Function parseRange

packages/server/src/routes/files.ts:333–357

Parse a `Range: bytes=start-end` header against the file size. Returns * undefined for a missing / malformed / unsatisfiable range, in which case the * caller serves the whole file with 200 (browsers accept that response).

(header: string | undefined, size: number)

 *  undefined for a missing / malformed / unsatisfiable range, in which case the
 *  caller serves the whole file with 200 (browsers accept that response). */
function parseRange(header: string | undefined, size: number): ByteRange | undefined {
  if (!header || size <= 0) return undefined;
  const m = /^bytes=(\d*)-(\d*)$/i.exec(header.trim());
  if (!m) return undefined;
  const startStr = m[1]!;
  const endStr = m[2]!;
  if (startStr === '' && endStr === '') return undefined;

  let start: number;
  let end: number;
  if (startStr === '') {
    // Suffix range: `bytes=-N` → the last N bytes.
    const suffix = Number(endStr);
    if (!Number.isFinite(suffix) || suffix <= 0) return undefined;
    start = Math.max(size - suffix, 0);
    end = size - 1;
  } else {
    start = Number(startStr);
    if (!Number.isFinite(start) || start < 0 || start >= size) return undefined;
    end = endStr === '' ? size - 1 : Number(endStr);
    if (!Number.isFinite(end) || end < 0) return undefined;
  }
  if (start > end) return undefined;
  return { start, end: Math.min(end, size - 1) };
}