1 files changed, 41 insertions, 0 deletions
diff --git a/yday.c b/yday.c
new file mode 100644
index 0000000..d8b169f
--- /dev/null
+++ b/yday.c
@@ -0,0 +1,41 @@
+#include <time.h>
+#include <stdbool.h>
+#include <stdio.h>
+
+bool leapyear(unsigned year) {
+  /* All years that are divisible by 4 are leap years,
+     unless they start a new century, provided they
+     are not divisible by 400. */
+  return !(year % 4) && ((year % 100) || !(year % 400));
+}
+
+#define DAYS_BEFORE                             \
+(int const[12]){                                \
+  [0] = 0, [1] = 31, [2] = 59, [3] = 90,        \
+  [4] = 120, [5] = 151, [6] = 181, [7] = 212,   \
+  [8] = 243, [9] = 273, [10] = 304, [11] = 334, \
+}
+
+struct tm time_set_yday(struct tm t) {
+  // tm_mdays starts at 1.
+  t.tm_yday += DAYS_BEFORE[t.tm_mon] + t.tm_mday - 1;
+  // Takes care of leap years
+  if ((t.tm_mon > 1) && leapyear(t.tm_year+1900))
+    ++t.tm_yday;
+  return t;
+}
+
+int main(void) {
+  struct tm today = {
+    .tm_year = 2019-1900,
+    .tm_mon  = 4-1,
+    .tm_mday = 3,
+    .tm_hour = 10,
+    .tm_min  = 0,
+    .tm_sec  = 47,
+  };
+  printf("this year is %d, next year will be %d\n",
+         today.tm_year+1900, today.tm_year+1900+1);
+  today = time_set_yday(today);
+  printf("day of the year is %d\n", today.tm_yday);
+}