/**
 * Lightweight iCalendar (.ics) generation for skate tours & events.
 *
 * No external deps — we build a spec-compliant VEVENT string and hand it to the
 * browser as a downloadable file so attendees can drop a tour straight into
 * Apple Calendar, Google Calendar, Outlook, etc.
 */

export interface CalendarEventInput {
  /** Stable unique id (we use the Nostr addressable coordinate). */
  uid: string;
  title: string;
  description?: string;
  location?: string;
  /** Start time in unix seconds. */
  start: number;
  /** Optional end time in unix seconds. If omitted, defaults to start + 2h. */
  end?: number;
  /** Optional URL to include in the event body. */
  url?: string;
  /** When true, the event is treated as an all-day / multi-day date range. */
  allDay?: boolean;
}

/** Escape text per RFC 5545 (commas, semicolons, backslashes, newlines). */
function escapeICS(text: string): string {
  return text
    .replace(/\\/g, '\\\\')
    .replace(/;/g, '\\;')
    .replace(/,/g, '\\,')
    .replace(/\r?\n/g, '\\n');
}

/** Format unix seconds to a UTC timestamp: YYYYMMDDTHHMMSSZ */
function toICSDateTime(secs: number): string {
  const d = new Date(secs * 1000);
  return (
    d.getUTCFullYear().toString().padStart(4, '0') +
    (d.getUTCMonth() + 1).toString().padStart(2, '0') +
    d.getUTCDate().toString().padStart(2, '0') +
    'T' +
    d.getUTCHours().toString().padStart(2, '0') +
    d.getUTCMinutes().toString().padStart(2, '0') +
    d.getUTCSeconds().toString().padStart(2, '0') +
    'Z'
  );
}

/** Format unix seconds to an all-day date value: YYYYMMDD (local calendar day). */
function toICSDate(secs: number): string {
  const d = new Date(secs * 1000);
  return (
    d.getFullYear().toString().padStart(4, '0') +
    (d.getMonth() + 1).toString().padStart(2, '0') +
    d.getDate().toString().padStart(2, '0')
  );
}

/** Build a full VCALENDAR document containing a single VEVENT. */
export function buildICS(input: CalendarEventInput): string {
  const now = toICSDateTime(Math.floor(Date.now() / 1000));

  const lines: string[] = [
    'BEGIN:VCALENDAR',
    'VERSION:2.0',
    'PRODID:-//SkateSpots//Tours//EN',
    'CALSCALE:GREGORIAN',
    'METHOD:PUBLISH',
    'BEGIN:VEVENT',
    `UID:${escapeICS(input.uid)}@skate-spots.org`,
    `DTSTAMP:${now}`,
  ];

  if (input.allDay) {
    // All-day: DTEND is exclusive, so add one day to the last calendar day.
    const endSecs = input.end ?? input.start;
    lines.push(`DTSTART;VALUE=DATE:${toICSDate(input.start)}`);
    lines.push(`DTEND;VALUE=DATE:${toICSDate(endSecs + 24 * 60 * 60)}`);
  } else {
    const endSecs = input.end ?? input.start + 2 * 60 * 60;
    lines.push(`DTSTART:${toICSDateTime(input.start)}`);
    lines.push(`DTEND:${toICSDateTime(endSecs)}`);
  }

  lines.push(`SUMMARY:${escapeICS(input.title)}`);

  const descParts: string[] = [];
  if (input.description) descParts.push(input.description);
  if (input.url) descParts.push(input.url);
  if (descParts.length > 0) {
    lines.push(`DESCRIPTION:${escapeICS(descParts.join('\n\n'))}`);
  }

  if (input.location) lines.push(`LOCATION:${escapeICS(input.location)}`);
  if (input.url) lines.push(`URL:${escapeICS(input.url)}`);

  lines.push('END:VEVENT', 'END:VCALENDAR');

  return lines.join('\r\n');
}

/** Trigger a browser download of an .ics file for the given event. */
export function downloadICS(input: CalendarEventInput, filename?: string): void {
  const ics = buildICS(input);
  const blob = new Blob([ics], { type: 'text/calendar;charset=utf-8' });
  const url = URL.createObjectURL(blob);
  const a = document.createElement('a');
  a.href = url;
  a.download = filename ?? `${slug(input.title) || 'skate-tour'}.ics`;
  document.body.appendChild(a);
  a.click();
  document.body.removeChild(a);
  // Give the browser a tick to start the download before revoking.
  setTimeout(() => URL.revokeObjectURL(url), 1000);
}

/** Build a Google Calendar "add event" URL as a fallback for the web. */
export function googleCalendarUrl(input: CalendarEventInput): string {
  const fmt = input.allDay ? toICSDate : toICSDateTime;
  const endSecs = input.allDay
    ? (input.end ?? input.start) + 24 * 60 * 60
    : input.end ?? input.start + 2 * 60 * 60;

  const params = new URLSearchParams({
    action: 'TEMPLATE',
    text: input.title,
    dates: `${fmt(input.start)}/${fmt(endSecs)}`,
  });
  if (input.description || input.url) {
    params.set('details', [input.description, input.url].filter(Boolean).join('\n\n'));
  }
  if (input.location) params.set('location', input.location);

  return `https://calendar.google.com/calendar/render?${params.toString()}`;
}

function slug(s: string): string {
  return s
    .toLowerCase()
    .replace(/[^a-z0-9]+/g, '-')
    .replace(/^-|-$/g, '')
    .slice(0, 50);
}