import { useEffect, useRef, useState } from 'react';

/**
 * Autosave a long form so it isn't lost on navigation or reload.
 *
 * - Persists `snapshot` to localStorage under `<prefix><key>` (content-debounced:
 *   the serialized string only changes when the draft actually changes).
 * - A saved draft is only offered as **resumable** when it *differs* from the
 *   current form state — so merely opening an existing record (whose prefill
 *   matches the saved draft) never shows a spurious "resume" banner, and a
 *   draft can never silently clobber a freshly loaded record.
 * - Call `clear()` after a successful publish.
 *
 * Used by the Lab profile forms (`useLabDraft`) and the create-spot form.
 */
export function useFormDraft<T>(
  prefix: string,
  key: string,
  snapshot: T,
  apply: (draft: T) => void,
) {
  const storageKey = prefix + key;
  const [loaded, setLoaded] = useState<T | null>(null);
  const firstRun = useRef(true);
  const serialized = JSON.stringify(snapshot);

  // Load any saved draft once.
  useEffect(() => {
    try {
      const raw = localStorage.getItem(storageKey);
      if (raw) setLoaded(JSON.parse(raw) as T);
    } catch {
      // ignore malformed/blocked storage
    }
  }, [storageKey]);

  // Persist on change (skip the initial empty paint).
  useEffect(() => {
    if (firstRun.current) {
      firstRun.current = false;
      return;
    }
    try {
      localStorage.setItem(storageKey, serialized);
    } catch {
      // storage may be full/blocked — a lost draft is not fatal
    }
  }, [serialized, storageKey]);

  const resume = () => {
    if (!loaded) return;
    apply(loaded);
    setLoaded(null);
  };

  const discard = () => {
    setLoaded(null);
    try { localStorage.removeItem(storageKey); } catch { /* ignore */ }
  };

  const clear = () => {
    try { localStorage.removeItem(storageKey); } catch { /* ignore */ }
  };

  const resumable = loaded !== null && JSON.stringify(loaded) !== serialized;
  return { resumable, resume, discard, clear };
}