From 5931868b6299d33c3b2ea7576d5bb1c98b7317ab Mon Sep 17 00:00:00 2001
From: Erik Winter <erik@publicsonar.com>
Date: Wed, 3 Mar 2021 11:54:36 +0100
Subject: [PATCH] fix overflow bug with adding months

---
 internal/task/date.go       | 10 +++++++++-
 internal/task/recur.go      |  3 +--
 internal/task/recur_test.go |  8 ++++++++
 3 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/internal/task/date.go b/internal/task/date.go
index 7498d6d..6fa6cf2 100644
--- a/internal/task/date.go
+++ b/internal/task/date.go
@@ -53,7 +53,7 @@ type Date struct {
 
 func NewDate(year, month, day int) Date {
 
-	if year == 0 && month == 0 && day == 0 {
+	if year == 0 || month == 0 || month > 12 || day == 0 {
 		return Date{}
 	}
 
@@ -209,6 +209,14 @@ func (d Date) Add(days int) Date {
 	return NewDate(year, int(month), day+days)
 }
 
+func (d Date) AddMonths(addMonths int) Date {
+	year, month, day := d.t.Date()
+	addYears := int((int(month) + addMonths) / 12)
+	newMonth := (int(month) + addMonths) % 12
+
+	return NewDate(year+addYears, newMonth, day)
+}
+
 func (d Date) Equal(ud Date) bool {
 	return d.t.Equal(ud.Time())
 }
diff --git a/internal/task/recur.go b/internal/task/recur.go
index 6bbbe5c..c5a2447 100644
--- a/internal/task/recur.go
+++ b/internal/task/recur.go
@@ -268,8 +268,7 @@ func (enm EveryNMonths) RecursOn(date Date) bool {
 		if tDate.After(date) {
 			return false
 		}
-		tYear, tMonth, tDay := tDate.Time().Date()
-		tDate = NewDate(tYear, int(tMonth)+enm.N, tDay)
+		tDate = tDate.AddMonths(enm.N)
 	}
 
 }
diff --git a/internal/task/recur_test.go b/internal/task/recur_test.go
index 18bf2b2..6addf2e 100644
--- a/internal/task/recur_test.go
+++ b/internal/task/recur_test.go
@@ -359,4 +359,12 @@ func TestEveryNMonths(t *testing.T) {
 			})
 		}
 	})
+
+	t.Run("recurs every year", func(t *testing.T) {
+		recur := task.EveryNMonths{
+			Start: task.NewDate(2021, 3, 1),
+			N:     12,
+		}
+		test.Equals(t, false, recur.RecursOn(task.NewDate(2021, 3, 9)))
+	})
 }