package main

import (
	"context"
	"encoding/xml"
	"fmt"
	"io"
	"log"
	"net/http"
	"net/url"
	"strings"
	"time"

	"github.com/kami/maven/internal/calendar"
	"github.com/kami/maven/internal/ipc"
	"github.com/kami/maven/internal/store"
)

// renderer is the write half of maven's own local calendar (Vikunja #127).
//
// It is a RENDER TARGET, not a store. sqlite stays canonical: every tick the
// renderer reads the pending reminders out of core and publishes each one as a
// single-event iCal resource in a CalDAV collection maven owns. Nothing is ever
// read back from that collection, and losing it costs nothing — the next tick
// rebuilds it.
//
// It structurally cannot write to a calendar maven only reads. The URL comes
// from its own flag, checked at startup against every read URL (see
// run in main.go), and the only paths it ever addresses carry
// calendar.ReminderUIDPrefix — so even pointed at the wrong collection it can
// only touch resources it created.
type renderer struct {
	core ipc.CoreAPI
	http *http.Client
	url  string
	user string
	pass string
	dur  time.Duration

	// published maps reminder id → the body last successfully PUT, so an
	// unchanged reminder costs nothing. Purely an optimisation: a restart
	// re-publishes every reminder once, which is idempotent.
	published map[int64]string

	// reconciled — whether the collection has been read once since start. It
	// has to be, because published is in-memory: withdrawal used to cover only
	// the reminders THIS process published, so a reminder that fired while the
	// daemon was down kept its event in the calendar forever, and nothing ever
	// revisited it.
	reconciled bool
}

func newRenderer(core ipc.CoreAPI, hc *http.Client, url, user, pass string, dur time.Duration) *renderer {
	return &renderer{
		core:      core,
		http:      hc,
		url:       strings.TrimRight(url, "/"),
		user:      user,
		pass:      pass,
		dur:       dur,
		published: make(map[int64]string),
	}
}

// renderOnce publishes every pending reminder and withdraws the ones that are
// no longer pending. Errors are logged and skipped: a calendar maven cannot
// reach must never break the reminder itself, which lives in sqlite.
func (r *renderer) renderOnce(ctx context.Context) {
	reminders, err := r.core.ListReminders(ctx, renderMaxReminders)
	if err != nil {
		log.Printf("mavcaldav: list reminders: %v", err)
		return
	}

	live := make(map[int64]bool, len(reminders))
	for _, rem := range reminders {
		if rem.Status != store.ReminderPending {
			continue
		}
		live[rem.ID] = true
		e := calendar.ReminderEvent(rem.ID, fireTime(rem), rem.Payload, r.dur)
		body := calendar.RenderICal([]calendar.Event{e})
		if r.published[rem.ID] == body {
			continue
		}
		if err := r.put(ctx, calendar.ReminderPath(rem.ID), body); err != nil {
			log.Printf("mavcaldav: render reminder %d: %v", rem.ID, err)
			continue
		}
		r.published[rem.ID] = body
		log.Printf("mavcaldav: rendered reminder %d (%s)", rem.ID, e.Summary)
	}

	stale := make(map[int64]bool)
	for id := range r.published {
		if !live[id] {
			stale[id] = true
		}
	}
	if !r.reconciled {
		remote, err := r.listPublished(ctx)
		if err != nil {
			// Try again next tick. A collection maven cannot read is not a
			// reason to stop publishing to it.
			log.Printf("mavcaldav: reconcile: %v", err)
		} else {
			r.reconciled = true
			for _, id := range remote {
				if !live[id] {
					stale[id] = true
				}
			}
		}
	}
	for id := range stale {
		if err := r.delete(ctx, calendar.ReminderPath(id)); err != nil {
			log.Printf("mavcaldav: withdraw reminder %d: %v", id, err)
			continue
		}
		delete(r.published, id)
		log.Printf("mavcaldav: withdrew reminder %d", id)
	}
}

// listPublished PROPFINDs the collection and returns the reminder ids maven has
// events for in it. Only resources carrying calendar.ReminderUIDPrefix are
// reported, so a reconciliation pass can never propose deleting a file maven
// did not create — the same bound every other path in this file has.
func (r *renderer) listPublished(ctx context.Context) ([]int64, error) {
	const body = `<?xml version="1.0" encoding="utf-8"?>` +
		`<D:propfind xmlns:D="DAV:"><D:prop><D:resourcetype/></D:prop></D:propfind>`
	req, err := http.NewRequestWithContext(ctx, "PROPFIND", r.url+"/", strings.NewReader(body))
	if err != nil {
		return nil, err
	}
	req.SetBasicAuth(r.user, r.pass)
	req.Header.Set("Content-Type", "application/xml; charset=utf-8")
	req.Header.Set("Depth", "1")

	resp, err := r.http.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	raw, err := io.ReadAll(io.LimitReader(resp.Body, 4<<20))
	if err != nil {
		return nil, err
	}
	if resp.StatusCode != http.StatusMultiStatus && (resp.StatusCode < 200 || resp.StatusCode >= 300) {
		return nil, fmt.Errorf("PROPFIND %s: %s", r.url, resp.Status)
	}

	var ms struct {
		Responses []struct {
			Href string `xml:"href"`
		} `xml:"response"`
	}
	if err := xml.Unmarshal(raw, &ms); err != nil {
		return nil, fmt.Errorf("PROPFIND %s: %w", r.url, err)
	}
	var ids []int64
	for _, resp := range ms.Responses {
		href, err := url.PathUnescape(strings.TrimSpace(resp.Href))
		if err != nil {
			continue
		}
		if id, ok := calendar.ReminderIDFromPath(href); ok {
			ids = append(ids, id)
		}
	}
	return ids, nil
}

// renderMaxReminders bounds the read. Reminders past this count are older than
// anything a calendar view is useful for.
const renderMaxReminders = 200

// fireTime prefers NextFireTs — for a recurring reminder that is the occurrence
// worth showing; FireTs is the original statement.
func fireTime(rem ipc.Reminder) time.Time {
	if !rem.NextFireTs.IsZero() {
		return rem.NextFireTs
	}
	return rem.FireTs
}

func (r *renderer) put(ctx context.Context, name, body string) error {
	req, err := http.NewRequestWithContext(ctx, http.MethodPut, r.url+"/"+name, strings.NewReader(body))
	if err != nil {
		return err
	}
	req.SetBasicAuth(r.user, r.pass)
	req.Header.Set("Content-Type", "text/calendar; charset=utf-8")
	return r.do(req, name)
}

func (r *renderer) delete(ctx context.Context, name string) error {
	req, err := http.NewRequestWithContext(ctx, http.MethodDelete, r.url+"/"+name, nil)
	if err != nil {
		return err
	}
	req.SetBasicAuth(r.user, r.pass)
	return r.do(req, name)
}

// do runs the request and treats any 2xx, plus 404 on a DELETE, as success —
// a resource that is already gone is the state the caller wanted.
func (r *renderer) do(req *http.Request, name string) error {
	resp, err := r.http.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	io.Copy(io.Discard, io.LimitReader(resp.Body, 1<<16))
	switch {
	case resp.StatusCode >= 200 && resp.StatusCode < 300:
		return nil
	case req.Method == http.MethodDelete && resp.StatusCode == http.StatusNotFound:
		return nil
	}
	return fmt.Errorf("%s %s: %s", req.Method, name, resp.Status)
}