package calendar

import (
	"bytes"
	"context"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"net/http"
	"strings"
	"time"

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

	"aether/internal/auth"
)

// Pfadschema. Zwei Eigenheiten der Bibliothek bestimmen es:
//
//  1. Zurückgegebene Pfade (CurrentUserPrincipal, CalendarHomeSetPath,
//     Calendar.Path, ...) vergleicht sie direkt mit r.URL.Path – also
//     inklusive Präfix. Deshalb tragen alle hier gebauten Pfade davPrefix
//     von Anfang an mit.
//
//  2. Welche Art Ressource ein Pfad ist, entscheidet sie allein an der
//     Anzahl der Pfadsegmente hinter dem Präfix (resourceTypeAtPath in
//     server.go) – nicht am Namen. Ein Segment = Principal, zwei = Calendar-
//     Home-Set, drei = Kalender, vier = Objekt. Deshalb steckt der
//     Nutzername beim Principal *in* dem einen Segment ("principals-max"),
//     nicht als eigenes Unterverzeichnis – sonst läse die Bibliothek ihn als
//     Calendar-Home-Set und die PROPFIND-Antwort bliebe leer.
//
//     /caldav/principals-{username}/                          – 1 Segment
//     /caldav/calendars/{username}/                            – 2 Segmente, Calendar-Home-Set
//     /caldav/calendars/{username}/{calendar-id}/               – 3 Segmente, ein Kalender
//     /caldav/calendars/{username}/{calendar-id}/{uid}.ics       – 4 Segmente, ein Objekt
const davPrefix = "/caldav"

type contextKey int

const keyUser contextKey = iota

func contextWithUser(ctx context.Context, u *auth.User) context.Context {
	return context.WithValue(ctx, keyUser, u)
}

func userFromContext(ctx context.Context) (*auth.User, error) {
	u, _ := ctx.Value(keyUser).(*auth.User)
	if u == nil {
		return nil, webdav.NewHTTPError(http.StatusUnauthorized, errors.New("no authenticated user"))
	}
	return u, nil
}

// Backend implementiert caldav.Backend. Phase 1: jeder Nutzer sieht nur die
// eigenen Kalender – Freigaben (cal_shares) kommen erst in einer späteren
// Phase, siehe kalender-plan.md.
type Backend struct {
	store *Store
}

func NewBackend(store *Store) *Backend {
	return &Backend{store: store}
}

func (b *Backend) CurrentUserPrincipal(ctx context.Context) (string, error) {
	u, err := userFromContext(ctx)
	if err != nil {
		return "", err
	}
	return davPrefix + "/principals-" + u.Username + "/", nil
}

func (b *Backend) CalendarHomeSetPath(ctx context.Context) (string, error) {
	u, err := userFromContext(ctx)
	if err != nil {
		return "", err
	}
	return davPrefix + "/calendars/" + u.Username + "/", nil
}

func calendarPath(username string, id uuid.UUID) string {
	return davPrefix + "/calendars/" + username + "/" + id.String() + "/"
}

func objectPath(username string, id uuid.UUID, uid string) string {
	return calendarPath(username, id) + uid + ".ics"
}

// parseCalendarPath erwartet "/caldav/calendars/{username}/{calendar-id}(/...)"
// und liefert die Kalender-ID. Der Benutzername im Pfad wird nicht gegen den
// angemeldeten Nutzer geprüft – Phase 1 kennt ohnehin nur den eigenen Pfad,
// spätere Freigaben ändern das gezielt in resolveCalendar.
func parseCalendarPath(p string) (calID uuid.UUID, rest string, err error) {
	parts := strings.SplitN(strings.TrimPrefix(p, davPrefix+"/calendars/"), "/", 3)
	if len(parts) < 2 || parts[1] == "" {
		return uuid.UUID{}, "", fmt.Errorf("calendar: malformed path %q", p)
	}
	id, err := uuid.Parse(parts[1])
	if err != nil {
		return uuid.UUID{}, "", fmt.Errorf("calendar: malformed calendar id in %q: %w", p, err)
	}
	if len(parts) == 3 {
		rest = parts[2]
	}
	return id, rest, nil
}

func (b *Backend) resolveCalendar(ctx context.Context, p string) (*auth.User, *Calendar, error) {
	u, err := userFromContext(ctx)
	if err != nil {
		return nil, nil, err
	}
	calID, _, err := parseCalendarPath(p)
	if err != nil {
		return nil, nil, webdav.NewHTTPError(http.StatusNotFound, err)
	}
	cal, err := b.store.GetCalendar(ctx, u.ID, calID)
	if errors.Is(err, ErrNotFound) {
		return nil, nil, webdav.NewHTTPError(http.StatusNotFound, err)
	}
	if err != nil {
		return nil, nil, err
	}
	return u, cal, nil
}

func toCalDAVCalendar(username string, c *Calendar) caldav.Calendar {
	comps := []string{"VEVENT"}
	if c.SupportsTodo {
		comps = append(comps, "VTODO")
	}
	return caldav.Calendar{
		Path:                  calendarPath(username, c.ID),
		Name:                  c.Name,
		Description:           c.Description,
		MaxResourceSize:       5 * 1024 * 1024,
		SupportedComponentSet: comps,
	}
}

func (b *Backend) CreateCalendar(ctx context.Context, cal *caldav.Calendar) error {
	u, err := userFromContext(ctx)
	if err != nil {
		return err
	}
	name := cal.Name
	if name == "" {
		name = "Kalender"
	}
	created, err := b.store.CreateCalendar(ctx, u.ID, name, "", cal.Description)
	if err != nil {
		return fmt.Errorf("create calendar: %w", err)
	}
	cal.Path = calendarPath(u.Username, created.ID)
	return nil
}

func (b *Backend) ListCalendars(ctx context.Context) ([]caldav.Calendar, error) {
	u, err := userFromContext(ctx)
	if err != nil {
		return nil, err
	}
	if _, err := b.store.EnsureDefaultCalendar(ctx, u.ID); err != nil {
		return nil, fmt.Errorf("ensure default calendar: %w", err)
	}

	all, err := b.store.ListCalendars(ctx, u.ID)
	if err != nil {
		return nil, fmt.Errorf("list calendars: %w", err)
	}
	out := make([]caldav.Calendar, len(all))
	for i, c := range all {
		out[i] = toCalDAVCalendar(u.Username, &c)
	}
	return out, nil
}

func (b *Backend) GetCalendar(ctx context.Context, p string) (*caldav.Calendar, error) {
	u, cal, err := b.resolveCalendar(ctx, p)
	if err != nil {
		return nil, err
	}
	c := toCalDAVCalendar(u.Username, cal)
	return &c, nil
}

func toCalendarObject(username string, calID uuid.UUID, o *Object) (*caldav.CalendarObject, error) {
	dec := ical.NewDecoder(strings.NewReader(o.Data))
	data, err := dec.Decode()
	if err != nil {
		return nil, fmt.Errorf("decode object %s: %w", o.UID, err)
	}
	return &caldav.CalendarObject{
		Path:          objectPath(username, calID, o.UID),
		ModTime:       o.UpdatedAt,
		ContentLength: int64(len(o.Data)),
		ETag:          o.ETag,
		Data:          data,
	}, nil
}

func (b *Backend) GetCalendarObject(ctx context.Context, p string, _ *caldav.CalendarCompRequest) (*caldav.CalendarObject, error) {
	u, cal, err := b.resolveCalendar(ctx, p)
	if err != nil {
		return nil, err
	}
	_, rest, err := parseCalendarPath(p)
	if err != nil {
		return nil, webdav.NewHTTPError(http.StatusNotFound, err)
	}
	uid := strings.TrimSuffix(rest, ".ics")
	obj, err := b.store.GetObjectByUID(ctx, cal.ID, uid)
	if errors.Is(err, ErrNotFound) {
		return nil, webdav.NewHTTPError(http.StatusNotFound, err)
	}
	if err != nil {
		return nil, err
	}
	return toCalendarObject(u.Username, cal.ID, obj)
}

func (b *Backend) ListCalendarObjects(ctx context.Context, p string, _ *caldav.CalendarCompRequest) ([]caldav.CalendarObject, error) {
	u, cal, err := b.resolveCalendar(ctx, p)
	if err != nil {
		return nil, err
	}
	objs, err := b.store.ListObjects(ctx, cal.ID)
	if err != nil {
		return nil, fmt.Errorf("list objects: %w", err)
	}
	out := make([]caldav.CalendarObject, 0, len(objs))
	for i := range objs {
		co, err := toCalendarObject(u.Username, cal.ID, &objs[i])
		if err != nil {
			// Eine kaputte Einzeldatei darf nicht die gesamte Synchronisation
			// blockieren; der Client bekommt die übrigen Termine trotzdem.
			continue
		}
		out = append(out, *co)
	}
	return out, nil
}

func (b *Backend) QueryCalendarObjects(ctx context.Context, p string, query *caldav.CalendarQuery) ([]caldav.CalendarObject, error) {
	all, err := b.ListCalendarObjects(ctx, p, &query.CompRequest)
	if err != nil {
		return nil, err
	}
	return caldav.Filter(query, all)
}

// firstUID/firstComponent lesen UID und Komponententyp (VEVENT/VTODO) aus
// dem ersten Termin-/Aufgaben-Kind der .ics-Datei. VTIMEZONE-Kinder werden
// übersprungen, die tragen keine UID.
func firstUIDAndComponent(cal *ical.Calendar) (uid, component string, err error) {
	for _, child := range cal.Children {
		if child.Name != "VEVENT" && child.Name != "VTODO" {
			continue
		}
		v, err := child.Props.Text("UID")
		if err != nil || v == "" {
			return "", "", fmt.Errorf("calendar object missing UID")
		}
		return v, child.Name, nil
	}
	return "", "", fmt.Errorf("calendar object has no VEVENT/VTODO component")
}

func timeRange(cal *ical.Calendar) (start, end *time.Time) {
	for _, ev := range cal.Events() {
		if t, err := ev.DateTimeStart(time.Local); err == nil {
			start = &t
		}
		if t, err := ev.DateTimeEnd(time.Local); err == nil {
			end = &t
		}
		break
	}
	return
}

func etagFor(data string) string {
	sum := sha256.Sum256([]byte(data))
	return hex.EncodeToString(sum[:])
}

func (b *Backend) PutCalendarObject(ctx context.Context, p string, cal *ical.Calendar, opts *caldav.PutCalendarObjectOptions) (*caldav.CalendarObject, error) {
	u, calRow, err := b.resolveCalendar(ctx, p)
	if err != nil {
		return nil, err
	}
	uid, component, err := firstUIDAndComponent(cal)
	if err != nil {
		return nil, webdav.NewHTTPError(http.StatusBadRequest, err)
	}

	existing, err := b.store.GetObjectByUID(ctx, calRow.ID, uid)
	if err != nil && !errors.Is(err, ErrNotFound) {
		return nil, err
	}
	if opts != nil {
		if opts.IfNoneMatch.IsSet() && opts.IfNoneMatch.IsWildcard() && existing != nil {
			return nil, webdav.NewHTTPError(http.StatusPreconditionFailed, errors.New("object already exists"))
		}
		if opts.IfMatch.IsSet() {
			if existing == nil {
				return nil, webdav.NewHTTPError(http.StatusPreconditionFailed, errors.New("object not found"))
			}
			ok, err := opts.IfMatch.MatchETag(existing.ETag)
			if err != nil || !ok {
				return nil, webdav.NewHTTPError(http.StatusPreconditionFailed, errors.New("etag mismatch"))
			}
		}
	}

	var buf bytes.Buffer
	if err := ical.NewEncoder(&buf).Encode(cal); err != nil {
		return nil, fmt.Errorf("encode object: %w", err)
	}
	data := buf.String()
	etag := etagFor(data)
	start, end := timeRange(cal)

	obj, err := b.store.PutObject(ctx, calRow.ID, uid, component, data, etag, start, end)
	if err != nil {
		return nil, fmt.Errorf("put object: %w", err)
	}
	return toCalendarObject(u.Username, calRow.ID, obj)
}

func (b *Backend) DeleteCalendarObject(ctx context.Context, p string) error {
	_, calRow, err := b.resolveCalendar(ctx, p)
	if err != nil {
		return err
	}
	_, rest, err := parseCalendarPath(p)
	if err != nil {
		return webdav.NewHTTPError(http.StatusNotFound, err)
	}
	uid := strings.TrimSuffix(rest, ".ics")
	if err := b.store.DeleteObject(ctx, calRow.ID, uid); err != nil {
		if errors.Is(err, ErrNotFound) {
			return webdav.NewHTTPError(http.StatusNotFound, err)
		}
		return fmt.Errorf("delete object: %w", err)
	}
	return nil
}