package caldav

import (
	"crypto/sha1"
	"encoding/hex"
	"fmt"
	"net/http"
	"strings"
	"time"

	ical "github.com/emersion/go-ical"
	"github.com/emersion/go-webdav"
	"github.com/emersion/go-webdav/caldav"

	"git.arnef.de/arnef/nidus/internal/db"
)

// defaultICSColor is the display color for an ICS/webcal subscription
// calendar when the user hasn't chosen one, matching internal/web's.
const defaultICSColor = "#0ea5e9"

// icsSubscriptionCalendarMeta returns the caldav.Calendar metadata for
// owner's ICS subscription sub, exposed under its own chosen name.
func (b *Backend) icsSubscriptionCalendarMeta(owner string, sub db.ICSSubscription) caldav.Calendar {
	return caldav.Calendar{
		Path:                  calHomePath() + sub.Name + "/",
		Name:                  sub.Name,
		Description:           "Read-only subscription: " + sub.URL,
		SupportedComponentSet: []string{"VEVENT"},
		MaxResourceSize:       256 * 1024,
	}
}

// icsObjectUID returns the UID a fetched VEVENT should be addressed by:
// its own UID property if it has one, otherwise a stable hash of its
// position so it still round-trips consistently between requests.
func icsObjectUID(ev ical.Event, fallback string) string {
	if p := ev.Props.Get(ical.PropUID); p != nil && p.Value != "" {
		return p.Value
	}
	return fallback
}

// icsObjID builds the object ID (file-name-like, ".ics" suffixed) used to
// address a fetched VEVENT within its subscription calendar, derived from
// its UID so it stays stable across fetches of the same feed.
func icsObjID(uid string) string {
	sum := sha1.Sum([]byte(uid))
	return hex.EncodeToString(sum[:]) + ".ics"
}

// listICSSubscriptionCalendarObjects fetches sub's remote calendar (via
// b.icsCache) and returns one caldav.CalendarObject per VEVENT.
func (b *Backend) listICSSubscriptionCalendarObjects(localName string, sub db.ICSSubscription) ([]caldav.CalendarObject, error) {
	cal, err := b.icsCache.Get(sub.URL)
	if err != nil {
		return nil, fmt.Errorf("fetching ics subscription %q: %w", sub.Name, err)
	}

	var objs []caldav.CalendarObject
	for i, ev := range cal.Events() {
		obj, err := b.encodeICSObject(localName, ev, fmt.Sprintf("event-%d", i))
		if err != nil {
			continue
		}
		objs = append(objs, *obj)
	}
	return objs, nil
}

// icsSubscriptionCalendarObject fetches sub's remote calendar and returns
// the single VEVENT whose derived object ID matches objID.
func (b *Backend) icsSubscriptionCalendarObject(localName, objID string, sub db.ICSSubscription) (*caldav.CalendarObject, error) {
	cal, err := b.icsCache.Get(sub.URL)
	if err != nil {
		return nil, webdav.NewHTTPError(http.StatusNotFound, fmt.Errorf("fetching ics subscription: %w", err))
	}
	for i, ev := range cal.Events() {
		uid := icsObjectUID(ev, fmt.Sprintf("event-%d", i))
		if icsObjID(uid) != objID {
			continue
		}
		return b.encodeICSObject(localName, ev, fmt.Sprintf("event-%d", i))
	}
	return nil, webdav.NewHTTPError(http.StatusNotFound, fmt.Errorf("ics subscription event not found"))
}

// encodeICSObject wraps a single fetched VEVENT ev into its own
// caldav.CalendarObject, encoding it as a standalone one-event calendar
// the same way every other calendar object in this backend is
// represented. fallbackUID is used to derive the object ID/UID if ev has
// no UID property of its own.
func (b *Backend) encodeICSObject(localName string, ev ical.Event, fallbackUID string) (*caldav.CalendarObject, error) {
	uid := icsObjectUID(ev, fallbackUID)
	objID := icsObjID(uid)

	event := ical.NewEvent()
	event.Props = ev.Props
	if event.Props.Get(ical.PropUID) == nil {
		event.Props.SetText(ical.PropUID, uid)
	}
	if event.Props.Get(ical.PropDateTimeStamp) == nil {
		event.Props.SetDateTime(ical.PropDateTimeStamp, time.Now().UTC())
	}

	out := ical.NewCalendar()
	out.Props.SetText(ical.PropVersion, "2.0")
	out.Props.SetText(ical.PropProductID, "-//nidus//ics-subscription//EN")
	out.Children = append(out.Children, event.Component)

	var buf strings.Builder
	if err := ical.NewEncoder(&buf).Encode(out); err != nil {
		return nil, fmt.Errorf("encoding ics subscription event: %w", err)
	}
	data := []byte(buf.String())

	return &caldav.CalendarObject{
		Path:          calObjectPath(localName, objID),
		ModTime:       time.Now(),
		ContentLength: int64(len(data)),
		ETag:          fmt.Sprintf(`"ics-%s"`, objID),
		Data:          out,
	}, nil
}