#include <stdio.h>
#include <math.h>

typedef struct
{
  unsigned year;
  unsigned is_leap;
  unsigned jday;
  unsigned month;
  unsigned mday;
  unsigned wday;
  unsigned hour;
  unsigned minute;
  unsigned second;
  unsigned nanosecond;
} UTC;

typedef struct
{
  unsigned hour;
  unsigned minute;
  unsigned second;
  unsigned nanosecond;
} ST;

static int
is_leap (unsigned year)
{
  if (year % 4)
    return 0;

  if (year % 100)
    return 1;

  if (year % 400)
    return 0;

  return 1;
}


double
time_utc_to_tjd (UTC u)
{
  unsigned y400;
  unsigned y100;
  unsigned y4;

  double ret, fd;
  unsigned jd;

  static int const mdays[] =
    { 0, 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };
  static int const lmdays[] =
    { 0, 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335 };

  u.is_leap = is_leap (u.year);

  if (u.year < 100)
    u.year += 2000;


  if (!u.jday)
    {
      if (u.is_leap)
        u.jday = u.mday + lmdays[u.month];
      else
        u.jday = u.mday + mdays[u.month];
    }

  u.year -= 1601;
  y400 = u.year / 400;
  u.year -= y400 * 400;
  y100 = u.year / 100;
  u.year -= y100 * 100;
  y4 = u.year / 4;
  u.year -= y4 * 4;

  jd = u.jday - 1;
  jd += u.year * 365;
  jd += y4 * 1461;
  jd += y100 * 36524;
  jd += y400 * 146097;


  jd += 2305813;
  jd -= 2451545;



  ret = (double) u.nanosecond;
  ret /= 1000000000.;
  ret = (double) u.second;
  ret /= 60.;
  ret += (double) u.minute;
  ret /= 60.;
  ret += (double) u.hour;
  ret /= 24.;

  ret += .5;

  ret += (double) jd;

  return ret;
}

double
time_utc_to_ra (UTC u)
{
  double tjd = time_utc_to_tjd (u);
  double T = tjd / 36525.;
  double theta0 =
    280.46061837 + (360.98564736629 * tjd) + (0.000387933 * T * T) -
    (T * T * T / 38710000.0);

  return remainder (theta0, 360.);
}


ST
time_ra_to_st (double ra)
{
  ST ret;
  unsigned i;

  while (ra < 0.)
    ra += 360.;
  if (ra >= 360.0)
    ra = remainder (ra, 360.);

  ra *= 240.0;

  i = (int) (floor (ra) + .5);
  ra -= (double) i;
  ra *= 1000000000.;

  ret.nanosecond = (unsigned) (ra + .5);
  ret.second = i % 60;
  i /= 60;
  ret.minute = i % 60;
  i /= 60;
  ret.hour = i;

  return ret;
}




int
main (int argc, char *argv[])
{

  UTC u = { 0 };
  double local_lon = 0.0;
  ST st;
  double ra;

  u.year = 2021;
  u.month = 2;
  u.mday = 12;
  u.hour = 13;
  u.minute =28;


  ra = time_utc_to_ra (u);
  st = time_ra_to_st (ra + local_lon);

  printf ("%d:%02d:%02d\n", st.hour, st.minute, st.second);
  return 0;
}