caldate_mjd.c

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#include "caldate.h"
 
static unsigned long times365[4] = { 0, 365, 730, 1095 } ;
static unsigned long times36524[4] = { 0, 36524UL, 73048UL, 109572UL } ;
static unsigned long montab[12] =
{ 0, 31, 61, 92, 122, 153, 184, 214, 245, 275, 306, 337 } ;
/* month length after february is (306 * m + 5) / 10 */
 
long caldate_mjd(struct caldate *cd)
{
  long y;
  long m;
  long d;
 
  d = cd->day - 678882L;
  m = cd->month - 1;
  y = cd->year;
 
  d += 146097L * (y / 400);
  y %= 400;
 
  if (m >= 2) m -= 2; else { m += 10; --y; }
 
  y += (m / 12);
  m %= 12;
  if (m < 0) { m += 12; --y; }
 
  d += montab[m];
 
  d += 146097L * (y / 400);
  y %= 400;
  if (y < 0) { y += 400; d -= 146097L; }
 
  d += times365[y & 3];
  y >>= 2;
 
  d += 1461L * (y % 25);
  y /= 25;
 
  d += times36524[y & 3];
 
  return d;
}