Random Number Generators

Random Number Generators

A Random Number Generator (RNG) is a system which produces seemingly random values based on an initial sequence and other sources of entropy. It is important to note that RNGs in games are not "truly" random; the same sequence of inputs and conditions will always produce the same results (and thus these constructs are technically Pseudo-Random Number Generators, or PRNGs). This means that in some cases a game's RNG can be manipulated to produce favorable results. The TASVideos page discusses RNGs in more detail, but this page is meant to discuss real-time strategies for manipulating RNGs to produce favorable outcomes.

Categories of Random Number Generators

Different games will require varying amounts of randomness and with different frequency. For example, consider a platformer such as Donkey Kong Country and a dungeon crawler such as Swords & Serpants. There might be very little randomness in DKC since most enemies move in fixed patterns, but nearly every action in Swords & Serpants has some element of randomness to it. Different RNG systems take different information as inputs as well. In general, game RNGs can be boiled down to a few types of categories. These categories have been adapted from the listing on TASVideos.

• Per-frame RNGs. This type changes every frame regardless of the actions taking place in the game. Events that require the RNG will use whatever value is available when they need it.
• Per-call RNGs. This type of RNG changes its value ("state") only when it is called for an event. This type of RNG is common in RPGs where significant randomness is needed only when deciding actions and in calculations for damage etc.
• Input-driven RNGs. This type collects entropy from player inputs to use as either the seeds or direct values for the RNG. This is uncommon, but can be used in games which expect input-heavy actions to drive the gameplay.
• Event-driven RNGs. Similar to input driven and per-call RNGs, event-driven RNGs advance on certain required actions in the gameplay even if the action has no randomness component to it. For example, some RPGs might advance the RNG each time a step is taken, regardless of whether or not the player is susceptible to random encounters.

While these are the main types of RNGs, they are not necessarily distinct from each other. For example, consider the RNG system used in Super Punch-Out, which relies on user inputs to seed a per-frame RNG. Suikoden II advances its RNG only when called for some periods of time before shifting to a per-frame version after timed "grace periods" have elapsed. Figuring out what type of RNG your game uses can be somewhat tricky, and a short guide is available in the Reverse Engineering section.