/*
 * showdate.c -- show date and time as per the command line options specified
 *
 * Written by Sandeep Sahore <ssahore@yahoo.com>
 *
 * The showdate program computes and prints the date in the past, present, or future
 * offset from the current date according to the command line options specified. The
 * default output format is to display the date and time according to the "%c" format
 * specification of the UNIX date(1) command however this can modified by giving the
 * "-f" switch along with a format string. If invoked without any command line options
 * the program abends and displays usage.
 *
 * $Id: showdate.c,v 1.6 2002/06/18 23:04:01 ssahore Exp $
 */

#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
#include <time.h>

void ckarg(char *, char);                      /* check arguments given to the command-line options */
char *progname(char *);                        /* get the basename of the program */
char *eofarg(char *);                          /* find the end of argument string */
void ckfarg(char *);                           /* check the validity of options in the format string */
void usage(char *);                            /* print usage upon error */
void ckopts(int);                              /* abend if both "-e" and "-f" options are given */

char *prog, *cp;                               /* store the command name in the variable prog */
time_t limit;                                  /* store the value when timekeeping overflows */

int main(int argc, char *argv[])
{
    int c, flag, epoch;
    struct tm *ptm;
    char *optarg, *farg, buf[BUFSIZ];
    time_t limit, esec, stp, *tp = &stp;

    epoch = flag = 0;
    prog = *argv;
    esec = time(tp);
    ptm = localtime(tp);

    prog = progname(prog);

    if (argc == 1)                             /* print usage and abend if invoked without arguments */
        usage(prog);

    limit = (~limit < 0) ? ~(~limit << (sizeof(time_t) * 8) - 1) : ~limit;

    while (--argc > 0 && (*++argv)[0] == '-') {

        /* abend if a standalone dash is encountered */
        if (!*(argv[0] + 1))
            usage(prog);

        while (c = *++argv[0])
            switch (c)
            {
                /* process the date and time options alongwith their arguments */
                case 'y': case 'm': case 'd': case 'h': case 'M': case 's':
                    if (*++argv[0])
                        optarg = argv[0];
                    else
                        optarg = *++argv, argc--;
                    ckarg(optarg, c);

                    if (c == 'y')
                        ptm->tm_year += atoi(optarg);
                    else if (c == 'm')
                        ptm->tm_mon += atoi(optarg);
                    else if (c == 'd')
                        ptm->tm_mday += atoi(optarg);
                    else if (c == 'h')
                        ptm->tm_hour += atoi(optarg);
                    else if (c == 'M')
                        ptm->tm_min += atoi(optarg);
                    else if (c == 's')
                        ptm->tm_sec += atoi(optarg);

                    argv[0] = eofarg(optarg);
                    break;

                /* display the seconds elapsed since the UNIX epoch */
                case 'e':
                    ckopts(flag);
                    epoch++;
                    break;

                /* formatting directives to tweak the default output */
                case 'f':
                    ckopts(epoch);
                    if (*++argv[0])
                        farg = argv[0];
                    else {
                        if (farg = *++argv)
                            argc--;
                        else
                            fprintf(stderr, "%s: argument required for  option -- %c\n",
                                prog, c), usage(prog);
                    }
                    ckfarg(farg);
                    argv[0] = eofarg(farg);
                    flag++;
                    break;

                default:
                    fprintf(stderr, "%s: illegal option -- %c\n", prog, c);
                    usage(prog);
            }
    }

    /* abend if arguments are leftover */
    if (argc)
        usage(prog);
/*
 * after checking system limits print desired date and time
 */
    if (mktime(ptm) < 0)
        fprintf(stderr, "%s: required time exceeds system limit\n", prog);
    else {
        if (flag)
            strftime(buf, sizeof buf, farg, ptm);
        else if (epoch)
            fprintf(stderr,"seconds since epoch %lu", mktime(ptm));
        else
            strftime(buf, sizeof buf, "%c", ptm);
        printf("%s\n", buf);
    }
    return 0;
}
/*
 * check arguments supplied to each of the command line options
 */
void ckarg(char *optarg, char c)
{
    char *temp = optarg;

    if (*optarg == '-' || *optarg == '+' || isdigit(*optarg))
            ++optarg;
    else if (!*optarg)
        fprintf(stderr, "%s: argument required for option -%c\n",
            prog, c), usage(prog);
    else
        fprintf(stderr, "%s: error in processing argument %s\n",
            prog, temp), usage(prog);

    while (*optarg) {
        if (isdigit(*optarg))
            ++optarg; 
        else
            fprintf(stderr, "%s: error in processing argument %s\n",
                prog, temp), usage(prog);
    }
}
/*
 * check arguments given to the "-f" command line option
 */
void ckfarg(char *farg)
{
    int c;

    while (c = *farg++)
        if (c == '%')
            if (c = *farg++)
                switch (c)
                {
                    case 'a': case 'A': case 'b': case 'B': case 'c': case 'C': case 'd': case 'e':
                    case 'E': case 'H': case 'I': case 'j': case 'm': case 'M': case 'n': case 'N':
                    case 'o': case 'p': case 'R': case 'S': case 't': case 'u': case 'U': case 'V':
                    case 'w': case 'W': case 'x': case 'X': case 'y': case 'Y': case 'Z': case '%':
                        break;
                    default:
                        fprintf(stderr, "%s: bad format character - %c\n", prog, c);
                        exit(2);
                }
}
/*
 * end abnormally if both "-e" and "-f" options are given
 */
void ckopts(int flag)
{
    if (flag)
        fprintf(stderr, "%s: cannot specify both -e and -f options\n",
            prog), usage(prog);
}
/*
 * find the end of the argument string
 */
char *eofarg(char *optarg)
{
    while(*optarg)
        ++optarg;
    return --optarg;
}
/*
 * get the basename of the program
 */
char *progname(char *cp)
{
    while (*cp)
        ++cp;
    --cp;

    while (*cp) {
        if (*cp == '/')
            return ++cp; 
        --cp;
    } 
    return ++cp;
}
/*
 * print usage and abend if an error is encountered during processing
 */
void usage(char *prog)
{
    fprintf(stderr, "usage: %s\n\t[-y [+|-]years]\n\t[-m [+|-]months]\n\t[-d [+|-]days]\n\t"
        "[-h [+|-]hours]\n\t[-M [+|-]minutes]\n\t[-s [+|-]seconds]\n\t[-e | -f format]\n", prog);
    exit(1);
}