Foundation for ScheduledOverrides (actions#513)

Adds two types `RecurrenceRule` and `Period` and one function `MatchSchedule` as the foundation for building the upcoming ScheduledOverrides feature.

Ref actions#484

controllers/horizontalrunnerautoscaler_scheduled_overrides.go

@@ -0,0 +1,122 @@
+package controllers
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/teambition/rrule-go"
+)
+
+type RecurrenceRule struct {
+	Frequency string
+	UntilTime time.Time
+}
+
+type Period struct {
+	StartTime time.Time
+	EndTime   time.Time
+}
+
+func (r *Period) String() string {
+	if r == nil {
+		return ""
+	}
+
+	return r.StartTime.Format(time.RFC3339) + "-" + r.EndTime.Format(time.RFC3339)
+}
+
+func MatchSchedule(now time.Time, startTime, endTime time.Time, recurrenceRule RecurrenceRule) (*Period, *Period, error) {
+	return calculateActiveAndUpcomingRecurringPeriods(
+		now,
+		startTime,
+		endTime,
+		recurrenceRule.Frequency,
+		recurrenceRule.UntilTime,
+	)
+}
+
+func calculateActiveAndUpcomingRecurringPeriods(now, startTime, endTime time.Time, frequency string, untilTime time.Time) (*Period, *Period, error) {
+	var freqValue rrule.Frequency
+
+	var freqDurationDay int
+	var freqDurationMonth int
+	var freqDurationYear int
+
+	switch frequency {
+	case "Daily":
+		freqValue = rrule.DAILY
+		freqDurationDay = 1
+	case "Weekly":
+		freqValue = rrule.WEEKLY
+		freqDurationDay = 7
+	case "Monthly":
+		freqValue = rrule.MONTHLY
+		freqDurationMonth = 1
+	case "Yearly":
+		freqValue = rrule.YEARLY
+		freqDurationYear = 1
+	case "":
+		if now.Before(startTime) {
+			return nil, &Period{StartTime: startTime, EndTime: endTime}, nil
+		}
+
+		if now.Before(endTime) {
+			return &Period{StartTime: startTime, EndTime: endTime}, nil, nil
+		}
+
+		return nil, nil, nil
+	default:
+		return nil, nil, fmt.Errorf(`invalid freq %q: It must be one of "Daily", "Weekly", "Monthly", and "Yearly"`, frequency)
+	}
+
+	freqDurationLater := time.Date(
+		now.Year()+freqDurationYear,
+		time.Month(int(now.Month())+freqDurationMonth),
+		now.Day()+freqDurationDay,
+		now.Hour(), now.Minute(), now.Second(), now.Nanosecond(), now.Location(),
+	)
+
+	freqDuration := freqDurationLater.Sub(now)
+
+	overrideDuration := endTime.Sub(startTime)
+	if overrideDuration > freqDuration {
+		return nil, nil, fmt.Errorf("override's duration %s must be equal to sor shorter than the duration implied by freq %q (%s)", overrideDuration, frequency, freqDuration)
+	}
+
+	rrule, err := rrule.NewRRule(rrule.ROption{
+		Freq:    freqValue,
+		Dtstart: startTime,
+		Until:   untilTime,
+	})
+	if err != nil {
+		return nil, nil, err
+	}
+
+	overrideDurationBefore := now.Add(-overrideDuration + 1)
+	activeOverrideStarts := rrule.Between(overrideDurationBefore, now, true)
+
+	var active *Period
+
+	if len(activeOverrideStarts) > 1 {
+		return nil, nil, fmt.Errorf("[bug] unexpted number of active overrides found: %v", activeOverrideStarts)
+	} else if len(activeOverrideStarts) == 1 {
+		active = &Period{
+			StartTime: activeOverrideStarts[0],
+			EndTime:   activeOverrideStarts[0].Add(overrideDuration),
+		}
+	}
+
+	oneSecondLater := now.Add(1)
+	upcomingOverrideStarts := rrule.Between(oneSecondLater, freqDurationLater, true)
+
+	var next *Period
+
+	if len(upcomingOverrideStarts) > 0 {
+		next = &Period{
+			StartTime: upcomingOverrideStarts[0],
+			EndTime:   upcomingOverrideStarts[0].Add(overrideDuration),
+		}
+	}
+
+	return active, next, nil
+}