/**
 * Rect registry — the contract from `wiki/SharedContracts.md` §7.
 *
 * Three subsystems publish DOMRects into a single registry so the SVG
 * visual-guide overlay can draw curves between them:
 *
 *   - "field"         — a form field (registered by FormRenderer)
 *   - "evidence-card" — a sidebar entry (registered by EvidenceSidebar / similar)
 *   - "highlight"     — a viewer highlight (registered by the PDF adapter)
 *
 * The overlay subscribes once and queries `getRect` on demand each frame.
 * **The contract MUST NOT change after CE-WP-0003-T03 lands** — that's the
 * whole point of pinning it now in §7. Future kinds (HTML-doc-section,
 * comment-anchor, …) are added by extending the `RectKind` union here AND
 * the doc together.
 */

export type RectKind = "field" | "evidence-card" | "highlight";

export interface RectRegistryRegistration {
  readonly kind: RectKind;
  readonly id: string;
  readonly getRect: () => DOMRect | null;
}

/**
 * A registry-change signal. Listeners typically don't care which
 * specific rect changed — they re-query everything they're tracking —
 * but the discriminator is kept so optimised consumers can filter.
 *
 * `rect-changed` with an undefined `kind`+`id` means "the registry is
 * advising you that *something* may have moved" (scroll, resize). That
 * is the dominant event in practice.
 */
export type RectRegistryEvent =
  | { readonly type: "registered"; readonly kind: RectKind; readonly id: string }
  | { readonly type: "unregistered"; readonly kind: RectKind; readonly id: string }
  | {
      readonly type: "rect-changed";
      readonly kind?: RectKind;
      readonly id?: string;
    };

export interface RectRegistry {
  register(
    kind: RectKind,
    id: string,
    getRect: () => DOMRect | null,
  ): () => void;
  getRect(kind: RectKind, id: string): DOMRect | null;
  subscribe(listener: (event: RectRegistryEvent) => void): () => void;
  /**
   * Imperative invalidation. Useful for the overlay's own
   * active-evidence-change trigger (which isn't observable via DOM
   * observers). Emits a `rect-changed` with no kind/id.
   */
  invalidate(): void;
  /**
   * Monotonically-increasing counter that bumps once per emit. Lets React
   * consumers use `useSyncExternalStore` without per-snapshot subscriber
   * leaks — the hook only adds one subscriber and reads `getVersion()`
   * for the snapshot.
   */
  getVersion(): number;
  /**
   * Debug-only — enumerate everything currently registered. Used in tests
   * and not part of the §7 contract; safe to use from `app/`.
   */
  list(): readonly { kind: RectKind; id: string }[];
}

function keyOf(kind: RectKind, id: string): string {
  return `${kind} ${id}`;
}

export function createRectRegistry(): RectRegistry {
  const entries = new Map<string, RectRegistryRegistration>();
  const listeners = new Set<(event: RectRegistryEvent) => void>();
  let version = 0;

  function emit(event: RectRegistryEvent) {
    version++;
    for (const l of listeners) {
      try {
        l(event);
      } catch {
        // Listener errors are swallowed: the overlay never wants a
        // single broken consumer to crash the others.
      }
    }
  }

  return {
    register(kind, id, getRect) {
      const key = keyOf(kind, id);
      entries.set(key, { kind, id, getRect });
      emit({ type: "registered", kind, id });
      return () => {
        const current = entries.get(key);
        // Only unregister if it's *this* registration. A re-register with
        // the same (kind,id) before our cleanup runs supersedes us.
        if (current && current.getRect === getRect) {
          entries.delete(key);
          emit({ type: "unregistered", kind, id });
        }
      };
    },
    getRect(kind, id) {
      const entry = entries.get(keyOf(kind, id));
      if (!entry) return null;
      try {
        return entry.getRect();
      } catch {
        return null;
      }
    },
    subscribe(listener) {
      listeners.add(listener);
      return () => {
        listeners.delete(listener);
      };
    },
    invalidate() {
      emit({ type: "rect-changed" });
    },
    getVersion() {
      return version;
    },
    list() {
      return [...entries.values()].map((e) => ({ kind: e.kind, id: e.id }));
    },
  };
}