emtime.c

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

#define usage() {printf("%s HH:MM\n", argv0); exit(1);}

const static char *argv0;

char *
itoa_base(u_int64_t n, int base)
{
    char *res, *tab;
    int i;

    if (n == 0)
        return strdup("0");
    tab = "0123456789abcdef";
    res = calloc(65, 1);
    i = 0;
    if (res)
    {
        while (n > 0)
        {
            res[i++] = tab[n % base];
            n /= base;
        }
        res[i] = 0;
        int len = strlen(res);
        for (int i = 0; i < len / 2; i++) {
            char tmp = res[i];
            res[i] = res[len-i-1];
            res[len-i-1] = tmp;
        }
    }
    return (res);
}

void
checkstr(const char *s)
{
    if (!isdigit(s[0])
    || !isdigit(s[1])
    || s[2] != ':'
    || !isdigit(s[3])
    || !isdigit(s[4])
    || s[5] != 0)
        usage();
}

int
maketime(const char *s)
{
    char num[3] = {0};
    int minutes, hours;

    memmove(num, s, 2);
    hours = atoi(num);
    memmove(num, s+3, 2);
    minutes = atoi(num);
    return minutes + (hours * 60);
}

int
putemtime(int time)
{
    static int em_min, em_hr;
    if (time >= 60) {
        em_hr++;
        return putemtime(time - 60);
    }
    if (time > 0) {
        em_min++;
        return putemtime(time - 1);
    }
    char *m = itoa_base(em_min, 6);
    char *h = itoa_base(em_hr, 6);
    printf("%s:%s\n", h, m);
    return 0;
}

int
main(int argc, char **argv)
{
    static int time;
    argv0 = argv[0]; // so we can have argv0 in usage

    if (argc != 2)
        usage();
    checkstr(argv[1]);
    time = maketime(argv[1]);
    time *= 1.5;
    putemtime(time);
}