(find_transition): Instead of a linear search try to guess the transition index,...

diff --git a/time/tzfile.c b/time/tzfile.c
index ea2d7cae4ca27115889db72d64dfc15fff05a6d8..0d48c8ca0cb318fe4d955d16583fcec937e311f7 100644 (file)
--- a/time/tzfile.c
+++ b/time/tzfile.c
@@ -548,13 +548,60 @@ find_transition (time_t timer)
       if (i == num_types)
        i = 0;
     }
+  else if (timer >= transitions[num_transitions - 1])
+    i = type_idxs[num_transitions - 1];
   else
     {
       /* Find the first transition after TIMER, and
         then pick the type of the transition before it.  */
-      for (i = 1; i < num_transitions; ++i)
-       if (timer < transitions[i])
-         break;
+      size_t lo = 0;
+      size_t hi = num_transitions - 1;
+      /* Assume that DST is changing twice a year and guess initial
+        search spot from it.
+        Half of a gregorian year has on average 365.2425 * 86400 / 2
+        = 15778476 seconds.  */
+      i = (transitions[num_transitions - 1] - timer) / 15778476;
+      if (i < num_transitions)
+       {
+         i = num_transitions - 1 - i;
+         if (timer < transitions[i])
+           {
+             if (i < 10 || timer >= transitions[i - 10])
+               {
+                 /* Linear search.  */
+                 while (timer < transitions[i - 1])
+                   --i;
+                 goto found;
+               }
+             hi = i - 10;
+           }
+         else
+           {
+             if (i + 10 >= num_transitions || timer < transitions[i + 10])
+               {
+                 /* Linear search.  */
+                 while (timer >= transitions[i])
+                   ++i;
+                 goto found;
+               }
+             lo = i + 10;
+           }
+       }
+
+      /* Binary search.  */
+      /* assert (timer >= transitions[lo] && timer < transitions[hi]); */
+      while (lo + 1 < hi)
+       {
+         i = (lo + hi) / 2;
+         if (timer < transitions[i])
+           hi = i;
+         else
+           lo = i;
+       }
+      i = hi;
+
+    found:
+      /* assert (timer >= transitions[i - 1] && timer < transitions[i]); */
       i = type_idxs[i - 1];
     }