Weighted Pools - PolyhedralDev/Terra Wiki
A weighted pool is a way to randomly select one of many objects from a "pool" based on "weight".
A pool is a list of objects from which selections will be pulled.
Weight is the relative probability that any object will be selected from the pool. Each object in the pool has a
weight, meaning the chance of object O being selected from pool P is
Here's an example pool:
OPTION_A: 2 OPTION_B: 1 OPTION_C: 1
In this YAML-based pool, the keys are the objects, and the values are the weights. If we want the probability that
OPTION_A will be selected, we sum all weights, and divide the weight of
OPTION_A by that sum.
2 + 1 + 1 = 4
Therefore the probability that OPTION_A will be selected is
2/4, or 50%.
Manipulating weights to get a desired probability
What if we want Option A to have a 60% chance of being selected? This requires manipulating the weights of all objects in the pool, since with B and C having a weight of 1, there is no integer weight that can be assigned to A that will give it a probability of 60%. What is usually done is that the weights are given a set sum that is easy to work with. Usually this is 100, since with a sum of 100, every object's weight is equal to the percent chance it will be selected. By applying this to the above example, we can produce this pool that has a 60% chance of A being selected:
OPTION_A: 60 OPTION_B: 20 OPTION_C: 20
Sometimes, it is undesirable to have an equal probability of all objects being selected, so you may want to "weight" some objects higher, so they are selected more often.