e50.scala

def isPrime(n: Int): Boolean = (2 to math.sqrt(n).toInt).forall(n % _ != 0)
val primes: Stream[Int] = Stream.from(2).filter(isPrime)

def primeProgressionLength(prime: Int, progression: Stream[Int] = primes): Option[Int] = {
  def sumLength(max: Int, seq: Seq[Int], sum: Int = 0, len: Int = 0): Option[Int] = {
    if (sum > max) return None
    if (sum == max) return Some(len)
    sumLength(max, seq.tail, sum + seq.head, len + 1)
  }

  if (progression.head >= (prime / 2)) return None
  sumLength(prime, progression) match {
    case Some(n) => return Some(n)
    case None    => primeProgressionLength(prime, progression.tail)
  }
}

val result = primes.takeWhile(_ < 1000000).map( p => p -> primeProgressionLength(p).getOrElse(0)).maxBy(_._2)
println(result) // => (997651,543)