TW Probability calculations are incorrect, 0 does not mean 0

[joshimoo]

Not sure where to post this, hopefully someone with contact to the developers can pass this information along.
Since this is happening in multiple places in the bannerlord code base and will lead to unexpected behavior for mods but also for the TW devs itself.

Please find a single example below with an explanation of the error:

bool isOffspringFemale = (double) MBRandom.RandomFloat <= (double) pregnancyModel.DeliveringFemaleOffspringProbability;

If you look at the above code you might think, that looks quite correct but it's not.
Since the underlying random number generator will return a number in the interval [0, 1)

/// <summary>Returns a random floating-point number that is greater than or equal to 0.0, and less than 1.0.</summary>
MBRandom.RandomFloat

For example if you wanted to disable Maternal mortality, you would assume that a probability of 0 would actually mean that this event can never happen.
But because of the way they check that probability there will still be a chance that this event can happen (rng includes 0 in distribution)
This will lead to unexpected behavior for the player / modder and funny bug reports

For example assuming you would want to disable Maternal mortability, you would actually need to set the propability below 0
Which sucks for the mod UI and player, since they will assume 0 means 0

bool killMother = MBRandom.RandomFloat <= pregnancyModel.MaternalMortalityProbabilityInLabor

 

[暗暗十分]

The chances of a RNG returning 0 is like 1 in 10^16.
It might take forever for all of MB2 players together to actually run into the issue one time.

 

[joshimoo]

The point is that as a dev (mod, native) one would want to avoid unexpected behavior and would want consistent probability calculations.

They could have done the below instead:

1 - MBRandom.RandomFloat <= pregnancyModel.DeliveringFemaleOffspringProbability;

They have a similar problem elsewhere, now if you always wanted to start a tournament it would never start
because of the rng interval [0, 1)

MBRandom.RandomFloat >= (double) Campaign.Current.Models.TournamentModel.GetTournamentStartChance(town)

As one can see their pattern for probability calculations are quite inconsistent and it would be useful to make them consistent

 

[MitchPTI]

They could have done the below instead:

1 - MBRandom.RandomFloat <= pregnancyModel.DeliveringFemaleOffspringProbability;

Would it not be easier to just change the less than or equal to to strictly less than? The random number cannot possibly be below 0.

 

[kalarhan]

Not sure where to post this

if you find bugs you can post them on the support section, its like a ticket system. Devs check stuff there and them transfer the actual bugs to their intern ticket system.

https://forums.taleworlds.com/index.php?forums/singleplayer.658/

 

[Timbough]

Is this why I see a negative probability of being able to sneak into a city sometimes?