Initial revision

scaldate.c

@@ -0,0 +1,55 @@
+/* +++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);
+}