(compute_change): Replace slow loop to compute T by simple algorithm.

diff --git a/time/tzset.c b/time/tzset.c
index aca1fe64a026891ba2ae00bfd75b8d80160b90cc..db08a47e86fc2b49c464594a8c7db6a48762aec3 100644 (file)
--- a/time/tzset.c
+++ b/time/tzset.c
@@ -418,16 +418,23 @@ compute_change (rule, year)
      int year;
 {
   register time_t t;
-  int y;
 
   if (year != -1 && rule->computed_for == year)
-    /* Operations on times in 1969 will be slower.  Oh well.  */
+    /* Operations on times in 2 BC will be slower.  Oh well.  */
     return 1;
 
   /* First set T to January 1st, 0:00:00 GMT in YEAR.  */
-  t = 0;
-  for (y = 1970; y < year; ++y)
-    t += SECSPERDAY * (__isleap (y) ? 366 : 365);
+  if (year > 1970)
+    t = ((year - 1970) * 365
+        + /* Compute the number of leapdays between 1970 and YEAR
+             (exclusive).  There is a leapday every 4th year ...  */
+        + ((year - 1) / 4 - 1970 / 4)
+        /* ... except every 100th year ... */
+        - ((year - 1) / 100 - 1970 / 100)
+        /* ... but still every 400th year.  */
+        + ((year - 1) / 400 - 1970 / 400)) * SECSPERDAY;
+  else
+    t = 0;
 
   switch (rule->type)
     {