You type two addresses into a navigation app. Within a fraction of a second, it returns a route. Not an approximate one. The shortest one, accounting for real road constraints, distances, and travel times. The road network it searched could span an entire continent. The graph underlying Europe's road network alone has hundreds of millions of nodes and edges. So how does the system search it so quickly? The answer isn't one clever algorithm. It's a progression of improvements, each one shif...