/**
 * Uploaded files the view draws with, keyed by file name. Nothing is evicted: file names are
 * content addressed, so a loaded file can never go stale. A miss is not an error – the subclass
 * starts the load and reports "not yet", and `onChange` fires when the file lands.
 */
export abstract class FileLibrary<T> {
  private base: string;
  private readonly loading = new Set<string>();
  /** Never retried, so one broken reference cannot spam the network. */
  private readonly failed = new Set<string>();
  private readonly listeners = new Set<() => void>();

  constructor(
    base: string,
    private readonly tag: string,
  ) {
    this.base = base;
  }

  /** Where the files are served from. Set once at start-up, before anything draws. */
  configure(base: string): void {
    this.base = base.replace(/\/+$/, '');
  }

  /** Runs `fn` whenever a file has arrived; returns the unsubscribe. */
  onChange(fn: () => void): () => void {
    this.listeners.add(fn);
    return () => this.listeners.delete(fn);
  }

  /** Runs the listeners: something they draw from has changed without a file arriving here. */
  notifyChange(): void {
    for (const listener of this.listeners) listener();
  }

  protected abstract fetchFile(url: string, file: string): Promise<T>;

  /** Keeps what arrived; the listeners run right after. */
  protected abstract keep(file: string, loaded: T): void;

  /** A file that could not be loaded; it is not asked for again. */
  protected hasFailed(file: string): boolean {
    return this.failed.has(file);
  }

  protected startLoading(file: string): void {
    if (this.loading.has(file) || this.failed.has(file)) return;
    this.loading.add(file);
    void this.fetchFile(`${this.base}/${file}`, file).then(
      (loaded) => {
        this.loading.delete(file);
        this.keep(file, loaded);
        this.notifyChange();
      },
      (error: unknown) => {
        this.loading.delete(file);
        this.failed.add(file);
        console.error(`[${this.tag}] cannot load ${file}`, error);
      },
    );
  }
}