package calendar

import (
	"context"
	"fmt"
	"io"
	"io/fs"
	"net/http"
	"os"
	"slices"
	"time"

	ical "github.com/arran4/golang-ical"
	"gitlab.com/tozd/go/errors"
	"go.alanpearce.eu/x/log"

	"go.alanpearce.eu/homestead/internal/cache"
	"go.alanpearce.eu/homestead/internal/config"
)

const Filename = "calendar.ics"
const Refresh = 30 * time.Minute

type Options struct {
	URL      config.URL
	Timezone config.Timezone
	Cache    bool
}

type Calendar struct {
	opts   *Options
	log    *log.Logger
	client *http.Client

	*ical.Calendar
}

type Busy struct {
	ical.VBusy
	StartTime Date
	EndTime   Date
}

type Date struct {
	time.Time
}

type CalendarDate struct {
	Date
	BusyPeriods []*Busy
}

func New(opts *Options, logger *log.Logger) *Calendar {
	if opts.URL.Scheme == "webcal" {
		opts.URL.Scheme = "https"
	}

	return &Calendar{
		opts: opts,
		log:  logger,
		client: &http.Client{
			Timeout: time.Second * 10,
		},
	}
}

func (c *Calendar) ValidCache() (bool, error) {
	if !c.opts.Cache {
		return false, nil
	}

	stat, err := cache.Root.Stat(Filename)
	if err != nil && !errors.Is(err, fs.ErrNotExist) {
		return false, errors.WithMessage(err, "could not stat calendar file")
	}

	return stat != nil && time.Since(stat.ModTime()) < Refresh && stat.Size() > 0, nil
}

func (c *Calendar) FetchIfNeeded(ctx context.Context) error {
	var err error

	if valid, err := c.ValidCache(); err != nil {
		return err
	} else if !valid {
		err := c.fetch(ctx)
		if err != nil {
			return err
		}
	}

	f, err := c.open()
	if err != nil {
		return err
	}
	defer f.Close()

	c.Calendar, err = ical.ParseCalendar(f)
	if err != nil {
		c.log.Warn("error parsing calendar", "error", err)

		return errors.WithMessage(err, "could not parse calendar")
	}

	return err
}

func (c *Calendar) open() (*os.File, errors.E) {
	f, err := cache.Root.Open(Filename)
	if err != nil {
		return nil, errors.WithMessage(err, "could not open calendar file")
	}

	return f, nil
}

func (c *Calendar) fetch(ctx context.Context) errors.E {
	c.log.Debug("fetching calendar", "url", c.opts.URL.String())

	f, err := cache.Root.OpenFile(Filename, os.O_RDWR|os.O_CREATE, 0o600)
	if err != nil {
		return errors.WithMessage(err, "could not create temp file")
	}
	defer f.Close()

	req, err := http.NewRequestWithContext(ctx, http.MethodGet, c.opts.URL.String(), nil)
	if err != nil {
		return errors.WithMessage(err, "could not create request")
	}

	res, err := c.client.Do(req)
	if err != nil {
		return errors.WithMessage(err, "could not fetch calendar")
	}
	defer res.Body.Close()

	if res.StatusCode != http.StatusOK {
		return errors.New(fmt.Sprintf("unexpected status code %d", res.StatusCode))
	}

	if _, err := io.Copy(f, res.Body); err != nil {
		return errors.WithMessage(err, "could not write calendar to file")
	}

	err = f.Sync()
	if err != nil {
		return errors.WithMessage(err, "could not sync file")
	}

	return nil
}

func (c *Calendar) EventsBetween(from time.Time, to time.Time) ([]*Busy, error) {
	if c.Calendar == nil {
		return nil, errors.New("calendar not initialised")
	}

	in := c.Busys()
	bs := make([]*Busy, 0, len(in))

	for _, b := range in {
		st, err := b.GetStartAt()
		if err != nil {
			c.log.Warn("could not get start time", "event", b.Id(), "error", err)

			continue
		}

		et, err := b.GetEndAt()
		if err != nil {
			c.log.Warn("could not get end time", "event", b.Id(), "error", err)

			continue
		}

		if st.Before(from) && et.Before(from) ||
			st.After(to) && et.After(to) {
			continue
		}

		bs = append(bs, &Busy{
			VBusy:     *b,
			StartTime: Date{Time: st},
			EndTime:   Date{Time: et},
		})
	}

	slices.SortFunc(bs, func(a, b *Busy) int {
		return a.StartTime.Compare(b.StartTime.Time)
	})

	return bs, nil
}

func (c *Calendar) Weeks(count int) []Date {
	now := time.Now()
	weekday := int(now.Weekday()-time.Monday) % 7
	start := time.Date(
		now.Year(),
		now.Month(),
		now.Day()-(weekday),
		0,
		0,
		0,
		0,
		c.opts.Timezone.Location,
	)
	days := count*7 + weekday + 1
	dates := make([]Date, 0, days)

	for i := start; i.Before(start.AddDate(0, 0, days)); i = i.AddDate(0, 0, 1) {
		dates = append(dates, Date{Time: i})
	}

	return dates
}

func (c *Calendar) Availability(weeks int) ([]*CalendarDate, error) {
	if !c.opts.Cache {
		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
		err := c.FetchIfNeeded(ctx)
		cancel()
		if err != nil {
			return nil, err
		}
	}

	cds := make([]*CalendarDate, 0, weeks*7)
	dates := c.Weeks(weeks)

	for _, date := range dates {
		c.log.Debug("processing date", "date", date.DateOnly())

		cd := &CalendarDate{Date: date}

		evs, err := c.EventsBetween(date.Add(1*time.Second), date.EndOfDay().Time)
		if err != nil {
			return nil, errors.WithMessage(err, "could not get events")
		}

		for _, ev := range evs {
			c.log.Debug("processing event", "id", ev.Id())

			cd.BusyPeriods = append(cd.BusyPeriods, ev)
		}

		cds = append(cds, cd)
	}

	return cds, nil
}

func (d Date) Between(lower, upper Date) bool {
	return d.After(lower) && d.Before(upper)
}

func (d Date) Key() int {
	return d.Year()*10000 +
		int(d.Month())*100 +
		d.Day()
}

func (d Date) BeginningOfDay() Date {
	return Date{
		Time: time.Date(
			d.Year(),
			d.Month(),
			d.Day(),
			0,
			0,
			0,
			0,
			d.Location(),
		),
	}
}

func (d Date) EndOfDay() Date {
	return Date{
		Time: time.Date(
			d.Year(),
			d.Month(),
			d.Day(),
			23,
			59,
			59,
			999999999,
			d.Location(),
		),
	}
}

func (d Date) DateOnly() string {
	return d.Format(time.DateOnly)
}

func (d Date) NextDay() Date {
	return Date{
		Time: d.AddDate(0, 0, 1),
	}
}

func (d Date) IsToday() bool {
	return d.Key() == Date{time.Now()}.Key()
}

func (d Date) Before(other Date) bool {
	return d.Time.Before(other.Time)
}

func (d Date) After(other Date) bool {
	return d.Time.After(other.Time)
}