fix dst bug in rangePerDates (#15)

static.go

@@ -97,6 +97,9 @@ func gtfsDate(offset time.Duration) string {
 	h := int(offset.Hours())
 	m := int(offset.Minutes()) - h*60
 	s := int(offset.Seconds()) - h*3600 - m*60
+	if h < 0 || m < 0 || s < 0 {
+		return ""
+	}
 	return fmt.Sprintf("%02d%02d%02d", h, m, s)
 }
 

whitebox_test.go

@@ -116,7 +116,31 @@ func TestStaticRangePerDate(t *testing.T) {
 			},
 		},
 
-		// TODO: write a test of overflow with DST change
+		{
+			"overflow with change to standard time",
+			time.Date(2023, 11, 5, 00, 59, 0, 0, tzET),
+			2 * time.Minute,
+			29 * time.Hour,
+			[]span{
+				{"20231104", "245900", "250100"},
+				{"20231105", "", "000100"},
+			},
+		},
+
+		{
+			"overflow with change to daylight savings",
+			time.Date(2023, 3, 12, 00, 59, 0, 0, tzET),
+			2 * time.Minute,
+			29 * time.Hour,
+			[]span{
+				{"20230311", "245900", "250100"},
+				{"20230312", "015900", "020100"},
+			},
+		},
+
+		// NOTE: DST changes are really annoying. Very likely
+		// these tests are missing some edge cases.
+
 	} {
 		t.Run(tc.Name, func(t *testing.T) {
 			spans := rangePerDate(tc.Start, tc.Window, tc.Max)