Scala > Calculate percentile

Percentile.scala

package util

object Percentile {

  // @see http://commons.apache.org/proper/commons-math/javadocs/api-3.6/org/apache/commons/math3/stat/descriptive/rank/Percentile.html
  def apply(array: Seq[Double], p: Int) = {
    require(p > 0)
    require(p <= 100)
    require(array.nonEmpty)
    val n = array.length
    if (n == 1) {
      array.head
    } else {
      val sorted = array.sorted
      val pos = p * (n + 1) / 100.0
      val posFloor = math.floor(pos).toInt
      if (pos < 1) {
        sorted.head
      } else if (pos >= n) {
        sorted.last
      } else {
        val lower = sorted(posFloor)
        val upper = if (n > posFloor + 1) sorted(posFloor + 1) else sorted(posFloor)
        val d = pos - posFloor
        lower + d * (upper - lower)
      }
    }
  }
}