/* ============================================================================
   iCALENDAR (RFC 5545)
   ----------------------------------------------------------------------------
   Builds the file behind "Add to calendar". Pure, dependency-free and written
   against the spec rather than against one client's tolerance:

     * CRLF line endings, and content lines folded at 75 OCTETS - folded on
       character boundaries, so an Arabic title (2 bytes per letter) is never
       cut in half;
     * TEXT values escaped for \ ; , and newlines;
     * timed events written as UTC (`...Z`), all-day events as VALUE=DATE with
       a DTEND on the day AFTER the last day, which is what "exclusive end"
       means in the spec and what every calendar client expects;
     * a stable UID, so re-importing an updated file replaces the entry
       instead of duplicating it.

   No ORGANIZER property: it requires a real mailto: address, and the project
   inbox is still marked TBD in the draft copy. An invented one would end up in
   readers' calendars.
   ========================================================================= */

const CRLF = "\r\n";

/** RFC 5545 3.1: a content line is at most 75 octets, excluding the CRLF. */
const MAX_OCTETS = 75;

const encoder = new TextEncoder();

export type IcsStatus = "CONFIRMED" | "TENTATIVE" | "CANCELLED";

export interface IcsEvent {
  /** Globally unique and STABLE for this event. */
  readonly uid: string;
  /** ISO 8601. */
  readonly start: string;
  readonly end?: string | undefined;
  /** Write the all-day form (VALUE=DATE) instead of a timed one. */
  readonly allDay?: boolean;
  readonly summary: string;
  readonly description?: string | undefined;
  readonly location?: string | undefined;
  readonly url?: string | undefined;
  readonly status?: IcsStatus;
  /** DTSTAMP. Defaults to now; pass a fixed value in a test. */
  readonly stamp?: Date;
}

export interface IcsCalendarOptions {
  /** PRODID. Identifies the software that wrote the file. */
  readonly prodId?: string;
  /** X-WR-CALNAME, shown by some clients when a file is imported. */
  readonly name?: string;
}

const DEFAULT_PROD_ID = "-//SHARP-ER//Events Calendar//EN";

/* ---------------------------------------------------------------------------
   VALUE ENCODING
   ------------------------------------------------------------------------ */

/**
 * Control characters a TEXT value may not carry. TAB is legal and the two
 * newline characters are escaped before this runs, so neither is in the class.
 */
const CONTROL_CHARS =
  /[\u0000-\u0008\u000B\u000C\u000E-\u001F\u007F]/g;

/** RFC 5545 3.3.11 TEXT escaping, plus control characters dropped. */
export function escapeIcsText(value: string): string {
  return value
    .replace(/\\/g, "\\\\")
    .replace(/;/g, "\\;")
    .replace(/,/g, "\\,")
    .replace(/\r\n|\r|\n/g, "\\n")
    .replace(CONTROL_CHARS, "");
}

/** "20260312T140000Z" */
function toUtcStamp(date: Date): string {
  return date.toISOString().replace(/[-:]/g, "").replace(/\.\d{3}/, "");
}

/** "20260312" - the calendar day of a date-only value, read in UTC. */
function toDateValue(date: Date): string {
  return date.toISOString().slice(0, 10).replace(/-/g, "");
}

function addDays(date: Date, days: number): Date {
  return new Date(date.getTime() + days * 86_400_000);
}

/**
 * Folds one content line to 75 octets. Continuation lines begin with a single
 * space, which the parser strips again.
 */
export function foldIcsLine(line: string): string {
  if (encoder.encode(line).length <= MAX_OCTETS) return line;

  const parts: string[] = [];
  let current = "";
  let octets = 0;

  // Iterating the string yields whole code points, so a multi-byte character
  // is never split across two lines.
  for (const char of line) {
    const size = encoder.encode(char).length;
    if (octets + size > MAX_OCTETS) {
      parts.push(current);
      current = " ";
      octets = 1;
    }
    current += char;
    octets += size;
  }

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

/* ---------------------------------------------------------------------------
   BUILDER
   ------------------------------------------------------------------------ */

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

function eventLines(event: IcsEvent): string[] {
  const start = new Date(event.start);
  if (Number.isNaN(start.getTime())) return [];

  const parsedEnd = event.end ? new Date(event.end) : undefined;
  const end =
    parsedEnd && !Number.isNaN(parsedEnd.getTime()) ? parsedEnd : undefined;

  const lines: string[] = ["BEGIN:VEVENT"];
  lines.push(property("UID", escapeIcsText(event.uid)));
  lines.push(property("DTSTAMP", toUtcStamp(event.stamp ?? new Date())));

  if (event.allDay) {
    lines.push(`DTSTART;VALUE=DATE:${toDateValue(start)}`);
    // DTEND is exclusive: the day after the last day of the event.
    lines.push(`DTEND;VALUE=DATE:${toDateValue(addDays(end ?? start, 1))}`);
  } else {
    lines.push(property("DTSTART", toUtcStamp(start)));
    if (end && end.getTime() > start.getTime()) {
      lines.push(property("DTEND", toUtcStamp(end)));
    }
  }

  lines.push(property("SUMMARY", escapeIcsText(event.summary)));

  if (event.description) {
    lines.push(property("DESCRIPTION", escapeIcsText(event.description)));
  }
  if (event.location) {
    lines.push(property("LOCATION", escapeIcsText(event.location)));
  }
  if (event.url) {
    // A URI value is not TEXT and must not be escaped.
    lines.push(property("URL", event.url));
  }

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

  return lines;
}

/** A complete VCALENDAR document, CRLF-terminated as the spec requires. */
export function buildIcsCalendar(
  events: readonly IcsEvent[],
  options: IcsCalendarOptions = {},
): string {
  const lines: string[] = [
    "BEGIN:VCALENDAR",
    "VERSION:2.0",
    property("PRODID", options.prodId ?? DEFAULT_PROD_ID),
    "CALSCALE:GREGORIAN",
    "METHOD:PUBLISH",
  ];

  if (options.name) {
    lines.push(property("X-WR-CALNAME", escapeIcsText(options.name)));
  }

  for (const event of events) lines.push(...eventLines(event));

  lines.push("END:VCALENDAR");

  return `${lines.join(CRLF)}${CRLF}`;
}

/* ---------------------------------------------------------------------------
   RESPONSE
   ------------------------------------------------------------------------ */

/** ASCII-safe file name, with the UTF-8 original kept in `filename*`. */
function contentDisposition(filename: string): string {
  const ascii = filename.replace(/[^\w.-]+/g, "-").replace(/^-+|-+$/g, "");
  const fallback = ascii.length > 4 ? ascii : "event.ics";

  return `attachment; filename="${fallback}"; filename*=UTF-8''${encodeURIComponent(
    filename,
  )}`;
}

/** The whole HTTP response for a .ics download. */
export function icsResponse(body: string, filename: string): Response {
  return new Response(body, {
    status: 200,
    headers: {
      "content-type": "text/calendar; charset=utf-8",
      "content-disposition": contentDisposition(filename),
      // The file changes whenever the event does, and it is cheap to rebuild.
      "cache-control": "public, max-age=0, must-revalidate",
    },
  });
}