import type { PropertyBag } from '../model/Properties';
import { sumAlongChain } from '../model/walk';
import { enabledAlongChain } from '../properties/enabled';

/**
 * Marks a live entity apart from a struct value: both reach a data edge as an object. A symbol
 * rather than a field, so no struct field name can impersonate it – and it never serializes,
 * which is right for a thing that exists only while the game runs.
 */
export const ENTITY_HANDLE: unique symbol = Symbol('entity handle');

/**
 * What a graph sees of a running entity; the runtime's Entity satisfies this shape. Mutating
 * `x`/`y`/`props` is how actions take effect.
 */
export interface EntityHandle {
  readonly [ENTITY_HANDLE]: true;
  id: string;
  assetId: string;
  name: string;
  /** Position local to `parent` (world position for top-level entities). */
  x: number;
  y: number;
  z: number;
  props: PropertyBag;
  parent: EntityHandle | null;
}

export function worldPositionOf(handle: EntityHandle): { x: number; y: number; z: number } {
  return sumAlongChain(
    handle,
    (h) => h.parent,
    (h) => h,
  );
}

/** Whether a handle and every ancestor of it are switched on. */
export function isHandleEnabled(handle: EntityHandle): boolean {
  return enabledAlongChain(
    handle,
    (h) => h.props,
    (h) => h.parent,
  );
}

/** Position in world units. */
export interface PointerState {
  x: number;
  y: number;
  inside: boolean;
  down: boolean;
  /** Went down / up during this frame. */
  pressed: boolean;
  released: boolean;
}

/**
 * Everything the graph does to the world goes through here, so the core stays free of runtime
 * imports. Implemented by the runtime (BehaviorSystem) and by test doubles.
 */
export interface NodeContext {
  self: EntityHandle;
  /** The behavior's parent object – the one the graph acts on. */
  owner: EntityHandle | null;
  variables: PropertyBag;
  /** Seconds since the world started and since the last tick. */
  time: number;
  dt: number;
  findByName(name: string): EntityHandle | null;
  isKeyDown(key: string): boolean;
  pointer: PointerState;
  /**
   * Whether a world point lies on `target`: on its colliders when it has any, otherwise on its own
   * footprint or that of anything parented below it.
   */
  hitTest(target: EntityHandle, x: number, y: number): boolean;
  /** A collider entity stands for itself; any other entity for its direct collider children. */
  isTouching(a: EntityHandle, b: EntityHandle): boolean;
  /**
   * Moves `target` by (dx, dy) in local units, stopping solid colliders at other solid ones axis by
   * axis (x first, then y) so it slides along walls.
   */
  moveAndSlide(target: EntityHandle, dx: number, dy: number): { blockedX: boolean; blockedY: boolean };
  log(message: string): void;
  /**
   * The position is local to `parent` (world when none); null when the asset does not exist or
   * cannot be placed. A spawned behavior runs its `On start` on the next tick.
   */
  spawn(
    assetId: string,
    position: { x: number; y: number; z: number },
    parent: EntityHandle | null,
  ): EntityHandle | null;
  destroy(target: EntityHandle): void;
  stop(): void;
  /** Ends the run and starts it again from the saved scene (everything spawned is lost). */
  restart(): void;
}