in core/src/main/scala/com/spotify/featran/FeatureSpec.scala [501:516]
def multiFeatureValues[F](a: ARRAY, c: ARRAY, fbs: Array[FeatureBuilder[F]]): Unit = {
var i = 0
val counts = multiFeatureDimension(c)
while (i < fbs.length) {
fbs(i).init(counts(i))
i += 1
}
i = 0
while (i < n) {
val f = features(i)
val fb = fbs(mapping(f.transformer.name))
fb.prepare(f.transformer)
f.unsafeBuildFeatures(a(i), c(i), fb)
i += 1
}
}