import { MAX_SSE_FIELD_BYTES } from "./config";

/**
 * Truncates a string to at most n bytes of UTF-8 without splitting multi-byte
 * runes. Keeps one SSE `data:` field bounded.
 */
export function truncateUtf8(s: string, n: number = MAX_SSE_FIELD_BYTES): string {
  const buf = new TextEncoder().encode(s);
  if (buf.byteLength <= n) return s;
  let end = n;
  while (end > 0 && (buf[end]! & 0xc0) === 0x80) end--;
  return new TextDecoder().decode(buf.subarray(0, end));
}

/**
 * A single SSE block: `event` + JSON `data`, blank-line terminated. The `id`
 * line is written only for logged events — an out-of-band frame carries no seq,
 * and an empty `id:` would clear the client's resume cursor.
 */
export function sseBlock(e: { id?: string; event: string; data: unknown }): string {
  const id = e.id ? `id: ${e.id}\n` : "";
  return `event: ${e.event}\n${id}data: ${JSON.stringify(e.data)}\n\n`;
}