// Where you were last, as data. NO server imports -- LastPage.tsx is a client
// component and imports these as VALUES, so the same rule lib/note-types.ts
// spells out applies here: one `node:` import anywhere in this file's graph and
// the bundler follows it into the browser chunk and 500s every page.
//
// The interesting part of "come back to where I was" is not the storage, it is
// deciding WHERE to land when the remembered URL has gone stale -- a song gets
// renamed, a cut gets rebuilt away, and /browse/yoshi/wide stops existing. This
// app has no app/not-found.tsx, so a stale restore would open the tool on
// Next's bare 404, which has no nav on it and no way out but the back button.
// So the walk-up lives here, pure and testable without a browser.

/** Same `umtool:` prefix as SHORTLIST_EVENT -- one namespace for the app. */
export const LAST_PAGE_KEY = "umtool:last-page";

/**
 * The floor of the walk-up. It was /sort when / was a doorway into the song
 * piles; / is the dashboard now, and the page a stale URL degrades to is the
 * list of every project rather than one kind's judging pile.
 */
export const DEFAULT_PAGE = "/browse";

/** Long enough for any real query string, short enough to bound a bad write. */
const URL_LIMIT = 2000;

/**
 * Is this a URL we would both record and restore to?
 *
 * Deliberately the SAME guard on both sides. Storage is user-editable and
 * survives a reinstall of the app, so a value read back is untrusted input: a
 * stored `//evil.example` handed to router.replace() is an off-site redirect
 * from a URL the user typed themselves. Relative, same-origin, real page only.
 */
export function isRecordable(url: string): boolean {
  if (!url || url.length > URL_LIMIT) return false;
  if (!url.startsWith("/")) return false;
  // `//host` is protocol-relative -- it looks local and is not.
  if (url.startsWith("//")) return false;
  // A backslash is a slash to some URL parsers and not to others; refuse the
  // whole class rather than guess which one router.replace() feeds.
  if (url.includes("\\")) return false;
  // `/` itself is this very page. Restoring to it would be a loop.
  if (url === "/") return false;
  // The API is not a place you can be.
  if (url === "/api" || url.startsWith("/api/")) return false;
  return true;
}

/**
 * Where to try landing, best first.
 *
 * The full URL, then the same path without its query, then each parent, then
 * DEFAULT_PAGE. `/browse/yoshi/wide` gives
 * `["/browse/yoshi/wide", "/browse/yoshi", "/browse"]` -- so a cut
 * that no longer exists degrades to its song, a song that no longer exists
 * degrades to the list of songs, and nothing ever lands on a 404.
 */
export function restoreCandidates(url: string): string[] {
  const out: string[] = [];
  const add = (u: string) => {
    if (u && !out.includes(u)) out.push(u);
  };

  if (isRecordable(url)) {
    add(url);
    const q = url.search(/[?#]/);
    let path = q === -1 ? url : url.slice(0, q);
    add(path);
    // Stop BEFORE "/": it is this page, and isRecordable rejects it anyway.
    while (path.length > 1) {
      path = path.slice(0, path.lastIndexOf("/"));
      if (path.length > 1) add(path);
    }
  }

  add(DEFAULT_PAGE);
  return out;
}