smartgame/js/traits/TPhysics.js

import {Trait} from "../Entity.js";
import Vec2 from "../Vec2.js";

export default class TPhysics extends Trait {
	constructor(entity) {
		super(entity, "physics");

		this.gravity = 40;
		this.groundFriction = 6;
		this.airFriction = 2;

		this.velocity = new Vec2();
		this.relVelocity = new Vec2();

		this.onGround = false;
		this.prevRelativeTo = null;
		this.relativeTo = null;
	}

	update(dt) {
		let collider = this.entity.t.collider;

		this.prevRelativeTo = this.relativeTo;
		this.relativeTo = null;
		this.onGround = false;

		if (
				this.entity.has("collider") &&
				collider.collides &&
				this.relVelocity.y >= 0) {

			// Structures are static; just teleport us to the top of them
			if (collider.cStructure) {
				this.relVelocity.y = 0;
				this.entity.bounds.bottom = collider.cStructure.top;
				this.onGround = true;

			// If we're colliding with an entity, and that entity is
			// a platform, teleport us to the top of them.
			} else if (collider.cEntity.has("platform")) {
				this.relVelocity.y = 0;
				this.entity.bounds.bottom = collider.cEntity.bounds.top;
				this.onGround = true;

				if (collider.cEntity.has("physics")) {
					let cPhys = collider.cEntity.t.physics;
					this.relativeTo = cPhys;
				}
			}
		}

		this.relVelocity.y += this.gravity * dt;

		// If we just started riding something, adjust relative velocity
		if (!this.prevRelativeTo && this.relativeTo) {
			this.relVelocity.x = this.velocity.x - this.relativeTo.velocity.x;
			this.relVelocity.y = this.velocity.y - this.relativeTo.velocity.y;

		// If we just stopped riding something, adjust relative velocity
		} else if (this.prevRelativeTo && !this.relativeTo) {
			this.relVelocity.x = this.velocity.x + this.relativeTo.velocity.x;
			this.relVelocity.y = this.velocity.y + this.relativeTo.velocity.y;
		}

		// Apply friction
		var fric = this.onGround ? this.groundFriction : this.airFriction;
		var xRatio = 1 / (1 + (dt * fric));
		this.relVelocity.x *= xRatio;

		// Update velocity
		if (this.relativeTo) {
			this.velocity.x = this.relVelocity.x + this.relativeTo.velocity.x;
			this.velocity.y = this.relVelocity.y + this.relativeTo.velocity.y;
		} else {
			this.velocity.x = this.relVelocity.x;
			this.velocity.y = this.relVelocity.y;
		}
	}

	postUpdate(dt) {
		this.entity.bounds.pos.x += this.velocity.x * dt;
		this.entity.bounds.pos.y += this.velocity.y * dt;
	}
}