time: days_in_month()

vlib/time/time.v

@@ -6,6 +6,11 @@ module time
 
 import rand
 
+// https://en.wikipedia.org/wiki/Month#Julian_and_Gregorian_calendars
+const (
+	MonthDays = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
+)
+
 #include <time.h>
 struct Time {
 pub:
@@ -18,6 +23,7 @@ pub:
 	uni    int // TODO it's safe to use "unix" now
 }
 
+
 fn C.localtime(int) *C.tm
 
 fn remove_me_when_c_bug_is_fixed() { // TODO 
@@ -324,3 +330,16 @@ pub fn sleep_ms(n int) {
 pub fn is_leap_year(year int) bool {
 	return (year%4 == 0) && (year%100 != 0 || year%400 == 0)
 }
+
+// Returns number of days in month
+pub fn days_in_month(month, year int) ?int {
+	if month > 12 || month < 1 {
+		return error('Invalid month: $month')
+	}
+
+	if month == 2 {
+		return MonthDays[month-1] + if is_leap_year(year) {1} else {0}
+	} else {
+		return MonthDays[month-1]
+	}
+}

vlib/time/time_test.v

@@ -15,3 +15,25 @@ fn test_is_leap_year() {
     assert time.is_leap_year(1996) == true
     assert time.is_leap_year(1997) == false
 }
+
+fn check(month, year, expectedDays int) bool {
+	return time.days_in_month(month, year) or {
+		return false
+	}
+}
+
+fn test_days_in_month() {
+	assert check(1, 2001, 31) // January
+	assert check(2, 2001, 28) // February
+	assert check(2, 2000, 29) // February (leap)
+	assert check(3, 2001, 31) // March
+	assert check(4, 2001, 30) // April
+	assert check(5, 2001, 31) // May
+	assert check(6, 2001, 30) // June
+	assert check(7, 2001, 31) // July
+	assert check(8, 2001, 31) // August
+	assert check(9, 2001, 30) // September
+	assert check(10, 2001, 31) // October
+	assert check(11, 2001, 30) // November
+	assert check(12, 2001, 31) // December
+}