static int DGifDecompressInput(GifFileType * GifFile, int *Code)
{
GifFilePrivateType *Private = (GifFilePrivateType *) GifFile->Private;
GifByteType NextByte;
static unsigned short CodeMasks[] =
{ 0x0000, 0x0001, 0x0003, 0x0007, 0x000f, 0x001f, 0x003f, 0x007f, 0x00ff, 0x01ff, 0x03ff, 0x07ff, 0x0fff };
if (Private->RunningBits > LZ_BITS)
{
_GifError = D_GIF_ERR_IMAGE_DEFECT;
return GIF_ERROR;
}
while (Private->CrntShiftState < Private->RunningBits)
{
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 (Private->RunningCode < LZ_MAX_CODE + 2 && ++Private->RunningCode > Private->MaxCode1 && Private->RunningBits < LZ_BITS)
{
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(GifFilePrivateType *Private, int *Code)
{
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(Private, Private->Buf, &NextByte)
== GIF_ERROR) {
return GIF_ERROR;
}
Private->CrntShiftDWord |=
((u32) 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;
}
/******************************************************************************
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 successfully.
******************************************************************************/
static int
DGifDecompressInput(GifFileType *GifFile, int *Code)
{
static const unsigned short CodeMasks[] = {
0x0000, 0x0001, 0x0003, 0x0007,
0x000f, 0x001f, 0x003f, 0x007f,
0x00ff, 0x01ff, 0x03ff, 0x07ff,
0x0fff
};
GifFilePrivateType *Private = (GifFilePrivateType *)GifFile->Private;
GifByteType NextByte;
/* The image can't contain more than LZ_BITS per code. */
if (Private->RunningBits > LZ_BITS) {
GifFile->Error = D_GIF_ERR_IMAGE_DEFECT;
return GIF_ERROR;
}
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 cannot fit into RunningBits bits, must raise its size. Note
* however that codes above 4095 are used for special signaling.
* If we're using LZ_BITS bits already and we're at the max code, just
* keep using the table as it is, don't increment Private->RunningCode.
*/
if (Private->RunningCode < LZ_MAX_CODE + 2 &&
++Private->RunningCode > Private->MaxCode1 &&
Private->RunningBits < LZ_BITS) {
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 successfully.
******************************************************************************/
static int
DGifDecompressInput(GifFileType *GifFile, int *Code)
{
static const unsigned short CodeMasks[] = {
0x0000, 0x0001, 0x0003, 0x0007,
0x000f, 0x001f, 0x003f, 0x007f,
0x00ff, 0x01ff, 0x03ff, 0x07ff,
0x0fff
};
GifFilePrivateType *Private = (GifFilePrivateType *)GifFile->Private;
GifByteType NextByte;
/* The image can't contain more than LZ_BITS per code. */
if (Private->RunningBits > LZ_BITS) {
GifFile->Error = D_GIF_ERR_IMAGE_DEFECT;
return GIF_ERROR;
}
/*
*
* 流程
* crntShiftState = 0; RunningBits = 3; NextByte = 01001011B; CrntShiftDWord = 0000000000000000B;
*
* CrntShiftDWord |= ((unsigned long) NextByte) << CrntShiftState; ---- CrntShiftDWord == 0000000001001011B;
*
* CrntShiftState += 8 ---- CrntShiftState == 8;
*
* Code = CrntShiftDWord & CodeMasks[RunningBits] ---- Code = 0000000001001011B & CodeMasks[3] ---- Code = 0000000001001011B & 111B ---- Code = 011B
*
* CrntShiftDWord >>= RunningBits ---- 0000000001001011B >>= 3 ---- CrntShiftDWord == 0000000000001001B;
*
* CrntShiftState -= RunningBits ---- 8 -= 3 ---- CrntShiftState == 5
*
* 重复流程一次
*
* NextByte = 01011100B;
*
* 由于这次 CrntShiftState > RunningBits 没有进入While循环
*
* Code = CrntShiftDWord & CodeMasks[RunningBits] ---- Code = 0000000000001001B & CodeMasks[3] ---- Code = 0000000000001001B & 111B ---- Code = 001B
*
* CrntShiftDWord >>= RunningBits ---- 0000000000001001B >>= 3 ---- CrntShiftDWord == 0000000000000001B;
*
* CrntShiftState -= RunningBits ---- 5 -= 3 ---- CrntShiftState == 2;
*
* 重复流程第二次
*
* NextByte = 10111111B;
*
* CrntShiftDWord |= ((unsigned long) NextByte) << CrntShiftState; ---- 0000000000000001B |= 10111111B << 2 ---- CrntShiftDWord == 0000001011111101B
*
* CrntShiftState += 8 ---- CrntShiftState = 10;
*
* Code = CrntShiftDWord & CodeMasks[RunningBits] ---- Code = 0000001011111101B & CodeMasks[3] ---- Code = 0000001011111101B & 111B ---- Code = 101B;
*
* CrntShiftDWord >>= RunningBits ---- 0000001011111101B >>= 3 ---- CrntShiftDWord == 0000000001011111B;
*
* CrntShiftState -= RunningBits ---- 10 -= 3 ---- CrntShiftState == 7;
*
* 由于LZW的每个code不是以字节为单位的， 在读取的时候使用了以上的技巧 用CrntShiftDWord作为读取出来的原始字节缓存 CrntShiftState作为读取Code的指针,
* 每次读code的时候，如果缓存(CrntShiftDWord)不够code的长度(用CrntShiftState来做长度的判断),就读一个字节并放入缓存中(左移CrntShiftState位来防止覆盖掉原来缓存里的数据),
* 缓存数据够的时候，用CodeMasks[RunningBits]来截取缓存中Code需要的长度, 并把缓存右移RunningBits位来清除掉已经读取的Code
*/
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 cannot fit into RunningBits bits, must raise its size. Note
* however that codes above 4095 are used for special signaling.
* If we're using LZ_BITS bits already and we're at the max code, just
* keep using the table as it is, don't increment Private->RunningCode.
*/
if (Private->RunningCode < LZ_MAX_CODE + 2 &&
++Private->RunningCode > Private->MaxCode1 &&
Private->RunningBits < LZ_BITS) {
Private->MaxCode1 <<= 1;
Private->RunningBits++;
}
return GIF_OK;
}