Numbers to English words

NumberUtils.scala

object NumberUtils {
  private def numberUnderThousandToEnglish(number: Int) = {
    val hundreds = number / 100
    val tensAndOnes = number % 100
    val tens = tensAndOnes / 10
    val ones = number % 10

    val hundredsStr = if (hundreds > 0) Some(numNames(hundreds) + " hundred") else None

    val restStr = numNames.get(tensAndOnes).map(s => List(Some(s))).getOrElse(
      List(
        if (tens > 0) Some(tensNames(tens)) else None,
        if (ones > 0) Some(numNames(ones)) else None
      )
    )

    (hundredsStr :: restStr).flatten.mkString(" ")
  }

  def numberToEnglish(number: Int): String = {
    if (number == 0) return "zero"

    val isPositive = number > 0
    val absNumber = math.abs(number)

    // 23456 -> List(456, 23). We group starting from the right rather than from the left.
    val magnitudeChunks = absNumber.toString.reverse.grouped(3).toList.map(_.reverse.toInt)

    val magnitudeStrings = magnitudeChunks.zip(None :: magnitudesByThree.map(Some(_))).filter(_._1 > 0).map {
      case (quantity, magStr) => List(Some(numberUnderThousandToEnglish(quantity)), magStr).flatten.mkString(" ")
    }

    (if (isPositive) "" else "minus ") + magnitudeStrings.reverse.mkString(" ")
  }

  private val tensNames = (1 to 9).zip(List("ten", "twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety")).toMap
  private val numNames =
    (1 to 19).zip(List("one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten", "eleven", "twelve", "thirteen", "fourteen",
      "fifteen", "sixteen", "seventeen", "eighteen", "nineteen")).toMap

  private val magnitudesByThree = List(
    "thousand",
    "million",
    "billion"
  )
}