We introduce a probability `Estimator` interface which is implemented by
the current apriori probability estimator. A second implementation, the
bimodal probability estimator follows.
* we rename the current probability estimator to be the "apriori"
probability estimator to distinguish from a different implementation
later
* the AprioriEstimator is exported to later be able to type switch
* getLocalPairProbability -> LocalPairProbabiltiy (later part of an
exported interface)
* getPairProbability -> getPairProbabiltiy (later part of an exported
interface)
We multiply the apriori probability with a factor to take capacity into
account:
P *= 1 - 1 / [1 + exp(-(amount - cutoff)/smearing)]
The factor is a function value between 1 (small amount) and 0 (high
amount). The zero limit may not be reached exactly depending on the
smearing and cutoff combination. The function is a logistic function
mirrored about the y-axis. The cutoff determines the amount at which a
significant reduction in probability takes place and the smearing
parameter defines how smooth the transition from 1 to 0 is. Both, the
cutoff and smearing parameters are defined in terms of fixed fractions
of the capacity.
Extends the pathfinder with a capacity argument for later usage.
In tests, the inserted testCapacity has no effect, but will be used
later to estimate reduced probabilities from it.
Previously we used the a priori probability also for our own untried
channels. This led to local channels that had seen a success already
being prioritized over untried local channels. In some cases, depending
on the configured payment attempt cost, this could lead to the payment
taking a two hop route while a direct payment was also possible.
This commit changes mission control to partially base the estimated
probability for untried connections on historical results obtained in
previous payment attempts. This incentivizes routing nodes to keep all
of their channels in good shape.
