systemic-game-design-course/course/08-push-chains/reference/ts/src/engine/puzzle/movement.ts

// Movement: validate, then commit.
//
// A move is planned before anything on the board changes. The `Planner` asks
// whether one object can step one tile; to answer, it runs every movement
// response content provides for that step, and a response may ask the planner
// to move other objects too (a push), or to destroy objects. If any response
// says no, the planner forgets everything that step added to the plan, so a
// failed push leaves no trace. Only a plan that succeeded is committed.
//
// The engine knows the board, its edge, and the order of the steps. What each
// property does when something moves (`PUSH`, `STOP`, `OPEN`/`SHUT`, ...) is a
// response in content, run in the order content lists them.

import type { Board, ObjectId } from "./grid";

/** One of the four directions a move can take. */
export type Dir = "up" | "down" | "left" | "right";

export type Pos = readonly [number, number];

/** One tile in direction `dir`, as `[dx, dy]`; `y` grows down. */
export function delta(dir: Dir): Pos {
  switch (dir) {
    case "up":
      return [0, -1];
    case "down":
      return [0, 1];
    case "left":
      return [-1, 0];
    case "right":
      return [1, 0];
  }
}

/** One object stepping one tile, from `from` to `to`, in direction `dir`. */
export interface Step {
  mover: ObjectId;
  from: Pos;
  to: Pos;
  dir: Dir;
}

/**
 * A movement response: content's rule for what happens when an object steps.
 * It returns `false` to block the step. It may plan other moves or destroy
 * objects through the planner.
 */
export type Response = (p: Planner, s: Step) => boolean;

/**
 * What a move will do: the objects that step one tile, and the objects destroyed.
 * A destroyed object doesn't step, even if it was pushed.
 */
export interface Plan {
  moving: Set<ObjectId>;
  destroyed: Set<ObjectId>;
}

/** What the planner reads: the board, the properties in force, the responses. */
export interface MoveContext {
  readonly board: Board;
  hasProperty(id: ObjectId, property: string): boolean;
  responses(): readonly Response[];
}

/** Plans one move on a board it only reads. */
export class Planner {
  private plan: Plan = { moving: new Set(), destroyed: new Set() };

  constructor(
    private readonly level: MoveContext,
    private readonly dir: Dir,
  ) {}

  /** Does object `id` have `property`, under the rules in force before the move? */
  has(id: ObjectId, property: string): boolean {
    return this.level.hasProperty(id, property);
  }

  /**
   * The objects that will still be on tile `at` when the plan so far is
   * committed: those there now, less those planned to move away or be destroyed.
   * Oldest first.
   */
  stayingAt([x, y]: Pos): ObjectId[] {
    return this.level.board
      .at(x, y)
      .map((o) => o.id)
      .filter((id) => !this.plan.moving.has(id) && !this.plan.destroyed.has(id));
  }

  /** Plan object `id`'s destruction. */
  destroy(id: ObjectId): void {
    this.plan.destroyed.add(id);
  }

  /**
   * Plan object `id` stepping one tile in the move's direction, with whatever
   * the responses add. Returns whether it can. If it can't, the plan is left as
   * it was before the call. An object already planned to move, or to be
   * destroyed, is out of the way: the answer is yes, and nothing is added.
   */
  tryMove(id: ObjectId): boolean {
    if (this.plan.moving.has(id) || this.plan.destroyed.has(id)) return true;
    const o = this.level.board.get(id);
    const [dx, dy] = delta(this.dir);
    const to: Pos = [o.x + dx, o.y + dy];
    // The edge of the level blocks every step (the course's choice; see the lesson).
    if (!this.level.board.inBounds(...to)) return false;
    const before: Plan = { moving: new Set(this.plan.moving), destroyed: new Set(this.plan.destroyed) };
    this.plan.moving.add(id);
    const step: Step = { mover: id, from: [o.x, o.y], to, dir: this.dir };
    const allowed = this.level.responses().every((respond) => respond(this, step));
    if (!allowed) this.plan = before;
    return allowed;
  }

  result(): Plan {
    return this.plan;
  }
}