date converter excercise

k&r/05-pointers-and-arrays/05-08.c

@@ -0,0 +1,68 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#define HELP_STR "usage: name year [month] day"
+
+int month_day(int year, int day, int *pmonth);
+int day_of_year(int year, int month, int day);
+
+/* converts arguments year yearday to year month day or
+ * year month day to year yearday */
+int main(int argc, char *argv[])
+{
+    int y, m, d, yd;
+    if (argc == 3) { /* year day -> month day */
+        y = atoi(argv[1]), yd = atoi(argv[2]);
+        if ((d = month_day(y, yd, &m))) {
+            printf("%d %d %d\n", y, m, d);
+            return 0;
+        }
+        printf("error: invalid yearday %d\n", yd);
+    } else if (argc == 4) { /* year month day -> year day */
+        y = atoi(argv[1]), m = atoi(argv[2]), d = atoi(argv[3]);
+        if ((yd = day_of_year(y, m, d))) {
+            printf("%d %d\n", y, yd);
+            return 0;
+        }
+        printf("error: invalid month %d or day %d\n", m, d);
+    }
+    puts(HELP_STR);
+    return 1;
+}
+
+static char daytab[][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}
+};
+
+#define IS_LEAP(Y) (Y%4 == 0 && Y%100 != 0 || Y%400 == 0)
+#define IS_MONTH(M) (0 < M && M <= 12)
+#define IS_D_OF_M(LEAP, M, D) (0 < D && D <= daytab[LEAP][M])
+#define IS_YD_OF_Y(LEAP, YD) (0 < YD && YD <= 365+LEAP)
+
+/* day_of_year: set day of year from month & day 
+ * return day of year or 0 on error */
+int day_of_year(int year, int month, int day)
+{
+    int m, leap = IS_LEAP(year);
+    if (!IS_MONTH(month) || !IS_D_OF_M(leap, month, day)) 
+        /* validity check */
+        return 0;
+    for (m = 1; m < month; m++)
+        day += daytab[leap][m];
+    return day;
+}
+
+/* month_day: set month, day from day of year 
+ * return day of month or 0 on error, set pmonth to month */
+int month_day(int year, int yearday, int *pmonth)
+{
+    int m, leap = IS_LEAP(year);
+    if (!IS_YD_OF_Y(leap, yearday)) /* validity check */
+        return 0;
+    for (m = 1; yearday > daytab[leap][m]; m++)
+        yearday -= daytab[leap][m];
+    *pmonth = m;
+    return yearday;
+}
+