path: root/generic/tkImgGIF.c

/*
 * tkImgGIF.c --
 *
 *	A photo image file handler for GIF files. Reads 87a and 89a GIF files.
 *	At present, there only is a file write function. GIF images may be
 *	read using the -data option of the photo image. The data may be given
 *	as a binary string in a Tcl_Obj or by representing the data as BASE64
 *	encoded ascii. Derived from the giftoppm code found in the pbmplus
 *	package and tkImgFmtPPM.c in the tk4.0b2 distribution.
 *
 * Copyright © Reed Wade (wade@cs.utk.edu), University of Tennessee
 * Copyright © 1995-1997 Sun Microsystems, Inc.
 * Copyright © 1997 Australian National University
 * Copyright © 2005-2010 Donal K. Fellows
 *
 * See the file "license.terms" for information on usage and redistribution of
 * this file, and for a DISCLAIMER OF ALL WARRANTIES.
 *
 * This file also contains code from the giftoppm program, which is
 * copyrighted as follows:
 *
 * +--------------------------------------------------------------------+
 * | Copyright 1990, David Koblas.					|
 * |   Permission to use, copy, modify, and distribute this software	|
 * |   and its documentation for any purpose and without fee is hereby	|
 * |   granted, provided that the above copyright notice appear in all	|
 * |   copies and that both that copyright notice and this permission	|
 * |   notice appear in supporting documentation. This software is	|
 * |   provided "as is" without express or implied warranty.		|
 * +--------------------------------------------------------------------+
 */

#include "tkInt.h"

/*
 * GIF's are represented as data in either binary or base64 format. base64
 * strings consist of 4 6-bit characters -> 3 8 bit bytes. A-Z, a-z, 0-9, +
 * and / represent the 64 values (in order). '=' is a trailing padding char
 * when the un-encoded data is not a multiple of 3 bytes. We'll ignore white
 * space when encountered. Any other invalid character is treated as an EOF
 */

#define GIF_SPECIAL	(256)
#define GIF_PAD		(GIF_SPECIAL+1)
#define GIF_SPACE	(GIF_SPECIAL+2)
#define GIF_BAD		(GIF_SPECIAL+3)
#define GIF_DONE	(GIF_SPECIAL+4)

/*
 * structure to hold the data of a Graphic Control Extension block.
 */

typedef struct {
    int blockPresent;		/* if 1, the block was read and is in scope */
    int transparent;		/* Transparency index */
    int delayTime;		/* update delay time in 10ms */
    int disposalMethod;		/* disposal method 0-3 */
    int userInteraction;	/* user interaction 0/1 */
} GIFGraphicControlExtensionBlock;

/*
 * structure to "mimic" FILE for Mread, so we can look like fread. The decoder
 * state keeps track of which byte we are about to read, or EOF.
 */

typedef struct mFile {
    unsigned char *data;	/* mmencoded source string */
    int c;			/* bits left over from previous character */
    int state;			/* decoder state (0-4 or GIF_DONE) */
    size_t length;			/* Total amount of bytes in data */
} MFile;

/*
 * Non-ASCII encoding support:
 * Most data in a GIF image is binary and is treated as such. However, a few
 * key bits are stashed in ASCII. If we try to compare those pieces to the
 * char they represent, it will fail on any non-ASCII (eg, EBCDIC) system. To
 * accommodate these systems, we test against the numeric value of the ASCII
 * characters instead of the characters themselves. This is encoding
 * independent.
 */

static const char GIF87a[] = {			/* ASCII GIF87a */
    0x47, 0x49, 0x46, 0x38, 0x37, 0x61, 0x00
};
static const char GIF89a[] = {			/* ASCII GIF89a */
    0x47, 0x49, 0x46, 0x38, 0x39, 0x61, 0x00
};
#define GIF_TERMINATOR	0x3b			/* ASCII ; */
#define GIF_EXTENSION	0x21			/* ASCII ! */
#define GIF_START	0x2c			/* ASCII , */

/*
 * Flags used to notify that we've got inline data instead of a file to read
 * from. Note that we need to figure out which type of inline data we've got
 * before handing off to the GIF reading code; this is done in StringReadGIF.
 */

#define INLINE_DATA_BINARY	((const char *) 0x01)
#define INLINE_DATA_BASE64	((const char *) 0x02)

/*
 *		HACK ALERT!!  HACK ALERT!!  HACK ALERT!!
 * This code is hard-wired for reading from files. In order to read from a
 * data stream, we'll trick fread so we can reuse the same code. 0==from file;
 * 1==from base64 encoded data; 2==from binary data
 */

typedef struct {
    const char *fromData;
    unsigned char workingBuffer[280];
    struct {
	int bytes;
	int done;
	unsigned int window;
	int bitsInWindow;
	unsigned char *c;
    } reader;
} GIFImageConfig;

/*
 * Type of a function used to do the writing to a file or buffer when
 * serializing in the GIF format.
 */

typedef size_t (WriteBytesFunc) (ClientData clientData, const char *bytes,
			    size_t byteCount);

/*
 * The format record for the GIF file format:
 */

static int		FileMatchGIF(Tcl_Interp *interp, Tcl_Channel chan,
			    const char *fileName, Tcl_Obj *format,
			    Tcl_Obj *metadataInObj, int *widthPtr,
			    int *heightPtr, Tcl_Obj *metadataOutObj);
static int		FileReadGIF(Tcl_Interp *interp, Tcl_Channel chan,
			    const char *fileName, Tcl_Obj *format,
			    Tcl_Obj *metadataInObj, Tk_PhotoHandle imageHandle,
			    int destX, int destY, int width, int height,
			    int srcX, int srcY, Tcl_Obj *metadataOutObj);
static int		StringMatchGIF(Tcl_Interp *interp, Tcl_Obj *dataObj,
			    Tcl_Obj *format, Tcl_Obj *metadataInObj,
			    int *widthPtr, int *heightPtr,
			    Tcl_Obj *metadataOutObj);
static int		StringReadGIF(Tcl_Interp *interp, Tcl_Obj *dataObj,
			    Tcl_Obj *format, Tcl_Obj *metadataInObj,
			    Tk_PhotoHandle imageHandle,
			    int destX, int destY, int width, int height,
			    int srcX, int srcY, Tcl_Obj *metadataOutObj);
static int		FileWriteGIF(Tcl_Interp *interp, const char *filename,
			    Tcl_Obj *format, Tcl_Obj *metadataInObj,
			    Tk_PhotoImageBlock *blockPtr);
static int		StringWriteGIF(Tcl_Interp *interp, Tcl_Obj *format,
			    Tcl_Obj *metadataInObj,
			    Tk_PhotoImageBlock *blockPtr);
static int		CommonWriteGIF(Tcl_Interp *interp,
			    ClientData clientData,
			    WriteBytesFunc *writeProc, Tcl_Obj *format,
			    Tcl_Obj *metadataInObj,
			    Tk_PhotoImageBlock *blockPtr);

Tk_PhotoImageFormatVersion3 tkImgFmtGIF = {
    "gif",		/* name */
    FileMatchGIF,	/* fileMatchProc */
    StringMatchGIF,	/* stringMatchProc */
    FileReadGIF,	/* fileReadProc */
    StringReadGIF,	/* stringReadProc */
    FileWriteGIF,	/* fileWriteProc */
    StringWriteGIF,	/* stringWriteProc */
    NULL
};

#define INTERLACE		0x40
#define LOCALCOLORMAP		0x80
#define BitSet(byte, bit)	(((byte) & (bit)) == (bit))
#define MAXCOLORMAPSIZE		256
#define CM_RED			0
#define CM_GREEN		1
#define CM_BLUE			2
#define CM_ALPHA		3
#define MAX_LWZ_BITS		12
#define LM_to_uint(a,b)		(((b)<<8)|(a))

/*
 * Prototypes for local functions defined in this file:
 */

static int		ReadOneByte(Tcl_Interp *interp,
			    GIFImageConfig *gifConfPtr, Tcl_Channel chan);
static int		DoExtension(GIFImageConfig *gifConfPtr,
			    Tcl_Channel chan, int label, unsigned char *buffer,
			    GIFGraphicControlExtensionBlock
			    *gifGraphicControlExtensionBlock,
			    Tcl_Obj *metadataOutObj);
static int		GetCode(Tcl_Channel chan, int code_size, int flag,
			    GIFImageConfig *gifConfPtr);
static int		GetDataBlock(GIFImageConfig *gifConfPtr,
			    Tcl_Channel chan, unsigned char *buf);
static int		ReadColorMap(GIFImageConfig *gifConfPtr,
			    Tcl_Channel chan, int number,
			    unsigned char buffer[MAXCOLORMAPSIZE][4]);
static int		ReadGIFHeader(GIFImageConfig *gifConfPtr,
			    Tcl_Channel chan, int *widthPtr, int *heightPtr);
static int		ReadImage(GIFImageConfig *gifConfPtr,
			    Tcl_Interp *interp, unsigned char *imagePtr,
			    Tcl_Channel chan, int len, int rows,
			    unsigned char cmap[MAXCOLORMAPSIZE][4], int srcX,
			    int srcY, int interlace, int transparent);

/*
 * these are for the BASE64 image reader code only
 */

static size_t		Fread(GIFImageConfig *gifConfPtr, unsigned char *dst,
			    size_t size, size_t count, Tcl_Channel chan);
static size_t		Mread(unsigned char *dst, size_t size, size_t count,
			    MFile *handle);
static int		Mgetc(MFile *handle);
static int		char64(int c);
static void		mInit(unsigned char *string, MFile *handle,
			    size_t length);

/*
 * Types, defines and variables needed to write and compress a GIF.
 */

#define LSB(a)		((unsigned char) (((short)(a)) & 0x00FF))
#define MSB(a)		((unsigned char) (((short)(a)) >> 8))

#define GIFBITS		12
#define HSIZE		5003	/* 80% occupancy */

#define DEFAULT_BACKGROUND_VALUE	0xD9

typedef struct {
    int ssize;
    int csize;
    int rsize;
    unsigned char *pixelOffset;
    int pixelSize;
    int pixelPitch;
    int greenOffset;
    int blueOffset;
    int alphaOffset;
    int num;
    unsigned char mapa[MAXCOLORMAPSIZE][3];
} GifWriterState;

typedef int (* ifunptr) (GifWriterState *statePtr);

/*
 * Support for compression of GIFs.
 */

#define MAXCODE(numBits)	(((long) 1 << (numBits)) - 1)

#ifdef SIGNED_COMPARE_SLOW
#define U(x)	((unsigned) (x))
#else
#define U(x)	(x)
#endif

typedef struct {
    int numBits;		/* Number of bits/code. */
    long maxCode;		/* Maximum code, given numBits. */
    int hashTable[HSIZE];
    unsigned int codeTable[HSIZE];
    long hSize;			/* For dynamic table sizing. */

    /*
     * To save much memory, we overlay the table used by compress() with those
     * used by decompress(). The tab_prefix table is the same size and type as
     * the codeTable. The tab_suffix table needs 2**GIFBITS characters. We get
     * this from the beginning of hashTable. The output stack uses the rest of
     * hashTable, and contains characters. There is plenty of room for any
     * possible stack (stack used to be 8000 characters).
     */

    int freeEntry;		/* First unused entry. */

    /*
     * Block compression parameters.  After all codes are used up, and
     * compression rate changes, start over.
     */

    int clearFlag;

    int offset;
    unsigned int inCount;	/* Length of input */
    unsigned int outCount;	/* # of codes output (for debugging) */

    /*
     * Algorithm: use open addressing double hashing (no chaining) on the
     * prefix code / next character combination. We do a variant of Knuth's
     * algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
     * secondary probe. Here, the modular division first probe is gives way to
     * a faster exclusive-or manipulation. Also do block compression with an
     * adaptive reset, whereby the code table is cleared when the compression
     * ratio decreases, but after the table fills. The variable-length output
     * codes are re-sized at this point, and a special CLEAR code is generated
     * for the decompressor. Late addition: construct the table according to
     * file size for noticeable speed improvement on small files. Please
     * direct questions about this implementation to ames!jaw.
     */

    int initialBits;
    ClientData destination;
    WriteBytesFunc *writeProc;

    int clearCode;
    int eofCode;

    unsigned long currentAccumulated;
    int currentBits;

    /*
     * Number of characters so far in this 'packet'
     */

    int accumulatedByteCount;

    /*
     * Define the storage for the packet accumulator
     */

    unsigned char packetAccumulator[256];
} GIFState_t;

/*
 * Definition of new functions to write GIFs
 */

static int		ColorNumber(GifWriterState *statePtr,
			    int red, int green, int blue);
static void		Compress(int initBits, ClientData handle,
			    WriteBytesFunc *writeProc, ifunptr readValue,
			    GifWriterState *statePtr);
static int		IsNewColor(GifWriterState *statePtr,
			    int red, int green, int blue);
static void		SaveMap(GifWriterState *statePtr,
			    Tk_PhotoImageBlock *blockPtr);
static int		ReadValue(GifWriterState *statePtr);
static WriteBytesFunc	WriteToChannel;
static WriteBytesFunc	WriteToByteArray;
static void		Output(GIFState_t *statePtr, long code);
static void		ClearForBlock(GIFState_t *statePtr);
static void		ClearHashTable(GIFState_t *statePtr, int hSize);
static void		CharInit(GIFState_t *statePtr);
static void		CharOut(GIFState_t *statePtr, int c);
static void		FlushChar(GIFState_t *statePtr);

/*
 *----------------------------------------------------------------------
 *
 * FileMatchGIF --
 *
 *	This function is invoked by the photo image type to see if a file
 *	contains image data in GIF format.
 *
 * Results:
 *	The return value is 1 if the first characters in file f look like GIF
 *	data, and 0 otherwise.
 *
 * Side effects:
 *	The access position in f may change.
 *
 *----------------------------------------------------------------------
 */

static int
FileMatchGIF(
    TCL_UNUSED(Tcl_Interp *),	/* not used */
    Tcl_Channel chan,		/* The image file, open for reading. */
    TCL_UNUSED(const char *),	/* The name of the image file. */
    TCL_UNUSED(Tcl_Obj *),	/* User-specified format object, or NULL. */
    TCL_UNUSED(Tcl_Obj *),	/* metadata input, may be NULL */
    int *widthPtr, int *heightPtr,
				/* The dimensions of the image are returned
				 * here if the file is a valid raw GIF file. */
    TCL_UNUSED(Tcl_Obj *))	/* metadata return dict, may be NULL */
{
    GIFImageConfig gifConf;

    memset(&gifConf, 0, sizeof(GIFImageConfig));
    return ReadGIFHeader(&gifConf, chan, widthPtr, heightPtr);
}

/*
 *----------------------------------------------------------------------
 *
 * FileReadGIF --
 *
 *	This function is called by the photo image type to read GIF format
 *	data from a file and write it into a given photo image.
 *
 * Results:
 *	A standard TCL completion code. If TCL_ERROR is returned then an error
 *	message is left in the interp's result.
 *
 * Side effects:
 *	The access position in file f is changed, and new data is added to the
 *	image given by imageHandle.
 *
 *----------------------------------------------------------------------
 */

static int
FileReadGIF(
    Tcl_Interp *interp,		/* Interpreter to use for reporting errors. */
    Tcl_Channel chan,		/* The image file, open for reading. */
    const char *fileName,	/* The name of the image file. */
    Tcl_Obj *format,		/* User-specified format object, or NULL. */
    TCL_UNUSED(Tcl_Obj *),	/* metadata input, may be NULL */
    Tk_PhotoHandle imageHandle,	/* The photo image to write into. */
    int destX, int destY,	/* Coordinates of top-left pixel in photo
				 * image to be written to. */
    int width, int height,	/* Dimensions of block of photo image to be
				 * written to. */
    int srcX, int srcY,		/* Coordinates of top-left pixel to be used in
				 * image being read. */
    Tcl_Obj *metadataOutObj)	/* metadata return dict, may be NULL */
{
    int fileWidth, fileHeight, imageWidth, imageHeight;
    unsigned int nBytes;
    int index = 0, result = TCL_ERROR;
    Tcl_Size argc = 0, i;
    Tcl_Obj **objv;
    unsigned char buf[100];
    unsigned char *trashBuffer = NULL;
    int bitPixel;
    int gifLabel;
    unsigned char colorMap[MAXCOLORMAPSIZE][4];
    GIFGraphicControlExtensionBlock gifGraphicControlExtensionBlock;
    static const char *const optionStrings[] = {
	"-index", NULL
    };
    GIFImageConfig gifConf, *gifConfPtr = &gifConf;

    gifGraphicControlExtensionBlock.blockPresent = 0;
    /*
     * Decode the magic used to convey when we're sourcing data from a string
     * source and not a file.
     */

    memset(colorMap, 0, MAXCOLORMAPSIZE*4);
    memset(gifConfPtr, 0, sizeof(GIFImageConfig));
    if (fileName == INLINE_DATA_BINARY || fileName == INLINE_DATA_BASE64) {
	gifConfPtr->fromData = fileName;
	fileName = "inline data";
    }

    /*
     * Parse the format string to get options.
     */

    if (format && Tcl_ListObjGetElements(interp, format,
	    &argc, &objv) != TCL_OK) {
	return TCL_ERROR;
    }
    for (i = 1; i < argc; i++) {
	int optionIdx;
	if (Tcl_GetIndexFromObjStruct(interp, objv[i], optionStrings,
		sizeof(char *), "option name", 0, &optionIdx) != TCL_OK) {
	    return TCL_ERROR;
	}
	if (i == (argc-1)) {
	    Tcl_SetObjResult(interp, Tcl_ObjPrintf(
		    "no value given for \"%s\" option",
		    Tcl_GetString(objv[i])));
	    Tcl_SetErrorCode(interp, "TK", "IMAGE", "GIF", "OPT_VALUE", NULL);
	    return TCL_ERROR;
	}
	if (Tcl_GetIntFromObj(interp, objv[++i], &index) != TCL_OK) {
	    return TCL_ERROR;
	}
    }

    /*
     * Read the GIF file header and check for some sanity.
     */

    if (!ReadGIFHeader(gifConfPtr, chan, &fileWidth, &fileHeight)) {
	Tcl_SetObjResult(interp, Tcl_ObjPrintf(
		"couldn't read GIF header from file \"%s\"", fileName));
	Tcl_SetErrorCode(interp, "TK", "IMAGE", "GIF", "HEADER", NULL);
	return TCL_ERROR;
    }
    if ((fileWidth <= 0) || (fileHeight <= 0)) {
	Tcl_SetObjResult(interp, Tcl_ObjPrintf(
		"GIF image file \"%s\" has dimension(s) <= 0", fileName));
	Tcl_SetErrorCode(interp, "TK", "IMAGE", "GIF", "BOGUS_SIZE", NULL);
	return TCL_ERROR;
    }

    /*
     * Get the general colormap information.
     */

    if (Fread(gifConfPtr, buf, 1, 3, chan) != 3) {
	return TCL_OK;
    }
    bitPixel = 2 << (buf[0] & 0x07);

    if (BitSet(buf[0], LOCALCOLORMAP)) {	/* Global Colormap */
	if (!ReadColorMap(gifConfPtr, chan, bitPixel, colorMap)) {
	    Tcl_SetObjResult(interp, Tcl_NewStringObj(
		    "error reading color map", -1));
	    Tcl_SetErrorCode(interp, "TK", "IMAGE", "GIF", "COLOR_MAP", NULL);
	    return TCL_ERROR;
	}
    }

    if ((srcX + width) > fileWidth) {
	width = fileWidth - srcX;
    }
    if ((srcY + height) > fileHeight) {
	height = fileHeight - srcY;
    }
    if ((width <= 0) || (height <= 0)
	    || (srcX >= fileWidth) || (srcY >= fileHeight)) {
	return TCL_OK;
    }

    /*
     * Make sure we have enough space in the photo image to hold the data from
     * the GIF.
     */

    if (Tk_PhotoExpand(interp, imageHandle,
	    destX + width, destY + height) != TCL_OK) {
	return TCL_ERROR;
    }

    /*
     * -------------------------------------------------------------------------
     * From here on, go to error to not leave memory leaks
     * -------------------------------------------------------------------------
     */

    /*
     * Search for the frame from the GIF to display.
     */

    while (1) {
	if (-1 == (gifLabel = ReadOneByte( interp, gifConfPtr, chan ) ) ) {
	    goto error;
	}

	switch (gifLabel) {
	case GIF_TERMINATOR:
	    Tcl_SetObjResult(interp, Tcl_NewStringObj(
		    "no image data for this index", -1));
	    Tcl_SetErrorCode(interp, "TK", "IMAGE", "GIF", "NO_DATA", NULL);
	    goto error;

	case GIF_EXTENSION:
	    /*
	     * This is a GIF extension.
	     */

	    if (-1 == (gifLabel = ReadOneByte( interp, gifConfPtr, chan ) ) ) {
		goto error;
	    }
	    if (DoExtension(gifConfPtr, chan, gifLabel,
		    gifConfPtr->workingBuffer, &gifGraphicControlExtensionBlock,
		    metadataOutObj)
		    < 0) {
		Tcl_SetObjResult(interp, Tcl_NewStringObj(
			"error reading extension in GIF image", -1));
		Tcl_SetErrorCode(interp, "TK", "IMAGE", "GIF", "BAD_EXT",
			NULL);
		goto error;
	    }
	    continue;
	case GIF_START:
	    if (Fread(gifConfPtr, buf, 1, 9, chan) != 9) {
		Tcl_SetObjResult(interp, Tcl_NewStringObj(
			"couldn't read left/top/width/height in GIF image",
			-1));
		Tcl_SetErrorCode(interp, "TK", "IMAGE", "GIF", "DIMENSIONS",
			NULL);
		goto error;
	    }
	    break;
	default:
	    /*
	     * Not a valid start character; ignore it.
	     */

	    continue;
	}

	/*
	 * We've read the header for a GIF frame. Work out what we are going
	 * to do about it.
	 */

	imageWidth = LM_to_uint(buf[4], buf[5]);
	imageHeight = LM_to_uint(buf[6], buf[7]);
	bitPixel = 1 << ((buf[8] & 0x07) + 1);

	if (index--) {
	    /*
	     * This is not the GIF frame we want to read: skip it.
	     */

	    if (BitSet(buf[8], LOCALCOLORMAP)) {
		if (!ReadColorMap(gifConfPtr, chan, bitPixel, colorMap)) {
		    Tcl_SetObjResult(interp, Tcl_NewStringObj(
			    "error reading color map", -1));
		    Tcl_SetErrorCode(interp, "TK", "IMAGE", "GIF",
			    "COLOR_MAP", NULL);
		    goto error;
		}
	    }

	    /*
	     * If we've not yet allocated a trash buffer, do so now.
	     */

	    if (trashBuffer == NULL) {
		if (fileWidth > (int)((UINT_MAX/3)/fileHeight)) {
		    goto error;
		}
		nBytes = fileWidth * fileHeight * 3;
		trashBuffer = (unsigned char *)ckalloc(nBytes);
		if (trashBuffer) {
		    memset(trashBuffer, 0, nBytes);
		}
	    }

	    /*
	     * Slurp! Process the data for this image and stuff it in a trash
	     * buffer.
	     *
	     * Yes, it might be more efficient here to *not* store the data
	     * (we're just going to throw it away later). However, I elected
	     * to implement it this way for good reasons. First, I wanted to
	     * avoid duplicating the (fairly complex) LWZ decoder in
	     * ReadImage. Fine, you say, why didn't you just modify it to
	     * allow the use of a NULL specifier for the output buffer? I
	     * tried that, but it negatively impacted the performance of what
	     * I think will be the common case: reading the first image in the
	     * file. Rather than marginally improve the speed of the less
	     * frequent case, I chose to maintain high performance for the
	     * common case.
	     */

	    if (ReadImage(gifConfPtr, interp, trashBuffer, chan, imageWidth,
		    imageHeight, colorMap, 0, 0, 0, -1) != TCL_OK) {
		goto error;
	    }

	    /*
	     * This extension starts a new scope, so Graphic control Extension
	     * data should be cleared
	     */
	    gifGraphicControlExtensionBlock.blockPresent = 0;

	    continue;
	}
	break;
    }

    /*
     * Found the frame we want to read. Next, check for a local color map for
     * this frame.
     */

    if (BitSet(buf[8], LOCALCOLORMAP)) {
	if (!ReadColorMap(gifConfPtr, chan, bitPixel, colorMap)) {
	    Tcl_SetObjResult(interp, Tcl_NewStringObj(
		    "error reading color map", -1));
	    Tcl_SetErrorCode(interp, "TK", "IMAGE", "GIF", "COLOR_MAP", NULL);
	    goto error;
	}
    }

    /*
     * Extract the location within the overall visible image to put the data
     * in this frame, together with the size of this frame.
     */

    index = LM_to_uint(buf[0], buf[1]);
    srcX -= index;
    if (srcX<0) {
	destX -= srcX; width += srcX;
	srcX = 0;
    }

    if (width > imageWidth) {
	width = imageWidth;
    }

    index = LM_to_uint(buf[2], buf[3]);
    srcY -= index;
    if (index > srcY) {
	destY -= srcY; height += srcY;
	srcY = 0;
    }
    if (height > imageHeight) {
	height = imageHeight;
    }

    if ((width > 0) && (height > 0)) {
	Tk_PhotoImageBlock block;
	int transparent = -1;
	if (gifGraphicControlExtensionBlock.blockPresent) {
	    transparent = gifGraphicControlExtensionBlock.transparent;
	}
	/*
	 * Read the data and put it into the photo buffer for display by the
	 * general image machinery.
	 */

	block.width = width;
	block.height = height;
	block.pixelSize = (transparent>=0) ? 4 : 3;
	block.offset[0] = 0;
	block.offset[1] = 1;
	block.offset[2] = 2;
	block.offset[3] = (transparent>=0) ? 3 : 0;
	if (imageWidth > INT_MAX/block.pixelSize) {
	    goto error;
	}
	block.pitch = block.pixelSize * imageWidth;
	if (imageHeight > (int)(UINT_MAX/block.pitch)) {
	    goto error;
	}
	nBytes = block.pitch * imageHeight;
	block.pixelPtr = (unsigned char *)ckalloc(nBytes);
	if (block.pixelPtr) {
	    memset(block.pixelPtr, 0, nBytes);
	}

	if (ReadImage(gifConfPtr, interp, block.pixelPtr, chan, imageWidth,
		imageHeight, colorMap, srcX, srcY, BitSet(buf[8], INTERLACE),
		transparent) != TCL_OK) {
	    ckfree(block.pixelPtr);
	    goto error;
	}
	if (Tk_PhotoPutBlock(interp, imageHandle, &block, destX, destY,
		width, height, TK_PHOTO_COMPOSITE_SET) != TCL_OK) {
	    ckfree(block.pixelPtr);
	    goto error;
	}
	ckfree(block.pixelPtr);
    }

    /*
     * Update the metadata dictionary with current image data
     */

    if (NULL != metadataOutObj) {

	/*
	 * Save the update box, if not the whole image
	 */

	if ( width != fileWidth || height != fileHeight) {
	    Tcl_Obj *itemList[4];
	    itemList[0] = Tcl_NewIntObj(destX);
	    itemList[1] = Tcl_NewIntObj(destY);
	    itemList[2] = Tcl_NewIntObj(width);
	    itemList[3] = Tcl_NewIntObj(height);
	    if ( TCL_OK != Tcl_DictObjPut(interp, metadataOutObj,
		    Tcl_NewStringObj("update region",-1),
		    Tcl_NewListObj(4, itemList) )) {
		result = TCL_ERROR;
		goto error;
	    }
	}

	/*
	 * Copy the Graphic Control Extension Block data to the metadata
	 * dictionary
	 */

	if (gifGraphicControlExtensionBlock.blockPresent) {
	    if ( gifGraphicControlExtensionBlock.delayTime != 0) {
		if ( TCL_OK != Tcl_DictObjPut(interp, metadataOutObj,
			Tcl_NewStringObj("delay time",-1),
			Tcl_NewIntObj(gifGraphicControlExtensionBlock.delayTime)
			)) {
		    result = TCL_ERROR;
		    goto error;
		}
	    }
	    switch ( gifGraphicControlExtensionBlock.disposalMethod ) {
	    case 1: /* Do not dispose */
		if ( TCL_OK != Tcl_DictObjPut(interp, metadataOutObj,
			Tcl_NewStringObj("disposal method",-1),
			Tcl_NewStringObj("do not dispose",-1))) {
		    result = TCL_ERROR;
		    goto error;
		}
		break;
	    case 2: /* Restore to background color */
		if ( TCL_OK != Tcl_DictObjPut(interp, metadataOutObj,
			Tcl_NewStringObj("disposal method",-1),
			Tcl_NewStringObj("restore to background color",-1))) {
		    result = TCL_ERROR;
		    goto error;
		}
		break;
	    case 3: /* Restore to previous */
		if ( TCL_OK != Tcl_DictObjPut(interp, metadataOutObj,
			Tcl_NewStringObj("disposal method",-1),
			Tcl_NewStringObj("restore to previous",-1))) {
		    result = TCL_ERROR;
		    goto error;
		}
		break;
	    }
	    if ( gifGraphicControlExtensionBlock.userInteraction != 0) {
		if ( TCL_OK != Tcl_DictObjPut(interp, metadataOutObj,
			Tcl_NewStringObj("user interaction",-1),
			Tcl_NewBooleanObj(1))) {
		    result = TCL_ERROR;
		    goto error;
		}
	    }
	}
    }

    /*
     * We've successfully read the GIF frame (or there was nothing to read,
     * which suits as well). We're done.
     */

    while (1) {
	if (-1 == (gifLabel = ReadOneByte( interp, gifConfPtr, chan ) ) ) {
	    goto error;
	}
	switch (gifLabel) {
	case GIF_TERMINATOR:
	    break;

	case GIF_EXTENSION:
	    /*
	     * This is a GIF extension.
	     */

	    if (-1 == (gifLabel = ReadOneByte( interp, gifConfPtr, chan ) ) ) {
		goto error;
	    }
	    if (DoExtension(gifConfPtr, chan, gifLabel,
		    gifConfPtr->workingBuffer, &gifGraphicControlExtensionBlock,
		    metadataOutObj)
		    < 0) {
		Tcl_SetObjResult(interp, Tcl_NewStringObj(
			"error reading extension in GIF image", -1));
		Tcl_SetErrorCode(interp, "TK", "IMAGE", "GIF", "BAD_EXT",
			NULL);
		goto error;
	    }
	    continue;
	case GIF_START:
	    /*
	     * There should not be a second image block - bail out without error
	     */
	    break;
	default:
	    /*
	     * Not a valid start character; ignore it.
	     */

	    continue;
	}
	break;
    }

    Tcl_SetObjResult(interp, Tcl_NewStringObj(tkImgFmtGIF.name, -1));
    result = TCL_OK;

error:

    /*
     * If a trash buffer has been allocated, free it now.
     */

    if (trashBuffer != NULL) {
	ckfree(trashBuffer);
    }
    return result;
}


/*
 *----------------------------------------------------------------------
 *
 * Read one Byte --
 *
 *	Read one byte (label byte) from the image stream.
 *
 * Results:
 *	The return value is 1 if the first characters in the data are like GIF
 *	data, and 0 otherwise.
 *
 * Side effects:
 *	The access position in the source is incremented.
 *
 *----------------------------------------------------------------------
 */

static int
ReadOneByte(
    Tcl_Interp *interp,		/* Interpreter to use for reporting errors. */
    GIFImageConfig *gifConfPtr,
    Tcl_Channel chan		/* The image file, open for reading. */
    )
{
    unsigned char buf[2];
    if (Fread(gifConfPtr, buf, 1, 1, chan) != 1) {
        /*
         * Premature end of image.
         */

        Tcl_SetObjResult(interp, Tcl_NewStringObj(
		"premature end of image data", -1));
	Tcl_SetErrorCode(interp, "TK", "IMAGE", "GIF", "PREMATURE_END", NULL);
	return -1;
    }
    return buf[0];
}

/*
 *----------------------------------------------------------------------
 *
 * StringMatchGIF --
 *
 *	This function is invoked by the photo image type to see if an object
 *	contains image data in GIF format.
 *
 * Results:
 *	The return value is 1 if the first characters in the data are like GIF
 *	data, and 0 otherwise.
 *
 * Side effects:
 *	The size of the image is placed in widthPtr and heightPtr.
 *
 *----------------------------------------------------------------------
 */


static int
StringMatchGIF(
    TCL_UNUSED(Tcl_Interp *),		/* not used */
    Tcl_Obj *dataObj,		/* the object containing the image data */
    TCL_UNUSED(Tcl_Obj *),		/* the image format object, or NULL */
    TCL_UNUSED(Tcl_Obj *),	/* metadata input, may be NULL */
    int *widthPtr,		/* where to put the string width */
    int *heightPtr,		/* where to put the string height */
    TCL_UNUSED(Tcl_Obj *))	/* metadata return dict, may be NULL */
{
    unsigned char *data, header[10];
    Tcl_Size got, length;
    MFile handle;

    data = Tcl_GetByteArrayFromObj(dataObj, &length);

    /*
     * Header is a minimum of 10 bytes.
     */

    if (length < 10) {
	return 0;
    }

    /*
     * Check whether the data is Base64 encoded.
     */

    if ((strncmp(GIF87a, (char *) data, 6) != 0) &&
	    (strncmp(GIF89a, (char *) data, 6) != 0)) {
	/*
	 * Try interpreting the data as Base64 encoded
	 */

	mInit((unsigned char *) data, &handle, length);
	got = Mread(header, 10, 1, &handle);
	if (got != 10 ||
		((strncmp(GIF87a, (char *) header, 6) != 0)
		&& (strncmp(GIF89a, (char *) header, 6) != 0))) {
	    return 0;
	}
    } else {
	memcpy(header, data, 10);
    }
    *widthPtr = LM_to_uint(header[6], header[7]);
    *heightPtr = LM_to_uint(header[8], header[9]);
    return 1;
}

/*
 *----------------------------------------------------------------------
 *
 * StringReadGIF --
 *
 *	This function is called by the photo image type to read GIF format
 *	data from an object, optionally base64 encoded, and give it to the
 *	photo image.
 *
 * Results:
 *	A standard TCL completion code. If TCL_ERROR is returned then an error
 *	message is left in the interp's result.
 *
 * Side effects:
 *	New data is added to the image given by imageHandle. This function
 *	calls FileReadGIF by redefining the operation of fprintf temporarily.
 *
 *----------------------------------------------------------------------
 */

static int
StringReadGIF(
    Tcl_Interp *interp,		/* interpreter for reporting errors in */
    Tcl_Obj *dataObj,		/* object containing the image */
    Tcl_Obj *format,		/* format object, or NULL */
    Tcl_Obj *metadataInObj,	/* metadata input, may be NULL */
    Tk_PhotoHandle imageHandle,	/* the image to write this data into */
    int destX, int destY,	/* The rectangular region of the */
    int width, int height,	/* image to copy */
    int srcX, int srcY,
    Tcl_Obj *metadataOutObj)	/* metadata return dict, may be NULL */
{
    MFile handle, *hdlPtr = &handle;
    Tcl_Size length;
    const char *xferFormat;
    unsigned char *data = Tcl_GetByteArrayFromObj(dataObj, &length);

    mInit(data, hdlPtr, length);

    /*
     * Check whether the data is Base64 encoded by doing a character-by-
     * charcter comparison with the binary-format headers; BASE64-encoded
     * never matches (matching the other way is harder because of potential
     * padding of the BASE64 data).
     */

    if (strncmp(GIF87a, (char *) data, 6)
	    && strncmp(GIF89a, (char *) data, 6)) {
	xferFormat = INLINE_DATA_BASE64;
    } else {
	xferFormat = INLINE_DATA_BINARY;
    }

    /*
     * Fall through to the file reader now that we have a correctly-configured
     * pseudo-channel to pull the data from.
     */

    return FileReadGIF(interp, (Tcl_Channel) hdlPtr, xferFormat, format,
	    metadataInObj, imageHandle, destX, destY, width, height, srcX, srcY,
	    metadataOutObj);
}

/*
 *----------------------------------------------------------------------
 *
 * ReadGIFHeader --
 *
 *	This function reads the GIF header from the beginning of a GIF file
 *	and returns the dimensions of the image.
 *
 * Results:
 *	The return value is 1 if file "f" appears to start with a valid GIF
 *	header, 0 otherwise. If the header is valid, then *widthPtr and
 *	*heightPtr are modified to hold the dimensions of the image.
 *
 * Side effects:
 *	The access position in f advances.
 *
 *----------------------------------------------------------------------
 */

static int
ReadGIFHeader(
    GIFImageConfig *gifConfPtr,
    Tcl_Channel chan,		/* Image file to read the header from */
    int *widthPtr, int *heightPtr)
				/* The dimensions of the image are returned
				 * here. */
{
    unsigned char buf[7];

    if ((Fread(gifConfPtr, buf, 1, 6, chan) != 6)
	    || ((strncmp(GIF87a, (char *) buf, 6) != 0)
	    && (strncmp(GIF89a, (char *) buf, 6) != 0))) {
	return 0;
    }

    if (Fread(gifConfPtr, buf, 1, 4, chan) != 4) {
	return 0;
    }

    *widthPtr = LM_to_uint(buf[0], buf[1]);
    *heightPtr = LM_to_uint(buf[2], buf[3]);
    return 1;
}

/*
 *-----------------------------------------------------------------
 * The code below is copied from the giftoppm program and modified just
 * slightly.
 *-----------------------------------------------------------------
 */

static int
ReadColorMap(
    GIFImageConfig *gifConfPtr,
    Tcl_Channel chan,
    int number,
    unsigned char buffer[MAXCOLORMAPSIZE][4])
{
    int i;
    unsigned char rgb[3];

    for (i = 0; i < number; ++i) {
	if (((size_t)Fread(gifConfPtr, rgb, sizeof(rgb), 1, chan) + 1) < 2) {
	    return 0;
	}

	if (buffer) {
	    buffer[i][CM_RED] = rgb[0];
	    buffer[i][CM_GREEN] = rgb[1];
	    buffer[i][CM_BLUE] = rgb[2];
	    buffer[i][CM_ALPHA] = 255;
	}
    }
    return 1;
}

/*
*----------------------------------------------------------------------
*
* DoExtension --
*
*	Process a GIF extension block
*
* Results:
*	-1 to trigger an extension read error
*       >= 0 ok
*
* Side effects:
*       The gifGraphicControlExtensionBlock is set if present in current
*       extensions
*       The data of the following extensions are saved to the metadata dict:
*       - Application extension
*         - Comment extension in key "comment"
*       Plain text extensions are currently ignored.
*
*----------------------------------------------------------------------
*/

static int
DoExtension(
    GIFImageConfig *gifConfPtr,
    Tcl_Channel chan,
    int label,
    unsigned char *buf, /* defined as 280 byte working buffer */
    GIFGraphicControlExtensionBlock *gifGraphicControlExtensionBlock,
    Tcl_Obj *metadataOutObj)
{
    int count;
    /* Prepare extension name
     * Maximum string size: "comment" + Code(3) + trailing zero
     */
    char extensionStreamName[8];
    extensionStreamName[0] = '\0';

    switch (label) {
    case 0x01:			/* Plain Text Extension */
	/*
	 * This extension starts a new scope, so Graphic control Extension
	 * data should be cleared
	 */
	gifGraphicControlExtensionBlock->blockPresent = 0;
	/* this extension is ignored, skip below */
	break;
    case 0xf9:			/* Graphic Control Extension */
	count = GetDataBlock(gifConfPtr, chan, buf);
	if (count < 0) {
	    return -1;
	}
	gifGraphicControlExtensionBlock->blockPresent=1;
	/* save disposal method */
	gifGraphicControlExtensionBlock->disposalMethod
		= ((buf[0] & 0x1C) >> 2);
	/* save disposal method */
	gifGraphicControlExtensionBlock->userInteraction = ((buf[0] & 2) >> 1);
	/* save delay time */
	gifGraphicControlExtensionBlock->delayTime
		= LM_to_uint(buf[1], buf[2]);
	/* save transparent index if given */
	if ((buf[0] & 0x1) == 0) {
	    gifGraphicControlExtensionBlock->transparent = -1;
	} else {
	    gifGraphicControlExtensionBlock->transparent = buf[3];
	}
	break;
    case 0xfe:			/* Comment Extension */
	strcpy(extensionStreamName,"comment");
        /* copy the extension data below */
	break;
    }
    /* Add extension to dict */
    if (NULL != metadataOutObj
	    && extensionStreamName[0] != '\0' ) {
	Tcl_Obj *ValueObj = NULL;
	int length = 0;
	for (;;) {
	    count = GetDataBlock(gifConfPtr, chan, buf);
	    switch (count) {
	    case -1: /* error */
		return -1;
	    case 0: /* end of data */
		if (length > 0) {
		    if ( TCL_OK != Tcl_DictObjPut(NULL, metadataOutObj,
			    Tcl_NewByteArrayObj(
				(unsigned char *)extensionStreamName,
			    strlen(extensionStreamName)), ValueObj)) {
			return -1;
		    }
		}
		/* return success */
		return 0;
	    default: /* block received */
		if (length == 0) {
		    /* first block */
		    ValueObj = Tcl_NewByteArrayObj(buf, count);
		    length = count;
		} else {
		    /* consecutive block */
		    unsigned char *bytePtr;
		    bytePtr = Tcl_SetByteArrayLength(ValueObj, length+count);
		    memcpy(bytePtr+length,buf,count);
		    length += count;
		}
		break;
	    }
	} /* for */
    }
    /* skip eventual remaining data block bytes */
    do {
	count = GetDataBlock(gifConfPtr, chan, buf);
    } while (count > 0);
    return count; /* this may be -1 for error or 0 */
}

static int
GetDataBlock(
    GIFImageConfig *gifConfPtr,
    Tcl_Channel chan,
    unsigned char *buf)
{
    unsigned char count;

    if (((size_t)Fread(gifConfPtr, &count, 1, 1, chan) + 1) < 2) {
	return -1;
    }

    if ((count != 0) && (((size_t)Fread(gifConfPtr, buf, count, 1, chan) + 1) < 2)) {
	return -1;
    }

    return count;
}

/*
 *----------------------------------------------------------------------
 *
 * ReadImage --
 *
 *	Process a GIF image from a given source, with a given height, width,
 *	transparency, etc.
 *
 *	This code is based on the code found in the ImageMagick GIF decoder,
 *	which is (c) 2000 ImageMagick Studio.
 *
 *	Some thoughts on our implementation:
 *	It sure would be nice if ReadImage didn't take 11 parameters! I think
 *	that if we were smarter, we could avoid doing that.
 *
 *	Possible further optimizations: we could pull the GetCode function
 *	directly into ReadImage, which would improve our speed.
 *
 * Results:
 *	Processes a GIF image and loads the pixel data into a memory array.
 *
 * Side effects:
 *	None.
 *
 *----------------------------------------------------------------------
 */

static int
ReadImage(
    GIFImageConfig *gifConfPtr,
    Tcl_Interp *interp,
    unsigned char *imagePtr,
    Tcl_Channel chan,
    int len, int rows,
    unsigned char cmap[MAXCOLORMAPSIZE][4],
    TCL_UNUSED(int),
    TCL_UNUSED(int),
    int interlace,
    int transparent)
{
    unsigned char initialCodeSize;
    int xpos = 0, ypos = 0, pass = 0, i, count;
    unsigned char *pixelPtr;
    static const int interlaceStep[] = { 8, 8, 4, 2 };
    static const int interlaceStart[] = { 0, 4, 2, 1 };
    unsigned short prefix[(1 << MAX_LWZ_BITS)];
    unsigned char append[(1 << MAX_LWZ_BITS)];
    unsigned char stack[(1 << MAX_LWZ_BITS)*2];
    unsigned char *top;
    int codeSize, clearCode, inCode, endCode, oldCode, maxCode;
    int code, firstCode, v;

    /*
     * Initialize the decoder
     */

    if (((size_t)Fread(gifConfPtr, &initialCodeSize, 1, 1, chan) + 1) < 2) {
	Tcl_SetObjResult(interp, Tcl_ObjPrintf(
		"error reading GIF image: %s", Tcl_PosixError(interp)));
	return TCL_ERROR;
    }

    if (initialCodeSize > MAX_LWZ_BITS) {
	Tcl_SetObjResult(interp, Tcl_NewStringObj("malformed image", -1));
	Tcl_SetErrorCode(interp, "TK", "IMAGE", "GIF", "MALFORMED", NULL);
	return TCL_ERROR;
    }

    if (transparent != -1) {
	cmap[transparent][CM_RED] = 0;
	cmap[transparent][CM_GREEN] = 0;
	cmap[transparent][CM_BLUE] = 0;
	cmap[transparent][CM_ALPHA] = 0;
    }

    pixelPtr = imagePtr;

    /*
     * Initialize the decoder.
     *
     * Set values for "special" numbers:
     * clear code	reset the decoder
     * end code		stop decoding
     * code size	size of the next code to retrieve
     * max code		next available table position
     */

    clearCode = 1 << (int) initialCodeSize;
    endCode = clearCode + 1;
    codeSize = (int) initialCodeSize + 1;
    maxCode = clearCode + 2;
    oldCode = -1;
    firstCode = -1;

    memset(prefix, 0, (1 << MAX_LWZ_BITS) * sizeof(short));
    memset(append, 0, (1 << MAX_LWZ_BITS) * sizeof(char));
    for (i = 0; i < clearCode; i++) {
	append[i] = i;
    }
    top = stack;

    GetCode(chan, 0, 1, gifConfPtr);

    /*
     * Read until we finish the image
     */

    for (i = 0, ypos = 0; i < rows; i++) {
	for (xpos = 0; xpos < len; ) {
	    if (top == stack) {
		/*
		 * Bummer - our stack is empty. Now we have to work!
		 */

		code = GetCode(chan, codeSize, 0, gifConfPtr);
		if (code < 0) {
		    return TCL_OK;
		}

		if (code > maxCode || code == endCode) {
		    /*
		     * If we're doing things right, we should never receive a
		     * code that is greater than our current maximum code. If
		     * we do, bail, because our decoder does not yet have that
		     * code set up.
		     *
		     * If the code is the magic endCode value, quit.
		     */

		    return TCL_OK;
		}

		if (code == clearCode) {
		    /*
		     * Reset the decoder.
		     */

		    codeSize = initialCodeSize + 1;
		    maxCode = clearCode + 2;
		    oldCode = -1;
		    continue;
		}

		if (oldCode == -1) {
		    /*
		     * Last pass reset the decoder, so the first code we see
		     * must be a singleton. Seed the stack with it, and set up
		     * the old/first code pointers for insertion into the
		     * codes table. We can't just roll this into the clearCode
		     * test above, because at that point we have not yet read
		     * the next code.
		     */

		    *top++ = append[code];
		    oldCode = code;
		    firstCode = code;
		    continue;
		}

		inCode = code;

		if ((code == maxCode) && (maxCode < (1 << MAX_LWZ_BITS))) {
		    /*
		     * maxCode is always one bigger than our highest assigned
		     * code. If the code we see is equal to maxCode, then we
		     * are about to add a new entry to the codes table.
		     */

		    *top++ = firstCode;
		    code = oldCode;
		}

		while (code > clearCode) {
		    /*
		     * Populate the stack by tracing the code in the codes
		     * table from its tail to its head
		     */

		    *top++ = append[code];
		    code = prefix[code];
		}
		firstCode = append[code];

	        /*
	         * Push the head of the code onto the stack.
	         */

	        *top++ = firstCode;

                if (maxCode < (1 << MAX_LWZ_BITS)) {
		    /*
		     * If there's still room in our codes table, add a new entry.
		     * Otherwise don't, and keep using the current table.
                     * See DEFERRED CLEAR CODE IN LZW COMPRESSION in the GIF89a
                     * specification.
		     */

		    prefix[maxCode] = oldCode;
		    append[maxCode] = firstCode;
		    maxCode++;
                }

		/*
		 * maxCode tells us the maximum code value we can accept. If
		 * we see that we need more bits to represent it than we are
		 * requesting from the unpacker, we need to increase the
		 * number we ask for.
		 */

		if ((maxCode >= (1 << codeSize))
			&& (maxCode < (1<<MAX_LWZ_BITS))) {
		    codeSize++;
		}
		oldCode = inCode;
	    }

	    /*
	     * Pop the next color index off the stack.
	     */

	    v = *(--top);
	    if (v < 0) {
		return TCL_OK;
	    }

	    /*
	     * If pixelPtr is null, we're skipping this image (presumably
	     * there are more in the file and we will be called to read one of
	     * them later)
	     */

	    *pixelPtr++ = cmap[v][CM_RED];
	    *pixelPtr++ = cmap[v][CM_GREEN];
	    *pixelPtr++ = cmap[v][CM_BLUE];
	    if (transparent >= 0) {
		*pixelPtr++ = cmap[v][CM_ALPHA];
	    }
	    xpos++;

	}

	/*
	 * If interlacing, the next ypos is not just +1.
	 */

	if (interlace) {
	    ypos += interlaceStep[pass];
	    while (ypos >= rows) {
		pass++;
		if (pass > 3) {
		    return TCL_OK;
		}
		ypos = interlaceStart[pass];
	    }
	} else {
	    ypos++;
	}
	pixelPtr = imagePtr + (ypos) * len * ((transparent>=0)?4:3);
    }

    /*
     * Now read until the final zero byte.
     * It was observed that there might be 1 length blocks
     * (test imgPhoto-14.1) which are not read.
     *
     * The field "stack" is abused for temporary buffer. it has 4096 bytes
     * and we need 256.
     *
     * Loop until we hit a 0 length block which is the end sign.
     */
    while ( 0 < (count = GetDataBlock(gifConfPtr, chan, stack)))
    {
	if (-1 == count ) {
	    Tcl_SetObjResult(interp, Tcl_ObjPrintf(
		    "error reading GIF image: %s", Tcl_PosixError(interp)));
	    return TCL_ERROR;
	}
    }
    return TCL_OK;
}

/*
 *----------------------------------------------------------------------
 *
 * GetCode --
 *
 *	Extract the next compression code from the file. In GIF's, the
 *	compression codes are between 3 and 12 bits long and are then packed
 *	into 8 bit bytes, left to right, for example:
 *		bbbaaaaa
 *		dcccccbb
 *		eeeedddd
 *		...
 *	We use a byte buffer read from the file and a sliding window to unpack
 *	the bytes. Thanks to ImageMagick for the sliding window idea.
 *	args:  chan	    the channel to read from
 *	       code_size    size of the code to extract
 *	       flag	    boolean indicating whether the extractor should be
 *			    reset or not
 *
 * Results:
 *	code		    the next compression code
 *
 * Side effects:
 *	May consume more input from chan.
 *
 *----------------------------------------------------------------------
 */

static int
GetCode(
    Tcl_Channel chan,
    int code_size,
    int flag,
    GIFImageConfig *gifConfPtr)
{
    int ret;

    if (flag) {
	/*
	 * Initialize the decoder.
	 */

	gifConfPtr->reader.bitsInWindow = 0;
	gifConfPtr->reader.bytes = 0;
	gifConfPtr->reader.window = 0;
	gifConfPtr->reader.done = 0;
	gifConfPtr->reader.c = NULL;
	return 0;
    }

    while (gifConfPtr->reader.bitsInWindow < code_size) {
	/*
	 * Not enough bits in our window to cover the request.
	 */

	if (gifConfPtr->reader.done) {
	    return -1;
	}
	if (gifConfPtr->reader.bytes == 0) {
	    /*
	     * Not enough bytes in our buffer to add to the window.
	     */

	    gifConfPtr->reader.bytes =
		    GetDataBlock(gifConfPtr, chan, gifConfPtr->workingBuffer);
	    gifConfPtr->reader.c = gifConfPtr->workingBuffer;
	    if (gifConfPtr->reader.bytes <= 0) {
		gifConfPtr->reader.done = 1;
		break;
	    }
	}

	/*
	 * Tack another byte onto the window, see if that's enough.
	 */

	gifConfPtr->reader.window +=
		(*gifConfPtr->reader.c) << gifConfPtr->reader.bitsInWindow;
	gifConfPtr->reader.c++;
	gifConfPtr->reader.bitsInWindow += 8;
	gifConfPtr->reader.bytes--;
    }

    /*
     * The next code will always be the last code_size bits of the window.
     */

    ret = gifConfPtr->reader.window & ((1 << code_size) - 1);

    /*
     * Shift data in the window to put the next code at the end.
     */

    gifConfPtr->reader.window >>= code_size;
    gifConfPtr->reader.bitsInWindow -= code_size;
    return ret;
}

/*
 *----------------------------------------------------------------------
 *
 * Minit -- --
 *
 *	This function initializes a base64 decoder handle
 *
 * Results:
 *	None
 *
 * Side effects:
 *	The base64 handle is initialized
 *
 *----------------------------------------------------------------------
 */

static void
mInit(
    unsigned char *string,	/* string containing initial mmencoded data */
    MFile *handle,		/* mmdecode "file" handle */
    size_t length)			/* Number of bytes in string */
{
    handle->data = string;
    handle->state = 0;
    handle->c = 0;
    handle->length = length;
}

/*
 *----------------------------------------------------------------------
 *
 * Mread --
 *
 *	This function is invoked by the GIF file reader as a temporary
 *	replacement for "fread", to get GIF data out of a string (using
 *	Mgetc).
 *
 * Results:
 *	The return value is the number of characters "read"
 *
 * Side effects:
 *	The base64 handle will change state.
 *
 *----------------------------------------------------------------------
 */

static size_t
Mread(
    unsigned char *dst,		/* where to put the result */
    size_t chunkSize,		/* size of each transfer */
    size_t numChunks,		/* number of chunks */
    MFile *handle)		/* mmdecode "file" handle */
{
    int c;
    size_t i, count = chunkSize * numChunks;

    for (i=0; i<count && (c=Mgetc(handle)) != GIF_DONE; i++) {
	*dst++ = c;
    }
    return i;
}

/*
 *----------------------------------------------------------------------
 *
 * Mgetc --
 *
 *	This function gets the next decoded character from an mmencode handle.
 *	This causes at least 1 character to be "read" from the encoded string.
 *
 * Results:
 *	The next byte (or GIF_DONE) is returned.
 *
 * Side effects:
 *	The base64 handle will change state.
 *
 *----------------------------------------------------------------------
 */

static int
Mgetc(
    MFile *handle)		/* Handle containing decoder data and state */
{
    int c;
    int result = 0;		/* Initialization needed only to prevent gcc
				 * compiler warning. */

    if (handle->state == GIF_DONE) {
	return GIF_DONE;
    }

    do {
	if (handle->length-- <= 0) {
	    return GIF_DONE;
	}
	c = char64(*handle->data);
	handle->data++;
    } while (c == GIF_SPACE);

    if (c > GIF_SPECIAL) {
	handle->state = GIF_DONE;
	return handle->c;
    }

    switch (handle->state++) {
    case 0:
	handle->c = c<<2;
	result = Mgetc(handle);
	break;
    case 1:
	result = handle->c | (c>>4);
	handle->c = (c&0xF)<<4;
	break;
    case 2:
	result = handle->c | (c>>2);
	handle->c = (c&0x3) << 6;
	break;
    case 3:
	result = handle->c | c;
	handle->state = 0;
	break;
    }
    return result;
}

/*
 *----------------------------------------------------------------------
 *
 * char64 --
 *
 *	This function converts a base64 ascii character into its binary
 *	equivalent. This code is a slightly modified version of the char64
 *	function in N. Borenstein's metamail decoder.
 *
 * Results:
 *	The binary value, or an error code.
 *
 * Side effects:
 *	None.
 *
 *----------------------------------------------------------------------
 */

static int
char64(
    int c)
{
    switch(c) {
    case 'A': return 0;  case 'B': return 1;  case 'C': return 2;
    case 'D': return 3;  case 'E': return 4;  case 'F': return 5;
    case 'G': return 6;  case 'H': return 7;  case 'I': return 8;
    case 'J': return 9;  case 'K': return 10; case 'L': return 11;
    case 'M': return 12; case 'N': return 13; case 'O': return 14;
    case 'P': return 15; case 'Q': return 16; case 'R': return 17;
    case 'S': return 18; case 'T': return 19; case 'U': return 20;
    case 'V': return 21; case 'W': return 22; case 'X': return 23;
    case 'Y': return 24; case 'Z': return 25; case 'a': return 26;
    case 'b': return 27; case 'c': return 28; case 'd': return 29;
    case 'e': return 30; case 'f': return 31; case 'g': return 32;
    case 'h': return 33; case 'i': return 34; case 'j': return 35;
    case 'k': return 36; case 'l': return 37; case 'm': return 38;
    case 'n': return 39; case 'o': return 40; case 'p': return 41;
    case 'q': return 42; case 'r': return 43; case 's': return 44;
    case 't': return 45; case 'u': return 46; case 'v': return 47;
    case 'w': return 48; case 'x': return 49; case 'y': return 50;
    case 'z': return 51; case '0': return 52; case '1': return 53;
    case '2': return 54; case '3': return 55; case '4': return 56;
    case '5': return 57; case '6': return 58; case '7': return 59;
    case '8': return 60; case '9': return 61; case '+': return 62;
    case '/': return 63;

    case ' ': case '\t': case '\n': case '\r': case '\f':
	return GIF_SPACE;
    case '=':
	return GIF_PAD;
    case '\0':
	return GIF_DONE;
    default:
	return GIF_BAD;
    }
}

/*
 *----------------------------------------------------------------------
 *
 * Fread --
 *
 *	This function calls either fread or Mread to read data from a file or
 *	a base64 encoded string.
 *
 * Results: - same as POSIX fread() or Tcl Tcl_Read()
 *
 *----------------------------------------------------------------------
 */

static size_t
Fread(
    GIFImageConfig *gifConfPtr,
    unsigned char *dst,		/* where to put the result */
    size_t hunk, size_t count,	/* how many */
    Tcl_Channel chan)
{
    if (gifConfPtr->fromData == INLINE_DATA_BASE64) {
	return Mread(dst, hunk, count, (MFile *) chan);
    }

    if (gifConfPtr->fromData == INLINE_DATA_BINARY) {
	MFile *handle = (MFile *) chan;

	if ((handle->length + 1 < 2) || (handle->length < hunk*count)) {
	    return (size_t)-1;
	}
	memcpy(dst, handle->data, hunk * count);
	handle->data += hunk * count;
	handle->length -= hunk * count;
	return hunk * count;
    }

    /*
     * Otherwise we've got a real file to read.
     */

    return Tcl_Read(chan, (char *) dst, hunk * count);
}

/*
 * ChanWriteGIF - writes a image in GIF format.
 *-------------------------------------------------------------------------
 * Author:		Lolo
 *			Engeneering Projects Area
 *			Department of Mining
 *			University of Oviedo
 * e-mail		zz11425958@zeus.etsimo.uniovi.es
 *			lolo@pcsig22.etsimo.uniovi.es
 * Date:		Fri September 20 1996
 *
 * Modified for transparency handling (gif89a)
 * by Jan Nijtmans <nijtmans@users.sourceforge.net>
 *
 *----------------------------------------------------------------------
 * FileWriteGIF-
 *
 *	This function is called by the photo image type to write GIF format
 *	data from a photo image into a given file
 *
 * Results:
 *	A standard TCL completion code.  If TCL_ERROR is returned then an
 *	error message is left in the interp's result.
 *
 *----------------------------------------------------------------------
 */


static int
FileWriteGIF(
    Tcl_Interp *interp,		/* Interpreter to use for reporting errors. */
    const char *filename,
    Tcl_Obj *format,
    Tcl_Obj *metadataInObj,
    Tk_PhotoImageBlock *blockPtr)
{
    Tcl_Channel chan = NULL;
    int result;

    chan = Tcl_OpenFileChannel(interp, (char *) filename, "w", 0644);
    if (!chan) {
	return TCL_ERROR;
    }
    if (Tcl_SetChannelOption(interp, chan, "-translation",
	    "binary") != TCL_OK) {
	Tcl_Close(NULL, chan);
	return TCL_ERROR;
    }

    result = CommonWriteGIF(interp, chan, WriteToChannel, format, metadataInObj,
	    blockPtr);

    if (Tcl_Close(interp, chan) == TCL_ERROR) {
	return TCL_ERROR;
    }
    return result;
}

static int
StringWriteGIF(
    Tcl_Interp *interp,		/* Interpreter to use for reporting errors and
				 * returning the GIF data. */
    Tcl_Obj *format,
    Tcl_Obj *metadataInObj,
    Tk_PhotoImageBlock *blockPtr)
{
    int result;
    Tcl_Obj *objPtr = Tcl_NewObj();

    Tcl_IncrRefCount(objPtr);
    result = CommonWriteGIF(interp, objPtr, WriteToByteArray, format,
	    metadataInObj, blockPtr);
    if (result == TCL_OK) {
	Tcl_SetObjResult(interp, objPtr);
    }
    Tcl_DecrRefCount(objPtr);
    return result;
}

static size_t
WriteToChannel(
    ClientData clientData,
    const char *bytes,
    size_t byteCount)
{
    Tcl_Channel handle = (Tcl_Channel)clientData;

    return Tcl_Write(handle, bytes, byteCount);
}

static size_t
WriteToByteArray(
    ClientData clientData,
    const char *bytes,
    size_t byteCount)
{
    Tcl_Obj *objPtr = (Tcl_Obj *)clientData;
    Tcl_Obj *tmpObj = Tcl_NewByteArrayObj((unsigned char *) bytes, byteCount);

    Tcl_IncrRefCount(tmpObj);
    Tcl_AppendObjToObj(objPtr, tmpObj);
    Tcl_DecrRefCount(tmpObj);
    return byteCount;
}

static int
CommonWriteGIF(
    Tcl_Interp *interp,
    ClientData handle,
    WriteBytesFunc *writeProc,
    TCL_UNUSED(Tcl_Obj *),
    Tcl_Obj *metadataInObj,
    Tk_PhotoImageBlock *blockPtr)
{
    GifWriterState state;
    int resolution;
    long width, height, x;
    unsigned char c;
    unsigned int top, left;

    top = 0;
    left = 0;

    memset(&state, 0, sizeof(state));

    state.pixelSize = blockPtr->pixelSize;
    state.greenOffset = blockPtr->offset[1]-blockPtr->offset[0];
    state.blueOffset = blockPtr->offset[2]-blockPtr->offset[0];
    state.alphaOffset = blockPtr->offset[0];
    if (state.alphaOffset < blockPtr->offset[2]) {
	state.alphaOffset = blockPtr->offset[2];
    }
    if (++state.alphaOffset < state.pixelSize) {
	state.alphaOffset -= blockPtr->offset[0];
    } else {
	state.alphaOffset = 0;
    }

    writeProc(handle, (char *) (state.alphaOffset ? GIF89a : GIF87a), 6);

    for (x = 0; x < MAXCOLORMAPSIZE ;x++) {
	state.mapa[x][CM_RED] = 255;
	state.mapa[x][CM_GREEN] = 255;
	state.mapa[x][CM_BLUE] = 255;
    }

    width = blockPtr->width;
    height = blockPtr->height;
    state.pixelOffset = blockPtr->pixelPtr + blockPtr->offset[0];
    state.pixelPitch = blockPtr->pitch;
    SaveMap(&state, blockPtr);
    if (state.num >= MAXCOLORMAPSIZE) {
	Tcl_SetObjResult(interp, Tcl_NewStringObj("too many colors", -1));
	Tcl_SetErrorCode(interp, "TK", "IMAGE", "GIF", "COLORFUL", NULL);
	return TCL_ERROR;
    }
    if (state.num<2) {
	state.num = 2;
    }
    c = LSB(width);
    writeProc(handle, (char *) &c, 1);
    c = MSB(width);
    writeProc(handle, (char *) &c, 1);
    c = LSB(height);
    writeProc(handle, (char *) &c, 1);
    c = MSB(height);
    writeProc(handle, (char *) &c, 1);

    resolution = 0;
    while (state.num >> resolution) {
	resolution++;
    }
    c = 111 + resolution * 17;
    writeProc(handle, (char *) &c, 1);

    state.num = 1 << resolution;

    /*
     * Background color
     */

    c = 0;
    writeProc(handle, (char *) &c, 1);

    /*
     * Zero for future expansion.
     */

    writeProc(handle, (char *) &c, 1);

    for (x = 0; x < state.num; x++) {
	c = state.mapa[x][CM_RED];
	writeProc(handle, (char *) &c, 1);
	c = state.mapa[x][CM_GREEN];
	writeProc(handle, (char *) &c, 1);
	c = state.mapa[x][CM_BLUE];
	writeProc(handle, (char *) &c, 1);
    }

    /*
     * Write out extension for transparent colour index, if necessary.
     */

    if (state.alphaOffset) {
	c = GIF_EXTENSION;
	writeProc(handle, (char *) &c, 1);
	writeProc(handle, "\371\4\1\0\0\0", 7);
    }

    c = GIF_START;
    writeProc(handle, (char *) &c, 1);
    c = LSB(top);
    writeProc(handle, (char *) &c, 1);
    c = MSB(top);
    writeProc(handle, (char *) &c, 1);
    c = LSB(left);
    writeProc(handle, (char *) &c, 1);
    c = MSB(left);
    writeProc(handle, (char *) &c, 1);

    c = LSB(width);
    writeProc(handle, (char *) &c, 1);
    c = MSB(width);
    writeProc(handle, (char *) &c, 1);

    c = LSB(height);
    writeProc(handle, (char *) &c, 1);
    c = MSB(height);
    writeProc(handle, (char *) &c, 1);

    c = 0;
    writeProc(handle, (char *) &c, 1);
    c = resolution;
    writeProc(handle, (char *) &c, 1);

    state.ssize = state.rsize = blockPtr->width;
    state.csize = blockPtr->height;
    Compress(resolution+1, handle, writeProc, ReadValue, &state);

    c = 0;
    writeProc(handle, (char *) &c, 1);
    /*
     * Check for metadata keys to add to file
     */
    if (NULL != metadataInObj) {
	Tcl_Obj *itemData;

	/*
	 * Check and code comment block
	 */

	if (TCL_ERROR == Tcl_DictObjGet(interp, metadataInObj,
		Tcl_NewStringObj("comment",-1),
		&itemData)) {
	    return TCL_ERROR;
	}
	if (itemData != NULL) {
	    int length;
	    unsigned char *comment;
	    comment = Tcl_GetByteArrayFromObj(itemData, &length);
	    if (length > 0) {
		/* write comment header */
		writeProc(handle, (char *) "\x21\xfe", 2);
		/* write comment blocks */
		for (;length > 0;) {
		    int blockLength;
		    unsigned char blockLengthChar;
		    if (length > 255) {
			length -=255;
			blockLength = 255;
		    } else {
			blockLength = length;
			length = 0;
		    }
		    blockLengthChar = (unsigned char) blockLength;
		    writeProc(handle, (char *) &blockLengthChar, 1);
		    writeProc(handle, (char *) comment, blockLength);
		    comment += blockLength;
		}
		/* Block terminator */
		c = 0;
		writeProc(handle, (char *) &c, 1);
	    }
	}
    }
    c = GIF_TERMINATOR;
    writeProc(handle, (char *) &c, 1);

    return TCL_OK;
}

static int
ColorNumber(
    GifWriterState *statePtr,
    int red, int green, int blue)
{
    int x = (statePtr->alphaOffset != 0);

    for (; x <= MAXCOLORMAPSIZE; x++) {
	if ((statePtr->mapa[x][CM_RED] == red) &&
		(statePtr->mapa[x][CM_GREEN] == green) &&
		(statePtr->mapa[x][CM_BLUE] == blue)) {
	    return x;
	}
    }
    return -1;
}

static int
IsNewColor(
    GifWriterState *statePtr,
    int red, int green, int blue)
{
    int x = (statePtr->alphaOffset != 0);

    for (; x<=statePtr->num ; x++) {
	if ((statePtr->mapa[x][CM_RED] == red) &&
		(statePtr->mapa[x][CM_GREEN] == green) &&
		(statePtr->mapa[x][CM_BLUE] == blue)) {
	    return 0;
	}
    }
    return 1;
}

static void
SaveMap(
    GifWriterState *statePtr,
    Tk_PhotoImageBlock *blockPtr)
{
    unsigned char *colores;
    int x, y;
    unsigned char red, green, blue;

    if (statePtr->alphaOffset) {
	statePtr->num = 0;
	statePtr->mapa[0][CM_RED] = DEFAULT_BACKGROUND_VALUE;
	statePtr->mapa[0][CM_GREEN] = DEFAULT_BACKGROUND_VALUE;
	statePtr->mapa[0][CM_BLUE] = DEFAULT_BACKGROUND_VALUE;
    } else {
	statePtr->num = -1;
    }

    for (y=0 ; y<blockPtr->height ; y++) {
	colores = blockPtr->pixelPtr + blockPtr->offset[0] + y*blockPtr->pitch;
	for (x=0 ; x<blockPtr->width ; x++) {
	    if (!statePtr->alphaOffset || colores[statePtr->alphaOffset]!=0) {
		red = colores[0];
		green = colores[statePtr->greenOffset];
		blue = colores[statePtr->blueOffset];
		if (IsNewColor(statePtr, red, green, blue)) {
		    statePtr->num++;
		    if (statePtr->num >= MAXCOLORMAPSIZE) {
			return;
		    }
		    statePtr->mapa[statePtr->num][CM_RED] = red;
		    statePtr->mapa[statePtr->num][CM_GREEN] = green;
		    statePtr->mapa[statePtr->num][CM_BLUE] = blue;
		}
	    }
	    colores += statePtr->pixelSize;
	}
    }
}

static int
ReadValue(
    GifWriterState *statePtr)
{
    unsigned int col;

    if (statePtr->csize == 0) {
	return EOF;
    }
    if (statePtr->alphaOffset
	    && (statePtr->pixelOffset[statePtr->alphaOffset]==0)) {
	col = 0;
    } else {
	col = ColorNumber(statePtr, statePtr->pixelOffset[0],
		statePtr->pixelOffset[statePtr->greenOffset],
		statePtr->pixelOffset[statePtr->blueOffset]);
    }
    statePtr->pixelOffset += statePtr->pixelSize;
    if (--statePtr->ssize <= 0) {
	statePtr->ssize = statePtr->rsize;
	statePtr->csize--;
	statePtr->pixelOffset += statePtr->pixelPitch
		- (statePtr->rsize * statePtr->pixelSize);
    }

    return col;
}

/*
 * GIF Image compression - modified 'Compress'
 *
 * Based on: compress.c - File compression ala IEEE Computer, June 1984.
 *
 * By Authors:  Spencer W. Thomas       (decvax!harpo!utah-cs!utah-gr!thomas)
 *              Jim McKie               (decvax!mcvax!jim)
 *              Steve Davies            (decvax!vax135!petsd!peora!srd)
 *              Ken Turkowski           (decvax!decwrl!turtlevax!ken)
 *              James A. Woods          (decvax!ihnp4!ames!jaw)
 *              Joe Orost               (decvax!vax135!petsd!joe)
 */

static void
Compress(
    int initialBits,
    ClientData handle,
    WriteBytesFunc *writeProc,
    ifunptr readValue,
    GifWriterState *statePtr)
{
    long fcode, ent, disp, hSize, i = 0;
    int c, hshift;
    GIFState_t state;

    memset(&state, 0, sizeof(state));

    /*
     * Set up the globals:  initialBits - initial number of bits
     *			    outChannel	- pointer to output file
     */

    state.initialBits = initialBits;
    state.destination = handle;
    state.writeProc = writeProc;

    /*
     * Set up the necessary values.
     */

    state.offset = 0;
    state.hSize = HSIZE;
    state.outCount = 0;
    state.clearFlag = 0;
    state.inCount = 1;
    state.maxCode = MAXCODE(state.numBits = state.initialBits);
    state.clearCode = 1 << (initialBits - 1);
    state.eofCode = state.clearCode + 1;
    state.freeEntry = state.clearCode + 2;
    CharInit(&state);

    ent = readValue(statePtr);

    hshift = 0;
    for (fcode = (long) state.hSize;  fcode < 65536L;  fcode *= 2L) {
	hshift++;
    }
    hshift = 8 - hshift;		/* Set hash code range bound */

    hSize = state.hSize;
    ClearHashTable(&state, (int) hSize); /* Clear hash table */

    Output(&state, (long) state.clearCode);

    while (U(c = readValue(statePtr)) != U(EOF)) {
	state.inCount++;

	fcode = (long) (((long) c << GIFBITS) + ent);
	i = ((long)c << hshift) ^ ent;	/* XOR hashing */

	if (state.hashTable[i] == fcode) {
	    ent = state.codeTable[i];
	    continue;
	} else if ((long) state.hashTable[i] < 0) {	/* Empty slot */
	    goto nomatch;
	}

	disp = hSize - i;		/* Secondary hash (after G. Knott) */
	if (i == 0) {
	    disp = 1;
	}

    probe:
	if ((i -= disp) < 0) {
	    i += hSize;
	}

	if (state.hashTable[i] == fcode) {
	    ent = state.codeTable[i];
	    continue;
	}
	if ((long) state.hashTable[i] > 0) {
	    goto probe;
	}

    nomatch:
	Output(&state, (long) ent);
	state.outCount++;
	ent = c;
	if (U(state.freeEntry) < U((long)1 << GIFBITS)) {
	    state.codeTable[i] = state.freeEntry++; /* code -> hashtable */
	    state.hashTable[i] = fcode;
	} else {
	    ClearForBlock(&state);
	}
    }

    /*
     * Put out the final code.
     */

    Output(&state, (long) ent);
    state.outCount++;
    Output(&state, (long) state.eofCode);
}

/*****************************************************************
 * Output --
 *	Output the given code.
 *
 * Inputs:
 *	code:	A numBits-bit integer. If == -1, then EOF. This assumes that
 *		numBits =< (long) wordsize - 1.
 * Outputs:
 *	Outputs code to the file.
 * Assumptions:
 *	Chars are 8 bits long.
 * Algorithm:
 *	Maintain a GIFBITS character long buffer (so that 8 codes will fit in
 *	it exactly). Use the VAX insv instruction to insert each code in turn.
 *	When the buffer fills up empty it and start over.
 */

static void
Output(
    GIFState_t *statePtr,
    long code)
{
    static const unsigned long masks[] = {
	0x0000,
	0x0001, 0x0003, 0x0007, 0x000F,
	0x001F, 0x003F, 0x007F, 0x00FF,
	0x01FF, 0x03FF, 0x07FF, 0x0FFF,
	0x1FFF, 0x3FFF, 0x7FFF, 0xFFFF
    };

    statePtr->currentAccumulated &= masks[statePtr->currentBits];
    if (statePtr->currentBits > 0) {
	statePtr->currentAccumulated |= ((long) code << statePtr->currentBits);
    } else {
	statePtr->currentAccumulated = code;
    }
    statePtr->currentBits += statePtr->numBits;

    while (statePtr->currentBits >= 8) {
	CharOut(statePtr, (unsigned) (statePtr->currentAccumulated & 0xff));
	statePtr->currentAccumulated >>= 8;
	statePtr->currentBits -= 8;
    }

    /*
     * If the next entry is going to be too big for the code size, then
     * increase it, if possible.
     */

    if ((statePtr->freeEntry > statePtr->maxCode) || statePtr->clearFlag) {
	if (statePtr->clearFlag) {
	    statePtr->maxCode = MAXCODE(
		    statePtr->numBits = statePtr->initialBits);
	    statePtr->clearFlag = 0;
	} else {
	    statePtr->numBits++;
	    if (statePtr->numBits == GIFBITS) {
		statePtr->maxCode = (long)1 << GIFBITS;
	    } else {
		statePtr->maxCode = MAXCODE(statePtr->numBits);
	    }
	}
    }

    if (code == statePtr->eofCode) {
	/*
	 * At EOF, write the rest of the buffer.
	 */

	while (statePtr->currentBits > 0) {
	    CharOut(statePtr,
		    (unsigned) (statePtr->currentAccumulated & 0xff));
	    statePtr->currentAccumulated >>= 8;
	    statePtr->currentBits -= 8;
	}
	FlushChar(statePtr);
    }
}

/*
 * Clear out the hash table
 */

static void
ClearForBlock(			/* Table clear for block compress. */
    GIFState_t *statePtr)
{
    ClearHashTable(statePtr, (int) statePtr->hSize);
    statePtr->freeEntry = statePtr->clearCode + 2;
    statePtr->clearFlag = 1;

    Output(statePtr, (long) statePtr->clearCode);
}

static void
ClearHashTable(			/* Reset code table. */
    GIFState_t *statePtr,
    int hSize)
{
    int *hashTablePtr = statePtr->hashTable + hSize;
    long i;
    long m1 = -1;

    i = hSize - 16;
    do {			/* might use Sys V memset(3) here */
	*(hashTablePtr-16) = m1;
	*(hashTablePtr-15) = m1;
	*(hashTablePtr-14) = m1;
	*(hashTablePtr-13) = m1;
	*(hashTablePtr-12) = m1;
	*(hashTablePtr-11) = m1;
	*(hashTablePtr-10) = m1;
	*(hashTablePtr-9) = m1;
	*(hashTablePtr-8) = m1;
	*(hashTablePtr-7) = m1;
	*(hashTablePtr-6) = m1;
	*(hashTablePtr-5) = m1;
	*(hashTablePtr-4) = m1;
	*(hashTablePtr-3) = m1;
	*(hashTablePtr-2) = m1;
	*(hashTablePtr-1) = m1;
	hashTablePtr -= 16;
    } while ((i -= 16) >= 0);

    for (i += 16; i > 0; i--) {
	*--hashTablePtr = m1;
    }
}

/*
 *****************************************************************************
 *
 * GIF Specific routines
 *
 *****************************************************************************
 */

/*
 * Set up the 'byte output' routine
 */

static void
CharInit(
    GIFState_t *statePtr)
{
    statePtr->accumulatedByteCount = 0;
    statePtr->currentAccumulated = 0;
    statePtr->currentBits = 0;
}

/*
 * Add a character to the end of the current packet, and if it is 254
 * characters, flush the packet to disk.
 */

static void
CharOut(
    GIFState_t *statePtr,
    int c)
{
    statePtr->packetAccumulator[statePtr->accumulatedByteCount++] = c;
    if (statePtr->accumulatedByteCount >= 254) {
	FlushChar(statePtr);
    }
}

/*
 * Flush the packet to disk, and reset the accumulator
 */

static void
FlushChar(
    GIFState_t *statePtr)
{
    unsigned char c;

    if (statePtr->accumulatedByteCount > 0) {
	c = statePtr->accumulatedByteCount;
	statePtr->writeProc(statePtr->destination, (const char *) &c, 1);
	statePtr->writeProc(statePtr->destination,
		(const char *) statePtr->packetAccumulator,
		statePtr->accumulatedByteCount);
	statePtr->accumulatedByteCount = 0;
    }
}

/* The End */

/*
 * Local Variables:
 * mode: c
 * c-basic-offset: 4
 * fill-column: 78
 * End:
 */