Fast Fibonacci implementation

Fib.scala

def fibFast1(num: Int): BigInt = {
   var a: BigInt = 1
   var b: BigInt = 0
   var c: BigInt = 1
   for(bit <- bits(num)){
     bit match {
        case 1 => {
           a = (a+c)*b
           b = b*b + c*c
        }
        case 0 => {
           val prevA = a
           a = a*a + b*b
           b = (prevA+c)*b
        }
     }
     c = a + b	
   }
   return b
}

def bits(num: Int, builder: Builder[Int, Seq[Int]] = Seq.newBuilder[Int]): Seq[Int] = {
   if(num > 0){
      bits(num / 2, builder += (num % 2))
   }		
   return builder.result.reverse
}