package schedule

import (
	"fmt"
	"strings"
	"time"

	cron "github.com/robfig/cron/v3"
)

var cronParser = cron.NewParser(
	cron.SecondOptional |
		cron.Minute |
		cron.Hour |
		cron.Dom |
		cron.Month |
		cron.Dow |
		cron.Descriptor,
)

func Parse(cronExpr string) (cron.Schedule, error) {
	trimmed := strings.TrimSpace(cronExpr)
	if trimmed == "" {
		return nil, fmt.Errorf("cron expression is required")
	}

	schedule, err := cronParser.Parse(trimmed)
	if err != nil {
		return nil, fmt.Errorf("parse cron expression %q: %w", trimmed, err)
	}
	return schedule, nil
}

func ComputeNextRun(cronExpr string, startAt, endAt *time.Time, now time.Time) (*time.Time, error) {
	schedule, err := Parse(cronExpr)
	if err != nil {
		return nil, err
	}

	notBefore := now.In(time.Local)
	if startAt != nil {
		start := startAt.In(time.Local)
		if start.After(notBefore) {
			notBefore = start
		}
	}

	next := schedule.Next(notBefore.Add(-time.Nanosecond)).Round(0)
	if next.IsZero() {
		return nil, nil
	}
	if endAt != nil && next.After(endAt.In(time.Local)) {
		return nil, nil
	}

	return &next, nil
}

func IsRunAllowed(runAt time.Time, startAt, endAt *time.Time) bool {
	runAt = runAt.In(time.Local)
	if startAt != nil && runAt.Before(startAt.In(time.Local)) {
		return false
	}
	if endAt != nil && runAt.After(endAt.In(time.Local)) {
		return false
	}
	return true
}