export const MAX_SAFE_NAME_BYTES = 120;
const FALLBACK = "attachment";
const WINDOWS_RESERVED = /^(con|prn|aux|nul|com[0-9]|lpt[0-9])(\..*)?$/i;

function truncateUtf8(text: string, maxBytes: number): string {
  let out = "";
  for (const ch of text) {
    if (Buffer.byteLength(out + ch, "utf8") > maxBytes) break;
    out += ch;
  }
  return out;
}

/**
 * Library-safe single path segment: no directories, control or reserved
 * characters, leading dots, or unbounded length. Extension is preserved when
 * truncating; a sniffed extension is appended when the name has none.
 */
export function safeFileName(raw: string, sniffedExtension: string | null = null): string {
  const basename = raw.normalize("NFKC").split(/[/\\]/).pop() ?? "";
  let name = basename
    .replace(/[\p{Cc}\p{Cf}]/gu, "")
    .replace(/[<>:"|?*#%{}^~[\]`]/g, "_")
    .replace(/\s+/g, " ")
    .replace(/_+/g, "_")
    .trim()
    .replace(/^[.\s_-]+/, "")
    .replace(/[.\s]+$/, "");
  if (name === "") name = FALLBACK;
  if (WINDOWS_RESERVED.test(name)) name = `_${name}`;

  const dot = name.lastIndexOf(".");
  let stem = dot > 0 ? name.slice(0, dot) : name;
  let ext = dot > 0 ? name.slice(dot + 1).toLowerCase() : "";
  if (ext === "" && sniffedExtension !== null) ext = sniffedExtension;
  if (Buffer.byteLength(ext, "utf8") > 16) {
    stem = name;
    ext = sniffedExtension ?? "";
  }
  const suffix = ext === "" ? "" : `.${ext}`;
  stem = truncateUtf8(stem, MAX_SAFE_NAME_BYTES - Buffer.byteLength(suffix, "utf8")).trim();
  if (stem === "") stem = FALLBACK;
  return `${stem}${suffix}`;
}