AK+LibC+LibCore: Have fewer implementations of day_of_week

The implementation in LibC did a timestamp->day-of-week conversion
which looks like a valuable thing to have. But we only need it in
time_to_tm, where we already computed year/month/day -- so let's
consolidate on the day_of_week function in DateTime (which is
getting extracted to AK).

AK/Time.cpp

@@ -51,4 +51,14 @@ int days_in_month(int year, unsigned month)
     bool is_long_month = (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12);
     return is_long_month ? 31 : 30;
 }
+
+unsigned day_of_week(int year, unsigned month, int day)
+{
+    ASSERT(month >= 1 && month <= 12);
+    static const int seek_table[] = { 0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4 };
+    if (month < 3)
+        --year;
+
+    return (year + year / 4 - year / 100 + year / 400 + seek_table[month - 1] + day) % 7;
+}
 }

AK/Time.h

@@ -28,6 +28,12 @@
 
 namespace AK {
 
+// Month and day start at 1. Month must be >= 1 and <= 12.
+// The return value is 0-indexed, that is 0 is Sunday, 1 is Monday, etc.
+// Day may be negative or larger than the number of days
+// in the given month.
+unsigned day_of_week(int year, unsigned month, int day);
+
 // Month and day start at 1. Month must be >= 1 and <= 12.
 // The return value is 0-indexed, that is Jan 1 is day 0.
 // Day may be negative or larger than the number of days
@@ -176,6 +182,7 @@ inline bool operator!=(const TimespecType& a, const TimespecType& b)
 
 }
 
+using AK::day_of_week;
 using AK::day_of_year;
 using AK::days_in_month;
 using AK::days_in_year;

Libraries/LibC/time.cpp

@@ -64,11 +64,6 @@ static const int __seconds_per_day = 60 * 60 * 24;
 
 static void time_to_tm(struct tm* tm, time_t t)
 {
-    tm->tm_wday = (4 * __seconds_per_day + t) % (7 * __seconds_per_day); // 1970-01-01 was a Thursday.
-    if (tm->tm_wday < 0)
-        tm->tm_wday += 7 * __seconds_per_day;
-    tm->tm_wday /= __seconds_per_day;
-
     int year = 1970;
     for (; t >= days_in_year(year) * __seconds_per_day; ++year)
         t -= days_in_year(year) * __seconds_per_day;
@@ -89,8 +84,9 @@ static void time_to_tm(struct tm* tm, time_t t)
     for (month = 1; month < 12 && days >= days_in_month(year, month); ++month)
         days -= days_in_month(year, month);
 
-    tm->tm_mon = month - 1;
     tm->tm_mday = days + 1;
+    tm->tm_wday = day_of_week(year, month, tm->tm_mday);
+    tm->tm_mon = month - 1;
 }
 
 static time_t tm_to_time(struct tm* tm, long timezone_adjust_seconds)

Libraries/LibCore/DateTime.cpp

@@ -61,12 +61,7 @@ DateTime DateTime::from_timestamp(time_t timestamp)
 
 unsigned DateTime::weekday() const
 {
-    int target_year = m_year;
-    static const int seek_table[] = { 0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4 };
-    if (m_month < 3)
-        --target_year;
-
-    return (target_year + target_year / 4 - target_year / 100 + target_year / 400 + seek_table[m_month - 1] + m_day) % 7;
+    return ::day_of_week(m_year, m_month, m_day);
 }
 
 unsigned DateTime::days_in_month() const