package caldav

import (
	"context"
	"fmt"
	"log/slog"
	"net/http"
	"path"
	"strings"
	"time"

	ical "github.com/emersion/go-ical"
	"github.com/emersion/go-webdav"
	"github.com/emersion/go-webdav/caldav"
	"github.com/yourusername/caldav-server/internal/auth"
	"github.com/yourusername/caldav-server/internal/config"
	"github.com/yourusername/caldav-server/internal/store"
)

// Backend implements caldav.Backend using a filesystem store.
type Backend struct {
	cfg    *config.Config
	store  *store.Store
	logger *slog.Logger
}

// NewBackend creates a CalDAV backend.
func NewBackend(cfg *config.Config, st *store.Store, logger *slog.Logger) *Backend {
	return &Backend{cfg: cfg, store: st, logger: logger}
}

// NewHandler returns an http.Handler for the /cal/ prefix.
func NewHandler(cfg *config.Config, st *store.Store, logger *slog.Logger) http.Handler {
	b := NewBackend(cfg, st, logger)
	return &caldav.Handler{Backend: b}
}

// -------- caldav.Backend interface --------

func (b *Backend) CurrentUserPrincipal(ctx context.Context) (string, error) {
	p := auth.FromContext(ctx)
	if p == nil {
		return "", webdav.NewHTTPError(http.StatusUnauthorized, fmt.Errorf("not authenticated"))
	}
	return principalPath(p.Username), nil
}

func (b *Backend) CalendarHomeSetPath(ctx context.Context) (string, error) {
	p := auth.FromContext(ctx)
	if p == nil {
		return "", webdav.NewHTTPError(http.StatusUnauthorized, fmt.Errorf("not authenticated"))
	}
	return calHomePath(p.Username), nil
}

func (b *Backend) ListCalendars(ctx context.Context) ([]caldav.Calendar, error) {
	p := auth.FromContext(ctx)
	if p == nil {
		return nil, webdav.NewHTTPError(http.StatusUnauthorized, fmt.Errorf("not authenticated"))
	}

	user, ok := b.cfg.Users[p.Username]
	if !ok {
		return nil, fmt.Errorf("user not found")
	}

	var cals []caldav.Calendar
	for _, name := range user.Calendars {
		if err := b.store.EnsureCollection(p.Username, "cal-"+name); err != nil {
			b.logger.Warn("ensuring calendar directory", "calendar", name, "error", err)
			continue
		}
		cals = append(cals, b.calendarMeta(p.Username, name))
	}

	// Also include any extra calendars that exist on disk but aren't in config
	disk, _ := b.store.ListCollections(p.Username)
	configured := make(map[string]bool)
	for _, n := range user.Calendars {
		configured["cal-"+n] = true
	}
	for _, dir := range disk {
		if strings.HasPrefix(dir, "cal-") && !configured[dir] {
			name := strings.TrimPrefix(dir, "cal-")
			cals = append(cals, b.calendarMeta(p.Username, name))
		}
	}

	return cals, nil
}

func (b *Backend) GetCalendar(ctx context.Context, calPath string) (*caldav.Calendar, error) {
	user, name, err := b.parseCalPath(ctx, calPath)
	if err != nil {
		return nil, err
	}
	if _, err := b.store.GetCollection(user, "cal-"+name); err != nil {
		return nil, webdav.NewHTTPError(http.StatusNotFound, err)
	}
	cal := b.calendarMeta(user, name)
	return &cal, nil
}

func (b *Backend) GetCalendarObject(ctx context.Context, objPath string, req *caldav.CalendarCompRequest) (*caldav.CalendarObject, error) {
	user, calName, objID, err := b.parseObjPath(ctx, objPath)
	if err != nil {
		return nil, err
	}

	data, err := b.store.GetObject(user, "cal-"+calName, objID)
	if err != nil {
		return nil, webdav.NewHTTPError(http.StatusNotFound, err)
	}

	return b.decodeObject(objPath, data)
}

func (b *Backend) ListCalendarObjects(ctx context.Context, calPath string, req *caldav.CalendarCompRequest) ([]caldav.CalendarObject, error) {
	user, calName, err := b.parseCalPath(ctx, calPath)
	if err != nil {
		return nil, err
	}

	ids, err := b.store.ListObjects(user, "cal-"+calName)
	if err != nil {
		return nil, err
	}

	var objs []caldav.CalendarObject
	for _, id := range ids {
		data, err := b.store.GetObject(user, "cal-"+calName, id)
		if err != nil {
			continue
		}
		obj, err := b.decodeObject(calObjectPath(user, calName, id), data)
		if err != nil {
			b.logger.Warn("decoding calendar object", "id", id, "error", err)
			continue
		}
		objs = append(objs, *obj)
	}
	return objs, nil
}

func (b *Backend) QueryCalendarObjects(ctx context.Context, calPath string, query *caldav.CalendarQuery) ([]caldav.CalendarObject, error) {
	// List all and filter – sufficient for small collections.
	all, err := b.ListCalendarObjects(ctx, calPath, &query.CompRequest)
	if err != nil {
		return nil, err
	}
	return caldav.Filter(query, all)
}

func (b *Backend) CreateCalendar(ctx context.Context, calendar *caldav.Calendar) error {
	p := auth.FromContext(ctx)
	if p == nil {
		return webdav.NewHTTPError(http.StatusUnauthorized, fmt.Errorf("not authenticated"))
	}
	// Derive collection name from the trailing path segment.
	name := path.Base(strings.TrimSuffix(calendar.Path, "/"))
	return b.store.EnsureCollection(p.Username, "cal-"+name)
}

func (b *Backend) DeleteCalendar(ctx context.Context, calPath string) error {
	user, name, err := b.parseCalPath(ctx, calPath)
	if err != nil {
		return err
	}
	return b.store.DeleteCollection(user, "cal-"+name)
}

func (b *Backend) PutCalendarObject(ctx context.Context, objPath string, calendar *ical.Calendar, opts *caldav.PutCalendarObjectOptions) (*caldav.CalendarObject, error) {
	user, calName, objID, err := b.parseObjPath(ctx, objPath)
	if err != nil {
		return nil, err
	}

	var buf strings.Builder
	enc := ical.NewEncoder(&buf)
	if err := enc.Encode(calendar); err != nil {
		return nil, fmt.Errorf("encoding calendar: %w", err)
	}

	data := []byte(buf.String())
	if err := b.store.PutObject(user, "cal-"+calName, objID, data); err != nil {
		return nil, fmt.Errorf("storing calendar object: %w", err)
	}

	return b.decodeObject(objPath, data)
}

func (b *Backend) DeleteCalendarObject(ctx context.Context, objPath string) error {
	user, calName, objID, err := b.parseObjPath(ctx, objPath)
	if err != nil {
		return err
	}
	if err := b.store.DeleteObject(user, "cal-"+calName, objID); err != nil {
		return webdav.NewHTTPError(http.StatusNotFound, err)
	}
	return nil
}

// -------- helpers --------

func (b *Backend) calendarMeta(user, name string) caldav.Calendar {
	return caldav.Calendar{
		Path:                  calHomePath(user) + name + "/",
		Name:                  name,
		Description:           fmt.Sprintf("%s's %s calendar", user, name),
		SupportedComponentSet: []string{"VEVENT", "VTODO", "VJOURNAL"},
		MaxResourceSize:       10 * 1024 * 1024, // 10 MiB
	}
}

func (b *Backend) decodeObject(objPath string, data []byte) (*caldav.CalendarObject, error) {
	cal, err := ical.NewDecoder(strings.NewReader(string(data))).Decode()
	if err != nil {
		return nil, fmt.Errorf("decoding ical: %w", err)
	}

	etag := fmt.Sprintf(`"%x"`, hashBytes(data))

	return &caldav.CalendarObject{
		Path:          objPath,
		ModTime:       time.Now(),
		ContentLength: int64(len(data)),
		ETag:          etag,
		Data:          cal,
	}, nil
}

func (b *Backend) parseCalPath(ctx context.Context, calPath string) (user, calName string, err error) {
	p := auth.FromContext(ctx)
	if p == nil {
		return "", "", webdav.NewHTTPError(http.StatusUnauthorized, fmt.Errorf("not authenticated"))
	}
	user = p.Username
	parts := strings.Split(strings.Trim(calPath, "/"), "/")
	// expected: cal/<user>/<calname>/
	if len(parts) < 3 {
		return "", "", webdav.NewHTTPError(http.StatusBadRequest, fmt.Errorf("invalid calendar path"))
	}
	calName = parts[2]
	return user, calName, nil
}

func (b *Backend) parseObjPath(ctx context.Context, objPath string) (user, calName, objID string, err error) {
	p := auth.FromContext(ctx)
	if p == nil {
		return "", "", "", webdav.NewHTTPError(http.StatusUnauthorized, fmt.Errorf("not authenticated"))
	}
	user = p.Username
	parts := strings.Split(strings.Trim(objPath, "/"), "/")
	// expected: cal/<user>/<calname>/<objid>
	if len(parts) < 4 {
		return "", "", "", webdav.NewHTTPError(http.StatusBadRequest, fmt.Errorf("invalid object path"))
	}
	calName = parts[2]
	objID = path.Base(objPath)
	return user, calName, objID, nil
}

func principalPath(user string) string {
	return fmt.Sprintf("/principals/%s/", user)
}

func calHomePath(user string) string {
	return fmt.Sprintf("/cal/%s/", user)
}

func calObjectPath(user, calName, objID string) string {
	return fmt.Sprintf("/cal/%s/%s/%s", user, calName, objID)
}

func hashBytes(data []byte) uint64 {
	var h uint64 = 14695981039346656037
	for _, b := range data {
		h ^= uint64(b)
		h *= 1099511628211
	}
	return h
}