Every system that tracks value eventually needs a ledger, and most start with a balance column and a sense of optimism. The fix is double-entry bookkeeping: five hundred years old, and small enough to build in an afternoon.
A modern chess engine can evaluate millions of positions per second, yet most of its strength comes from surprisingly simple ideas: search, pruning, caching, and a relentless obsession with efficiency.
Modern financial exchanges move billions of dollars a day, yet the core matching engine comes down to a handful of data structures and a few unglamorous rules.