How can a game fit billions of worlds into a few numbers?

Procedural generation means making a game's content with an algorithm instead of by hand, usually by combining human-made pieces with rules and computer-generated randomness. The trick that makes it work is the seed. A map seed is a relatively short number or string used to procedurally create the game world. The same seed always gives the same world, so the world does not need to be stored: it is regenerated whenever you need it. You can even reset it or share it with another player just by sharing the seed.

The idea is older than video games. Advanced Dungeons & Dragons let a dungeon master build dungeons and terrain with random die rolls. Early computer games such as Beneath Apple Manor (1978) and Rogue (1980) did it on screen, producing dungeons randomly, and the genre was named roguelikes after Rogue.

The classic example is Elite. A single seed number is run through a fixed algorithm and produces a sequence of numbers that determine each planet's composition, including prices and even a name and description. This means no extra memory is needed to store planets, yet each is unique and fixed. Elite was originally planned to contain about 282 trillion galaxies, but the publisher feared that such a gigantic universe would cause disbelief, and eight were chosen.

Today No Man's Sky features a universe of 18 quintillion planets, and Minecraft's 64-bit seeds allow 18 quintillion maps. The catch is that quantity is not variety. Writer Kate Compton called the risk procedural oatmeal: thousands of worlds that are mathematically different but feel the same.