package schedule

import (
	"time"

	"golang.org/x/xerrors"

	"github.com/coder/coder/v2/coderd/schedule/cron"
)

var ErrNoAllowedAutostart = xerrors.New("no allowed autostart")

// NextAutostart takes the workspace and template schedule and returns the next autostart schedule
// after "at". The boolean returned is if the autostart should be allowed to start based on the template
// schedule.
func NextAutostart(at time.Time, wsSchedule string, templateSchedule TemplateScheduleOptions) (time.Time, bool) {
	sched, err := cron.Weekly(wsSchedule)
	if err != nil {
		return time.Time{}, false
	}

	// Round down to the nearest minute, as this is the finest granularity cron supports.
	// Truncate is probably not necessary here, but doing it anyway to be sure.
	nextTransition := sched.Next(at).Truncate(time.Minute)

	// The nextTransition is when the auto start should kick off. If it lands on a
	// forbidden day, do not allow the auto start. We use the time location of the
	// schedule to determine the weekday. So if "Saturday" is disallowed, the
	// definition of "Saturday" depends on the location of the schedule.
	zonedTransition := nextTransition.In(sched.Location())
	allowed := templateSchedule.AutostartRequirement.DaysMap()[zonedTransition.Weekday()]

	return zonedTransition, allowed
}

func NextAllowedAutostart(at time.Time, wsSchedule string, templateSchedule TemplateScheduleOptions) (time.Time, error) {
	next := at

	// Our cron schedules work on a weekly basis, so to ensure we've exhausted all
	// possible autostart times we need to check up to 7 days worth of autostarts.
	for next.Sub(at) < 7*24*time.Hour {
		var valid bool
		next, valid = NextAutostart(next, wsSchedule, templateSchedule)
		if valid {
			return next, nil
		}
	}

	return time.Time{}, ErrNoAllowedAutostart
}