// Generates permutations of reference and active window values to determine
// whether or not data is anomalous. The number of permutations desired defaults
// to 500 but can be increased for more precision. A comparison function above
// can be specified to create Rank and ReverseRank tests.
func rankTest(vector govector.Vector, conf AnomalyzerConf, comparison compare) float64 {
// Rank the elements of a vector
ranks := vector.Rank()
_, active, err := extractWindows(ranks, conf.referenceSize, conf.ActiveSize, conf.ActiveSize)
if err != nil {
return NA
}
// Consider the sum of the ranks across the active data. This is the sum that
// we will compare our permutations to.
activeSum := active.Sum()
i := 0
significant := 0
// Permute the active and reference data and compute the sums across the tail
// (from the length of the reference data to the full length).
for i < conf.PermCount {
permRanks := vector.Shuffle().Rank()
_, permActive, _ := extractWindows(permRanks, conf.referenceSize, conf.ActiveSize, conf.ActiveSize)
// If we find a sum that is less than the initial sum across the active data,
// this implies our initial sum might be uncharacteristically high. We increment
// our count.
permSum := permActive.Sum()
if comparison(permSum, activeSum) {
significant++
}
i++
}
// We return the percentage of the number of iterations where we found our initial
// sum to be high.
return float64(significant) / float64(conf.PermCount)
}
