recur every n months

internal/task/date.go

@@ -201,6 +201,10 @@ func (d Date) Weekday() time.Weekday {
 	return d.t.Weekday()
 }
 
+func (d Date) Day() int {
+	return d.t.Day()
+}
+
 func (d Date) Add(days int) Date {
 	year, month, day := d.t.Date()
 	return NewDate(year, int(month), day+days)

internal/task/date_test.go

@@ -195,13 +195,13 @@ func TestDateString(t *testing.T) {
 		},
 		{
 			name: "normal",
-			date: task.NewDate(2021, 1, 30),
+			date: task.NewDate(2021, 5, 30),
-			exp:  "2021-01-30 (saturday)",
+			exp:  "2021-05-30 (sunday)",
 		},
 		{
 			name: "normalize",
-			date: task.NewDate(2021, 1, 32),
+			date: task.NewDate(2021, 5, 32),
-			exp:  "2021-02-01 (monday)",
+			exp:  "2021-06-01 (tuesday)",
 		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {

internal/task/recur.go

@@ -37,6 +37,9 @@ func NewRecurrer(recurStr string) Recurrer {
 	if recur, ok := ParseEveryNWeeks(start, terms); ok {
 		return recur
 	}
+	if recur, ok := ParseEveryNMonths(start, terms); ok {
+		return recur
+	}
 
 	return nil
 }
@@ -191,7 +194,7 @@ type EveryNWeeks struct {
 
 // yyyy-mm-dd, every 3 weeks
 func ParseEveryNWeeks(start Date, terms []string) (Recurrer, bool) {
-	if len(terms) < 1 || len(terms) > 1 {
+	if len(terms) != 1 {
 		return nil, false
 	}
 
@@ -225,3 +228,41 @@ func (enw EveryNWeeks) RecursOn(date Date) bool {
 func (enw EveryNWeeks) String() string {
 	return fmt.Sprintf("%s, every %d weeks", enw.Start.String(), enw.N)
 }
+
+type EveryNMonths struct {
+	Start Date
+	N     int
+}
+
+// yyyy-mm-dd, every 3 months
+func ParseEveryNMonths(start Date, terms []string) (Recurrer, bool) {
+	if len(terms) != 1 {
+		return nil, false
+	}
+
+	terms = strings.Split(terms[0], " ")
+	if len(terms) != 3 || terms[0] != "every" || terms[2] != "months" {
+		return nil, false
+	}
+	n, err := strconv.Atoi(terms[1])
+	if err != nil {
+		return nil, false
+	}
+
+	return EveryNMonths{
+		Start: start,
+		N:     n,
+	}, true
+}
+
+func (enm EveryNMonths) RecursOn(date Date) bool {
+	if enm.Start.After(date) {
+		return false
+	}
+
+	return enm.Start.Day() == date.Day()
+}
+
+func (enm EveryNMonths) String() string {
+	return fmt.Sprintf("%s, every %d months", enm.Start.String(), enm.N)
+}

internal/task/recur_test.go

@@ -300,3 +300,50 @@ func TestEveryNWeeks(t *testing.T) {
 		}
 	})
 }
+
+func TestEveryNMonths(t *testing.T) {
+	everyNMonths := task.EveryNMonths{
+		Start: task.NewDate(2021, 2, 3),
+		N:     3,
+	}
+	everyNMonthsStr := "2021-02-03 (wednesday), every 3 months"
+
+	t.Run("parse", func(t *testing.T) {
+		test.Equals(t, everyNMonths, task.NewRecurrer(everyNMonthsStr))
+	})
+
+	t.Run("string", func(t *testing.T) {
+		test.Equals(t, everyNMonthsStr, everyNMonths.String())
+	})
+
+	t.Run("recurs on", func(t *testing.T) {
+		for _, tc := range []struct {
+			name string
+			date task.Date
+			exp  bool
+		}{
+			{
+				name: "before start",
+				date: task.NewDate(2021, 1, 27),
+			},
+			{
+				name: "on start",
+				date: task.NewDate(2021, 2, 3),
+				exp:  true,
+			},
+			{
+				name: "8 weeks after",
+				date: task.NewDate(2021, 3, 31),
+			},
+			{
+				name: "one month",
+				date: task.NewDate(2021, 3, 3),
+				exp:  true,
+			},
+		} {
+			t.Run(tc.name, func(t *testing.T) {
+				test.Equals(t, tc.exp, everyNMonths.RecursOn(tc.date))
+			})
+		}
+	})
+}