Function isWithinDir

src/utils.ts:73–81

* Whether `child` is `parent` itself or sits underneath it. Case-insensitive on * Windows — NTFS is case-insensitive, and realpathSync can hand back a different * case than the lexical root, which would otherwise false-reject a valid file.

(child: string, parent: string)

 * case than the lexical root, which would otherwise false-reject a valid file.
 */
function isWithinDir(child: string, parent: string): boolean {
  let c = child;
  let p = parent;
  if (process.platform === 'win32') {
    c = c.toLowerCase();
    p = p.toLowerCase();
  }
  return c === p || c.startsWith(p + path.sep);
}

/**
 * Validate that a file path stays within the project root, resolving symlinks.