/*----------------------------------------------------------------------------
ANSI C program to set display time of each frame in an animated GIF.
Chris Giese	<ChrisGiese@SisAndHappy.com>
http://SisAndHappy.com/ChrisGiese/

I, the copyright holder of this work, hereby release it into the
public domain. This applies worldwide. If this is not legally possible:
I grant any entity the right to use this work for any purpose,
without any conditions, unless such conditions are required by law.

15 June 2019: initial version.
----------------------------------------------------------------------------*/
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#if 0
#include <stdint.h>
#else
typedef unsigned char	uint8_t;
typedef unsigned short	uint16_t;
typedef unsigned long	uint32_t;
#endif
/****************************************************************************/
static void write_le16(void *buf0, unsigned val)
{
	uint8_t *buf = buf0;

	buf[0] = val;
	val >>= 8;
	buf[1] = val;
}
/****************************************************************************/
int main(int arg_c, char *arg_v[])
{
	uint8_t buf[13];
	uint32_t ms;
	unsigned j;
	char *s;
	FILE *f;
	int i;

	if(arg_c != 3)
	{
		printf(	"Sets display time of each frame in an animated .GIF file\n"
			"Usage: giftime file.gif milliseconds\n");
		return 1;
	}
/* Number of milliseconds to display each frame in the animated .GIF

The time value is stored in the .GIF file as a 16-bit unsigned value in
units of 10 milliseconds, so each frame can be displayed for no longer than
65535 * 10 = 655350 milliseconds = 655 seconds = ~11 minutes. */
	ms = strtoul(arg_v[2], &s, 10);
	if(s == arg_v[2] || ms == 0 || ms > 655350L)
	{
		printf("Error: invalid display time %lu ms\n", ms);
		return 2;
	}
/* open .GIF file, validate */
	if((f = fopen(arg_v[1], "r+b")) == NULL)
	{
		printf("Error: can't open file '%s'\n", arg_v[1]);
		return 3;
	}
	if(fread(buf, 1, 13, f) != 13 || memcmp(buf, "GIF8", 4))
	{
		printf("Error: file '%s' is not a .GIF file\n", arg_v[1]);
		fclose(f);
		return 4;
	}
/* skip global palette, if any */
	if(buf[10] & 0x80)
	{
		j = buf[10] & 7;
		j = 2 << j;
		fseek(f, j * 3, SEEK_CUR);
	}
	while((i = fgetc(f)) != EOF)
	{
printf("%9lu %c\t", ftell(f) - 1, i);
/* end of .GIF file */
		if(i == ';')
			break;
/* image descriptor */
		else if(i == ',')
		{
			if(fread(buf, 1, 9, f) != 9)
READ_ERR:		{
				printf("Error reading .GIF file '%s'\n",
					arg_v[1]);
				fclose(f);
				return 5;
			}
/* skip local palette, if any */
			if(buf[8] & 0x80)
			{
				j = buf[8] & 7;
				j = 2 << j;
				fseek(f, j * 3, SEEK_CUR);
			}
/* skip initial LZW code size */
			if(fgetc(f) == EOF)
				goto READ_ERR;
/* skip blocks of LZW-compressed data (1-255 bytes each) */
			while(1)
			{
				if((i = fgetc(f)) == EOF)
					goto READ_ERR;
				if(i == 0)
					break;
				fseek(f, i, SEEK_CUR);
			}
		}
/* extension blocks */
		else if(i == '!')
		{
/* type of the block */
			if((i = fgetc(f)) == EOF)
				goto READ_ERR;
/* skip blocks other than graphic control extension (type 249) */
			if(i != 249)
			{
				while(1)
				{
					if((i = fgetc(f)) == EOF)
						goto READ_ERR;
					if(i == 0)
						break;
					fseek(f, i, SEEK_CUR);
				}
			}
			else
			{
				uint32_t save;

				if(fread(buf, 1, 6, f) != 6)
					goto READ_ERR;
				if(buf[0] != 4 || buf[5] != 0)
					goto FMT_ERR;
/* display time in the GCE block is in units of 10 milliseconds */
				write_le16(&buf[2], (unsigned)(ms / 10));
				save = ftell(f);
				fseek(f, -6, SEEK_CUR);
printf("Changing display time to %lu ms", ms);
				if(fwrite(buf, 1, 6, f) != 6)
				{
					printf("Error writing .GIF file "
						"'%s'\n", arg_v[1]);
					fclose(f);
					return 6;
				}
				fseek(f, save, SEEK_SET);
			}
		}
		else
FMT_ERR:	{
			printf("Error in .GIF file '%s'\n", arg_v[1]);
			fclose(f);
			return 7;
		}
printf("\n");
	}
printf("Success\n");
	fclose(f);
	return 0;
}