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tkImgGIF.c

/*
 * tkImgGIF.c --
 *
 *    A photo image file handler for GIF files. Reads 87a and 89a GIF
 *    files. At present there is no write function.  GIF images may be
 *    read using the -data option of the photo image 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 (c) Reed Wade (wade@cs.utk.edu), University of Tennessee
 * Copyright (c) 1995-1997 Sun Microsystems, Inc.
 *
 * 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.           |
 * +-------------------------------------------------------------------+
 *
 * RCS: @(#) $Id: tkImgGIF.c,v 1.2 1998/09/14 18:23:12 stanton Exp $
 */

/*
 * GIF's are represented as data in 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 "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) */
} MFile;

#include "tkInt.h"
#include "tkPort.h"

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

static int      FileMatchGIF _ANSI_ARGS_((Tcl_Channel chan, char *fileName,
                char *formatString, int *widthPtr, int *heightPtr));
static int      FileReadGIF  _ANSI_ARGS_((Tcl_Interp *interp,
                Tcl_Channel chan, char *fileName, char *formatString,
                Tk_PhotoHandle imageHandle, int destX, int destY,
                int width, int height, int srcX, int srcY));
static int  StringMatchGIF _ANSI_ARGS_(( char *string,
                char *formatString, int *widthPtr, int *heightPtr));
static int  StringReadGIF _ANSI_ARGS_((Tcl_Interp *interp, char *string,
                char *formatString, Tk_PhotoHandle imageHandle,
                int destX, int destY, int width, int height,
                int srcX, int srcY));

Tk_PhotoImageFormat tkImgFmtGIF = {
      "GIF",                  /* name */
      FileMatchGIF,   /* fileMatchProc */
      StringMatchGIF, /* stringMatchProc */
      FileReadGIF,    /* fileReadProc */
      StringReadGIF,  /* stringReadProc */
      NULL,           /* fileWriteProc */
      NULL,           /* stringWriteProc */
};

#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))
#define ReadOK(file,buffer,len)     (Fread(buffer, len, 1, file) != 0)

/*
 *                 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
 */
 
static int fromData=0;

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

static int        DoExtension _ANSI_ARGS_((Tcl_Channel chan, int label,
                      int *transparent));
static int        GetCode _ANSI_ARGS_((Tcl_Channel chan, int code_size,
                      int flag));
static int        GetDataBlock _ANSI_ARGS_((Tcl_Channel chan,
                      unsigned char *buf));
static int        LWZReadByte _ANSI_ARGS_((Tcl_Channel chan, int flag,
                      int input_code_size));
static int        ReadColorMap _ANSI_ARGS_((Tcl_Channel chan, int number,
                      unsigned char buffer[MAXCOLORMAPSIZE][4]));
static int        ReadGIFHeader _ANSI_ARGS_((Tcl_Channel chan,
                      int *widthPtr, int *heightPtr));
static int        ReadImage _ANSI_ARGS_((Tcl_Interp *interp,
                      char *imagePtr, Tcl_Channel chan,
                      int len, int rows,
                      unsigned char cmap[MAXCOLORMAPSIZE][4],
                      int width, int height, int srcX, int srcY,
                      int interlace, int transparent));

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

static int        Fread _ANSI_ARGS_((unsigned char *dst, size_t size,
                      size_t count, Tcl_Channel chan));
static int        Mread _ANSI_ARGS_((unsigned char *dst, size_t size,
                      size_t count, MFile *handle));
static int        Mgetc _ANSI_ARGS_((MFile *handle));
static int        char64 _ANSI_ARGS_((int c));
static void       mInit _ANSI_ARGS_((unsigned char *string,
                      MFile *handle));

/*
 *----------------------------------------------------------------------
 *
 * FileMatchGIF --
 *
 *    This procedure 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(chan, fileName, formatString, widthPtr, heightPtr)
    Tcl_Channel chan;         /* The image file, open for reading. */
    char *fileName;           /* The name of the image file. */
    char *formatString;       /* User-specified format string, or NULL. */
    int *widthPtr, *heightPtr;      /* The dimensions of the image are
                         * returned here if the file is a valid
                         * raw GIF file. */
{
      return ReadGIFHeader(chan, widthPtr, heightPtr);
}

/*
 *----------------------------------------------------------------------
 *
 * FileReadGIF --
 *
 *    This procedure 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 interp->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(interp, chan, fileName, formatString, imageHandle, destX, destY,
      width, height, srcX, srcY)
    Tcl_Interp *interp;       /* Interpreter to use for reporting errors. */
    Tcl_Channel chan;         /* The image file, open for reading. */
    char *fileName;           /* The name of the image file. */
    char *formatString;       /* User-specified format string, or NULL. */
    Tk_PhotoHandle imageHandle;     /* The photo image to write into. */
    int destX, destY;         /* Coordinates of top-left pixel in
                         * photo image to be written to. */
    int width, height;        /* Dimensions of block of photo image to
                         * be written to. */
    int srcX, srcY;           /* Coordinates of top-left pixel to be used
                         * in image being read. */
{
    int fileWidth, fileHeight;
    int nBytes;
    Tk_PhotoImageBlock block;
    unsigned char buf[100];
    int bitPixel;
    unsigned char colorMap[MAXCOLORMAPSIZE][4];
    int transparent = -1;

    if (!ReadGIFHeader(chan, &fileWidth, &fileHeight)) {
      Tcl_AppendResult(interp, "couldn't read GIF header from file \"",
            fileName, "\"", NULL);
      return TCL_ERROR;
    }
    if ((fileWidth <= 0) || (fileHeight <= 0)) {
      Tcl_AppendResult(interp, "GIF image file \"", fileName,
            "\" has dimension(s) <= 0", (char *) NULL);
      return TCL_ERROR;
    }

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

    if (BitSet(buf[0], LOCALCOLORMAP)) {    /* Global Colormap */
      if (!ReadColorMap(chan, bitPixel, colorMap)) {
          Tcl_AppendResult(interp, "error reading color map",
                (char *) 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;
    }

    Tk_PhotoExpand(imageHandle, destX + width, destY + height);

    block.width = width;
    block.height = height;
    block.pixelSize = 4;
    block.pitch = block.pixelSize * block.width;
    block.offset[0] = 0;
    block.offset[1] = 1;
    block.offset[2] = 2;
    nBytes = height * block.pitch;
    block.pixelPtr = (unsigned char *) ckalloc((unsigned) nBytes);

    while (1) {
      if (Fread(buf, 1, 1, chan) != 1) {
          /*
           * Premature end of image.  We should really notify
           * the user, but for now just show garbage.
           */

          break;
      }

      if (buf[0] == ';') {
          /*
           * GIF terminator.
           */

          break;
      }

      if (buf[0] == '!') {
          /*
           * This is a GIF extension.
           */

          if (Fread(buf, 1, 1, chan) != 1) {
            interp->result =
                  "error reading extension function code in GIF image";
            goto error;
          }
          if (DoExtension(chan, buf[0], &transparent) < 0) {
            interp->result = "error reading extension in GIF image";
            goto error;
          }
          continue;
      }

      if (buf[0] != ',') {
          /*
           * Not a valid start character; ignore it.
           */
          continue;
      }

      if (Fread(buf, 1, 9, chan) != 9) {
          interp->result = "couldn't read left/top/width/height in GIF image";
          goto error;
      }

      bitPixel = 1<<((buf[8]&0x07)+1);

      if (BitSet(buf[8], LOCALCOLORMAP)) {
          if (!ReadColorMap(chan, bitPixel, colorMap)) {
                Tcl_AppendResult(interp, "error reading color map", 
                      (char *) NULL);
                goto error;
          }
      }
      if (ReadImage(interp, (char *) block.pixelPtr, chan, width,
            height, colorMap, fileWidth, fileHeight, srcX, srcY,
            BitSet(buf[8], INTERLACE), transparent) != TCL_OK) {
          goto error;
      }
      break;
   }

    if (transparent == -1) {
      Tk_PhotoPutBlock(imageHandle, &block, destX, destY, width, height);
    } else {
      int x, y, end;
      unsigned char *imagePtr, *rowPtr, *pixelPtr;

      imagePtr = rowPtr = block.pixelPtr;
      for (y = 0; y < height; y++) {
          x = 0;
          pixelPtr = rowPtr;
          while(x < width) {
            /* search for first non-transparent pixel */
            while ((x < width) && !(pixelPtr[CM_ALPHA])) {
                x++; pixelPtr += 4;
            }
            end = x;
            /* search for first transparent pixel */
            while ((end < width) && pixelPtr[CM_ALPHA]) {
                end++; pixelPtr += 4;
            }
            if (end > x) {
                block.pixelPtr = rowPtr + 4 * x;
                Tk_PhotoPutBlock(imageHandle, &block, destX+x,
                      destY+y, end-x, 1);
            }
            x = end;
          }
          rowPtr += block.pitch;
      }
      block.pixelPtr = imagePtr;
    }
    ckfree((char *) block.pixelPtr);
    return TCL_OK;

    error:
    ckfree((char *) block.pixelPtr);
    return TCL_ERROR;

}

/*
 *----------------------------------------------------------------------
 *
 * StringMatchGIF --
 *
 *  This procedure is invoked by the photo image type to see if
 *  a string contains image data in GIF format.
 *
 * Results:
 *  The return value is 1 if the first characters in the string
 *  like GIF data, and 0 otherwise.
 *
 * Side effects:
 *  the size of the image is placed in widthPre and heightPtr.
 *
 *----------------------------------------------------------------------
 */

static int
StringMatchGIF(string, formatString, widthPtr, heightPtr)
    char *string;       /* the string containing the image data */
    char *formatString;       /* the image format string */
    int *widthPtr;            /* where to put the string width */
    int *heightPtr;           /* where to put the string height */
{
    unsigned char header[10];
    int got;
    MFile handle;
    mInit((unsigned char *) string, &handle);
    got = Mread(header, 10, 1, &handle);
    if (got != 10
          || ((strncmp("GIF87a", (char *) header, 6) != 0)
          && (strncmp("GIF89a", (char *) header, 6) != 0))) {
      return 0;
    }
    *widthPtr = LM_to_uint(header[6],header[7]);
    *heightPtr = LM_to_uint(header[8],header[9]);
    return 1;
}

/*
 *----------------------------------------------------------------------
 *
 * StringReadGif -- --
 *
 *    This procedure is called by the photo image type to read
 *    GIF format data from a base64 encoded string, 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 interp->result.
 *
 * Side effects:
 *    new data is added to the image given by imageHandle.  This
 *    procedure calls FileReadGif by redefining the operation of
 *    fprintf temporarily.
 *
 *----------------------------------------------------------------------
 */

static int
StringReadGIF(interp,string,formatString,imageHandle,
      destX, destY, width, height, srcX, srcY)
    Tcl_Interp *interp;       /* interpreter for reporting errors in */
    char *string;       /* string containing the image */
    char *formatString;       /* format string if any */
    Tk_PhotoHandle imageHandle;     /* the image to write this data into */
    int destX, destY;         /* The rectangular region of the  */
    int  width, height;       /*   image to copy */
    int srcX, srcY;
{
      int result;
      MFile handle;
      mInit((unsigned char *)string,&handle);
      fromData = 1;
      result = FileReadGIF(interp, (Tcl_Channel) &handle, "inline data",
            formatString, imageHandle, destX, destY, width, height,
            srcX, srcY);
      fromData = 0;
      return(result);
}

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

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

    if (Fread(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(chan, number, buffer)
     Tcl_Channel chan;
     int number;
     unsigned char buffer[MAXCOLORMAPSIZE][4];
{
      int i;
      unsigned char rgb[3];

      for (i = 0; i < number; ++i) {
          if (! ReadOK(chan, rgb, sizeof(rgb))) {
            return 0;
          }
          
          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;
}



static int
DoExtension(chan, label, transparent)
     Tcl_Channel chan;
     int label;
     int *transparent;
{
    static unsigned char buf[256];
    int count;

    switch (label) {
      case 0x01:      /* Plain Text Extension */
          break;
          
      case 0xff:      /* Application Extension */
          break;

      case 0xfe:      /* Comment Extension */
          do {
            count = GetDataBlock(chan, (unsigned char*) buf);
          } while (count > 0);
          return count;

      case 0xf9:      /* Graphic Control Extension */
          count = GetDataBlock(chan, (unsigned char*) buf);
          if (count < 0) {
            return 1;
          }
          if ((buf[0] & 0x1) != 0) {
            *transparent = buf[3];
          }

          do {
            count = GetDataBlock(chan, (unsigned char*) buf);
          } while (count > 0);
          return count;
    }

    do {
      count = GetDataBlock(chan, (unsigned char*) buf);
    } while (count > 0);
    return count;
}

static int ZeroDataBlock = 0;

static int
GetDataBlock(chan, buf)
     Tcl_Channel chan;
     unsigned char *buf;
{
    unsigned char count;

    if (! ReadOK(chan, &count,1)) {
      return -1;
    }

    ZeroDataBlock = count == 0;

    if ((count != 0) && (! ReadOK(chan, buf, count))) {
      return -1;
    }

    return count;
}


static int
ReadImage(interp, imagePtr, chan, len, rows, cmap,
      width, height, srcX, srcY, interlace, transparent)
     Tcl_Interp *interp;
     char *imagePtr;
     Tcl_Channel chan;
     int len, rows;
     unsigned char cmap[MAXCOLORMAPSIZE][4];
     int width, height;
     int srcX, srcY;
     int interlace;
     int transparent;
{
    unsigned char c;
    int v;
    int xpos = 0, ypos = 0, pass = 0;
    char *pixelPtr;


    /*
     *  Initialize the Compression routines
     */
    if (! ReadOK(chan, &c, 1))  {
      Tcl_AppendResult(interp, "error reading GIF image: ",
            Tcl_PosixError(interp), (char *) NULL);
      return TCL_ERROR;
    }

    if (LWZReadByte(chan, 1, c) < 0) {
      interp->result = "format error in GIF image";
      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;
    while ((v = LWZReadByte(chan, 0, c)) >= 0 ) {

      if ((xpos>=srcX) && (xpos<srcX+len) &&
            (ypos>=srcY) && (ypos<srcY+rows)) {
          *pixelPtr++ = cmap[v][CM_RED];
          *pixelPtr++ = cmap[v][CM_GREEN];
          *pixelPtr++ = cmap[v][CM_BLUE];
          *pixelPtr++ = cmap[v][CM_ALPHA];
      }

      ++xpos;
      if (xpos == width) {
          xpos = 0;
          if (interlace) {
            switch (pass) {
                case 0:
                case 1:
                  ypos += 8; break;
                case 2:
                  ypos += 4; break;
                case 3:
                  ypos += 2; break;
            }
            
            while (ypos >= height) {
                ++pass;
                switch (pass) {
                  case 1:
                      ypos = 4; break;
                  case 2:
                      ypos = 2; break;
                  case 3:
                      ypos = 1; break;
                  default:
                      return TCL_OK;
                }
            }
          } else {
            ++ypos;
          }
          pixelPtr = imagePtr + (ypos-srcY) * len * 4;
      }
      if (ypos >= height)
          break;
    }
    return TCL_OK;
}

static int
LWZReadByte(chan, flag, input_code_size)
     Tcl_Channel chan;
     int flag;
     int input_code_size;
{
    static int  fresh = 0;
    int code, incode;
    static int code_size, set_code_size;
    static int max_code, max_code_size;
    static int firstcode, oldcode;
    static int clear_code, end_code;
    static int table[2][(1<< MAX_LWZ_BITS)];
    static int stack[(1<<(MAX_LWZ_BITS))*2], *sp;
    register int    i;

    if (flag) {
      set_code_size = input_code_size;
      code_size = set_code_size+1;
      clear_code = 1 << set_code_size ;
      end_code = clear_code + 1;
      max_code_size = 2*clear_code;
      max_code = clear_code+2;

      GetCode(chan, 0, 1);

      fresh = 1;

      for (i = 0; i < clear_code; ++i) {
          table[0][i] = 0;
          table[1][i] = i;
      }
      for (; i < (1<<MAX_LWZ_BITS); ++i) {
          table[0][i] = table[1][0] = 0;
      }

      sp = stack;

      return 0;
    } else if (fresh) {
      fresh = 0;
      do {
          firstcode = oldcode = GetCode(chan, code_size, 0);
      } while (firstcode == clear_code);
      return firstcode;
    }

    if (sp > stack) {
      return *--sp;
    }

    while ((code = GetCode(chan, code_size, 0)) >= 0) {
      if (code == clear_code) {
          for (i = 0; i < clear_code; ++i) {
            table[0][i] = 0;
            table[1][i] = i;
          }
          
          for (; i < (1<<MAX_LWZ_BITS); ++i) {
            table[0][i] = table[1][i] = 0;
          }

          code_size = set_code_size+1;
          max_code_size = 2*clear_code;
          max_code = clear_code+2;
          sp = stack;
          firstcode = oldcode = GetCode(chan, code_size, 0);
          return firstcode;

      } else if (code == end_code) {
          int count;
          unsigned char buf[260];

          if (ZeroDataBlock) {
            return -2;
          }
          
          while ((count = GetDataBlock(chan, buf)) > 0)
            /* Empty body */;

          if (count != 0) {
            return -2;
          }
      }

      incode = code;

      if (code >= max_code) {
          *sp++ = firstcode;
          code = oldcode;
      }

      while (code >= clear_code) {
          *sp++ = table[1][code];
          if (code == table[0][code]) {
            return -2;

            /*
             * Used to be this instead, Steve Ball suggested
             * the change to just return.
             printf("circular table entry BIG ERROR\n");
             */
          }
          code = table[0][code];
      }

      *sp++ = firstcode = table[1][code];

      if ((code = max_code) <(1<<MAX_LWZ_BITS)) {
          table[0][code] = oldcode;
          table[1][code] = firstcode;
          ++max_code;
          if ((max_code>=max_code_size) && (max_code_size < (1<<MAX_LWZ_BITS))) {
            max_code_size *= 2;
            ++code_size;
          }
      }

      oldcode = incode;

      if (sp > stack)
          return *--sp;
      }
      return code;
}


static int
GetCode(chan, code_size, flag)
     Tcl_Channel chan;
     int code_size;
     int flag;
{
    static unsigned char buf[280];
    static int curbit, lastbit, done, last_byte;
    int i, j, ret;
    unsigned char count;

    if (flag) {
      curbit = 0;
      lastbit = 0;
      done = 0;
      return 0;
    }


    if ( (curbit+code_size) >= lastbit) {
      if (done) {
          /* ran off the end of my bits */
          return -1;
      }
      if (last_byte >= 2) {
          buf[0] = buf[last_byte-2];
      }
      if (last_byte >= 1) {
          buf[1] = buf[last_byte-1];
      }

      if ((count = GetDataBlock(chan, &buf[2])) == 0) {
          done = 1;
      }

      last_byte = 2 + count;
      curbit = (curbit - lastbit) + 16;
      lastbit = (2+count)*8 ;
    }

    ret = 0;
    for (i = curbit, j = 0; j < code_size; ++i, ++j) {
      ret |= ((buf[ i / 8 ] & (1 << (i % 8))) != 0) << j;
    }

    curbit += code_size;

    return ret;
}

/*
 *----------------------------------------------------------------------
 *
 * Minit -- --
 *
 *  This procedure initializes a base64 decoder handle
 *
 * Results:
 *  none
 *
 * Side effects:
 *  the base64 handle is initialized
 *
 *----------------------------------------------------------------------
 */

static void
mInit(string, handle)
   unsigned char *string;     /* string containing initial mmencoded data */
   MFile *handle;       /* mmdecode "file" handle */
{
   handle->data = string;
   handle->state = 0;
}

/*
 *----------------------------------------------------------------------
 *
 * Mread --
 *
 *    This procedure 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 int
Mread(dst, chunkSize, numChunks, handle)  
   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 */
{
   register int i, c;
   int count = chunkSize * numChunks;

   for(i=0; i<count && (c=Mgetc(handle)) != GIF_DONE; i++) {
      *dst++ = c;
   }
   return i;
}

/*
 * get the next decoded character from an mmencode handle
 * This causes at least 1 character to be "read" from the encoded string
 */

/*
 *----------------------------------------------------------------------
 *
 * Mgetc --
 *
 *  This procedure decodes and returns the next byte from a base64
 *  encoded string.
 *
 * Results:
 *  The next byte (or GIF_DONE) is returned.
 *
 * Side effects:
 *  The base64 handle will change state.
 *
 *----------------------------------------------------------------------
 */

static int
Mgetc(handle)
   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 {
      c = char64(*handle->data);
      handle->data++;
    } while (c==GIF_SPACE);

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

    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 procedure converts a base64 ascii character into its binary
 *    equivalent.  This code is a slightly modified version of the
 *    char64 proc in N. Borenstein's metamail decoder.
 *
 * Results:
 *    The binary value, or an error code.
 *
 * Side effects:
 *    None.
 *----------------------------------------------------------------------
 */

static int
char64(c)
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 procedure calls either fread or Mread to read data
 *  from a file or a base64 encoded string.
 *
 * Results: - same as fread
 *
 *----------------------------------------------------------------------
 */

static int
Fread(dst, hunk, count, chan)
    unsigned char *dst;       /* where to put the result */
    size_t hunk,count;        /* how many */
    Tcl_Channel chan;
{
    if (fromData) {
      return(Mread(dst, hunk, count, (MFile *) chan));
    } else {
      return Tcl_Read(chan, (char *) dst, (int) (hunk * count));
    }
}

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