dgif_lib.c

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/******************************************************************************
*   "Gif-Lib" - Yet another gif library.                                      *
*                                                                             *
* Written by:  Gershon Elber                    IBM PC Ver 1.1, Aug. 1990     *
*******************************************************************************
* The kernel of the GIF Decoding process can be found here.                   *
*******************************************************************************
* History:                                                                    *
* 16 Jun 89 - Version 1.0 by Gershon Elber.                                   *
*  3 Sep 90 - Version 1.1 by Gershon Elber (Support for Gif89, Unique names). *
******************************************************************************/

#ifdef _WIN32
#include "../win32/config.h"
#else
#include "../config.h"
#endif

#if defined (__MSDOS__) && !defined(__DJGPP__) && !defined(__GNUC__)
#include <alloc.h>
#include <sys\stat.h>
#else
#include <sys/types.h>
#include <sys/stat.h>
#endif /* __MSDOS__ */

#ifdef HAVE_UNISTD_H
#include <unistd.h>
#endif
#ifdef HAVE_STDLIB_H
#include <stdlib.h>
#endif
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include "gif_lib.h"
#include "gif_lib_private.h"

#define COMMENT_EXT_FUNC_CODE   0xfe /* Extension function code for comment. */

/* avoid extra function call in case we use fread (TVT) */
#define READ(_gif,_buf,_len)                                     \
  (((GifFilePrivateType*)_gif->Private)->Read ?                   \
    ((GifFilePrivateType*)_gif->Private)->Read(_gif,_buf,_len) : \
    fread(_buf,1,_len,((GifFilePrivateType*)_gif->Private)->File))

static int DGifGetWord(GifFileType *GifFile, int *Word);
static int DGifSetupDecompress(GifFileType *GifFile);
static int DGifDecompressLine(GifFileType *GifFile, GifPixelType *Line,
                                                                int LineLen);
static int DGifGetPrefixChar(unsigned int *Prefix, int Code, int ClearCode);
static int DGifDecompressInput(GifFileType *GifFile, int *Code);
static int DGifBufferedInput(GifFileType *GifFile, GifByteType *Buf,
                                                     GifByteType *NextByte);

/******************************************************************************
* GifFileType constructor with user supplied input function (TVT)             *
*                                                                             *
******************************************************************************/
GifFileType *DGifOpen( void* userData, InputFunc readFunc )
{
    unsigned char Buf[GIF_STAMP_LEN+1];
    GifFileType *GifFile;
    GifFilePrivateType *Private;


    if ((GifFile = (GifFileType *) malloc(sizeof(GifFileType))) == NULL) {
          _GifError = D_GIF_ERR_NOT_ENOUGH_MEM;
          return NULL;
    }

    memset(GifFile, '\0', sizeof(GifFileType));

    if (!(Private = (GifFilePrivateType*) malloc(sizeof(GifFilePrivateType)))){
          _GifError = D_GIF_ERR_NOT_ENOUGH_MEM;
          free((char *) GifFile);
          return NULL;
    }

    GifFile->Private = (VoidPtr) Private;
    Private->FileHandle = 0;
    Private->File = 0;
    Private->FileState = FILE_STATE_READ;

        Private->Read     = readFunc;  /* TVT */
        GifFile->UserData = userData;    /* TVT */

    /* Lets see if this is a GIF file: */
    if ( READ( GifFile, Buf, GIF_STAMP_LEN) != GIF_STAMP_LEN) {
          _GifError = D_GIF_ERR_READ_FAILED;
          free((char *) Private);
          free((char *) GifFile);
          return NULL;
    }

    /* The GIF Version number is ignored at this time. Maybe we should do    */
    /* something more useful with it.                                        */
    Buf[GIF_STAMP_LEN] = 0;
    if (strncmp(GIF_STAMP, (char*)Buf, GIF_VERSION_POS) != 0) {
          _GifError = D_GIF_ERR_NOT_GIF_FILE;
          free((char *) Private);
          free((char *) GifFile);
          return NULL;
    }

    if (DGifGetScreenDesc(GifFile) == GIF_ERROR) {
          free((char *) Private);
          free((char *) GifFile);
          return NULL;
    }

    _GifError = 0;

    return GifFile;
}

/******************************************************************************
*   This routine should be called before any other DGif calls. Note that      *
* this routine is called automatically from DGif file open routines.          *
******************************************************************************/
int DGifGetScreenDesc(GifFileType *GifFile)
{
    int i, BitsPerPixel;
    GifByteType Buf[3];
    GifFilePrivateType *Private = (GifFilePrivateType *) GifFile->Private;

    if (!IS_READABLE(Private)) {
        /* This file was NOT open for reading: */
        _GifError = D_GIF_ERR_NOT_READABLE;
        return GIF_ERROR;
    }

    /* Put the screen descriptor into the file: */
    if (DGifGetWord(GifFile, &GifFile->SWidth) == GIF_ERROR ||
        DGifGetWord(GifFile, &GifFile->SHeight) == GIF_ERROR)
        return GIF_ERROR;

    if (READ( GifFile, Buf, 3) != 3) {
        _GifError = D_GIF_ERR_READ_FAILED;
        return GIF_ERROR;
    }
    GifFile->SColorResolution = (((Buf[0] & 0x70) + 1) >> 4) + 1;
    BitsPerPixel = (Buf[0] & 0x07) + 1;
    GifFile->SBackGroundColor = Buf[1];
    if (Buf[0] & 0x80) {                     /* Do we have global color map? */

        GifFile->SColorMap = MakeMapObject(1 << BitsPerPixel, NULL);

        /* Get the global color map: */
        for (i = 0; i < GifFile->SColorMap->ColorCount; i++) {
            if (READ(GifFile, Buf, 3) != 3) {
                _GifError = D_GIF_ERR_READ_FAILED;
                return GIF_ERROR;
            }
            GifFile->SColorMap->Colors[i].Red = Buf[0];
            GifFile->SColorMap->Colors[i].Green = Buf[1];
            GifFile->SColorMap->Colors[i].Blue = Buf[2];
        }
    }

    return GIF_OK;
}

/******************************************************************************
*   This routine should be called before any attemp to read an image.         *
******************************************************************************/
int DGifGetRecordType(GifFileType *GifFile, GifRecordType *Type)
{
    GifByteType Buf;
    GifFilePrivateType *Private = (GifFilePrivateType *) GifFile->Private;

    if (!IS_READABLE(Private)) {
        /* This file was NOT open for reading: */
        _GifError = D_GIF_ERR_NOT_READABLE;
        return GIF_ERROR;
    }

    if (READ( GifFile, &Buf, 1) != 1) {
        _GifError = D_GIF_ERR_READ_FAILED;
        return GIF_ERROR;
    }

    switch (Buf) {
        case ',':
            *Type = IMAGE_DESC_RECORD_TYPE;
            break;
        case '!':
            *Type = EXTENSION_RECORD_TYPE;
            break;
        case ';':
            *Type = TERMINATE_RECORD_TYPE;
            break;
        default:
            *Type = UNDEFINED_RECORD_TYPE;
            _GifError = D_GIF_ERR_WRONG_RECORD;
            return GIF_ERROR;
    }

    return GIF_OK;
}

/******************************************************************************
*   This routine should be called before any attemp to read an image.         *
*   Note it is assumed the Image desc. header (',') has been read.            *
******************************************************************************/
int DGifGetImageDesc(GifFileType *GifFile)
{
    int i, BitsPerPixel;
    GifByteType Buf[3];
    GifFilePrivateType *Private = (GifFilePrivateType *) GifFile->Private;
        SavedImage      *sp;

    if (!IS_READABLE(Private)) {
        /* This file was NOT open for reading: */
        _GifError = D_GIF_ERR_NOT_READABLE;
        return GIF_ERROR;
    }
    if (DGifGetWord(GifFile, &GifFile->Image.Left) == GIF_ERROR ||
        DGifGetWord(GifFile, &GifFile->Image.Top) == GIF_ERROR ||
        DGifGetWord(GifFile, &GifFile->Image.Width) == GIF_ERROR ||
        DGifGetWord(GifFile, &GifFile->Image.Height) == GIF_ERROR)
        return GIF_ERROR;
    if (READ(GifFile,Buf, 1) != 1) {
        _GifError = D_GIF_ERR_READ_FAILED;
        return GIF_ERROR;
    }
    BitsPerPixel = (Buf[0] & 0x07) + 1;
    GifFile->Image.Interlace = (Buf[0] & 0x40);
    if (Buf[0] & 0x80) {            /* Does this image have local color map? */

        if (GifFile->Image.ColorMap && GifFile->SavedImages == NULL)
            FreeMapObject(GifFile->Image.ColorMap);

        GifFile->Image.ColorMap = MakeMapObject(1 << BitsPerPixel, NULL);
        /* Get the image local color map: */
        for (i = 0; i < GifFile->Image.ColorMap->ColorCount; i++) {
            if (READ(GifFile,Buf, 3) != 3) {
                _GifError = D_GIF_ERR_READ_FAILED;
                return GIF_ERROR;
            }
            GifFile->Image.ColorMap->Colors[i].Red = Buf[0];
            GifFile->Image.ColorMap->Colors[i].Green = Buf[1];
            GifFile->Image.ColorMap->Colors[i].Blue = Buf[2];
        }
    }
    if (GifFile->SavedImages) {
            if ((GifFile->SavedImages = (SavedImage *)realloc(GifFile->SavedImages,
                     sizeof(SavedImage) * (GifFile->ImageCount + 1))) == NULL) {
                _GifError = D_GIF_ERR_NOT_ENOUGH_MEM;
                return GIF_ERROR;
            }
    } else {
        if ((GifFile->SavedImages =
             (SavedImage *)malloc(sizeof(SavedImage))) == NULL) {
            _GifError = D_GIF_ERR_NOT_ENOUGH_MEM;
            return GIF_ERROR;
        }
    }

        sp = &GifFile->SavedImages[GifFile->ImageCount];
        memcpy(&sp->ImageDesc, &GifFile->Image, sizeof(GifImageDesc));
    if (GifFile->Image.ColorMap != NULL) {
        sp->ImageDesc.ColorMap = MakeMapObject(
                GifFile->Image.ColorMap->ColorCount,
                GifFile->Image.ColorMap->Colors);
    }
        sp->RasterBits = (char *)NULL;
        sp->ExtensionBlockCount = 0;
        sp->ExtensionBlocks = (ExtensionBlock *)NULL;

    GifFile->ImageCount++;

    Private->PixelCount = (long) GifFile->Image.Width *
                            (long) GifFile->Image.Height;

    DGifSetupDecompress(GifFile);  /* Reset decompress algorithm parameters. */

    return GIF_OK;
}

/******************************************************************************
*  Get one full scanned line (Line) of length LineLen from GIF file.          *
******************************************************************************/
int DGifGetLine(GifFileType *GifFile, GifPixelType *Line, int LineLen)
{
    GifByteType *Dummy;
    GifFilePrivateType *Private = (GifFilePrivateType *) GifFile->Private;

    if (!IS_READABLE(Private)) {
        /* This file was NOT open for reading: */
        _GifError = D_GIF_ERR_NOT_READABLE;
        return GIF_ERROR;
    }

    if (!LineLen) LineLen = GifFile->Image.Width;

#if defined(__MSDOS__) || defined(__GNUC__)
    if ((Private->PixelCount -= LineLen) > 0xffff0000UL) {
#else
    if ((Private->PixelCount -= LineLen) > 0xffff0000) {
#endif /* __MSDOS__ */
        _GifError = D_GIF_ERR_DATA_TOO_BIG;
        return GIF_ERROR;
    }

    if (DGifDecompressLine(GifFile, Line, LineLen) == GIF_OK) {
        if (Private->PixelCount == 0) {
            /* We probably would not be called any more, so lets clean       */
            /* everything before we return: need to flush out all rest of    */
            /* image until empty block (size 0) detected. We use GetCodeNext.*/
            do if (DGifGetCodeNext(GifFile, &Dummy) == GIF_ERROR)
                return GIF_ERROR;
            while (Dummy != NULL);
        }
        return GIF_OK;
    }
    else
        return GIF_ERROR;
}

/******************************************************************************
*   Get an extension block (see GIF manual) from gif file. This routine only  *
* returns the first data block, and DGifGetExtensionNext shouldbe called      *
* after this one until NULL extension is returned.                            *
*   The Extension should NOT be freed by the user (not dynamically allocated).*
*   Note it is assumed the Extension desc. header ('!') has been read.        *
******************************************************************************/
int DGifGetExtension(GifFileType *GifFile, int *ExtCode,
                                                    GifByteType **Extension)
{
    GifByteType Buf;
    GifFilePrivateType *Private = (GifFilePrivateType *) GifFile->Private;

    if (!IS_READABLE(Private)) {
        /* This file was NOT open for reading: */
        _GifError = D_GIF_ERR_NOT_READABLE;
        return GIF_ERROR;
    }

    if (READ(GifFile,&Buf, 1) != 1) {
        _GifError = D_GIF_ERR_READ_FAILED;
        return GIF_ERROR;
    }
    *ExtCode = Buf;

    return DGifGetExtensionNext(GifFile, Extension);
}

/******************************************************************************
*   Get a following extension block (see GIF manual) from gif file. This      *
* routine should be called until NULL Extension is returned.                  *
*   The Extension should NOT be freed by the user (not dynamically allocated).*
******************************************************************************/
int DGifGetExtensionNext(GifFileType *GifFile, GifByteType **Extension)
{
    GifByteType Buf;
    GifFilePrivateType *Private = (GifFilePrivateType *) GifFile->Private;

    if (READ(GifFile,&Buf, 1) != 1) {
        _GifError = D_GIF_ERR_READ_FAILED;
        return GIF_ERROR;
    }
    if (Buf > 0) {
        *Extension = Private->Buf;           /* Use private unused buffer. */
        (*Extension)[0] = Buf;  /* Pascal strings notation (pos. 0 is len.). */
        if (READ(GifFile,&((*Extension)[1]), Buf) != Buf) {
            _GifError = D_GIF_ERR_READ_FAILED;
            return GIF_ERROR;
        }
    }
    else
        *Extension = NULL;

    return GIF_OK;
}

/******************************************************************************
*   This routine should be called last, to close the GIF file.                *
******************************************************************************/
int DGifCloseFile(GifFileType *GifFile)
{
    GifFilePrivateType *Private;
    FILE *File;
        int ret = GIF_OK;

    if (GifFile == NULL) return GIF_ERROR;

    Private = (GifFilePrivateType *) GifFile->Private;

    if (!IS_READABLE(Private)) {
                /* This file was NOT open for reading: */
                _GifError = D_GIF_ERR_NOT_READABLE;
                ret = GIF_ERROR; /* we have to free everything regardless */
    }

    File = Private->File;

    if (GifFile->Image.ColorMap)
    {
                FreeMapObject(GifFile->Image.ColorMap);
        GifFile->Image.ColorMap = NULL;
    }

    if (GifFile->SColorMap)
    {
                FreeMapObject(GifFile->SColorMap);
                GifFile->SColorMap = NULL;
    }

    if (Private)
    {
                free((char *) Private);
                GifFile->Private = NULL;
    }

    if (GifFile->SavedImages)
    {
                FreeSavedImages(GifFile);
                GifFile->SavedImages = NULL;
    }

    free(GifFile);

    if ( File && (fclose(File) != 0)) 
        {
                _GifError = D_GIF_ERR_CLOSE_FAILED;
                ret = GIF_ERROR;
    }
    return ret;
}

/******************************************************************************
*   Get 2 bytes (word) from the given file:                                   *
******************************************************************************/
static int DGifGetWord(GifFileType *GifFile, int *Word)
{
    unsigned char c[2];

    if (READ(GifFile,c, 2) != 2) {
        _GifError = D_GIF_ERR_READ_FAILED;
        return GIF_ERROR;
    }

    *Word = (((unsigned int) c[1]) << 8) + c[0];
    return GIF_OK;
}

/******************************************************************************
*   Get the image code in compressed form.  his routine can be called if the  *
* information needed to be piped out as is. Obviously this is much faster     *
* than decoding and encoding again. This routine should be followed by calls  *
* to DGifGetCodeNext, until NULL block is returned.                           *
*   The block should NOT be freed by the user (not dynamically allocated).    *
******************************************************************************/
int DGifGetCode(GifFileType *GifFile, int *CodeSize, GifByteType **CodeBlock)
{
    GifFilePrivateType *Private = (GifFilePrivateType *) GifFile->Private;

    if (!IS_READABLE(Private)) {
        /* This file was NOT open for reading: */
        _GifError = D_GIF_ERR_NOT_READABLE;
        return GIF_ERROR;
    }

    *CodeSize = Private->BitsPerPixel;

    return DGifGetCodeNext(GifFile, CodeBlock);
}

/******************************************************************************
*   Continue to get the image code in compressed form. This routine should be *
* called until NULL block is returned.                                        *
*   The block should NOT be freed by the user (not dynamically allocated).    *
******************************************************************************/
int DGifGetCodeNext(GifFileType *GifFile, GifByteType **CodeBlock)
{
    GifByteType Buf;
    GifFilePrivateType *Private = (GifFilePrivateType *) GifFile->Private;
    if (READ(GifFile,&Buf, 1) != 1) {
        _GifError = D_GIF_ERR_READ_FAILED;
        return GIF_ERROR;
    }
    if (Buf > 0) {
                int ret ; 
        *CodeBlock = Private->Buf;             /* Use private unused buffer. */
        (*CodeBlock)[0] = Buf;  /* Pascal strings notation (pos. 0 is len.). */
        ret = READ(GifFile,&((*CodeBlock)[1]), Buf);
        if ( ret != Buf) 
        {
                if( Buf == 59 ) 
                {
                        fseek( GifFile->UserData, -1, SEEK_END );       
                        *CodeBlock = NULL ; 
                        return GIF_OK;
                }        
            _GifError = D_GIF_ERR_READ_FAILED;
            return GIF_ERROR;
        }
    }
    else {
        *CodeBlock = NULL;
        Private->Buf[0] = 0;               /* Make sure the buffer is empty! */
        Private->PixelCount = 0;   /* And local info. indicate image read. */
    }

    return GIF_OK;
}

/******************************************************************************
*   Setup the LZ decompression for this image:                                *
******************************************************************************/
static int DGifSetupDecompress(GifFileType *GifFile)
{
    int i, BitsPerPixel;
    GifByteType CodeSize;
    unsigned int *Prefix;
    GifFilePrivateType *Private = (GifFilePrivateType *) GifFile->Private;

    READ(GifFile,&CodeSize, 1);    /* Read Code size from file. */
    BitsPerPixel = CodeSize;

    Private->Buf[0] = 0;                              /* Input Buffer empty. */
    Private->BitsPerPixel = BitsPerPixel;
    Private->ClearCode = (1 << BitsPerPixel);
    Private->EOFCode = Private->ClearCode + 1;
    Private->RunningCode = Private->EOFCode + 1;
    Private->RunningBits = BitsPerPixel + 1;     /* Number of bits per code. */
    Private->MaxCode1 = 1 << Private->RunningBits;     /* Max. code + 1. */
    Private->StackPtr = 0;                  /* No pixels on the pixel stack. */
    Private->LastCode = NO_SUCH_CODE;
    Private->CrntShiftState = 0;        /* No information in CrntShiftDWord. */
    Private->CrntShiftDWord = 0;

    Prefix = Private->Prefix;
    for (i = 0; i <= LZ_MAX_CODE; i++) Prefix[i] = NO_SUCH_CODE;

    return GIF_OK;
}

/******************************************************************************
*   The LZ decompression routine:                                             *
*   This version decompress the given gif file into Line of length LineLen.   *
*   This routine can be called few times (one per scan line, for example), in *
* order the complete the whole image.                                         *
******************************************************************************/
static int DGifDecompressLine(GifFileType *GifFile, GifPixelType *Line,
                                                                int LineLen)
{
    int i = 0, j, CrntCode, EOFCode, ClearCode, CrntPrefix, LastCode, StackPtr;
    GifByteType *Stack, *Suffix;
    unsigned int *Prefix;
    GifFilePrivateType *Private = (GifFilePrivateType *) GifFile->Private;

    StackPtr = Private->StackPtr;
    Prefix = Private->Prefix;
    Suffix = Private->Suffix;
    Stack = Private->Stack;
    EOFCode = Private->EOFCode;
    ClearCode = Private->ClearCode;
    LastCode = Private->LastCode;

    if (StackPtr != 0) {
        /* Let pop the stack off before continueing to read the gif file: */
        while (StackPtr != 0 && i < LineLen) Line[i++] = Stack[--StackPtr];
    }

    while (i < LineLen) {                           /* Decode LineLen items. */
        if (DGifDecompressInput(GifFile, &CrntCode) == GIF_ERROR)
            return GIF_ERROR;

        if (CrntCode == EOFCode) {
            /* Note however that usually we will not be here as we will stop */
            /* decoding as soon as we got all the pixel, or EOF code will    */
            /* not be read at all, and DGifGetLine/Pixel clean everything.   */
            if (i != LineLen - 1 || Private->PixelCount != 0) {
                _GifError = D_GIF_ERR_EOF_TOO_SOON;
                return GIF_ERROR;
            }
            i++;
        }
        else if (CrntCode == ClearCode) {
            /* We need to start over again: */
            for (j = 0; j <= LZ_MAX_CODE; j++) Prefix[j] = NO_SUCH_CODE;
            Private->RunningCode = Private->EOFCode + 1;
            Private->RunningBits = Private->BitsPerPixel + 1;
            Private->MaxCode1 = 1 << Private->RunningBits;
            LastCode = Private->LastCode = NO_SUCH_CODE;
        }
        else {
            /* Its regular code - if in pixel range simply add it to output  */
            /* stream, otherwise trace to codes linked list until the prefix */
            /* is in pixel range:                                            */
            if (CrntCode < ClearCode) {
                /* This is simple - its pixel scalar, so add it to output:   */
                Line[i++] = CrntCode;
            }
            else {
                /* Its a code to needed to be traced: trace the linked list  */
                /* until the prefix is a pixel, while pushing the suffix     */
                /* pixels on our stack. If we done, pop the stack in reverse */
                /* (thats what stack is good for!) order to output.          */
                if (Prefix[CrntCode] == NO_SUCH_CODE) {
                    /* Only allowed if CrntCode is exactly the running code: */
                    /* In that case CrntCode = XXXCode, CrntCode or the      */
                    /* prefix code is last code and the suffix char is       */
                    /* exactly the prefix of last code!                      */
                    if (CrntCode == Private->RunningCode - 2) {
                        CrntPrefix = LastCode;
                        Suffix[Private->RunningCode - 2] =
                        Stack[StackPtr++] = DGifGetPrefixChar(Prefix,
                                                        LastCode, ClearCode);
                    }
                    else {
                        /*fprintf( stderr, "error at line %d, CrntCode = %d,  Private->RunningCode - 2 = %d\n", __LINE__, CrntCode, Private->RunningCode - 2 ); */
                        _GifError = D_GIF_ERR_IMAGE_DEFECT;
                        return GIF_ERROR;
                    }
                }
                else
                    CrntPrefix = CrntCode;

                /* Now (if image is O.K.) we should not get an NO_SUCH_CODE  */
                /* During the trace. As we might loop forever, in case of    */
                /* defective image, we count the number of loops we trace    */
                /* and stop if we got LZ_MAX_CODE. obviously we can not      */
                /* loop more than that.                                      */
                j = 0;
                while (j++ <= LZ_MAX_CODE &&
                       CrntPrefix > ClearCode &&
                       CrntPrefix <= LZ_MAX_CODE) {
                    Stack[StackPtr++] = Suffix[CrntPrefix];
                    CrntPrefix = Prefix[CrntPrefix];
                }
                if (j >= LZ_MAX_CODE || CrntPrefix > LZ_MAX_CODE) {
                        /* fprintf( stderr,  "error at line %d, j = %d, CrntPrefix = %d\n", __LINE__, j, CrntPrefix ); */
                    _GifError = D_GIF_ERR_IMAGE_DEFECT;
                    return GIF_ERROR;
                }
                /* Push the last character on stack: */
                Stack[StackPtr++] = CrntPrefix;

                /* Now lets pop all the stack into output: */
                while (StackPtr != 0 && i < LineLen)
                    Line[i++] = Stack[--StackPtr];
            }
            if (LastCode != NO_SUCH_CODE) {
                Prefix[Private->RunningCode - 2] = LastCode;

                if (CrntCode == Private->RunningCode - 2) {
                    /* Only allowed if CrntCode is exactly the running code: */
                    /* In that case CrntCode = XXXCode, CrntCode or the      */
                    /* prefix code is last code and the suffix char is       */
                    /* exactly the prefix of last code!                      */
                    Suffix[Private->RunningCode - 2] =
                        DGifGetPrefixChar(Prefix, LastCode, ClearCode);
                }
                else {
                    Suffix[Private->RunningCode - 2] =
                        DGifGetPrefixChar(Prefix, CrntCode, ClearCode);
                }
            }
            LastCode = CrntCode;
        }
    }

    Private->LastCode = LastCode;
    Private->StackPtr = StackPtr;

    return GIF_OK;
}

/******************************************************************************
* Routine to trace the Prefixes linked list until we get a prefix which is    *
* not code, but a pixel value (less than ClearCode). Returns that pixel value.*
* If image is defective, we might loop here forever, so we limit the loops to *
* the maximum possible if image O.k. - LZ_MAX_CODE times.                     *
******************************************************************************/
static int DGifGetPrefixChar(unsigned int *Prefix, int Code, int ClearCode)
{
    int i = 0;

    while (Code > ClearCode && i++ <= LZ_MAX_CODE) Code = Prefix[Code];
    return Code;
}

/******************************************************************************
*   The LZ decompression input routine:                                       *
*   This routine is responsable for the decompression of the bit stream from  *
* 8 bits (bytes) packets, into the real codes.                                *
*   Returns GIF_OK if read succesfully.                                       *
******************************************************************************/
static int DGifDecompressInput(GifFileType *GifFile, int *Code)
{
    GifFilePrivateType *Private = (GifFilePrivateType *)GifFile->Private;

    GifByteType NextByte;
    static unsigned int CodeMasks[] = {
        0x0000, 0x0001, 0x0003, 0x0007,
        0x000f, 0x001f, 0x003f, 0x007f,
        0x00ff, 0x01ff, 0x03ff, 0x07ff,
        0x0fff
    };

    while (Private->CrntShiftState < Private->RunningBits) {
        /* Needs to get more bytes from input stream for next code: */
        if (DGifBufferedInput(GifFile, Private->Buf, &NextByte)
            == GIF_ERROR) {
            return GIF_ERROR;
        }
        Private->CrntShiftDWord |=
                ((unsigned long) NextByte) << Private->CrntShiftState;
        Private->CrntShiftState += 8;
    }
    *Code = Private->CrntShiftDWord & CodeMasks[Private->RunningBits];

    Private->CrntShiftDWord >>= Private->RunningBits;
    Private->CrntShiftState -= Private->RunningBits;

    /* If code cannt fit into RunningBits bits, must raise its size. Note */
    /* however that codes above 4095 are used for special signaling.      */
    if (++Private->RunningCode > Private->MaxCode1 &&
        Private->RunningBits < LZ_BITS) {
        Private->MaxCode1 <<= 1;
        Private->RunningBits++;
    }
    return GIF_OK;
}

/******************************************************************************
*   This routines read one gif data block at a time and buffers it internally *
* so that the decompression routine could access it.                          *
*   The routine returns the next byte from its internal buffer (or read next  *
* block in if buffer empty) and returns GIF_OK if succesful.                  *
******************************************************************************/
static int DGifBufferedInput(GifFileType *GifFile, GifByteType *Buf,
                                                      GifByteType *NextByte)
{
    if (Buf[0] == 0) {
        /* Needs to read the next buffer - this one is empty: */
        if (READ(GifFile, Buf, 1) != 1)
        {
            _GifError = D_GIF_ERR_READ_FAILED;
            return GIF_ERROR;
        }
        if (READ(GifFile,&Buf[1], Buf[0]) != Buf[0])
        {
            _GifError = D_GIF_ERR_READ_FAILED;
            return GIF_ERROR;
        }
        *NextByte = Buf[1];
        Buf[1] = 2;        /* We use now the second place as last char read! */
        Buf[0]--;
    }
    else {
        *NextByte = Buf[Buf[1]++];
        Buf[0]--;
    }

    return GIF_OK;
}

---