Made of little things.

import { BoxGeometry, InstancedMesh, Matrix4, MeshBasicMaterial, Quaternion, Vector3 } from 'three';
import type { Bullet } from '../sim/guns.ts';

const MAX_TRACERS = 512;

/** Every round in flight drawn as one glowing streak, in a single draw call. */
export function createTracers() {
  const mesh = new InstancedMesh(
    new BoxGeometry(0.12, 0.12, 4.5),
    new MeshBasicMaterial({ color: 0xffd36b }),
    MAX_TRACERS,
  );
  mesh.frustumCulled = false;
  mesh.count = 0;
  const matrix = new Matrix4();
  const turn = new Quaternion();
  const dir = new Vector3();
  const zAxis = new Vector3(0, 0, 1);
  const unit = new Vector3(1, 1, 1);
  return {
    mesh,
    update(bullets: readonly Bullet[]) {
      const count = Math.min(MAX_TRACERS, bullets.length);
      for (let i = 0; i < count; i++) {
        const bullet = bullets[i]!;
        dir.copy(bullet.velocity).normalize();
        turn.setFromUnitVectors(zAxis, dir);
        matrix.compose(bullet.position, turn, unit);
        mesh.setMatrixAt(i, matrix);
      }
      mesh.count = count;
      mesh.instanceMatrix.needsUpdate = true;
    },
  };
}