CodeJam 問題C. ビット数 second attempt

codejam-large.scala

def calcAandB(N:BigInt):(BigInt,BigInt) = {
  def setIth(i: Int, a:BigInt, b:BigInt, carry:Boolean):(BigInt, BigInt, Boolean) =
            (N.bitLength-i, carry) match {
    case (n,_) if n<1 => (a,b,carry) // finished all bits of N
    case (1,true) => (a,b,carry) // finished all bit of N except msb, and there is no carry
    case (n,_) => { // recurse
      // handle the carry
      val (n_i, c) = (N.testBit(i), carry) match {
        case (true, true) => (false, false)
        case (false, true) => (true, true)
        case (i,c) => (i,c)
      }
      if (n_i)
        setIth(i+1, a.setBit(i), b, c)
      else
        setIth(i+1, a.setBit(i), b.setBit(i), true)
    }
  }
  val (a,b,carry) = setIth(0, BigInt(0), BigInt(0), false)
  (a,b)
}

def findMax(N:BigInt):Int = {
  val (a,b) = calcAandB(N)
  a.bitCount + b.bitCount
}

import scala.io.Source
val lines = Source.fromFile("C-large-practice.in").getLines()

lines.toList.tail.zipWithIndex.foreach{ valAndIndex => println("Case #"+(valAndIndex._2+1)+": "+findMax(BigInt(valAndIndex._1))) }

The question is here: http://code.google.com/codejam/contest/dashboard?c=889487#s=p2