1 files changed, 58 insertions, 0 deletions

diff --git a/5/9.c b/5/9.c
new file mode 100644
index 0000000..5bcd213
--- /dev/null
+++ b/5/9.c
@@ -0,0 +1,58 @@
+#include <stdio.h>
+
+static char daytab[2][13] = {
+    {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
+    {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}};
+
+/* Sets day of year from month and day */
+int day_of_year(int year, int month, int day) {
+  int i, leap;
+
+  // leap year logic is only valid if this is true.
+  if (year < 1752) {
+    printf("error: invalid year\n");
+    return -1;
+  }
+
+  if (month < 1 || month > 12) {
+    printf("error: invalid month\n");
+    return -1;
+  }
+
+  if (day < 1) {
+    printf("error: invalid day\n");
+    return -1;
+  }
+
+  leap = year % 4 == 0 && year % 100 != 0 || year % 400 == 0;
+
+  for (i = 1; i < month; i++)
+    day += daytab[leap][i];
+
+  return day;
+}
+
+/* Sets month and day from day of year */
+void month_day(int year, int yearday, int *pmonth, int *pday) {
+  int i, leap;
+
+  if (year < 0) {
+    *pmonth = *pday = -1;
+    printf("error: invalid year\n");
+    return;
+  }
+
+  if (yearday < 1) {
+    *pmonth = *pday = -1;
+    printf("error: invalid yearday\n");
+    return;
+  }
+
+  leap = year % 4 == 0 && year % 100 != 0 || year % 400 == 0;
+
+  for (i = 1; yearday > daytab[leap][i]; i++)
+    yearday -= daytab[leap][i];
+
+  *pmonth = i;
+  *pday = yearday;
+}
\ No newline at end of file