More NES emulation…
We didn’t like how this other RPG game was rolling random encounters. It happened on the first step too often. Very annoying. I think there needs to be some kind of step counter setup with a minimum value… I don’t know how many games do that. Is it possible they have arranged the statistics so that low rolls are less common? I haven’t checked this in a while.
This game has 4 terrain types, and up to 4 encounters per terrain type; on top of that it is organized into zones in the world map (thus further-away zones have harder encounter sets). There is only one monster per encounter, so encounter type = monster type. The data is organized this way:
struct EncounterDataForWorldZone
{
byte chances[4][4];
byte monsters[4][4];
}
EncounterDataForWorldZone encounterData[N]; //N zones in the game, N unknown
Now, for each step, the game gets a random number R from a flat distribution. It then considers chances[currterrain][0] = C as a C/256 chance of getting the encounter on that step. We see if R is less than C and do the encounter if so. Otherwise C is subtracted from the random number, so the next chance C’ from chances[currterrain][1] is a C’/(256-C) chance. And so on.
Generally the chances near the beginning of the game where we were testing, is, I dont know, 16-20 or so. So it’s easy to see why we get too many encounters… and why they can happen on the first step so often.
My first thought was to use the old “turn it into a normal distribution” trick by changing a 1d256 roll into a 2d128. This biases more normal rolls in the middle at the expense of rolls in the tails. So we did this for a while, and it felt okay… but we weren’t testing the whole game, and I got nervous.
Let’s take a simplified scenario where we have 2 encounter chances, each of value 64. Now the original way it was done, each of those would be a 64 in 256 chance (1 in 256). Well, kind of, because of how the subtraction happens in the algorithm. Anyway with two dice, there’s fewer ways of rolling the smallest 64 numbers than there are the next 64 numbers (just like there’s only one way to roll a 2 on two dice but there’s many ways to roll a 7). And yet since they’re adding up to 128, the new probability distribution is centered on 128, and the absolute probability of an encounter is still the same! All we accomplished was to bias the selection to the later encounters. This would change the character of the game in a way we don’t really have time to test and scrutinize.
So my romhacker came up with a better idea to add a tiny bit of logic to only do encounter checks on every other step. This way you won’t get them on successive steps under any circumstances, and we get the other desired result of just generally fewer encounters–precisely half as many, without having to grapple with the really thorny problem of how to do it, logically speaking, without affecting the design of the game by biasing certain encounters. Actually after a little more iteration we developed an approach to run it with any divisor so that I could give another option to reduce encounters by half again (so a divisor of 4) from the emulation menu, so people would feel a bit empowered to be masters of their own destinies instead of frustrated by the fixed design. (There’s also a cheat to disable encounters entirely)
