I build the systems games run on.

-- Pack a world delta into BITS, not bytes. A position + id costs ~5B.
function BitBuffer:writeUInt(value, bits)
    for _ = 1, bits do
        self.acc = bit32.bor(self.acc, bit32.lshift(value % 2, self.n))
        value, self.n = value // 2, self.n + 1
        if self.n == 8 then self.bytes[#self.bytes+1], self.acc, self.n = self.acc, 0, 0 end
    end
end
function BitBuffer:writeVarInt(v)              -- 7 bits + continue flag
    repeat local b = v % 128; v = v // 128; self:writeUInt(v > 0 and b+128 or b, 8) until v == 0
end

-- Archetype ECS: entities grouped by component signature for
-- cache-friendly iteration. A query is a bitmask test, not a lookup.
local mask = Component.Position | Component.Velocity
for _, arch in ipairs(world.archetypes) do
    if bit32.band(arch.signature, mask) == mask then
        local pos, vel = arch.cols.Position, arch.cols.Velocity
        for i = 1, arch.count do
            pos[i] = pos[i] + vel[i] * dt        -- tight contiguous loop
        end
    end
end

-- Server rewinds every player to the shooter's render time,
-- so a hit that looked true on the client is validated fairly.
local function validateShot(shooter, origin, dir, clientTime)
    local t = clamp(clientTime, now - MAX_REWIND, now)
    for _, target in ipairs(candidates(origin, dir)) do
        local box = rewindHitbox(target, t)    -- historical position
        local hit, dist = rayBox(origin, dir, box)
        if hit and dist <= weapon.range then
            if not lineOfSight(origin, box.center) then continue end
            local dmg = falloff(weapon, dist) * zoneMul(box.part)
            return { ok = true, id = target.id, dmg = dmg }
        end
    end
    return { ok = false }                       -- reject; never trust the client
end

// POST /api/match/:id/score - typed, validated, authenticated.
// One DB transaction: never half-write a leaderboard row.
const Body = z.object({ score: z.number().int().min(0), runId: z.string().uuid() });

export const submitScore: Handler = async (req, reply) => {
  const user = await requireUser(req);            // 401 if no valid session
  const body = Body.parse(req.body);              // 400 on bad shape
  const match = await db.match.findUnique({ where: { id: req.params.id } });
  if (!match || match.ownerId !== user.id) return reply.code(403).send();

  return db.$transaction(async (tx) => {
    await tx.score.upsert({
      where:  { runId: body.runId },                 // idempotent on retry
      create: { runId: body.runId, userId: user.id, value: body.score },
      update: { value: body.score },
    });
    const rank = await tx.score.count({ where: { value: { gt: body.score } } });
    return reply.send({ ok: true, rank: rank + 1 });
  });
};

// Optimistic React hook: paint the guess instantly, reconcile to the
// server, and roll back cleanly if the socket rejects the write.
export function useLiveQuery<T>(key: string, initial: T) {
  const [data, setData] = useState<T>(initial);
  const socket = useSocket();

  useEffect(() => socket.subscribe(key, setData), [key]);   // server is truth

  const mutate = useCallback(async (next: T) => {
    const prev = data;
    setData(next);                                // optimistic paint
    try {
      await socket.send(key, next);                 // await server ack
    } catch {
      setData(prev);                              // rollback on reject
    }
  }, [key, data]);

  return [data, mutate] as const;
}