bool platformCollision(float x, float y, int w, int h) {
    return y + h > platform.y &&
        x + 48 >= platform.x &&
        x + w - 48 <= platform.x + platform.w;
}

void update() {

    // Zombies
    for (int i = 0; i < ZOMBIE_COUNT; i++) {
        if (zombies[i].alive) {

            // Apply gravity
            zombies[i].vY += world.gravity;
            zombies[i].y += zombies[i].vY;

            // Motion
            zombies[i].x += zombies[i].vX;
            zombies[i].vX = zombieSpeed * zombies[i].dir;

            // Platform
            if (platformCollision(zombies[i].x, zombies[i].y, zombieWidth, zombieHeight)) {
                zombies[i].y = platform.y - zombieHeight;
                zombies[i].vY = 0;
            } else {
                zombies[i].vX = 0;
            }

            // Death
            if (zombies[i].y > SCREEN_HEIGHT) {
                zombies[i].alive = false;
            }

        }
    }

    // Spawn zombie every N seconds
    currentTime = SDL_GetTicks();
    if (currentTime > lastSpawnTime + SPAWN_FREQ * 1000) {
        spawnZombie();
        lastSpawnTime = currentTime;
    }
}