dgif_lib.c

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    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 {
			_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) {
		    _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;
}

/******************************************************************************
*   Interface for accessing the LZ codes directly. Set Code to the real code  *
* (12bits), or to -1 if EOF code is returned.				      *
******************************************************************************/
int DGifGetLZCodes(GifFileType *GifFile, int *Code)
{
    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;
    }

    if (DGifDecompressInput(GifFile, Code) == GIF_ERROR)
	return GIF_ERROR;

    if (*Code == Private->EOFCode) {
	/* Skip rest of codes (hopefully only NULL terminating block): */
	do if (DGifGetCodeNext(GifFile, &CodeBlock) == GIF_ERROR)
    	    return GIF_ERROR;
	while (CodeBlock != NULL);

	*Code = -1;
    }
    else if (*Code == Private->ClearCode) {
	/* We need to start over again: */
	Private->RunningCode = Private->EOFCode + 1;
	Private->RunningBits = Private->BitsPerPixel + 1;
	Private->MaxCode1 = 1 << Private->RunningBits;
    }

    return GIF_OK;
}

/******************************************************************************
*   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;
}

/******************************************************************************
* This routine reads an entire GIF into core, hanging all its state info off  *
* the GifFileType pointer.  Call DGifOpenFileName() or DGifOpenFileHandle()   *
* first to initialize I/O.  Its inverse is EGifSpew().			      *
* 
 ******************************************************************************/
int DGifSlurp(GifFileType *GifFile)
{
    int ImageSize;
    GifRecordType RecordType;
    SavedImage *sp;
    GifByteType *ExtData;
    SavedImage temp_save;

    temp_save.ExtensionBlocks=NULL;
    temp_save.ExtensionBlockCount=0;
  
    do {
	if (DGifGetRecordType(GifFile, &RecordType) == GIF_ERROR)
	    return(GIF_ERROR);

	switch (RecordType) {
	    case IMAGE_DESC_RECORD_TYPE:
		if (DGifGetImageDesc(GifFile) == GIF_ERROR)
		    return(GIF_ERROR);

		sp = &GifFile->SavedImages[GifFile->ImageCount-1];
		ImageSize = sp->ImageDesc.Width * sp->ImageDesc.Height;

		sp->RasterBits
		    = (char *) malloc(ImageSize * sizeof(GifPixelType));

		if (DGifGetLine(GifFile, sp->RasterBits, ImageSize)
		    == GIF_ERROR)
		    return(GIF_ERROR);

        if (temp_save.ExtensionBlocks) {
            sp->ExtensionBlocks = temp_save.ExtensionBlocks;
            sp->ExtensionBlockCount = temp_save.ExtensionBlockCount;

            temp_save.ExtensionBlocks = NULL;
            temp_save.ExtensionBlockCount=0;

        /* FIXME: The following is wrong.  It is left in only for backwards
         * compatibility.  Someday it should go away.  Use the
         * sp->ExtensionBlocks->Function variable instead.
         */
            sp->Function = sp->ExtensionBlocks[0].Function;

        }

		break;

	    case EXTENSION_RECORD_TYPE:
		if (DGifGetExtension(GifFile,&temp_save.Function,&ExtData)==GIF_ERROR)
		    return(GIF_ERROR);
		while (ExtData != NULL) {
            
            /* Create an extension block with our data */
            if (AddExtensionBlock(&temp_save, ExtData[0], &ExtData[1])
               == GIF_ERROR)
                return (GIF_ERROR); 
            
		    if (DGifGetExtensionNext(GifFile, &ExtData) == GIF_ERROR)
			    return(GIF_ERROR);
            temp_save.Function = 0;
		}
		break;

	    case TERMINATE_RECORD_TYPE:
		break;

	    default:	/* Should be trapped by DGifGetRecordType */
		break;
	}
    } while (RecordType != TERMINATE_RECORD_TYPE);

    /* Just in case the Gif has an extension block without an associated
     * image... (Should we save this into a savefile structure with no image
     * instead?  Have to check if the present writing code can handle that as
     * well....
     */
    if (temp_save.ExtensionBlocks)
        FreeExtension(&temp_save);
    
    return(GIF_OK);
}