package caldav

import (
	"fmt"
	"net/http"
	"path"
	"regexp"
	"strconv"
	"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/birthdays"
)

// birthdaysCalendarName is the fixed, reserved local calendar name the
// synthetic Birthdays calendar is exposed under (see
// db.reservedCalendarNames, which prevents a real calendar from ever
// colliding with it). It has no owner/shared-name segment: it's always
// computed from the requester's own contacts.
const birthdaysCalendarName = "birthdays"

// defaultBirthdayColor is the display color for the virtual birthdays
// calendar when the user hasn't chosen one, matching internal/web's.
const defaultBirthdayColor = "#ec4899"

// birthdayWindowPast/birthdayWindowFuture bound how many years around the
// current one the synthetic Birthdays calendar generates events for.
// CalDAV clients sync ahead of time (unlike the web view, which only
// computes the month currently being browsed), so a fixed window is
// generated upfront instead.
const birthdayWindowPast = 2
const birthdayWindowFuture = 8

// birthdayObjIDRe parses an object ID of the form
// "<book>~<contact-id-without-ext>~<year>.ics" back into its parts.
var birthdayObjIDRe = regexp.MustCompile(`^([^~]+)~([^~]+)~(\d{4})\.ics$`)

// birthdayObjID builds the stable object ID for contact c's birthday
// occurrence in year, encoding enough information to regenerate that
// exact VEVENT on demand in GetCalendarObject.
func birthdayObjID(c birthdays.Contact, year int) string {
	base := strings.TrimSuffix(c.ID, path.Ext(c.ID))
	return fmt.Sprintf("%s~%s~%d.ics", c.Book, base, year)
}

// birthdaysCalendarMeta returns the caldav.Calendar metadata for
// username's synthetic Birthdays calendar.
func (b *Backend) birthdaysCalendarMeta(username string) caldav.Calendar {
	return caldav.Calendar{
		Path:                  calHomePath() + birthdaysCalendarName + "/",
		Name:                  "Birthdays",
		Description:           "Birthdays computed from your contacts",
		SupportedComponentSet: []string{"VEVENT"},
		MaxResourceSize:       64 * 1024,
	}
}

// listBirthdayCalendarObjects generates one VEVENT per (contact, year)
// pair within the fixed rolling window for every contact of username's
// that has a parseable birthday.
func (b *Backend) listBirthdayCalendarObjects(username string) ([]caldav.CalendarObject, error) {
	contacts, err := birthdays.Collect(b.store, b.dbase, username)
	if err != nil {
		return nil, fmt.Errorf("collecting birthdays: %w", err)
	}

	now := time.Now()
	var objs []caldav.CalendarObject
	for _, c := range contacts {
		for year := now.Year() - birthdayWindowPast; year <= now.Year()+birthdayWindowFuture; year++ {
			obj, ok := b.encodeBirthdayObject(c, year)
			if !ok {
				continue
			}
			objs = append(objs, *obj)
		}
	}
	return objs, nil
}

// birthdayCalendarObject regenerates a single VEVENT for the (contact,
// year) pair encoded in objID.
func (b *Backend) birthdayCalendarObject(username, objPath, objID string) (*caldav.CalendarObject, error) {
	m := birthdayObjIDRe.FindStringSubmatch(objID)
	if m == nil {
		return nil, webdav.NewHTTPError(http.StatusNotFound, fmt.Errorf("unrecognized birthday object id"))
	}
	book, idBase, year := m[1], m[2], m[3]
	yearNum, err := strconv.Atoi(year)
	if err != nil {
		return nil, webdav.NewHTTPError(http.StatusNotFound, fmt.Errorf("invalid year"))
	}

	contacts, err := birthdays.Collect(b.store, b.dbase, username)
	if err != nil {
		return nil, fmt.Errorf("collecting birthdays: %w", err)
	}
	for _, c := range contacts {
		if c.Book != book {
			continue
		}
		if strings.TrimSuffix(c.ID, path.Ext(c.ID)) != idBase {
			continue
		}
		obj, ok := b.encodeBirthdayObject(c, yearNum)
		if !ok {
			break
		}
		obj.Path = objPath
		return obj, nil
	}
	return nil, webdav.NewHTTPError(http.StatusNotFound, fmt.Errorf("birthday event not found"))
}

// encodeBirthdayObject builds the all-day VEVENT for contact c's birthday
// occurrence in year, returning ok=false if that (month, day) combination
// doesn't exist in year (e.g. Feb 29 in a non-leap year).
func (b *Backend) encodeBirthdayObject(c birthdays.Contact, year int) (*caldav.CalendarObject, bool) {
	start, ok := birthdays.OccurrenceDate(c, year, time.UTC)
	if !ok {
		return nil, false
	}
	end := start.AddDate(0, 0, 1)

	event := ical.NewEvent()
	uid := birthdayObjID(c, year)
	event.Props.SetText(ical.PropUID, uid)
	event.Props.SetText(ical.PropSummary, birthdays.Summary(c, year))
	event.Props.SetDate(ical.PropDateTimeStart, start)
	event.Props.SetDate(ical.PropDateTimeEnd, end)
	event.Props.SetDateTime(ical.PropDateTimeStamp, time.Now().UTC())
	event.Props.SetText(ical.PropTransparency, "TRANSPARENT")

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

	var buf strings.Builder
	if err := ical.NewEncoder(&buf).Encode(cal); err != nil {
		return nil, false
	}

	objID := birthdayObjID(c, year)
	return &caldav.CalendarObject{
		Path:          calObjectPath(birthdaysCalendarName, objID),
		ModTime:       time.Now(),
		ContentLength: int64(buf.Len()),
		ETag:          fmt.Sprintf(`"bday-%s-%d"`, strings.TrimSuffix(c.ID, path.Ext(c.ID)), year),
		Data:          cal,
	}, true
}