// Path splitting and completion arithmetic for the working-directory input.

export function splitPath(value: string): { dir: string; prefix: string } {
  const slash = value.lastIndexOf("/");
  if (slash === -1) return { dir: "", prefix: value };
  return { dir: value.slice(0, slash + 1), prefix: value.slice(slash + 1) };
}

/** Longest common prefix of a list of strings (case-sensitive, preserving the
 * actual on-disk casing). */
export function longestCommonPrefix(items: string[]): string {
  if (items.length === 0) return "";
  let lcp = items[0];
  for (const s of items.slice(1)) {
    let i = 0;
    while (i < lcp.length && i < s.length && lcp[i] === s[i]) i++;
    lcp = lcp.slice(0, i);
    if (lcp === "") break;
  }
  return lcp;
}

/** Everything above the last segment of an absolute path: `/a/b/c` → `/a/b`.
 * A top-level path keeps `/`, and a trailing slash is ignored. */
export function parentDir(path: string): string {
  const trimmed = path.length > 1 && path.endsWith("/") ? path.slice(0, -1) : path;
  const slash = trimmed.lastIndexOf("/");
  if (slash <= 0) return slash === 0 ? "/" : "";
  return trimmed.slice(0, slash);
}