/*
 * Per-tab wiki page history.
 *
 * BreadcrumbTrail owns the ordered page slugs and their session-storage
 * representation. It deliberately knows nothing about page titles, routes or
 * DOM elements; the application supplies the current set of valid page slugs.
 */

const storageKey = "racket-wiki-breadcrumb-trail";
const trailLimit = 8;

class BreadcrumbTrail {
  /**
   * goal   : Create an empty breadcrumb history backed by session storage.
   * pre    : storage implements the Web Storage getItem/setItem interface.
   * post   : No stored value is read until load is called with valid pages.
   * result : A directly usable breadcrumb history.
   */
  constructor(storage) {
    this.storage = storage;
    this.pageSlugs = [];
  }

  /** Return a copy of the ordered non-home page slugs. */
  get slugs() {
    return this.pageSlugs.slice();
  }

  /**
   * goal   : Restore the trail and discard references to missing pages.
   * pre    : validSlugs contains the current readable page slugs.
   * post   : The normalized trail is stored again for this browser tab.
   * result : A copy of the restored trail.
   * internals: JSON is decoded first, then filtered through validSlugs so stale
   *            browser state cannot become application navigation.
   */
  load(validSlugs) {
    let stored = [];
    try {
      stored = JSON.parse(this.storage.getItem(storageKey) || "[]");
    } catch (_error) {
      stored = [];
    }

    const valid = new Set(validSlugs);
    if (Array.isArray(stored)) {
      this.pageSlugs = stored.filter((slug) =>
        typeof slug === "string" && valid.has(slug));
    } else {
      this.pageSlugs = [];
    }
    this.#save();
    return this.slugs;
  }

  /** Empty and persist the breadcrumb history. */
  clear() {
    this.pageSlugs = [];
    this.#save();
  }

  /**
   * goal   : Record navigation to one wiki page.
   * pre    : pageSlug identifies the visited page; homeSlug may be null.
   * post   : Home clears the trail, revisits truncate it, and new visits append.
   * result : A copy of the updated trail.
   * internals: The trail contains only non-home pages and retains at most the
   *            eight most recent entries, matching the visible breadcrumb.
   */
  record(pageSlug, homeSlug = null) {
    if (!pageSlug) return this.slugs;
    if (homeSlug && pageSlug === homeSlug) {
      this.clear();
      return this.slugs;
    }

    const existingIndex = this.pageSlugs.indexOf(pageSlug);
    if (existingIndex >= 0) {
      this.pageSlugs = this.pageSlugs.slice(0, existingIndex + 1);
    } else {
      this.pageSlugs.push(pageSlug);
      if (this.pageSlugs.length > trailLimit) {
        this.pageSlugs = this.pageSlugs.slice(-trailLimit);
      }
    }
    this.#save();
    return this.slugs;
  }

  /**
   * goal   : Discard breadcrumb entries that follow a selected trail page.
   * pre    : pageSlug is a breadcrumb destination; homeSlug may be null.
   * post   : Home clears the trail; an existing page becomes its final entry.
   * result : A copy of the updated trail.
   */
  truncate(pageSlug, homeSlug = null) {
    if (homeSlug && pageSlug === homeSlug) {
      this.clear();
      return this.slugs;
    }

    const index = this.pageSlugs.indexOf(pageSlug);
    if (index >= 0) {
      this.pageSlugs = this.pageSlugs.slice(0, index + 1);
      this.#save();
    }
    return this.slugs;
  }

  /** Persist the current representation for this browser tab. */
  #save() {
    this.storage.setItem(storageKey, JSON.stringify(this.pageSlugs));
  }
}

export { BreadcrumbTrail };