const CRLF = '\r\n';
const MAX_OCTETS = 75;
const CONTROL_CHARS = /[\u0000-\u0008\u000B\u000C\u000E-\u001F\u007F]/g;

export type CalendarEvent = {
  uid: string;
  start: string | Date;
  end?: string | Date | null;
  allDay?: boolean;
  summary: string;
  description?: string;
  location?: string;
  url?: string;
  status?: 'CONFIRMED' | 'TENTATIVE' | 'CANCELLED';
};

function escapeText(value: string): string {
  return value
    .replace(/\\/g, '\\\\')
    .replace(/;/g, '\\;')
    .replace(/,/g, '\\,')
    .replace(/\r\n|\r|\n/g, '\\n')
    .replace(CONTROL_CHARS, '');
}

function foldLine(line: string): string {
  if (Buffer.byteLength(line, 'utf8') <= MAX_OCTETS) return line;

  const folded: string[] = [];
  let current = '';
  let octets = 0;

  for (const character of line) {
    const size = Buffer.byteLength(character, 'utf8');
    if (octets + size > MAX_OCTETS) {
      folded.push(current);
      current = ' ';
      octets = 1;
    }
    current += character;
    octets += size;
  }

  folded.push(current);
  return folded.join(CRLF);
}

function property(name: string, value: string): string {
  return foldLine(`${name}:${value}`);
}

function parseDate(value: string | Date | null | undefined): Date | undefined {
  if (!value) return undefined;
  const date = value instanceof Date ? value : new Date(value);
  return Number.isNaN(date.getTime()) ? undefined : date;
}

function toUtcStamp(date: Date): string {
  return date.toISOString().replace(/[-:]/g, '').replace(/\.\d{3}/, '');
}

function toDateValue(date: Date): string {
  return date.toISOString().slice(0, 10).replace(/-/g, '');
}

export function buildEventCalendar(event: CalendarEvent): string {
  const start = parseDate(event.start);
  const end = parseDate(event.end);
  const lines = [
    'BEGIN:VCALENDAR',
    'VERSION:2.0',
    property('PRODID', '-//SHARP-ER//Events Calendar//EN'),
    'CALSCALE:GREGORIAN',
    'METHOD:PUBLISH',
    property('X-WR-CALNAME', 'SHARP-ER'),
  ];

  if (start) {
    lines.push('BEGIN:VEVENT');
    lines.push(property('UID', escapeText(event.uid)));
    lines.push(property('DTSTAMP', toUtcStamp(new Date())));

    if (event.allDay) {
      lines.push(`DTSTART;VALUE=DATE:${toDateValue(start)}`);
      const exclusiveEnd = new Date((end ?? start).getTime() + 86_400_000);
      lines.push(`DTEND;VALUE=DATE:${toDateValue(exclusiveEnd)}`);
    } else {
      lines.push(property('DTSTART', toUtcStamp(start)));
      if (end && end.getTime() > start.getTime()) {
        lines.push(property('DTEND', toUtcStamp(end)));
      }
    }

    lines.push(property('SUMMARY', escapeText(event.summary)));
    if (event.description) {
      lines.push(property('DESCRIPTION', escapeText(event.description)));
    }
    if (event.location) {
      lines.push(property('LOCATION', escapeText(event.location)));
    }
    if (event.url && /^https?:\/\//i.test(event.url)) {
      lines.push(property('URL', event.url));
    }

    lines.push(property('STATUS', event.status ?? 'CONFIRMED'));
    lines.push('END:VEVENT');
  }

  lines.push('END:VCALENDAR');
  return `${lines.join(CRLF)}${CRLF}`;
}