time.c

#include <time.h>

/* days per month -- nonleap! */
const short __spm[13] =
  { 0,
    (31),
    (31+28),
    (31+28+31),
    (31+28+31+30),
    (31+28+31+30+31),
    (31+28+31+30+31+30),
    (31+28+31+30+31+30+31),
    (31+28+31+30+31+30+31+31),
    (31+28+31+30+31+30+31+31+30),
    (31+28+31+30+31+30+31+31+30+31),
    (31+28+31+30+31+30+31+31+30+31+30),
    (31+28+31+30+31+30+31+31+30+31+30+31),
  };
static long int timezone;
static const char days[] = "Sun Mon Tue Wed Thu Fri Sat ";
static const char months[] = "Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ";

/* seconds per day */
#define SPD 24*60*60

int __isleap(int year) {
  /* every fourth year is a leap year except for century years that are
   * not divisible by 400. */
/*  return (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)); */
  return (!(year%4) && ((year%100) || !(year%400)));
}

struct tm *gmtime_r(const time_t *timep, struct tm *r) {
  time_t i;
  register time_t work=*timep%(SPD);
  r->tm_sec=work%60; work/=60;
  r->tm_min=work%60; r->tm_hour=work/60;
  work=*timep/(SPD);
  r->tm_wday=(4+work)%7;
  for (i=1970; ; ++i) {
    register time_t k=__isleap(i)?366:365;
    if (work>=k)
      work-=k;
    else
      break;
  }
  r->tm_year=i-1900;
  r->tm_yday=work;

  r->tm_mday=1;
  if (__isleap(i) && (work>58)) {
    if (work==59) r->tm_mday=2; /* 29.2. */
    work-=1;
  }

  for (i=11; i && (__spm[i]>work); --i) ;
  r->tm_mon=i;
  r->tm_mday+=work-__spm[i];
  return r;
}

struct tm* localtime_r(const time_t* t, struct tm* r) {
  time_t tmp;
  struct timezone tz;
  gettimeofday(0, &tz);
  timezone=tz.tz_minuteswest*60L;
  tmp=*t+timezone;
  return gmtime_r(&tmp,r);
}

struct tm* localtime(const time_t* t) {
  static struct tm tmp;
  return localtime_r(t,&tmp);
}

time_t timegm(struct tm *const t) {
  register time_t  day;
  register time_t  i;
  register time_t years = t->tm_year - 70;

  if (t->tm_sec>60) { t->tm_min += t->tm_sec/60; t->tm_sec%=60; }
  if (t->tm_min>60) { t->tm_hour += t->tm_min/60; t->tm_min%=60; }
  if (t->tm_hour>24) { t->tm_mday += t->tm_hour/24; t->tm_hour%=24; }
  if (t->tm_mon>12) { t->tm_year += t->tm_mon/12; t->tm_mon%=12; }
  while (t->tm_mday>__spm[1+t->tm_mon]) {
    if (t->tm_mon==1 && __isleap(t->tm_year+1900)) {
      --t->tm_mday;
    }
    t->tm_mday-=__spm[t->tm_mon];
    ++t->tm_mon;
    if (t->tm_mon>11) { t->tm_mon=0; ++t->tm_year; }
  }

  if (t->tm_year < 70)
    return (time_t) -1;

  /* Days since 1970 is 365 * number of years + number of leap years since 1970 */
  day  = years * 365 + (years + 1) / 4;

  /* After 2100 we have to substract 3 leap years for every 400 years
     This is not intuitive. Most mktime implementations do not support
     dates after 2059, anyway, so we might leave this out for it's
     bloat. */
  if ((years -= 131) >= 0) {
    years /= 100;
    day -= (years >> 2) * 3 + 1;
    if ((years &= 3) == 3) years--;
    day -= years;
  }

  day += t->tm_yday = __spm [t->tm_mon] + t->tm_mday-1 + ( __isleap (t->tm_year+1900)  &  (t->tm_mon > 1) );

  /* day is now the number of days since 'Jan 1 1970' */
  i = 7;
  t->tm_wday = (day + 4) % i;                        /* Sunday=0, Monday=1, ..., Saturday=6 */

  i = 24;
  day *= i;
  i = 60;
  return ((day + t->tm_hour) * i + t->tm_min) * i + t->tm_sec;
}

time_t mktime(register struct tm* const t) {
  time_t x=timegm(t);
  struct timezone tz;
  gettimeofday(0, &tz);
  timezone=tz.tz_minuteswest*60L;
  x+=timezone;
  return x;
}

static void num2str(char *c,int i) {
  c[0]=i/10+'0';
  c[1]=i%10+'0';
}

char *asctime_r(const struct tm *t, char *buf) {
  /* "Wed Jun 30 21:49:08 1993\n" */
  *(int*)buf=*(int*)(days+(t->tm_wday<<2));
  *(int*)(buf+4)=*(int*)(months+(t->tm_mon<<2));
  num2str(buf+8,t->tm_mday);
  if (buf[8]=='0') buf[8]=' ';
  buf[10]=' ';
  num2str(buf+11,t->tm_hour);
  buf[13]=':';
  num2str(buf+14,t->tm_min);
  buf[16]=':';
  num2str(buf+17,t->tm_sec);
  buf[19]=' ';
  num2str(buf+20,(t->tm_year+1900)/100);
  num2str(buf+22,(t->tm_year+1900)%100);
  buf[24]='\n';
  return buf;
}

char *asctime(const struct tm *timeptr) {
  static char buf[25];
  return asctime_r(timeptr,buf);
}

char *ctime(const time_t *timep) {
  return asctime(localtime(timep));
}

int gettimeofday (struct timeval *tp, void *ignore)
{
     time_t t;

     t = time(NULL);
     tp->tv_sec  = t;
     tp->tv_usec = 0;
     return 0;
}