feat: schedule: add schedule.Min function to determine minimum interval

johnstcn · johnstcn · commit 5ad048b39327 · 2022-06-03T16:13:55.000Z
diff --git a/coderd/autobuild/schedule/schedule.go b/coderd/autobuild/schedule/schedule.go
@@ -108,6 +108,36 @@ func (s Schedule) Next(t time.Time) time.Time {
 	return s.sched.Next(t)
 }
 
+var t0 = time.Date(1970, 1, 1, 1, 1, 1, 0, time.UTC)
+var tMax = t0.Add(168 * time.Hour)
+
+// Min returns the minimum duration of the schedule.
+// This is calculated as follows:
+//  - Let t(0) be a given point in time (1970-01-01T01:01:01Z00:00)
+//  - Let t(max) be 168 hours after t(0).
+//  - Let t(1) be the next scheduled time after t(0).
+//  - Let t(n) be the next scheduled time after t(n-1).
+//  - Then, the minimum duration of s d(min)
+//    = min( t(n) - t(n-1) ∀ n ∈ N, t(n) < t(max) )
+func (s Schedule) Min() time.Duration {
+	durMin := tMax.Sub(t0)
+	tPrev := s.Next(t0)
+	tCurr := s.Next(tPrev)
+	for {
+		dur := tCurr.Sub(tPrev)
+		if dur < durMin {
+			durMin = dur
+		}
+		tmp := tCurr
+		tCurr = s.Next(tmp)
+		tPrev = tmp
+		if tCurr.After(tMax) {
+			break
+		}
+	}
+	return durMin
+}
+
 // validateWeeklySpec ensures that the day-of-month and month options of
 // spec are both set to *
 func validateWeeklySpec(spec string) error {
diff --git a/coderd/autobuild/schedule/schedule_test.go b/coderd/autobuild/schedule/schedule_test.go
@@ -16,6 +16,7 @@ func Test_Weekly(t *testing.T) {
 		spec           string
 		at             time.Time
 		expectedNext   time.Time
+		expectedMin    time.Duration
 		expectedError  string
 		expectedCron   string
 		expectedTz     string
@@ -26,6 +27,7 @@ func Test_Weekly(t *testing.T) {
 			spec:           "CRON_TZ=US/Central 30 9 * * 1-5",
 			at:             time.Date(2022, 4, 1, 14, 29, 0, 0, time.UTC),
 			expectedNext:   time.Date(2022, 4, 1, 14, 30, 0, 0, time.UTC),
+			expectedMin:    24 * time.Hour,
 			expectedError:  "",
 			expectedCron:   "30 9 * * 1-5",
 			expectedTz:     "US/Central",
@@ -36,11 +38,34 @@ func Test_Weekly(t *testing.T) {
 			spec:           "30 9 * * 1-5",
 			at:             time.Date(2022, 4, 1, 9, 29, 0, 0, time.UTC),
 			expectedNext:   time.Date(2022, 4, 1, 9, 30, 0, 0, time.UTC),
+			expectedMin:    24 * time.Hour,
 			expectedError:  "",
 			expectedCron:   "30 9 * * 1-5",
 			expectedTz:     "UTC",
 			expectedString: "CRON_TZ=UTC 30 9 * * 1-5",
 		},
+		{
+			name:           "convoluted with timezone",
+			spec:           "CRON_TZ=US/Central */5 12-18 * * 1,3,6",
+			at:             time.Date(2022, 4, 1, 14, 29, 0, 0, time.UTC),
+			expectedNext:   time.Date(2022, 4, 2, 17, 0, 0, 0, time.UTC), // Apr 1 was a Friday in 2022
+			expectedMin:    5 * time.Minute,
+			expectedError:  "",
+			expectedCron:   "*/5 12-18 * * 1,3,6",
+			expectedTz:     "US/Central",
+			expectedString: "CRON_TZ=US/Central */5 12-18 * * 1,3,6",
+		},
+		{
+			name:           "another convoluted example",
+			spec:           "CRON_TZ=US/Central 10,20,40-50 * * * *",
+			at:             time.Date(2022, 4, 1, 14, 29, 0, 0, time.UTC),
+			expectedNext:   time.Date(2022, 4, 1, 14, 40, 0, 0, time.UTC),
+			expectedMin:    time.Minute,
+			expectedError:  "",
+			expectedCron:   "10,20,40-50 * * * *",
+			expectedTz:     "US/Central",
+			expectedString: "CRON_TZ=US/Central 10,20,40-50 * * * *",
+		},
 		{
 			name:          "time.Local will bite you",
 			spec:          "CRON_TZ=Local 30 9 * * 1-5",
@@ -104,6 +129,7 @@ func Test_Weekly(t *testing.T) {
 				require.Equal(t, testCase.expectedCron, actual.Cron())
 				require.Equal(t, testCase.expectedTz, actual.Timezone())
 				require.Equal(t, testCase.expectedString, actual.String())
+				require.Equal(t, testCase.expectedMin, actual.Min())
 			} else {
 				require.EqualError(t, err, testCase.expectedError)
 				require.Nil(t, actual)
