gifsave.c

完美的在线教育系统

    int  bitswritten = 0;
    Byte numbytes = 255;

    do {
        /* if the buffer is full, write it */
        if ((Index == 254 && !BitsLeft) || Index > 254) {
            if (WriteByte(numbytes) != GIF_OK)
                return -1;

            if (Write(Buffer, numbytes) != GIF_OK)
                return -1;

            Buffer[Index = 0] = 0;
            BitsLeft = 8;
        }

        /* now take care of the two specialcases */
        if (numbits <= BitsLeft) {
            Buffer[Index] |= (bits & ((1 << numbits) - 1)) << (8 - BitsLeft);
            bitswritten += numbits;
            BitsLeft -= numbits;
            numbits = 0;
        } else {
            Buffer[Index] |= (bits & ((1 << BitsLeft) - 1)) << (8 - BitsLeft);
            bitswritten += BitsLeft;
            bits >>= BitsLeft;
            numbits -= BitsLeft;

            Buffer[++Index] = 0;
            BitsLeft = 8;
        }
    } while (numbits);

    return bitswritten;
}



/*========================================================================*
 =                Routines to maintain an LZW-string table                =
 *========================================================================*/

/*-------------------------------------------------------------------------
 *
 *  NAME          FreeStrtab
 *
 *  DESCRIPTION   Free arrays used in string table routines
 */
static void
FreeStrtab(void)
{
    if (StrHsh) {
        free(StrHsh);
        StrHsh = NULL;
    }
    if (StrNxt) {
        free(StrNxt);
        StrNxt = NULL;
    }
    if (StrChr) {
        free(StrChr);
        StrChr = NULL;
    }
}



/*-------------------------------------------------------------------------
 *
 *  NAME          AllocStrtab
 *
 *  DESCRIPTION   Allocate arrays used in string table routines
 *
 *  RETURNS       GIF_OK     - OK
 *                GIF_OUTMEM - Out of memory
 */
static int
AllocStrtab(void)
{
    /* just in case */
    FreeStrtab();

    if ((StrChr = (Byte *) malloc(MAXSTR * sizeof(Byte))) == 0) {
        FreeStrtab();
        return GIF_OUTMEM;
    }
    if ((StrNxt = (Word *) malloc(MAXSTR * sizeof(Word))) == 0) {
        FreeStrtab();
        return GIF_OUTMEM;
    }
    if ((StrHsh = (Word *) malloc(HASHSIZE * sizeof(Word))) == 0) {
        FreeStrtab();
        return GIF_OUTMEM;
    }
    return GIF_OK;
}



/*-------------------------------------------------------------------------
 *
 *  NAME          AddCharString
 *
 *  DESCRIPTION   Add a string consisting of the string of index plus
 *                the byte b.
 *
 *                If a string of length 1 is wanted, the index should
 *                be 0xFFFF.
 *
 *  INPUT         index   index to first part of string, or 0xFFFF is
 *                        only 1 byte is wanted
 *                b       last byte in new string
 *
 *  RETURNS       Index to new string, or 0xFFFF if no more room
 */
static Word
AddCharString(Word index, Byte b)
{
    Word hshidx;

    /* check if there is more room */
    if (NumStrings >= MAXSTR)
        return 0xFFFF;

    /* search the string table until a free position is found */
    hshidx = HASH(index, b);
    while (StrHsh[hshidx] != 0xFFFF)
        hshidx = (hshidx + HASHSTEP) % HASHSIZE;

    /* insert new string */
    StrHsh[hshidx] = NumStrings;
    StrChr[NumStrings] = b;
    StrNxt[NumStrings] = (index != 0xFFFF) ? index : NEXT_FIRST;

    return NumStrings++;
}



/*-------------------------------------------------------------------------
 *
 *  NAME          FindCharString
 *
 *  DESCRIPTION   Find index of string consisting of the string of index
 *                plus the byte b.
 *
 *                If a string of length 1 is wanted, the index should
 *                be 0xFFFF.
 *
 *  INPUT         index   index to first part of string, or 0xFFFF is
 *                        only 1 byte is wanted
 *                b       last byte in string
 *
 *  RETURNS       Index to string, or 0xFFFF if not found
 */
static Word
FindCharString(Word index, Byte b)
{
    Word hshidx, nxtidx;

    /* check if index is 0xFFFF. in that case we need only return b,
     * since all one-character strings has their bytevalue as their
     * index */
    if (index == 0xFFFF)
        return b;

    /* search the string table until the string is found, or we find
     * HASH_FREE. in that case the string does not exist. */
    hshidx = HASH(index, b);
    while ((nxtidx = StrHsh[hshidx]) != 0xFFFF) {
        if (StrNxt[nxtidx] == index && StrChr[nxtidx] == b)
            return nxtidx;
        hshidx = (hshidx + HASHSTEP) % HASHSIZE;
    }

    /* no match is found */
    return 0xFFFF;
}



/*-------------------------------------------------------------------------
 *
 *  NAME          ClearStrtab
 *
 *  DESCRIPTION   Mark the entire table as free, enter the 2**codesize
 *                one-byte strings, and reserve the RES_CODES reserved
 *                codes.
 *
 *  INPUT         codesize
 *                        number of bits to encode one pixel
 */
static void
ClearStrtab(int codesize)
{
    int q, w;
    Word *wp;

    /* no strings currently in the table */
    NumStrings = 0;

    /* mark entire hashtable as free */
    wp = StrHsh;
    for (q = 0; q < HASHSIZE; q++)
        *wp++ = HASH_FREE;

    /* insert 2**codesize one-character strings, and reserved codes */
    w = (1 << codesize) + RES_CODES;
    for (q = 0; q < w; q++)
        AddCharString(0xFFFF, q);
}



/*========================================================================*
 =                        LZW compression routine                         =
 *========================================================================*/

/*-------------------------------------------------------------------------
 *
 *  NAME          LZW_Compress
 *
 *  DESCRIPTION   Perform LZW compression as specified in the
 *                GIF-standard.
 *
 *  INPUT         codesize
 *                         number of bits needed to represent
 *                         one pixelvalue.
 *                inputbyte
 *                         function that fetches each byte to compress.
 *                         must return -1 when no more bytes.
 *
 *  RETURNS       GIF_OK     - OK
 *                GIF_OUTMEM - Out of memory
 */
static int
LZW_Compress(int codesize, int (*inputbyte)(void))
{
    register int c;
    register Word index;
    int  clearcode, endofinfo, numbits, limit, errcode;
    Word prefix = 0xFFFF;

    /* set up the given outfile */
    InitBitFile();

    /* set up variables and tables */
    clearcode = 1 << codesize;
    endofinfo = clearcode + 1;

    numbits = codesize + 1;
    limit = (1 << numbits) - 1;

    if ((errcode = AllocStrtab()) != GIF_OK)
        return errcode;
    ClearStrtab(codesize);

    /* first send a code telling the unpacker to clear the stringtable */
    WriteBits(clearcode, numbits);

    /* pack image */
    while ((c = inputbyte()) != -1) {
        /* now perform the packing. check if the prefix + the new
         *  character is a string that exists in the table */
        if ((index = FindCharString(prefix, c)) != 0xFFFF) {
            /* the string exists in the table. make this string the
             * new prefix.  */
            prefix = index;
        } else {
            /* the string does not exist in the table. first write
             * code of the old prefix to the file. */
            WriteBits(prefix, numbits);

            /* add the new string (the prefix + the new character) to
             * the stringtable */
            if (AddCharString(prefix, c) > limit) {
                if (++numbits > 12) {
                    WriteBits(clearcode, numbits - 1);
                    ClearStrtab(codesize);
                    numbits = codesize + 1;
                }
                limit = (1 << numbits) - 1;
            }

            /* set prefix to a string containing only the character
             * read. since all possible one-character strings exists
             * int the table, there's no need to check if it is found. */
            prefix = c;
        }
    }

    /* end of info is reached. write last prefix. */
    if (prefix != 0xFFFF)
        WriteBits(prefix, numbits);

    /* erite end of info -mark, flush the buffer, and tidy up */
    WriteBits(endofinfo, numbits);
    ResetOutBitFile();
    FreeStrtab();

    return GIF_OK;
}



/*========================================================================*
 =                              Other routines                            =
 *========================================================================*/

/*-------------------------------------------------------------------------
 *
 *  NAME          BitsNeeded
 *
 *  DESCRIPTION   Calculates number of bits needed to store numbers
 *                between 0 and n - 1
 *
 *  INPUT         n       number of numbers to store (0 to n - 1)
 *
 *  RETURNS       Number of bits needed
 */
static int
BitsNeeded(Word n)
{
    int ret = 1;

    if (!n--)
        return 0;
    while (n >>= 1)
        ++ret;
    return ret;
}



/*-------------------------------------------------------------------------
 *
 *  NAME          InputByte
 *
 *  DESCRIPTION   Get next pixel from image. Called by the
 *                LZW_Compress()-function
 *
 *  RETURNS       Next pixelvalue, or -1 if no more pixels
 */
static int
InputByte(void)
{
    int ret;

    if (RelPixY >= ImageHeight)
        return -1;
    ret = GetPixel(ImageLeft + RelPixX, ImageTop + RelPixY);
    if (++RelPixX >= ImageWidth) {
        RelPixX = 0;
        ++RelPixY;
    }
    return ret;
}



/*-------------------------------------------------------------------------
 *
 *  NAME          WriteScreenDescriptor
 *
 *  DESCRIPTION   Output a screen descriptor to the current GIF-file
 *
 *  INPUT         sd      pointer to screen descriptor to output
 *
 *  RETURNS       GIF_OK       - OK
 *                GIF_ERRWRITE - Error writing to the file
 */
static int
WriteScreenDescriptor(ScreenDescriptor *sd)
{