package ical

import (
	"io"
	"strings"
	"time"
	"unicode/utf8"

	"usoscal/internal/store"
)

const (
	stamp     = "20060102T150405Z"
	lineLimit = 75
)

func Summary(event store.Event) string {
	if event.Kind == "" || event.Kind == event.Title {
		return event.Title
	}
	return event.Title + " (" + event.Kind + ")"
}

func Location(event store.Event) string {
	place := event.Room
	switch {
	case place != "" && event.Building != "":
		place += " (" + event.Building + ")"
	case place == "":
		place = event.Building
	}
	return joinNonEmpty(", ", place, event.Address)
}

func Description(event store.Event) string {
	about := event.Kind
	if event.Group != "" {
		about = joinNonEmpty(", ", about, "group "+event.Group)
	}
	lecturers := ""
	if event.Lecturers != "" {
		lecturers = "Lecturer: " + event.Lecturers
	}
	return joinNonEmpty("\n", about, lecturers, event.URL)
}

func Write(w io.Writer, name, host string, events []store.Event) error {
	var b strings.Builder
	line(&b, "BEGIN:VCALENDAR")
	line(&b, "VERSION:2.0")
	line(&b, "PRODID:-//usoscal//EN")
	line(&b, "CALSCALE:GREGORIAN")
	line(&b, "METHOD:PUBLISH")
	line(&b, "X-WR-CALNAME:"+escape(name))
	line(&b, "X-WR-TIMEZONE:Europe/Warsaw")
	line(&b, "REFRESH-INTERVAL;VALUE=DURATION:PT6H")
	line(&b, "X-PUBLISHED-TTL:PT6H")
	for _, event := range events {
		line(&b, "BEGIN:VEVENT")
		line(&b, "UID:"+event.UID+"@"+host)
		line(&b, "DTSTAMP:"+utc(event.UpdatedAt))
		line(&b, "LAST-MODIFIED:"+utc(event.UpdatedAt))
		line(&b, "DTSTART:"+utc(event.Start))
		line(&b, "DTEND:"+utc(event.End))
		line(&b, "SUMMARY:"+escape(Summary(event)))
		if location := Location(event); location != "" {
			line(&b, "LOCATION:"+escape(location))
		}
		if description := Description(event); description != "" {
			line(&b, "DESCRIPTION:"+escape(description))
		}
		if event.URL != "" {
			line(&b, "URL:"+event.URL)
		}
		line(&b, "END:VEVENT")
	}
	line(&b, "END:VCALENDAR")
	_, err := io.WriteString(w, b.String())
	return err
}

func utc(t time.Time) string {
	return t.UTC().Format(stamp)
}

func joinNonEmpty(separator string, parts ...string) string {
	kept := parts[:0:0]
	for _, part := range parts {
		if part != "" {
			kept = append(kept, part)
		}
	}
	return strings.Join(kept, separator)
}

var escaper = strings.NewReplacer(`\`, `\\`, ";", `\;`, ",", `\,`, "\r\n", `\n`, "\n", `\n`, "\r", "")

func escape(text string) string {
	return escaper.Replace(text)
}

func line(b *strings.Builder, content string) {
	for len(content) > lineLimit {
		cut := lineLimit
		for cut > 0 && !utf8.RuneStart(content[cut]) {
			cut--
		}
		b.WriteString(content[:cut])
		b.WriteString("\r\n ")
		content = content[cut:]
	}
	b.WriteString(content)
	b.WriteString("\r\n")
}