/* rickdate.c - small RickDate utility */
/* PUBLIC DOMAIN - NO COPYRIGHT CLAIMED - Jon Mayo - December 25, 2006 */
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <unistd.h>

static const char base36[36] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

/* displays a usage message */
static void usage(const char *argv0) {
	printf(
	"encode  : %s [-345u] [year] [month] [day]\n"
	"encode  : %s -s [-4u] [secondsUTC]\n"
	"decode  : %s -d [-s] [-f formatstr] [rickdate]\n"
	"help    : %s -h\n"
	"  -3    three character RickDate (default)\n"
	"  -4    four character RickDate\n"
	"  -5    five character RickDate\n"
	"  -s    use seconds (UTC only)\n"
	"  -u    use UTC instead of local time\n"
	"  -h    help\n"
	"  -f    output using format string (see also: man strftime)\n"
	"\n\nFurther information at http://www.yak.net/kablooey/RickDate.html\n"
	, argv0, argv0, argv0, argv0);
}

/* encodes a struct tm as a rickdate. output to string in out must be long
 * enough to hold max+1 characters. if out is NULL then returns a pointer to a
 * static string, else returns out. */
static const char *encode_rickdate(char *out, struct tm *tm, int max) {
	static char buf[6];
	char *dest;
	dest=out?out:buf;
	if(max==0) max=5;
	if(max>=5) *(dest++)=base36[(tm->tm_year/36/36) % 36];
	if(max>=4) *(dest++)=base36[(tm->tm_year/36) % 36];
	if(max>=3) *(dest++)=base36[tm->tm_year % 36];
	if(max>=2) *(dest++)=base36[(tm->tm_mon+1) % 36];
	if(max>=1) *(dest++)=base36[(tm->tm_mday) % 36];
	*dest=0;
	return out?out:buf;
}

/* returns the current year in the current locale */
static unsigned current_year(void) {
	time_t t;
	t=time(&t);
	return localtime(&t)->tm_year+1900;
}

/* decodes a RickDate into a struct tm. handles 5, 4 and 3 character RickDates.
 * If tm is NULL then a static struct tm is used and returned on success, else
 * tm is returned. on failured returns NULL. */
static struct tm *decode_rickdate(struct tm *tm, const char *rickdate) {
	static struct tm tm_static;
	struct tm *dest;
	unsigned long val;
	char *endptr;
	char len;
	int curr_year, tmp;
	dest=tm?tm:&tm_static;
	memset(dest, 0, sizeof *dest);
	
	len=strlen(rickdate);
	val=strtoul(rickdate, &endptr, 36);
	if(!endptr || *endptr) {
		fprintf(stderr, "Parse error on '%s' @ %s:%d!\n", rickdate, __FILE__, __LINE__);
		return 0; /* parse error */
	}
	
	tm->tm_mday=val%36;
	val/=36;
	tm->tm_mon=(val%36)-1;
	val/=36;

	curr_year=current_year();
	if(len==5) {
		tmp=curr_year-curr_year%(36*36*36);
	} else if(len==4) {
		tmp=curr_year-curr_year%(36*36);
	} else if(len==3) {
		tmp=curr_year-curr_year%36;
	} else {
		fprintf(stderr, "Parse error on '%s' @ %s:%d!\n", rickdate, __FILE__, __LINE__);
		return 0; /* parse error */
	}
	tm->tm_year=(val+tmp)-1900;

	if(tm->tm_mday < 1 || tm->tm_mon > 31) {
		fprintf(stderr, "Parse error on '%s' @ %s:%d!\n", rickdate, __FILE__, __LINE__);
		return 0; /* parse error */
	}

	if(tm->tm_mon < 0 || tm->tm_mon > 11) {
		fprintf(stderr, "Parse error on '%s' @ %s:%d!\n", rickdate, __FILE__, __LINE__);
		return 0; /* parse error */
	}

	/* TODO: a sanity check for year would be nice */

	return dest;
}

int main(int argc, char **argv) {
	int fl_decode = 0, fl_seconds = 0, fl_width = 3, fl_useutc = 0;
	int ch;
	const char *fl_format = "%Y-%m-%d";

	while ((ch = getopt(argc, argv, "f:hsu345d")) != -1) {
		switch(ch) {
			case 'd': fl_decode = 1; break;
			case 's': fl_seconds = 1; break;
			case '3': fl_width=3; break;
			case '4': fl_width=4; break;
			case '5': fl_width=5; break;
			case 'u': fl_useutc=1; break;
			case 'f': fl_format=optarg; break;
			case 'h':
			default:
				usage(argv[0]);
				return EXIT_FAILURE;
		}
	}
	if(fl_decode) {
		int i;
		struct tm tm;
		if(optind==argc) {
			usage(argv[0]);
			return EXIT_FAILURE;
		}
		for(i=optind;i<argc;i++) {
			if(!decode_rickdate(&tm, argv[i])) {
				return EXIT_FAILURE;
			}
			if(fl_format) {
				char buf[128];
				strftime(buf, sizeof buf, fl_format, &tm);
				printf("%s\n", buf);
			} else {
				printf("%s", asctime(&tm));
			}
		}
	} else { /* encode */
		struct tm tm = {0};
		if(optind<argc) {
			if(fl_seconds) {
				time_t t;
				if(argc-optind != 1) {
					usage(argv[0]);
					return EXIT_FAILURE;
				}
				t=atoi(argv[optind]);
				if(fl_useutc) {
					tm=*gmtime(&t);
				} else {
					tm=*localtime(&t);
				}
				
			} else {
				if(argc-optind != 3) {
					usage(argv[0]);
					return EXIT_FAILURE;
				}

				/* TODO: handle parse errors */
				tm.tm_year=atoi(argv[optind]);
				tm.tm_mon=atoi(argv[optind+1])-1;
				tm.tm_mday=atoi(argv[optind+2]);
			}
		} else {
			time_t t;
			time(&t);
			if(fl_useutc) {
				tm=*gmtime(&t);
			} else {
				tm=*localtime(&t);
			}
		}
		printf("%s\n", encode_rickdate(0, &tm, fl_width));
	}
	return EXIT_SUCCESS;
}