/* png-fix-itxt version 1.0.0

 *

 * Copyright 2015 Glenn Randers-Pehrson

 * Last changed in libpng 1.6.18 [July 23, 2015]

 *

 * This code is released under the libpng license.

 * For conditions of distribution and use, see the disclaimer

 * and license in png.h

 *

 * Usage:

 *

 *     png-fix-itxt.exe < bad.png > good.png

 *

 * Fixes a PNG file written with libpng-1.6.0 or 1.6.1 that has one or more

 * uncompressed iTXt chunks.  Assumes that the actual length is greater

 * than or equal to the value in the length byte, and that the CRC is

 * correct for the actual length.  This program hunts for the CRC and

 * adjusts the length byte accordingly.  It is not an error to process a

 * PNG file that has no iTXt chunks or one that has valid iTXt chunks;

 * such files will simply be copied.

 *

 * Requires zlib (for crc32 and Z_NULL); build with

 *

 *     gcc -O -o png-fix-itxt png-fix-itxt.c -lz

 *

 * If you need to handle iTXt chunks larger than 500000 kbytes you must

 * rebuild png-fix-itxt with a larger values of MAX_LENGTH (or a smaller value

 * if you know you will never encounter such huge iTXt chunks).

 */

#include <stdio.h></stdio.h>

#include <zlib.h></zlib.h>

#define MAX_LENGTH 500000

/* Read one character (inchar), also return octet (c), break if EOF */

#define GETBREAK inchar=getchar(); \

                 c=(inchar & 0xffU);\

                 if (inchar != c) break

int

main(void)

{

   unsigned int i;

   unsigned char buf[MAX_LENGTH];

   unsigned long crc;

   unsigned char c;

   int inchar;

/* Skip 8-byte signature */

   for (i=8; i; i--)

   {

      GETBREAK;

      putchar(c);

   }

if (inchar == c) /* !EOF */

for (;;)

 {

   /* Read the length */

   unsigned long length; /* must be 32 bits! */

   GETBREAK; buf[0] = c; length  = c; length <<= 8;

   GETBREAK; buf[1] = c; length += c; length <<= 8;

   GETBREAK; buf[2] = c; length += c; length <<= 8;

   GETBREAK; buf[3] = c; length += c;

   /* Read the chunkname */

   GETBREAK; buf[4] = c;

   GETBREAK; buf[5] = c;

   GETBREAK; buf[6] = c;

   GETBREAK; buf[7] = c;

   /* The iTXt chunk type expressed as integers is (105, 84, 88, 116) */

   if (buf[4] == 105 && buf[5] == 84 && buf[6] == 88 && buf[7] == 116)

   {

      if (length >= MAX_LENGTH-12)

         break;  /* To do: handle this more gracefully */

      /* Initialize the CRC */

      crc = crc32(0, Z_NULL, 0);

      /* Copy the data bytes */

      for (i=8; i < length + 12; i++)

      {

         GETBREAK; buf[i] = c;

      }

      if (inchar != c) /* EOF */

         break;

      /* Calculate the CRC */

      crc = crc32(crc, buf+4, (uInt)length+4);

      for (;;)

      {

        /* Check the CRC */

        if (((crc >> 24) & 0xffU) == buf[length+8] &&

            ((crc >> 16) & 0xffU) == buf[length+9] &&

            ((crc >>  8) & 0xffU) == buf[length+10] &&

            ((crc      ) & 0xffU) == buf[length+11])

           break;

        length++;

        if (length >= MAX_LENGTH-12)

           break;

        GETBREAK;

        buf[length+11] = c;

        /* Update the CRC */

        crc = crc32(crc, buf+7+length, 1);

      }

      if (inchar != c) /* EOF */

         break;

      /* Update length bytes */

      buf[0] = (unsigned char)((length >> 24) & 0xffU);

      buf[1] = (unsigned char)((length >> 16) & 0xffU);

      buf[2] = (unsigned char)((length >>  8) & 0xffU);

      buf[3] = (unsigned char)((length      ) & 0xffU);

      /* Write the fixed iTXt chunk (length, name, data, crc) */

      for (i=0; i

         putchar(buf[i]);

   }

   else

   {

      if (inchar != c) /* EOF */

         break;

      /* Copy bytes that were already read (length and chunk name) */

      for (i=0; i<8; i++)

         putchar(buf[i]);

      /* Copy data bytes and CRC */

      for (i=8; i< length+12; i++)

      {

         GETBREAK;

         putchar(c);

      }

      if (inchar != c) /* EOF */

      {

         break;

      }

   /* The IEND chunk type expressed as integers is (73, 69, 78, 68) */

      if (buf[4] == 73 && buf[5] == 69 && buf[6] == 78 && buf[7] == 68)

         break;

   }

   if (inchar != c) /* EOF */

      break;

   if (buf[4] == 73 && buf[5] == 69 && buf[6] == 78 && buf[7] == 68)

     break;

 }

 return 0;

}