// Incrementally fix particle distances.
for(auto i = 0; i < 20; i++) {
    foreach(j, p; taskPool.parallel(particles, numParticles/16)) {
        auto chg = vec2(0, 0);
        foreach(k, q; p.neighbors) {
            auto delta = q.pos - p.pos;
            auto dir = delta.normalize;
            auto mov = delta - dir * separation;
            chg += mov * 0.5;
        }
        changes[j] = chg;
    }
    foreach(j, chg; changes) {
        particles[j].pos += chg;
    }
}