package schedule

import (
	"errors"
	"fmt"
	"slices"
	"strings"
	"time"

	"g.fc.run/ai/byre/internal/api"
)

var ErrInvalid = errors.New("invalid schedule")

var Weekdays = []api.Weekday{api.Mon, api.Tue, api.Wed, api.Thu, api.Fri, api.Sat, api.Sun}

var goWeekdays = map[api.Weekday]time.Weekday{
	api.Mon: time.Monday,
	api.Tue: time.Tuesday,
	api.Wed: time.Wednesday,
	api.Thu: time.Thursday,
	api.Fri: time.Friday,
	api.Sat: time.Saturday,
	api.Sun: time.Sunday,
}

func invalid(format string, args ...any) error {
	return fmt.Errorf("%w: %s", ErrInvalid, fmt.Sprintf(format, args...))
}

func Timing(save api.ScheduleSave) (api.Schedule, error) {
	sc := api.Schedule{
		Title:    strings.TrimSpace(save.Title),
		Every:    save.Every,
		Weekdays: []api.Weekday{},
		Hour:     save.Hour,
		Minute:   save.Minute,
	}
	if sc.Title == "" {
		return sc, invalid("title is required")
	}
	if save.Minute < 0 || save.Minute > 59 {
		return sc, invalid("minute must be 0 to 59")
	}
	if save.Hour < 0 || save.Hour > 23 {
		return sc, invalid("hour must be 0 to 23")
	}
	switch save.Every {
	case api.Hour, api.Day, api.Week, api.Month:
	default:
		return sc, invalid("every must be hour, day, week or month")
	}
	if save.Every == api.Hour && save.Hour != 0 {
		return sc, invalid("an hourly schedule runs at a minute past every hour, so hour must be 0")
	}
	if (save.Weekdays != nil && len(*save.Weekdays) > 0) != (save.Every == api.Week) {
		return sc, invalid("weekdays are required for a weekly schedule, and only allowed for one")
	}
	if save.Weekdays != nil {
		for _, d := range Weekdays {
			if slices.Contains(*save.Weekdays, d) {
				sc.Weekdays = append(sc.Weekdays, d)
			}
		}
		for _, d := range *save.Weekdays {
			if _, found := goWeekdays[d]; !found {
				return sc, invalid("unknown weekday %q", d)
			}
		}
	}
	if (save.DayOfMonth != nil) != (save.Every == api.Month) {
		return sc, invalid("day_of_month is required for a monthly schedule, and only allowed for one")
	}
	if save.DayOfMonth != nil {
		if *save.DayOfMonth < 1 || *save.DayOfMonth > 31 {
			return sc, invalid("day_of_month must be 1 to 31")
		}
		sc.DayOfMonth = *save.DayOfMonth
	}
	return sc, nil
}

func Next(sc api.Schedule, after time.Time) time.Time {
	t := after.Local()
	y, m, d := t.Date()
	at := func(y int, m time.Month, d, h int) time.Time {
		return time.Date(y, m, d, h, sc.Minute, 0, 0, time.Local)
	}
	switch sc.Every {
	case api.Hour:
		c := at(y, m, d, t.Hour())
		for !c.After(t) {
			c = c.Add(time.Hour)
		}
		return c
	case api.Day:
		c := at(y, m, d, sc.Hour)
		if !c.After(t) {
			c = at(y, m, d+1, sc.Hour)
		}
		return c
	case api.Week:
		days := map[time.Weekday]bool{}
		for _, w := range sc.Weekdays {
			days[goWeekdays[w]] = true
		}
		for i := 0; i <= 7; i++ {
			c := at(y, m, d+i, sc.Hour)
			if days[c.Weekday()] && c.After(t) {
				return c
			}
		}
	case api.Month:
		for i := 0; i <= 1; i++ {
			first := time.Date(y, m+time.Month(i), 1, 0, 0, 0, 0, time.Local)
			last := first.AddDate(0, 1, -1).Day()
			c := at(first.Year(), first.Month(), min(sc.DayOfMonth, last), sc.Hour)
			if c.After(t) {
				return c
			}
		}
	}
	panic(fmt.Sprintf("schedule %s has no next run after %s", sc.Id, after))
}