odds and sods

[tridge/junkcode.git] / dst.c

diff --git a/dst.c b/dst.c
new file mode 100644 (file)
index 0000000..97ff9b3
--- /dev/null
+++ b/dst.c
@@ -0,0 +1,64 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/time.h>
+
+#define DAY (24*60*60)
+#define YEAR (365*DAY)
+
+void get_time_diff(time_t t,int *timediff,int *is_dst)
+{
+  struct tm tm_utc,tm_local;
+  tm_utc = *(gmtime(&t));
+  tm_local = *(localtime(&t));
+
+  *timediff = mktime(&tm_utc) - mktime(&tm_local);
+  *is_dst = tm_local.tm_isdst;
+}
+
+main()
+{
+  time_t tnow = time(NULL);
+  int is_dst,timediff;
+  int last_dst,lastdiff;
+  time_t endt = tnow + 2*YEAR;
+  time_t t = tnow - 2*YEAR;
+  time_t high,low;
+  int count=0;
+
+  while (1) {
+
+    get_time_diff(t,&lastdiff,&last_dst);
+    low = t;
+
+    /* run forward till we get a change */
+    for (timediff = lastdiff, is_dst = last_dst;
+        timediff == lastdiff && last_dst == is_dst && t < endt ; 
+        t += 10*DAY)
+      get_time_diff(t,&timediff,&is_dst);
+
+    if (t >= endt) break;
+
+    high = t;
+
+    /* find the change */
+    while (low+1 < high) {
+      t = (low+high)/2;
+      get_time_diff(t,&timediff,&is_dst);
+      if (timediff == lastdiff && is_dst == last_dst) 
+       low = t;
+      else
+       high = t;
+    }
+    t = high;
+
+    get_time_diff(t,&timediff,&is_dst);
+    printf("TimeDiff=%d is_dst=%d at %s",
+          timediff,is_dst,asctime(localtime(&t)));
+    count++;
+  }
+
+  if (!count)
+    printf("No timezone or DST changes?\n");
+
+  return(0);
+}