[Sync Iteration] c/meetup/1

solutions/c/meetup/1/meetup.c

@@ -0,0 +1,73 @@
+#include "meetup.h"
+#include <string.h>
+
+static inline int day_of_week(int y, int m, int d);
+static inline int get_day_index(const char *day);
+static inline int get_days_in_month(int year, int month);
+
+// There are six week values to consider: first, second, third, fourth, last, teenth.
+int meetup_day_of_month(unsigned int year, unsigned int month, const char *week, const char *day){
+    int days_in_month = get_days_in_month(year, month);
+    int target_day = get_day_index(day);
+    int first_day_of_month = day_of_week(year, month, 1);
+
+    int first_occurrence = (target_day - first_day_of_month + 7) % 7 + 1;
+
+    if (strcmp(week, "first") == 0)  return first_occurrence;
+    if (strcmp(week, "second") == 0) return first_occurrence + 7;
+    if (strcmp(week, "third") == 0)  return first_occurrence + 14;
+    if (strcmp(week, "fourth") == 0) return first_occurrence + 21;
+
+    if (strcmp(week, "teenth") == 0) {
+        if (first_occurrence + 7 >= 13) return first_occurrence + 7;
+        return first_occurrence + 14;
+    }
+
+    if (strcmp(week, "last") == 0) {
+        int res = first_occurrence + 21;
+        if (res + 7 <= days_in_month) res += 7;
+        return res;
+    }
+
+    return -1;
+}
+
+static inline int is_leap(int year) {
+    if (year % 400 == 0)
+        return 1;
+    if (year % 100 == 0)
+        return 0;
+    if (year % 4 == 0)
+        return 1;
+    return 0;
+}
+
+static inline int get_days_in_month(int year, int month) {
+    if (month == 2) {
+        return is_leap(year) ? 29 : 28;
+    }
+
+    if (month == 1 || month == 3 || month == 5 || month == 7 ||
+        month == 8 || month == 10 || month == 12) {
+        return 31;
+    }
+
+    return 30;
+}
+
+static inline int get_day_index(const char *day){
+    const char *days[] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
+
+    for (int i = 0; i < 7; i++){
+        if (strcmp(day, days[i]) == 0) return i;
+    }
+
+    return -1;
+}
+
+// Sakamoto's algorithm
+static inline int day_of_week(int y, int m, int d) {
+    static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
+    y -= m < 3;
+    return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;
+}

solutions/c/meetup/1/meetup.h

@@ -0,0 +1,7 @@
+#ifndef MEETUP_H
+#define MEETUP_H
+
+int meetup_day_of_month(unsigned int year, unsigned int month, const char *week,
+                        const char *day_of_week);
+
+#endif