package pl.scalania.euler

import scala.math._

object Euler04 {

  def largestPalindrome(digits:Int):Long = {

    def toDigits(n: Long): List[Char] = {
      def loopFunc(n: Long): List[Char] = {
        if (n <= 0) Nil
        else ('0'.toInt + n.toInt % 10).toChar :: loopFunc(n / 10)
      }
      loopFunc(n).reverse
    }

    def isPolindrome[T](ls: List[T]): Boolean = {
      def compare(l1: List[T], l2: List[T]): Boolean =
        if (l1.size != l2.size) false
        else l1 match {
          case head :: tail => (head == l2.head) && compare(tail, l2.tail)
          case _ => true
        }
      compare(ls.drop(ls.size/2), ls.reverse.drop(ls.size/2))
    }

    def traverseIndexes(min: Int, max: Int, start: (Int, Int), current: (Int, Int)): Stream[((Int, Int))] = current match {
      case (a, b) =>

        if ((a < min) || (b > max)) {
          start match {
            case (a, b) =>

              if (a == b)
                traverseIndexes(min, max, (a - 1, b), (a - 1, b))
              else
                traverseIndexes(min, max, (a, a), (a, a))
          }
        }
        else (a, b) #:: traverseIndexes(min, max, start, (a - 1, b + 1))
    }

    val fromNo = pow(10, digits - 1).toInt // 10 - for 2 digits
    val toNo = pow(10, digits).toInt - 1 // 99 - for 2 digits

    traverseIndexes(fromNo, toNo, (toNo, toNo), (toNo, toNo))
      .filter{ case (a, b) => isPolindrome(toDigits(a*b)) }
      .take(1).toList.map{ case (a, b) => a*b }.head

  }
}