Gameable Engine

Add an enemy ​

Goal ​

A second kind of enemy — a heavy: slower, twice the health, hits harder, and a different colour — sharing the existing chase-and-attack behaviour.

Files you will edit ​

  • src/prefabs.ts
  • src/game.ts

Steps ​

  1. Declare what a heavy is made of. A prefab is pure and safe at module scope; nothing is resolved until it is spawned.

    ts
    // src/prefabs.ts
/** A heavy: bigger, slower, and it takes four rifle magazines to notice. */
export const HeavyPrefab = prefab({
  name: 'heavy',
  body: {
    shape: 'capsule',
    dims: [0.5, 0.9],
    kind: 'character',
    mass: 140,
    layer: { enemy: true },
    mask: { staticGeometry: true, player: true, enemy: true },
    flags: { reportContacts: true, noSleep: true },
  },
  health: 120,
  components: [Enemy],
});

/** Height of a heavy capsule's centre above its feet. */
export const HEAVY_CENTRE = 0.5 + 0.9;

/** Heavy purple. */
export const HEAVY_COLOUR: readonly [number, number, number] = [0.44, 0.2, 0.62];

    The placeholder mesh is drawn from the body, so the capsule on screen is exactly the capsule the solver uses. Changing dims changes both.

    reportContacts is off unless a body asks for it, and a heavy is exactly the kind of body that should ask: the game reacts when one touches the player. begin and end are all it gets — the per-step stay manifold is a separate, rarer opt-in, and nothing here needs it.

  2. Spawn two of them, and let the existing AI drive them. HeavyPrefab carries the Enemy tag, so enemyAI already queries it and updateHud already counts it — nothing else has to change.

    diff
    // src/game.ts
-import { EnemyPrefab, ENEMY_CENTRE, ENEMY_COLOUR, ... } from './prefabs';
+import {
+  EnemyPrefab,
+  ENEMY_CENTRE,
+  ENEMY_COLOUR,
+  HeavyPrefab,
+  HEAVY_CENTRE,
+  HEAVY_COLOUR,
+  ...
+} from './prefabs';

 init: (ctx) => {
    for (const spawn of arenaSpawns.enemies) {
      at.x = spawn.position[0];
      at.y = spawn.position[1] + ENEMY_CENTRE;
      at.z = spawn.position[2];
      tint(ctx.spawn(EnemyPrefab, at), ENEMY_COLOUR[0], ENEMY_COLOUR[1], ENEMY_COLOUR[2]);
    }
+
+   // Two heavies, in the far corners.
+   for (const spawn of [arenaSpawns.enemies[0], arenaSpawns.enemies[1]]) {
+     at.x = spawn.position[0];
+     at.y = spawn.position[1] + HEAVY_CENTRE;
+     at.z = spawn.position[2] - 2;
+     tint(ctx.spawn(HeavyPrefab, at), HEAVY_COLOUR[0], HEAVY_COLOUR[1], HEAVY_COLOUR[2]);
+   }
 },

    To make heavies genuinely slower rather than merely tougher, give them their own speed in enemyAI — the state arrays there are indexed by entity id, so a aiSpeed: Float32Array written at spawn time is the natural place.

Verify ​

sh
npm test

The template's own test asserts six enemies; update it to eight and it should pass again:

ts
expect(livingEnemies(harness.guest)).toHaveLength(8);

Then npm run dev: the HUD should read enemies 8 at the start, and the two purple capsules should be visibly wider than the red ones and take five rifle hits instead of two.

See also ​