_ENDIAN) {
                        value=*(uint16_t *)p;
                    } else {
                        value=((uint32_t)*p<<8)|p[1];
                    }
                    /* EUC 16-bit fixed-length representation */
                    if(value<=0xff) {
                        length=1;
                    } else if((value&0x8000)==0) {
                        value|=0x8e8000;
                        length=3;
                    } else if((value&0x80)==0) {
                        value|=0x8f0080;
                        length=3;
                    } else {
                        length=2;
                    }
                    break;
                case MBCS_OUTPUT_4_EUC:
                    p=bytes+(16*(uint32_t)table[i]+(c&0xf))*3;
                    value=((uint32_t)*p<<16)|((uint32_t)p[1]<<8)|p[2];
                    /* EUC 16-bit fixed-length representation applied to the first two bytes */
                    if(value<=0xff) {
                        length=1;
                    } else if(value<=0xffff) {
                        length=2;
                    } else if((value&0x800000)==0) {
                        value|=0x8e800000;
                        length=4;
                    } else if((value&0x8000)==0) {
                        value|=0x8f008000;
                        length=4;
                    } else {
                        length=3;
                    }
                    break;
                default:
                    /* must not occur */
                    break;
                }

                /* is the codepage value really an "unassigned" indicator? */
                if(value==0 && c!=0 && (table[i-1]&(1<<(c&0xf)))==0) {
                    /*
                     * We allow a 0 byte output if the Unicode code point is
                     * U+0000 and also if the "assigned" bit is set for this entry.
                     * There is no way with this data structure for fallback output
                     * for other than U+0000 to be a zero byte.
                     */
                    /* callback(unassigned) */
                    reason=UCNV_UNASSIGNED;
                    *pErrorCode=U_INVALID_CHAR_FOUND;
                    goto callback;
                }
            } else {
                /* callback(unassigned) */
                reason=UCNV_UNASSIGNED;
                *pErrorCode=U_INVALID_CHAR_FOUND;
                goto callback;
            }

            /* write the output character bytes from value and length */
            if(length==1) {
                /* this is easy because we know that there is enough space */
                *target++=(uint8_t)value;
                if(offsets!=NULL) {
                    *offsets++=sourceIndex;
                }
                --targetCapacity;
            } else {
                /* from the first if in the loop we know that available>0 */
                if(length<=targetCapacity) {
                    switch(length) {
                        /* each branch falls through to the next one */
                    case 4:
                        *target++=(uint8_t)(value>>24);
                        if(offsets!=NULL) {
                            *offsets++=sourceIndex;
                        }
                    case 3:
                        *target++=(uint8_t)(value>>16);
                        if(offsets!=NULL) {
                            *offsets++=sourceIndex;
                        }
                    case 2:
                        *target++=(uint8_t)(value>>8);
                        if(offsets!=NULL) {
                            *offsets++=sourceIndex;
                        }
                    /* case 1: covered by above, but all branches also have to output this byte */
                        *target++=(uint8_t)value;
                        if(offsets!=NULL) {
                            *offsets++=sourceIndex;
                        }
                    default:
                        /* will never occur */
                        break;
                    }
                    targetCapacity-=length;
                } else {
                    uint8_t *p;

                    /*
                     * We actually do this backwards here:
                     * In order to save an intermediate variable, we output
                     * first to the overflow buffer what does not fit into the
                     * regular target.
                     */
                    /* we know that 1<=available<length<=4 */
                    length-=targetCapacity;
                    p=(uint8_t *)cnv->charErrorBuffer;
                    switch(length) {
                        /* each branch falls through to the next one */
                    case 3:
                        *p++=(uint8_t)(value>>16);
                    case 2:
                        *p++=(uint8_t)(value>>8);
                    case 1:
                        *p=(uint8_t)value;
                    default:
                        /* will never occur */
                        break;
                    }
                    cnv->charErrorBufferLength=(int8_t)length;

                    /* now output what fits into the regular target */
                    value>>=8*length; /* length was reduced by available */
                    switch(targetCapacity) {
                        /* each branch falls through to the next one */
                    case 3:
                        *target++=(uint8_t)(value>>16);
                        if(offsets!=NULL) {
                            *offsets++=sourceIndex;
                        }
                    case 2:
                        *target++=(uint8_t)(value>>8);
                        if(offsets!=NULL) {
                            *offsets++=sourceIndex;
                        }
                    case 1:
                        *target++=(uint8_t)value;
                        if(offsets!=NULL) {
                            *offsets++=sourceIndex;
                        }
                    default:
                        /* will never occur */
                        break;
                    }

                    /* target overflow */
                    targetCapacity=0;
                    *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;
                    c=0;
                    break;
                }
            }

            /* normal end of conversion: prepare for a new character */
            c=0;
            sourceIndex=nextSourceIndex;
            continue;

            /*
             * This is the same ugly trick as in ToUnicode(), for the
             * same reasons...
             */
callback:
            /* call the callback function with all the preparations and post-processing */
            /* update the arguments structure */
            pArgs->source=source;
            pArgs->target=(char *)target;
            pArgs->offsets=offsets;

            /* set the converter state in UConverter to deal with the next character */
            cnv->fromUSurrogateLead=0;

            /* write the code point as code units */
            i=0;
            UTF_APPEND_CHAR_UNSAFE(cnv->invalidUCharBuffer, i, c);
            cnv->invalidUCharLength=(int8_t)i;

            /* call the callback function */
            cnv->fromUCharErrorBehaviour(cnv->fromUContext, pArgs, cnv->invalidUCharBuffer, i, c, reason, pErrorCode);

            /* get the converter state from UConverter */
            c=cnv->fromUSurrogateLead;

            /* update target and deal with offsets if necessary */
            if(offsets!=NULL) {
                /* add the sourceIndex to the relative offsets that the callback wrote */
                if(sourceIndex>=0) {
                    while(target<(const uint8_t *)pArgs->target) {
                        *offsets+=sourceIndex;
                        ++offsets;
                        ++target;
                    }
                } else {
                    /* sourceIndex==-1, set -1 offsets */
                    while(target<(uint8_t *)pArgs->target) {
                        *offsets=-1;
                        ++offsets;
                        ++target;
                    }
                }
            } else {
                target=(uint8_t *)pArgs->target;
            }

            /* update the source pointer and index */
            sourceIndex=nextSourceIndex+(pArgs->source-source);
            source=pArgs->source;
            targetCapacity=(uint8_t *)pArgs->targetLimit-target;

            /* break on error */
            if(U_FAILURE(*pErrorCode)) {
                c=0;
                break;
            }

            /*
             * If the callback overflowed the target, then we need to
             * stop here with an overflow indication.
             */
            if(cnv->charErrorBufferLength>0) {
                /* target is full */
                *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;
                break;
            }

            /*
             * We do not need to repeat the statements from the normal
             * end of the conversion because we already updated all the
             * necessary variables.
             */
        } else {
            /* target is full */
            *pErrorCode=U_INDEX_OUTOFBOUNDS_ERROR;
            break;
        }
    }

    if(pArgs->flush && source>=sourceLimit) {
        /* reset the state for the next conversion */
        if(c!=0 && U_SUCCESS(*pErrorCode)) {
            /* a character byte sequence remains incomplete */
            *pErrorCode=U_TRUNCATED_CHAR_FOUND;
        }
        cnv->fromUSurrogateLead=0;
    } else {
        /* set the converter state back into UConverter */
        cnv->fromUSurrogateLead=(UChar)c;
    }

    /* write back the updated pointers */
    pArgs->source=source;
    pArgs->target=(char *)target;
    pArgs->offsets=offsets;
}

U_CFUNC void
_MBCSFromUnicode(UConverterFromUnicodeArgs *pArgs,
                 UErrorCode *pErrorCode) {
    _MBCSFromUnicodeWithOffsets(pArgs, pErrorCode);
}

static void
_MBCSGetStarters(const UConverter* cnv,
                 UBool starters[256],
                 UErrorCode *pErrorCode) {
    const int32_t *state0=cnv->sharedData->table->mbcs.stateTable[0];
    int i;

    for(i=0; i<256; ++i) {
        /* all bytes that cause a state transition from state 0 are lead bytes */
        starters[i]= (UBool)(state0[i]>=0);
    }
}

/*
 * This is an internal function that allows other converter implementations
 * to check whether a byte is a lead byte.
 */
U_CFUNC UBool
_MBCSIsLeadByte(UConverterSharedData *sharedData, char byte) {
    return (UBool)(sharedData->table->mbcs.stateTable[0][(uint8_t)byte]>=0);
}

static const UConverterImpl _MBCSImpl={
    UCNV_MBCS,

    _MBCSLoad,
    NULL,

    _MBCSOpen,
    NULL,
    _MBCSReset,

    _MBCSToUnicode,
    _MBCSToUnicodeWithOffsets,
    _MBCSFromUnicode,
    _MBCSFromUnicodeWithOffsets,
    _MBCSGetNextUChar,

    _MBCSGetStarters
};


/* Static data is in tools/makeconv/ucnvstat.c for data-based
 * converters. Be sure to update it as well.
 */

const UConverterSharedData _MBCSData={
    sizeof(UConverterSharedData), 1,
    NULL, NULL, NULL, FALSE, &_MBCSImpl, 
    0
};