/* +++Date last modified: 05-Jul-1997 */

/*
** scalar date routines    --    public domain by Ray Gardner
** Numerically, these will work over the range 1/01/01 thru 14699/12/31.
** Practically, these only work from the beginning of the Gregorian 
** calendar thru 14699/12/31.  The Gregorian calendar took effect in
** much of Europe in about 1582, some parts of Germany in about 1700, in
** England and the colonies in about 1752ff, and in Russia in 1918.
*/

#include "scaldate.h"

int isleap(unsigned yr)
{
  return yr % 400 == 0 || (yr % 4 == 0 && yr % 100 != 0);
}

static unsigned months_to_days(unsigned month)
{
  return (month * 3057 - 3007) / 100;
}

static long years_to_days(unsigned yr)
{
  return yr * 365L + yr / 4 - yr / 100 + yr / 400;
}

long ymd_to_scalar(unsigned yr, unsigned mo, unsigned day)
{
  long scalar;
  scalar = day + months_to_days(mo);
  if (mo > 2)                   /* adjust if past February */
    scalar -= isleap(yr) ? 1 : 2;
  yr--;
  scalar += years_to_days(yr);
  return scalar;
}

void scalar_to_ymd(long scalar, unsigned *yr, unsigned *mo, unsigned *day)
{
  unsigned n;                   /* compute inverse of years_to_days() */

  for (n = (unsigned) ((scalar * 400L) / 146097L);
       years_to_days(n) < scalar;)
    n++;                        /* 146097 == years_to_days(400) */
  *yr = n;
  n = (unsigned) (scalar - years_to_days(n - 1));
  if (n > 59) {                 /* adjust if past February */
    n += 2;
    if (isleap(*yr))
      n -= n > 62 ? 1 : 2;
  }
  *mo = (n * 100 + 3007) / 3057;        /* inverse of months_to_days() */
  *day = n - months_to_days(*mo);
}