/**
 * The UI camera split — the seam that lets a system effect (a camera
 * filter on the WORLD camera) warp the world while the HUD stays crisp.
 *
 * How Phaser 4 draws cameras: each camera is one render pass, in
 * `scene.cameras` array order (the array's FIRST camera draws first, so
 * later cameras draw ON TOP). Every pass can composite through its own
 * `filters.internal` list (WebGL). `camera.ignore(targets)` marks the
 * targets (and their container descendants) with the camera's id bit —
 * `object.willRender(camera)` then fails for that camera — so a target
 * simply does not exist for that pass. There is no un-ignore; the
 * partition is set once.
 *
 * The split this module owns:
 *   - `cameras[0]` = main — the WORLD camera (scrolls with the ship).
 *     The system effect's filter is attached here.
 *   - `cameras[1]` = `fx-ui` — the UI camera (fixed viewport). It is
 *     force-composited (rendered to a framebuffer, then blitted over
 *     the main pass) so it draws on top WITHOUT clearing the main
 *     camera's output — a second non-composited pass would clear the
 *     canvas and erase the world.
 *
 * Partition rule (heuristic, deliberate): a top-level scene object is
 * UI iff it is screen-pinned (scrollFactor 0 on either axis). That is
 * exactly this game's screen/world divide — the HUD, deck, panels and
 * toasts are scrollFactor 0; the starfield, worlds, gates, tethers,
 * the ship, even the world-anchored pop-ups (comms, mining) are not.
 * Screen-pinned objects are ignored by MAIN (they exist only for the UI
 * pass); world objects are ignored by the UI pass (they exist only for
 * the world pass, and are what the effect displaces — starfield
 * included, for free).
 *
 * Consequences to remember (audited, deliberate):
 *   - The UI no longer shakes with the world camera (shake is a
 *     camera effect — the UI camera is untouched).
 *   - Input is camera-aware: the interaction manager hit-tests each
 *     camera under the pointer, in REVERSE array order — the UI pass
 *     first — and an object is only a candidate where `willRender`
 *     holds. UI-over-world clicks therefore keep working, with the UI
 *     winning (the same topness the single-camera depth order gave).
 *
 * Lazily created objects (toasts, the jump clip, world markers, the
 * mining beam) are assigned explicitly at their creation sites via
 * assignUi()/assignWorld() below.
 */

/**
 * @param {object} scene Phaser scene
 * @returns {boolean} true if the object is screen-pinned (UI)
 */
export function isScreenPinned(obj) {
  return (
    typeof obj?.scrollFactorX === 'number' &&
    (obj.scrollFactorX === 0 || obj.scrollFactorY === 0)
  );
}

/**
 * Create (once) the fixed UI camera and partition the scene's
 * top-level objects between the two passes. Idempotent — subsequent
 * calls return the existing split.
 *
 * @param {object} scene Phaser scene
 * @returns {{ main: object, ui: object } | null} the split, or null if
 *   the scene has no camera plugin (defensive).
 */
export function ensureUiCameras(scene) {
  if (scene._fxUiCameras) return scene._fxUiCameras;
  const cameras = scene.cameras;
  if (!cameras?.cameras || cameras.cameras.length === 0) return null;

  const main = cameras.main;
  const ui = cameras.add(0, 0, scene.scale.width, scene.scale.height, false, 'fx-ui');
  // The UI pass must composite (FBO + blit over the main pass) — a
  // second DIRECT pass would clear() the canvas and erase the world.
  if (typeof ui.setForceComposite === 'function') ui.setForceComposite(true);

  const uiRoots = [];
  const worldRoots = [];
  const children = scene.sys.displayList?.getChildren?.() ?? [];
  for (const child of children) {
    if (isScreenPinned(child)) uiRoots.push(child);
    else worldRoots.push(child);
  }
  if (uiRoots.length) main.ignore(uiRoots); // UI: world pass never sees it
  if (worldRoots.length) ui.ignore(worldRoots); // world: UI pass never sees it

  scene._fxUiCameras = { main, ui, uiRoots: uiRoots.length, worldRoots: worldRoots.length };
  return scene._fxUiCameras;
}

/**
 * Assign a lazily created screen-pinned object to the UI pass (main
 * ignores it). No-op while the split does not exist — the single-camera
 * pipeline renders it exactly as before.
 *
 * @param {object} scene Phaser scene
 * @param {object} child object to attach to the UI pass
 */
export function assignUi(scene, child) {
  const split = scene._fxUiCameras;
  if (split && child) split.main.ignore(child);
}

/**
 * Assign a lazily created world object to the world pass (the UI camera
 * ignores it, so it is not drawn twice — semi-transparent world objects
 * would otherwise additively double). No-op while the split does not
 * exist.
 *
 * @param {object} scene Phaser scene
 * @param {object} child object to keep on the world pass
 */
export function assignWorld(scene, child) {
  const split = scene._fxUiCameras;
  if (split && child) split.ui.ignore(child);
}