// Package ical implements a minimal RFC 5545 iCalendar parser/builder — just
// VEVENT with the fields CalDAV needs: UID, SUMMARY, DTSTART, DTEND,
// DESCRIPTION, LOCATION. Not full RFC 5545 (no VTODO/VJOURNAL/VALARM, no
// RRULE recurrence) — enough for real calendar clients to create, fetch, and
// list single events, with recurrence and other component types as natural
// next additions once client compatibility testing calls for them.
package ical

import (
	"fmt"
	"strings"
	"time"
)

const icalTimeLayout = "20060102T150405Z"

type Event struct {
	UID         string
	Summary     string
	Description string
	Location    string
	DTStart     time.Time
	DTEnd       time.Time
}

// Parse reads a VCALENDAR containing one VEVENT.
func Parse(data string) (*Event, error) {
	lines := unfold(data)
	e := &Event{}
	inEvent := false

	for _, line := range lines {
		line = strings.TrimSpace(line)
		if line == "" {
			continue
		}
		upper := strings.ToUpper(line)
		switch {
		case upper == "BEGIN:VEVENT":
			inEvent = true
			continue
		case upper == "END:VEVENT":
			inEvent = false
			continue
		}
		if !inEvent {
			continue
		}

		name, value, found := splitProperty(line)
		if !found {
			continue
		}
		switch strings.ToUpper(name) {
		case "UID":
			e.UID = value
		case "SUMMARY":
			e.Summary = unescape(value)
		case "DESCRIPTION":
			e.Description = unescape(value)
		case "LOCATION":
			e.Location = unescape(value)
		case "DTSTART":
			if t, err := time.Parse(icalTimeLayout, value); err == nil {
				e.DTStart = t
			}
		case "DTEND":
			if t, err := time.Parse(icalTimeLayout, value); err == nil {
				e.DTEnd = t
			}
		}
	}

	if e.UID == "" {
		return nil, fmt.Errorf("ical missing required UID property")
	}
	return e, nil
}

// Build renders an Event back into a full VCALENDAR/VEVENT block, CRLF line
// endings per spec.
func (e *Event) Build() string {
	var b strings.Builder
	b.WriteString("BEGIN:VCALENDAR\r\n")
	b.WriteString("VERSION:2.0\r\n")
	b.WriteString("PRODID:-//GoMail//CalDAV//EN\r\n")
	b.WriteString("BEGIN:VEVENT\r\n")
	b.WriteString("UID:" + e.UID + "\r\n")
	if !e.DTStart.IsZero() {
		b.WriteString("DTSTART:" + e.DTStart.UTC().Format(icalTimeLayout) + "\r\n")
	}
	if !e.DTEnd.IsZero() {
		b.WriteString("DTEND:" + e.DTEnd.UTC().Format(icalTimeLayout) + "\r\n")
	}
	if e.Summary != "" {
		b.WriteString("SUMMARY:" + escape(e.Summary) + "\r\n")
	}
	if e.Description != "" {
		b.WriteString("DESCRIPTION:" + escape(e.Description) + "\r\n")
	}
	if e.Location != "" {
		b.WriteString("LOCATION:" + escape(e.Location) + "\r\n")
	}
	b.WriteString("END:VEVENT\r\n")
	b.WriteString("END:VCALENDAR\r\n")
	return b.String()
}

func splitProperty(line string) (name, value string, found bool) {
	colonIdx := strings.Index(line, ":")
	if colonIdx == -1 {
		return "", "", false
	}
	namePart := line[:colonIdx]
	value = line[colonIdx+1:]
	if semiIdx := strings.Index(namePart, ";"); semiIdx != -1 {
		namePart = namePart[:semiIdx]
	}
	return namePart, value, true
}

// unfold reverses RFC 5545 §3.1 line folding, same rule as vCard's.
func unfold(data string) []string {
	raw := strings.Split(strings.ReplaceAll(data, "\r\n", "\n"), "\n")
	var out []string
	for _, line := range raw {
		if len(line) > 0 && (line[0] == ' ' || line[0] == '\t') && len(out) > 0 {
			out[len(out)-1] += line[1:]
		} else {
			out = append(out, line)
		}
	}
	return out
}

func escape(s string) string {
	s = strings.ReplaceAll(s, "\\", "\\\\")
	s = strings.ReplaceAll(s, ",", "\\,")
	s = strings.ReplaceAll(s, ";", "\\;")
	s = strings.ReplaceAll(s, "\n", "\\n")
	return s
}

func unescape(s string) string {
	s = strings.ReplaceAll(s, "\\n", "\n")
	s = strings.ReplaceAll(s, "\\,", ",")
	s = strings.ReplaceAll(s, "\\;", ";")
	s = strings.ReplaceAll(s, "\\\\", "\\")
	return s
}