package calendar

import (
	"strings"
	"time"
)

// Event is a single entry rendered into an ICS feed. Fields are already
// resolved to display text; the renderer handles escaping and folding.
type Event struct {
	UID         string
	StartsAt    time.Time
	EndsAt      time.Time
	Stamp       time.Time
	Summary     string
	Description string
	Location    string
	Status      string // "confirmed" | "tentative" | "cancelled"
	Transparent bool   // TRANSP:TRANSPARENT — does not block free/busy
}

const icsTimestampLayout = "20060102T150405Z"

// RenderCalendar produces an RFC 5545 VCALENDAR body with CRLF line endings
// and 75-octet folding.
func RenderCalendar(name string, timezone string, events []Event) string {
	var b strings.Builder
	writeLine(&b, "BEGIN:VCALENDAR")
	writeLine(&b, "VERSION:2.0")
	writeLine(&b, "PRODID:-//Bookra//Bookra//EN")
	writeLine(&b, "CALSCALE:GREGORIAN")
	writeLine(&b, "METHOD:PUBLISH")
	writeLine(&b, "X-WR-CALNAME:"+escapeText(name))
	if timezone != "" {
		writeLine(&b, "X-WR-TIMEZONE:"+timezone)
	}
	for _, event := range events {
		writeLine(&b, "BEGIN:VEVENT")
		writeLine(&b, "UID:"+event.UID)
		stamp := event.Stamp
		if stamp.IsZero() {
			stamp = event.StartsAt
		}
		writeLine(&b, "DTSTAMP:"+stamp.UTC().Format(icsTimestampLayout))
		writeLine(&b, "DTSTART:"+event.StartsAt.UTC().Format(icsTimestampLayout))
		writeLine(&b, "DTEND:"+event.EndsAt.UTC().Format(icsTimestampLayout))
		writeLine(&b, "SUMMARY:"+escapeText(event.Summary))
		if event.Description != "" {
			writeLine(&b, "DESCRIPTION:"+escapeText(event.Description))
		}
		if event.Location != "" {
			writeLine(&b, "LOCATION:"+escapeText(event.Location))
		}
		switch event.Status {
		case "cancelled":
			writeLine(&b, "STATUS:CANCELLED")
		case "tentative":
			writeLine(&b, "STATUS:TENTATIVE")
		}
		if event.Transparent {
			writeLine(&b, "TRANSP:TRANSPARENT")
		}
		writeLine(&b, "END:VEVENT")
	}
	writeLine(&b, "END:VCALENDAR")
	return b.String()
}

// escapeText applies RFC 5545 §3.3.11 TEXT escaping.
func escapeText(value string) string {
	replacer := strings.NewReplacer(
		`\`, `\\`,
		";", `\;`,
		",", `\,`,
		"\n", `\n`,
		"\r", "",
	)
	return replacer.Replace(value)
}

// writeLine emits one content line folded at 75 octets per RFC 5545 §3.1.
// Continuation lines start with a single space; folds happen on rune
// boundaries so multi-byte characters are never split.
func writeLine(b *strings.Builder, line string) {
	const maxOctets = 75
	octets := 0
	first := true
	for _, r := range line {
		width := len(string(r))
		limit := maxOctets
		if !first {
			limit-- // continuation lines carry a leading space
		}
		if octets+width > limit {
			b.WriteString("\r\n ")
			octets = 0
			first = false
			limit = maxOctets - 1
		}
		b.WriteRune(r)
		octets += width
	}
	b.WriteString("\r\n")
}