Berlin clock kata (http://content.codersdojo.org/code-kata-catalogue/berlin-clock/) implemented in Scala when pairing with Alberto. By just letting the tests guide the design and merciless refactoring after every step we got to this terse code.

BerlinClock.scala

package berlin

object BerlinClock {

  sealed trait Light
  case object Off extends Light
  case object Yellow extends Light
  case object Red extends Light

  type Clock = List[List[Light]]

  def format(hours: Int, mins: Int, secs: Int): Clock = for {
    (n, lights) <- numberOfLights(hours, mins, secs) zip lightColors
  } yield makeLightsRow(n, lights)

  private def numberOfLights(hours: Int, mins: Int, secs: Int) = List(
    if (secs % 4 < 2) 1 else 0,
    hours / 5,
    hours % 5,
    mins / 5,
    mins % 5
  )

  private def makeLightsRow(turnedOn: Int, onLights: List[Light]) =
    onLights.take(turnedOn) ++ List.fill(onLights.size - turnedOn)(Off)

  private val lightColors = List(
    List(Yellow),
    List.fill(4)(Red),
    List.fill(4)(Red),
    List.tabulate(11)(index => if (index % 3 == 2) Red else Yellow),
    List.fill(4)(Yellow)
  )
}

BerlinClockTest.scala

package berlin

import org.scalatest.FlatSpec
import org.scalatest.matchers.MustMatchers
import berlin.BerlinClock._

class BerlinClockTest extends FlatSpec with MustMatchers {

  "At 00:00:02" must "return completely off" in {
     BerlinClock.format(0, 0, 2) must be (List(
       List(Off),
       List.fill(4)(Off),
       List.fill(4)(Off),
       List.fill(11)(Off),
       List.fill(4)(Off)
     ))
  }

  "First row's light" must "switch every two seconds" in {
    BerlinClock.format( 0, 10, 0).head must be (List(Yellow))
    BerlinClock.format(12,  0, 1).head must be (List(Yellow))
    BerlinClock.format(23, 30, 2).head must be (List(Off))
    BerlinClock.format(20, 17, 8).head must be (List(Yellow))
  }

  "Second row's lights" must "have a red light per 5 hours" in {
    BerlinClock.format( 0, 10, 10)(1) must be (List.fill(4)(Off))
    BerlinClock.format( 6, 15,  5)(1) must be (List(Red, Off, Off, Off))
    BerlinClock.format(10,  0, 10)(1) must be (List(Red, Red, Off, Off))
  }

  "Third row's lights" must "have a red light per hour not represented in the second row" in {
    BerlinClock.format( 1, 10, 10)(2) must be (List(Red, Off, Off, Off))
    BerlinClock.format(23, 30, 59)(2) must be (List(Red, Red, Red, Off))
  }

  "Forth row's lights" must "have a light on per 5 minutes" in {
    countLightsOn(BerlinClock.format(10,  0, 10)(3)) must be (0)
    countLightsOn(BerlinClock.format(15,  6,  5)(3)) must be (1)
    countLightsOn(BerlinClock.format( 0, 10, 10)(3)) must be (2)
  }

  "Forth row's light" must "follow a repeated yellow-yellow-red pattern when turned on" in {
    BerlinClock.format(23, 30, 10)(3) must be (List.tabulate(11)(index => {
      if (index >= 6) Off
      else if (index % 3 == 2) Red
      else Yellow
    }))
    BerlinClock.format(23, 59, 10)(3) must
      be (List.tabulate(11)(index => if (index % 3 == 2) Red else Yellow))
  }

  "Fifth row's lights" must "have a yellow light per minute not represented in the forth row" in {
    BerlinClock.format(23,  1, 10)(4) must be (List(Yellow, Off, Off, Off))
    BerlinClock.format(23, 59, 10)(4) must be (List.fill(4)(Yellow))
  }

  private def countLightsOn(row: List[Light]) = row.count(_ != Off)
}