// Package calendar builds the "add to calendar" artefacts shown on the RSVP
// confirmation flow: an RFC 5545 iCalendar (.ics) file, plus the three
// well-known provider deep-links (Google, Outlook, Yahoo). Tier 2 Block B.
//
// The .ics is the source of truth — Google/Outlook/Yahoo URLs are pure URL
// builders and don't need a server round-trip, but we serve them anyway so
// the frontend doesn't have to reimplement the encoding rules per provider.
package calendar

import (
	"fmt"
	"net/url"
	"strings"
	"time"

	"github.com/google/uuid"
)

// DefaultDuration is the assumed duration of an event when the host hasn't
// told us when it ends. Two hours covers most weddings, parties, and dinners
// without being so long it pollutes the guest's calendar if they don't tweak
// it themselves.
const DefaultDuration = 2 * time.Hour

// Event is the projection of domain.Event needed to render calendar artefacts.
// Kept narrow so this package doesn't depend on the broader domain model and
// can be unit-tested with synthetic data.
type Event struct {
	ID          uuid.UUID
	Name        string
	Venue       string
	Description string
	StartsAt    time.Time
	// EndsAt is optional. When zero, DefaultDuration is added to StartsAt.
	EndsAt time.Time
}

// ProviderLinks bundles the three add-to-calendar URLs plus the path to
// download the .ics. Embedded in the /access/{token} response so the
// frontend renders four ready-made buttons.
type ProviderLinks struct {
	Google  string `json:"google"`
	Outlook string `json:"outlook"`
	Yahoo   string `json:"yahoo"`
	ICS     string `json:"ics"`
}

// BuildLinks renders the four add-to-calendar URLs for the event. ICS is the
// publicly-reachable path on this server (typically /access/{token}/calendar.ics).
func BuildLinks(e Event, icsURL string) ProviderLinks {
	start, end := e.window()

	gq := url.Values{}
	gq.Set("action", "TEMPLATE")
	gq.Set("text", e.Name)
	gq.Set("dates", fmt.Sprintf("%s/%s", fmtUTC(start), fmtUTC(end)))
	if e.Description != "" {
		gq.Set("details", e.Description)
	}
	if e.Venue != "" {
		gq.Set("location", e.Venue)
	}

	oq := url.Values{}
	oq.Set("path", "/calendar/action/compose")
	oq.Set("rru", "addevent")
	oq.Set("subject", e.Name)
	oq.Set("startdt", start.UTC().Format(time.RFC3339))
	oq.Set("enddt", end.UTC().Format(time.RFC3339))
	if e.Description != "" {
		oq.Set("body", e.Description)
	}
	if e.Venue != "" {
		oq.Set("location", e.Venue)
	}

	yq := url.Values{}
	yq.Set("v", "60")
	yq.Set("title", e.Name)
	yq.Set("st", fmtUTC(start))
	yq.Set("et", fmtUTC(end))
	if e.Description != "" {
		yq.Set("desc", e.Description)
	}
	if e.Venue != "" {
		yq.Set("in_loc", e.Venue)
	}

	return ProviderLinks{
		Google:  "https://www.google.com/calendar/render?" + gq.Encode(),
		Outlook: "https://outlook.live.com/calendar/0/deeplink/compose?" + oq.Encode(),
		Yahoo:   "https://calendar.yahoo.com/?" + yq.Encode(),
		ICS:     icsURL,
	}
}

// BuildICS renders a minimal RFC 5545 VCALENDAR with a single VEVENT.
// The UID is deterministic — re-downloading replaces the same calendar
// entry instead of creating a duplicate. Timestamps are emitted in UTC
// (Z-suffixed); we don't bother with VTIMEZONE blocks because every modern
// client handles UTC correctly and the host already chose a specific
// instant in `event_date`.
//
// `now` is injected for deterministic testing — pass time.Now().UTC() in
// production.
func BuildICS(e Event, now time.Time) string {
	start, end := e.window()
	var b strings.Builder
	b.Grow(512)
	write := func(lines ...string) {
		for _, l := range lines {
			b.WriteString(l)
			b.WriteString("\r\n")
		}
	}

	write(
		"BEGIN:VCALENDAR",
		"VERSION:2.0",
		"PRODID:-//GuestGuard//RSVP//EN",
		"CALSCALE:GREGORIAN",
		"METHOD:PUBLISH",
		"BEGIN:VEVENT",
		"UID:"+uidFor(e.ID),
		"DTSTAMP:"+fmtUTC(now.UTC()),
		"DTSTART:"+fmtUTC(start),
		"DTEND:"+fmtUTC(end),
		"SUMMARY:"+escape(e.Name),
	)
	if e.Description != "" {
		write("DESCRIPTION:" + escape(e.Description))
	}
	if e.Venue != "" {
		write("LOCATION:" + escape(e.Venue))
	}
	write(
		"END:VEVENT",
		"END:VCALENDAR",
	)
	return b.String()
}

// FileName slugifies an event name into something safe for the
// Content-Disposition header. Falls back to "event.ics" when nothing usable
// is left after slugification.
func FileName(name string) string {
	out := make([]byte, 0, len(name))
	for _, r := range strings.ToLower(name) {
		switch {
		case r >= 'a' && r <= 'z', r >= '0' && r <= '9':
			out = append(out, byte(r))
		case r == ' ', r == '-', r == '_':
			out = append(out, '-')
		}
	}
	// Collapse repeated dashes.
	for strings.Contains(string(out), "--") {
		out = []byte(strings.ReplaceAll(string(out), "--", "-"))
	}
	s := strings.Trim(string(out), "-")
	if s == "" {
		return "event.ics"
	}
	return s + ".ics"
}

func (e Event) window() (start, end time.Time) {
	start = e.StartsAt.UTC()
	if e.EndsAt.IsZero() {
		end = start.Add(DefaultDuration)
	} else {
		end = e.EndsAt.UTC()
	}
	return
}

// uidFor produces a deterministic UID for an event. The @guestguard.k4scloud
// suffix is required by RFC 5545 ("globally unique identifier") and ties the
// UID to our origin so unrelated systems don't accidentally collide.
func uidFor(id uuid.UUID) string {
	return id.String() + "@guestguard.k4scloud"
}

// fmtUTC encodes a time as the basic-form UTC stamp the providers expect:
// "20260517T180000Z". time.Format with a literal layout avoids the locale
// surprises of time.RFC* constants.
func fmtUTC(t time.Time) string {
	return t.UTC().Format("20060102T150405Z")
}

// escape applies the small set of TEXT-value substitutions RFC 5545 calls
// out. Without these, a comma in a venue name silently truncates the field
// in some readers.
var textReplacer = strings.NewReplacer(
	`\`, `\\`,
	"\n", `\n`,
	"\r", "",
	",", `\,`,
	";", `\;`,
)

func escape(s string) string {
	return textReplacer.Replace(s)
}