tl;dr:"How can we scale the calculation engine 100x, from millions to billions of cells? By moving from maps to arrays. That may seem like an awfully pedestrian observation, but it certainly wasn’t obvious to us at the outset that this was the crux of the problem!"
tl;dr:A mental model for how a neural network works by building one from scratch. "The hardest part is acquiring the conceptual understanding of a system to come up with an equation for its performance."