jumpstorm/engine/systems/Collision.ts

import { SystemNames, System } from ".";
import {
  Mass,
  BoundingBox,
  ComponentNames,
  Jump,
  Velocity,
  Moment,
} from "../components";
import { Game } from "../Game";
import { PhysicsConstants } from "../config";
import { Entity } from "../entities";
import type { Dimension2D } from "../interfaces";
import { QuadTree } from "../structures";

export class Collision extends System {
  private static readonly COLLIDABLE_COMPONENT_NAMES = [
    ComponentNames.Collide,
    ComponentNames.TopCollidable,
  ];
  private static readonly QUADTREE_MAX_LEVELS = 10;
  private static readonly QUADTREE_SPLIT_THRESHOLD = 10;

  private quadTree: QuadTree;

  constructor(screenDimensions: Dimension2D) {
    super(SystemNames.Collision);

    this.quadTree = new QuadTree(
      { x: 0, y: 0 },
      screenDimensions,
      Collision.QUADTREE_MAX_LEVELS,
      Collision.QUADTREE_SPLIT_THRESHOLD
    );
  }

  public update(dt: number, game: Game) {
    // rebuild the quadtree
    this.quadTree.clear();

    const entitiesToAddToQuadtree: Entity[] = [];

    Collision.COLLIDABLE_COMPONENT_NAMES.map((componentName) =>
      game.componentEntities.get(componentName)
    ).forEach((entityIds: Set<number>) =>
      entityIds.forEach((id) => {
        const entity = game.entities.get(id);
        if (!entity.hasComponent(ComponentNames.BoundingBox)) {
          return;
        }
        entitiesToAddToQuadtree.push(entity);
      })
    );

    entitiesToAddToQuadtree.forEach((entity) => {
      const boundingBox = entity.getComponent<BoundingBox>(
        ComponentNames.BoundingBox
      );

      this.quadTree.insert(
        entity.id,
        boundingBox.dimension,
        boundingBox.center
      );
    });

    // find colliding entities and perform collisions
    const collidingEntities = this.getCollidingEntities(
      entitiesToAddToQuadtree,
      game.entities
    );

    collidingEntities.forEach(([entityAId, entityBId]) => {
      const [entityA, entityB] = [entityAId, entityBId].map((id) =>
        game.entities.get(id)
      );
      this.performCollision(entityA, entityB);
    });
  }

  private performCollision(entityA: Entity, entityB: Entity) {
    const [entityABoundingBox, entityBBoundingBox] = [entityA, entityB].map(
      (entity) => entity.getComponent<BoundingBox>(ComponentNames.BoundingBox)
    );

    let velocity: Velocity;
    if (entityA.hasComponent(ComponentNames.Velocity)) {
      velocity = entityA.getComponent<Velocity>(ComponentNames.Velocity);
    }

    if (
      entityA.hasComponent(ComponentNames.Collide) &&
      entityB.hasComponent(ComponentNames.TopCollidable) &&
      entityABoundingBox.center.y <= entityBBoundingBox.center.y &&
      velocity &&
      velocity.dCartesian.dy >= 0 // don't apply "floor" logic when coming through the bottom
    ) {
      if (entityBBoundingBox.rotation != 0) {
        throw new Error(
          `entity with id ${entityB.id} has TopCollidable component and a non-zero rotation. that is not (yet) supported.`
        );
      }

      // remove previous velocity in the y axis
      velocity.dCartesian.dy = 0;

      // apply normal force
      if (entityA.hasComponent(ComponentNames.Gravity)) {
        const mass = entityA.getComponent<Mass>(ComponentNames.Mass).mass;
        const F_n = -mass * PhysicsConstants.GRAVITY;

        entityA.getComponent<Forces>(ComponentNames.Forces).forces.push({
          fCartesian: { fy: F_n },
        });
      }

      // reset the entities' jump
      if (entityA.hasComponent(ComponentNames.Jump)) {
        entityA.getComponent<Jump>(ComponentNames.Jump).canJump = true;
      }

      entityABoundingBox.center.y =
        entityBBoundingBox.center.y -
        entityBBoundingBox.dimension.height / 2 -
        this.getDyToPushOutOfFloor(entityABoundingBox, entityBBoundingBox);
    }
  }

  private getCollidingEntities(
    collidableEntities: Entity[],
    entityMap: Map<number, Entity>
  ): [number, number][] {
    const collidingEntityIds: [number, number] = [];

    for (const entity of collidableEntities) {
      const boundingBox = entity.getComponent<BoundingBox>(
        ComponentNames.BoundingBox
      );

      this.quadTree
        .getNeighborIds({
          id: entity.id,
          dimension: boundingBox.dimension,
          center: boundingBox.center,
        })
        .filter((neighborId) => neighborId != entity.id)
        .forEach((neighborId) => {
          const neighborBoundingBox = entityMap
            .get(neighborId)
            .getComponent<BoundingBox>(ComponentNames.BoundingBox);

          if (boundingBox.isCollidingWith(neighborBoundingBox)) {
            collidingEntityIds.push([entity.id, neighborId]);
          }
        });
    }

    return collidingEntityIds;
  }

  // ramblings: https://excalidraw.com/#json=z-xD86Za4a3duZuV2Oky0,KaGe-5iHJu1Si8inEo4GLQ
  private getDyToPushOutOfFloor(
    entityBoundingBox: BoundingBox,
    floorBoundingBox: BoundingBox
  ): number {
    const {
      rotation,
      center: { x, y },
      dimension: { width, height },
    } = entityBoundingBox;

    let rads = rotation * (Math.PI / 180);
    if (rads >= Math.PI) {
      rads -= Math.PI; // we have symmetry so we can skip two cases
    }

    let boundedCollisionX = 0; // bounded x on the surface from width
    let clippedX = 0; // x coordinate of the vertex below the surface
    let outScribedRectangleHeight, dy, dx;

    if (rads <= Math.PI / 2) {
      dx = (width * Math.cos(rads) - height * Math.sin(rads)) / 2;
      outScribedRectangleHeight =
        width * Math.sin(rads) + height * Math.cos(rads);
    } else if (rads <= Math.PI) {
      rads -= Math.PI / 2;
      dx = (height * Math.cos(rads) - width * Math.sin(rads)) / 2;
      outScribedRectangleHeight =
        width * Math.cos(rads) + height * Math.sin(rads);
    }

    if (x >= floorBoundingBox.center.x) {
      clippedX = x + dx;
      boundedCollisionX = Math.min(
        floorBoundingBox.center.x + floorBoundingBox.dimension.width / 2,
        clippedX
      );
      return (
        outScribedRectangleHeight / 2 -
        Math.max((clippedX - boundedCollisionX) * Math.tan(rads), 0)
      );
    }

    clippedX = x - dx;
    boundedCollisionX = Math.max(
      floorBoundingBox.center.x - floorBoundingBox.dimension.width / 2,
      clippedX
    );

    return (
      outScribedRectangleHeight / 2 -
      Math.max((boundedCollisionX - clippedX) * Math.tan(rads), 0)
    );
  }
}