drive-between-the-lines/src/unitBodies.ts

/**
 * Gives the inhabited world a physical presence. Integration layer: Rapier on
 * one side, the pure unit sim on the other.
 *
 * Units are simulated as points that ignore each other, which is fine for
 * traffic going about its business and useless the moment the player wants to
 * interact with any of it. This is what makes them things you can hit.
 *
 * Two deliberately different treatments:
 *
 *  - **Vehicles get colliders.** Kinematic bodies, driven from the sim, so
 *    ramming a patrol is a real collision that shoves your car about.
 *  - **People do not.** A capsule collider would mean getting hung up on
 *    pedestrians, or launching them like skittles. Instead they are checked for
 *    proximity and simply go down. You drive *through* a person, not into them.
 */
import type RAPIER from '@dimforge/rapier3d-compat';
import type { PhysicsWorld } from './physics/physics';
import type { Unit, UnitState } from './sim/units';

/** Units further out than this get no body; the physics world stays small. */
const PHYSICAL_RANGE = 160;
/** Half-extents of a vehicle body. */
const CAR_HALF = { x: 0.9, y: 0.7, z: 2 };
/** How close the car has to pass to knock a person down. */
const RUN_OVER_RADIUS = 2.6;
/** Below this you are nudging past someone, not running them over. */
const RUN_OVER_SPEED = 3.5;
/** Closing speed at which a vehicle collision starts hurting the other car. */
const RAM_SPEED = 7;
const RAM_RADIUS = 4.6;

export interface ContactEvents {
  /** People knocked down by the player this step. */
  ranOver: Unit[];
  /** Vehicles the player rammed hard enough to damage. */
  rammed: Unit[];
}

export function createUnitBodies(physics: PhysicsWorld) {
  const bodies = new Map<number, RAPIER.RigidBody>();

  const drop = (id: number) => {
    const body = bodies.get(id);
    if (!body) return;
    physics.removeBody(body);
    bodies.delete(id);
  };

  return {
    /**
     * Syncs bodies to the sim and reports what the player just hit.
     * Call once per fixed step, after the units have moved.
     */
    sync(
      units: UnitState,
      player: { x: number; z: number },
      playerSpeed: number,
      dt: number,
    ): ContactEvents {
      const events: ContactEvents = { ranOver: [], rammed: [] };
      const alive = new Set<number>();
      const fast = Math.abs(playerSpeed);

      for (const unit of units.units) {
        const gap = Math.hypot(unit.x - player.x, unit.z - player.z);

        if (unit.kind === 'soldier') {
          // No collider — just whether the car went through them.
          if (gap < RUN_OVER_RADIUS && fast > RUN_OVER_SPEED && unit.hp > 0) {
            unit.hp = 0;
            events.ranOver.push(unit);
          }
          continue;
        }

        if (gap > PHYSICAL_RANGE) continue;
        alive.add(unit.id);

        let body = bodies.get(unit.id);
        if (!body) {
          body = physics.addKinematicBox({
            x: unit.x,
            z: unit.z,
            y: CAR_HALF.y,
            yaw: unit.heading,
            halfExtents: CAR_HALF,
          });
          bodies.set(unit.id, body);
        }
        // Kinematic: the sim decides where it is, and the physics engine works
        // out what that does to anything dynamic it meets — namely the player.
        body.setNextKinematicTranslation({ x: unit.x, y: CAR_HALF.y, z: unit.z });
        body.setNextKinematicRotation({
          x: 0,
          y: Math.sin(unit.heading / 2),
          z: 0,
          w: Math.cos(unit.heading / 2),
        });

        // Rapier will resolve the shove; the damage is ours to decide.
        if (gap < RAM_RADIUS && fast > RAM_SPEED) {
          unit.hp -= fast * 4 * dt;
          if (!events.rammed.includes(unit)) events.rammed.push(unit);
        }
      }

      for (const id of [...bodies.keys()]) if (!alive.has(id)) drop(id);
      return events;
    },

    get count() {
      return bodies.size;
    },
  };
}