package web

import (
	"context"
	"errors"
	"fmt"
	"net/http"
	"net/url"
	"sort"
	"strings"
	"time"

	"github.com/m/projax/caldav"
	"github.com/m/projax/store"
)

const refTypeCalDAV = "caldav-list"

// CalDAVDeps is the optional CalDAV integration. When nil, the /admin/caldav
// page renders a "not configured" notice and the detail page hides the Tasks
// section. main.go sets it from DAV_URL / DAV_USER / DAV_PASSWORD env.
type CalDAVDeps struct {
	Client *caldav.Client
}

// Suggestion pairs one calendar with its best-match projax item, if any.
type Suggestion struct {
	Calendar    caldav.Calendar
	Item        *store.Item // nil = no auto-match
	AlreadyLink *store.ItemLink
}

// CalDAVOverview is rendered by /admin/caldav.
type CalDAVOverview struct {
	Suggestions []Suggestion
	Items       []*store.Item // for the manual-link selector
}

// buildCalDAVOverview fetches the calendar list, looks up existing
// caldav-list links, and pairs each calendar with the best matching projax
// item by case-insensitive title/slug.
func (s *Server) buildCalDAVOverview(ctx context.Context) (*CalDAVOverview, error) {
	cals, err := s.CalDAV.Client.ListCalendars(ctx)
	if err != nil {
		return nil, fmt.Errorf("caldav list: %w", err)
	}
	items, err := s.Store.ListAll(ctx)
	if err != nil {
		return nil, err
	}
	links, err := s.Store.LinksByRefType(ctx, refTypeCalDAV)
	if err != nil {
		return nil, err
	}
	// Map calendar URL → existing link
	byURL := map[string]*store.ItemLink{}
	for _, l := range links {
		byURL[l.RefID] = l
	}
	// Lower-case lookup over title+slug for the heuristic.
	byKey := map[string]*store.Item{}
	for _, it := range items {
		byKey[strings.ToLower(it.Slug)] = it
		byKey[strings.ToLower(it.Title)] = it
	}

	sort.Slice(cals, func(i, j int) bool { return cals[i].DisplayName < cals[j].DisplayName })
	overview := &CalDAVOverview{Items: items}
	for _, c := range cals {
		s := Suggestion{Calendar: c}
		if l, ok := byURL[c.URL]; ok {
			s.AlreadyLink = l
			// surface the linked item
			for _, it := range items {
				if it.ID == l.ItemID {
					s.Item = it
					break
				}
			}
		} else {
			key := strings.ToLower(c.DisplayName)
			if it, ok := byKey[key]; ok {
				s.Item = it
			}
		}
		overview.Suggestions = append(overview.Suggestions, s)
	}
	return overview, nil
}

func (s *Server) handleCalDAVAdmin(w http.ResponseWriter, r *http.Request) {
	if s.CalDAV == nil {
		s.render(w, r, "caldav_disabled", map[string]any{"Title": "caldav"})
		return
	}
	ov, err := s.buildCalDAVOverview(r.Context())
	if err != nil {
		s.fail(w, r, err)
		return
	}
	s.render(w, r, "caldav_admin", map[string]any{
		"Title":       "caldav",
		"Suggestions": ov.Suggestions,
		"Items":       ov.Items,
	})
}

func (s *Server) handleCalDAVLink(w http.ResponseWriter, r *http.Request) {
	if s.CalDAV == nil {
		http.Error(w, "caldav not configured", http.StatusServiceUnavailable)
		return
	}
	if err := r.ParseForm(); err != nil {
		s.fail(w, r, err)
		return
	}
	itemID := strings.TrimSpace(r.FormValue("item_id"))
	calURL := strings.TrimSpace(r.FormValue("calendar_url"))
	note := strings.TrimSpace(r.FormValue("display_name"))
	color := strings.TrimSpace(r.FormValue("color"))
	if itemID == "" || calURL == "" {
		http.Error(w, "item_id + calendar_url required", http.StatusBadRequest)
		return
	}
	meta := map[string]any{
		"display_name":   note,
		"calendar_color": color,
		"linked_at":      time.Now().UTC().Format(time.RFC3339),
	}
	if _, err := s.Store.AddLink(r.Context(), itemID, refTypeCalDAV, calURL, "contains", meta); err != nil {
		s.fail(w, r, err)
		return
	}
	http.Redirect(w, r, "/admin/caldav", http.StatusSeeOther)
}

func (s *Server) handleCalDAVUnlink(w http.ResponseWriter, r *http.Request) {
	if err := r.ParseForm(); err != nil {
		s.fail(w, r, err)
		return
	}
	linkID := strings.TrimSpace(r.FormValue("link_id"))
	if linkID == "" {
		http.Error(w, "link_id required", http.StatusBadRequest)
		return
	}
	if err := s.Store.DeleteLink(r.Context(), linkID); err != nil {
		s.fail(w, r, err)
		return
	}
	http.Redirect(w, r, "/admin/caldav", http.StatusSeeOther)
}

// handleCalDAVCreate handles POST /i/{path}/caldav/create — MKCALENDAR on
// dav.msbls.de derived from the item slug, then the item_link insert.
func (s *Server) handleCalDAVCreate(w http.ResponseWriter, r *http.Request, path string) {
	if s.CalDAV == nil {
		http.Error(w, "caldav not configured", http.StatusServiceUnavailable)
		return
	}
	it, err := s.Store.GetByPath(r.Context(), path)
	if err != nil {
		s.fail(w, r, err)
		return
	}
	slug := safeCalendarSlug(it.Slug)
	calURL := s.CalDAV.Client.BaseURL + slug + "/"
	displayName := it.Title
	if displayName == "" {
		displayName = it.Slug
	}
	if err := s.CalDAV.Client.CreateCalendar(r.Context(), calURL, displayName, ""); err != nil {
		if errors.Is(err, caldav.ErrCalendarExists) {
			// Existing calendar — link instead.
			meta := map[string]any{"display_name": displayName, "linked_at": time.Now().UTC().Format(time.RFC3339)}
			if _, err := s.Store.AddLink(r.Context(), it.ID, refTypeCalDAV, calURL, "contains", meta); err != nil {
				s.fail(w, r, err)
				return
			}
			http.Redirect(w, r, "/i/"+it.PrimaryPath(), http.StatusSeeOther)
			return
		}
		s.fail(w, r, err)
		return
	}
	meta := map[string]any{
		"display_name": displayName,
		"created_at":   time.Now().UTC().Format(time.RFC3339),
	}
	if _, err := s.Store.AddLink(r.Context(), it.ID, refTypeCalDAV, calURL, "contains", meta); err != nil {
		s.fail(w, r, err)
		return
	}
	http.Redirect(w, r, "/i/"+it.PrimaryPath(), http.StatusSeeOther)
}

// safeCalendarSlug normalises a projax slug for use in a CalDAV URL segment.
// Slugs are already lowercase + no dots per the projax invariant, but we
// re-escape to be safe.
func safeCalendarSlug(slug string) string {
	return url.PathEscape(strings.ToLower(strings.TrimSpace(slug)))
}

// detailTodos pulls open + recently-completed VTODOs for the item by iterating
// every caldav-list link. Errors per-calendar are logged and skipped so one
// down calendar doesn't blank the whole section.
type calendarTasks struct {
	CalendarURL string
	DisplayName string
	Open        []caldav.Todo
	DoneRecent  []caldav.Todo
	// Error, when non-empty, surfaces a per-calendar problem (network,
	// upstream auth, parse) so the UI can show a banner instead of silently
	// blanking the calendar.
	Error string
}

func (s *Server) detailTodos(ctx context.Context, item *store.Item) ([]calendarTasks, error) {
	if s.CalDAV == nil {
		return nil, nil
	}
	links, err := s.Store.LinksByType(ctx, item.ID, refTypeCalDAV)
	if err != nil {
		return nil, err
	}
	cutoff := time.Now().AddDate(0, 0, -30)
	var out []calendarTasks
	for _, l := range links {
		todos, err := s.CalDAV.Client.ListTodos(ctx, l.RefID)
		if err != nil {
			s.Logger.Warn("caldav todos", "calendar", l.RefID, "err", err)
			continue
		}
		ct := calendarTasks{
			CalendarURL: l.RefID,
			DisplayName: linkDisplay(l),
		}
		for _, td := range todos {
			if td.Status == "COMPLETED" || td.Status == "CANCELLED" {
				if td.LastModified == nil || td.LastModified.After(cutoff) {
					ct.DoneRecent = append(ct.DoneRecent, td)
				}
				continue
			}
			ct.Open = append(ct.Open, td)
		}
		out = append(out, ct)
	}
	return out, nil
}

func linkDisplay(l *store.ItemLink) string {
	if v, ok := l.Metadata["display_name"].(string); ok && v != "" {
		return v
	}
	if l.Note != nil && *l.Note != "" {
		return *l.Note
	}
	return l.RefID
}

// handleCalDAVTodoAction dispatches POST /i/{path}/caldav/todo/{action}.
// action ∈ {complete, reopen, edit, delete, todo-create}. The handler reloads
// the live VTODO (to pick up the freshest ETag), applies the requested edit,
// PUTs / DELETEs against the server, then re-renders the tasks section so
// HTMX can swap it in. 412 responses surface as a banner so m can retry.
func (s *Server) handleCalDAVTodoAction(w http.ResponseWriter, r *http.Request, path, action string) {
	if s.CalDAV == nil {
		http.Error(w, "caldav not configured", http.StatusServiceUnavailable)
		return
	}
	it, err := s.Store.GetByPath(r.Context(), path)
	if err != nil {
		s.fail(w, r, err)
		return
	}
	if err := r.ParseForm(); err != nil {
		s.fail(w, r, err)
		return
	}
	calURL := strings.TrimSpace(r.FormValue("calendar_url"))
	if calURL == "" {
		http.Error(w, "calendar_url required", http.StatusBadRequest)
		return
	}
	// Guard: the calendar URL must be linked to this item — otherwise a
	// crafted form could route writes to arbitrary calendars.
	links, err := s.Store.LinksByType(r.Context(), it.ID, refTypeCalDAV)
	if err != nil {
		s.fail(w, r, err)
		return
	}
	var matchedLink *store.ItemLink
	for _, l := range links {
		if l.RefID == calURL {
			matchedLink = l
			break
		}
	}
	if matchedLink == nil {
		http.Error(w, "calendar not linked to this item", http.StatusForbidden)
		return
	}

	banner := ""
	switch action {
	case "todo-create":
		summary := strings.TrimSpace(r.FormValue("summary"))
		if summary == "" {
			banner = "Cannot create task with empty summary."
			break
		}
		edit := caldav.VTodoEdit{Summary: &summary}
		if dueStr := strings.TrimSpace(r.FormValue("due")); dueStr != "" {
			if t, ok := parseDueInput(dueStr); ok {
				edit.Due = &t
			}
		}
		uid := caldav.NewUID()
		ics := caldav.BuildVTodoICS(uid, edit)
		url := caldav.TodoURLFor(calURL, uid)
		if _, err := s.CalDAV.Client.PutTodo(r.Context(), url, ics, "", "*"); err != nil {
			banner = "Could not create task: " + err.Error()
		}
	case "complete", "reopen", "edit", "delete":
		uid := strings.TrimSpace(r.FormValue("uid"))
		if uid == "" {
			http.Error(w, "uid required", http.StatusBadRequest)
			return
		}
		// Refetch — ETags from the original page render may be stale, and we
		// also need the latest Raw ICS body for in-place edits that preserve
		// unknown fields.
		todos, err := s.CalDAV.Client.ListTodos(r.Context(), calURL)
		if err != nil {
			banner = "Could not reach calendar: " + err.Error()
			break
		}
		var current *caldav.Todo
		for i := range todos {
			if todos[i].UID == uid {
				current = &todos[i]
				break
			}
		}
		if current == nil {
			banner = "Task no longer exists on the server."
			break
		}
		switch action {
		case "complete":
			st := "COMPLETED"
			updated := caldav.ApplyVTodoEdit(current.Raw, caldav.VTodoEdit{Status: &st})
			if _, err := s.CalDAV.Client.PutTodo(r.Context(), current.URL, updated, current.ETag, ""); err != nil {
				banner = caldavBanner("complete", err)
			}
		case "reopen":
			st := "NEEDS-ACTION"
			updated := caldav.ApplyVTodoEdit(current.Raw, caldav.VTodoEdit{Status: &st})
			if _, err := s.CalDAV.Client.PutTodo(r.Context(), current.URL, updated, current.ETag, ""); err != nil {
				banner = caldavBanner("reopen", err)
			}
		case "edit":
			edit := caldav.VTodoEdit{}
			if v := r.FormValue("summary"); v != "" {
				vv := strings.TrimSpace(v)
				edit.Summary = &vv
			}
			if dueStr := strings.TrimSpace(r.FormValue("due")); dueStr != "" {
				if t, ok := parseDueInput(dueStr); ok {
					edit.Due = &t
				}
			} else if _, present := r.Form["due"]; present {
				// Field submitted but blank → user cleared it.
				edit.ClearDue = true
			}
			updated := caldav.ApplyVTodoEdit(current.Raw, edit)
			if _, err := s.CalDAV.Client.PutTodo(r.Context(), current.URL, updated, current.ETag, ""); err != nil {
				banner = caldavBanner("edit", err)
			}
		case "delete":
			if err := s.CalDAV.Client.DeleteTodo(r.Context(), current.URL, current.ETag); err != nil {
				banner = caldavBanner("delete", err)
			}
		}
	default:
		http.Error(w, "unknown action: "+action, http.StatusBadRequest)
		return
	}

	// Writeback may move a task on or off the timeline, so bust both caches.
	if s.dashboard != nil {
		s.dashboard.InvalidateAll()
	}
	if s.timeline != nil {
		s.timeline.InvalidateAll()
	}
	// Always re-render the tasks section so HTMX (or a plain redirect for
	// non-HTMX clients) sees the post-write state.
	if r.Header.Get("HX-Request") == "true" {
		s.renderTasksSection(w, r, it, banner)
		return
	}
	http.Redirect(w, r, "/i/"+it.PrimaryPath(), http.StatusSeeOther)
}

// caldavBanner formats an HTMX-banner string from a write error, distinguishing
// the 412-mismatch case ("task changed elsewhere") from generic upstream
// failures so m sees something actionable.
func caldavBanner(action string, err error) string {
	if errors.Is(err, caldav.ErrPreconditionFailed) {
		return "Task changed elsewhere since this page was loaded — refresh and retry the " + action + "."
	}
	if errors.Is(err, caldav.ErrNotFound) {
		return "Task is gone on the server. The list below is current."
	}
	return "Could not " + action + " task: " + err.Error()
}

// renderTasksSection re-runs detailTodos for the item and renders the
// tasks-section template fragment with an optional banner. Used by HTMX
// responses so swap operations stay in-place.
func (s *Server) renderTasksSection(w http.ResponseWriter, r *http.Request, it *store.Item, banner string) {
	tasks, err := s.detailTodos(r.Context(), it)
	if err != nil {
		s.fail(w, r, err)
		return
	}
	data := map[string]any{
		"Item":     it,
		"Tasks":    tasks,
		"CalDAVOn": s.CalDAV != nil,
		"Banner":   banner,
	}
	s.render(w, r, "tasks_section", data)
}

// parseDueInput accepts an HTML5 date-input value (`YYYY-MM-DD`) or a
// datetime-local value (`YYYY-MM-DDTHH:MM`), returning the corresponding UTC
// time. Dates with no clock component round-trip to a DUE;VALUE=DATE line.
func parseDueInput(s string) (time.Time, bool) {
	s = strings.TrimSpace(s)
	if s == "" {
		return time.Time{}, false
	}
	for _, layout := range []string{"2006-01-02T15:04", "2006-01-02T15:04:05", "2006-01-02"} {
		if t, err := time.Parse(layout, s); err == nil {
			return t, true
		}
	}
	return time.Time{}, false
}