/**
 * Helpers for "send calendar entry to my own calendar" flows.
 *
 * - `buildICS()` produces a single-event RFC-5545 string suitable for
 *   download. Apple Calendar, Outlook and Google all import it.
 * - `googleCalUrl()` returns a deep link to Google Calendar's event-
 *   create form pre-filled with the event details.
 * - `mapsUrl()` returns a Google Maps search URL for an address string.
 *
 * All client-only — no Node APIs touched. Safe in SSR contexts (the
 * functions just return strings; the actual download is triggered by
 * the caller via `URL.createObjectURL`).
 */

export type CalendarItemICS = {
  title: string;
  start: Date;
  /** Optional end date. If omitted, default duration is 1 hour. */
  end?: Date | null;
  description?: string;
  location?: string;
  url?: string;
  /** Stable identifier; falls back to start-timestamp + title hash. */
  uid?: string;
};

/** Folds long lines per RFC-5545 §3.1 (75 octets max). Most clients
 *  tolerate longer lines but Outlook is strict — better safe. */
function foldLine(line: string): string {
  if (line.length <= 75) return line;
  const out: string[] = [];
  let i = 0;
  while (i < line.length) {
    const chunk = line.slice(i, i + (i === 0 ? 75 : 74));
    out.push((i === 0 ? "" : " ") + chunk);
    i += i === 0 ? 75 : 74;
  }
  return out.join("\r\n");
}

/** Escape per RFC-5545 §3.3.11 — backslash, comma, semicolon, newline. */
function escapeText(s: string): string {
  return s
    .replace(/\\/g, "\\\\")
    .replace(/\n/g, "\\n")
    .replace(/,/g, "\\,")
    .replace(/;/g, "\\;");
}

/** UTC timestamp formatted as `YYYYMMDDTHHMMSSZ`. */
function fmtICSDate(d: Date): string {
  const pad = (n: number) => String(n).padStart(2, "0");
  return (
    d.getUTCFullYear().toString() +
    pad(d.getUTCMonth() + 1) +
    pad(d.getUTCDate()) +
    "T" +
    pad(d.getUTCHours()) +
    pad(d.getUTCMinutes()) +
    pad(d.getUTCSeconds()) +
    "Z"
  );
}

function deterministicUid(title: string, start: Date): string {
  let hash = 0;
  for (let i = 0; i < title.length; i++) {
    hash = (hash * 31 + title.charCodeAt(i)) | 0;
  }
  return `${start.getTime()}-${Math.abs(hash).toString(36)}@windwiderstand.de`;
}

export function buildICS(item: CalendarItemICS): string {
  const start = item.start;
  const end =
    item.end ??
    new Date(start.getTime() + 60 * 60 * 1000); // 1h default
  const uid = item.uid ?? deterministicUid(item.title, start);
  const now = new Date();

  const lines: string[] = [
    "BEGIN:VCALENDAR",
    "VERSION:2.0",
    "PRODID:-//windwiderstand.de//calendar//DE",
    "CALSCALE:GREGORIAN",
    "METHOD:PUBLISH",
    "BEGIN:VEVENT",
    `UID:${uid}`,
    `DTSTAMP:${fmtICSDate(now)}`,
    `DTSTART:${fmtICSDate(start)}`,
    `DTEND:${fmtICSDate(end)}`,
    foldLine(`SUMMARY:${escapeText(item.title)}`),
  ];
  if (item.description) {
    lines.push(foldLine(`DESCRIPTION:${escapeText(item.description)}`));
  }
  if (item.location) {
    lines.push(foldLine(`LOCATION:${escapeText(item.location)}`));
  }
  if (item.url) {
    lines.push(foldLine(`URL:${item.url}`));
  }
  lines.push("END:VEVENT", "END:VCALENDAR");
  return lines.join("\r\n");
}

/** Trigger a browser download of an ICS file. Caller should pass a
 *  filesystem-friendly filename; we still strip path separators
 *  defensively. No-op on the server (`window` undefined). */
export function downloadICS(item: CalendarItemICS, filename: string): void {
  if (typeof window === "undefined") return;
  const safeName = filename.replace(/[/\\:*?"<>|]/g, "_") || "event.ics";
  const ics = buildICS(item);
  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 = safeName.endsWith(".ics") ? safeName : `${safeName}.ics`;
  document.body.appendChild(a);
  a.click();
  document.body.removeChild(a);
  URL.revokeObjectURL(url);
}

/** Builds a VCALENDAR with multiple VEVENTs — for feed/bulk export. */
export function buildICSFeed(items: CalendarItemICS[], calName = "Windwiderstand Termine"): string {
  const now = new Date();
  const lines: string[] = [
    "BEGIN:VCALENDAR",
    "VERSION:2.0",
    "PRODID:-//windwiderstand.de//calendar//DE",
    "CALSCALE:GREGORIAN",
    "METHOD:PUBLISH",
    foldLine(`X-WR-CALNAME:${escapeText(calName)}`),
    "X-WR-TIMEZONE:Europe/Berlin",
    "REFRESH-INTERVAL;VALUE=DURATION:PT1H",
    "X-PUBLISHED-TTL:PT1H",
  ];
  for (const item of items) {
    const start = item.start;
    const end = item.end ?? new Date(start.getTime() + 60 * 60 * 1000);
    const uid = item.uid ?? deterministicUid(item.title, start);
    lines.push(
      "BEGIN:VEVENT",
      `UID:${uid}`,
      `DTSTAMP:${fmtICSDate(now)}`,
      `DTSTART:${fmtICSDate(start)}`,
      `DTEND:${fmtICSDate(end)}`,
      foldLine(`SUMMARY:${escapeText(item.title)}`),
    );
    if (item.description) lines.push(foldLine(`DESCRIPTION:${escapeText(item.description)}`));
    if (item.location) lines.push(foldLine(`LOCATION:${escapeText(item.location)}`));
    if (item.url) lines.push(foldLine(`URL:${item.url}`));
    lines.push("END:VEVENT");
  }
  lines.push("END:VCALENDAR");
  return lines.join("\r\n");
}

/** Triggers a browser download of a multi-event ICS file. */
export function downloadICSFeed(items: CalendarItemICS[], filename: string): void {
  if (typeof window === "undefined") return;
  const safeName = filename.replace(/[/\\:*?"<>|]/g, "_") || "termine.ics";
  const ics = buildICSFeed(items);
  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 = safeName.endsWith(".ics") ? safeName : `${safeName}.ics`;
  document.body.appendChild(a);
  a.click();
  document.body.removeChild(a);
  URL.revokeObjectURL(url);
}

/** Deep-link to Google Calendar's pre-filled create form. */
export function googleCalUrl(item: CalendarItemICS): string {
  const fmt = (d: Date) =>
    d.toISOString().replace(/[-:]/g, "").replace(/\.\d+Z$/, "Z");
  const end = item.end ?? new Date(item.start.getTime() + 60 * 60 * 1000);
  const params = new URLSearchParams({
    action: "TEMPLATE",
    text: item.title,
    dates: `${fmt(item.start)}/${fmt(end)}`,
  });
  if (item.description) params.set("details", item.description);
  if (item.location) params.set("location", item.location);
  return `https://calendar.google.com/calendar/render?${params}`;
}

/** OpenStreetMap search URL for an address or place name. */
export function mapsUrl(location: string): string {
  return `https://www.openstreetmap.org/search?query=${encodeURIComponent(location)}`;
}