오일러 프로젝트 문제 2: http://euler.synap.co.kr/prob_detail.php?id=2 피보나치 수열의 각 항은 바로 앞의 항 두 개를 더한 것이 됩니다. 1과 2로 시작하는 경우 이 수열은 아래와 같습니다. 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ... 짝수이면서 4백만 이하인 모든 항을 더하면 얼마가 됩니까?

Problem2.scala

package com.fupfin.euler.problem2

import scala.collection.immutable.Stream._
import scala.collection.GenIterable

object Problem2 {
  def fib(max: Int): Stream[Int] = 1 #:: 2 #:: fib(1, 2, max)
  private def fib(v1:Int, v2:Int, max:Int): Stream[Int] = if(v1 + v2 <= max) (v1 + v2) #:: fib(v2, v1 + v2, max) else Empty
  
  def sum(that:GenIterable[Int], p: Int => Boolean): Int = {
    def loop(that: GenIterable[Int], acc: Int): Int = {
      if(that.isEmpty) acc
      else loop(that.tail, acc + (if (p(that.head)) that.head else 0))
    }
    loop(that, 0)
  }
  
  def sumOfEvenFibonacciNumbersLessThan4000000 = sum(fib(4000000), _ % 2 == 0)
}

Problem2Spec.scala

package com.fupfin.euler.problem2

import org.scalatest.FlatSpec
import org.junit.runner.RunWith
import org.scalatest.junit.JUnitRunner
import org.scalatest.matchers.ShouldMatchers

@RunWith(classOf[JUnitRunner])
class Problem2Spec extends FlatSpec with ShouldMatchers {
  import Problem2._

  "first 7 fibonacci numbers" should "be 1, 2, 3, 5, 8, 13, 21" in {
    List(1, 2, 3, 5, 8, 13, 21).zip(fib(30)).forall(v => v._1 == v._2) should equal (true)
  }
  
  "sum of 1, 2, 3, 4, 5" should "be 15" in {
    sum(List(1, 2, 3, 4, 5), _ => true) should equal (15)
  }
  
  "sum of fibonacci numbers" should "be 19" in {
    sum(fib(10), _ => true) should equal (19)
  }
  
  "sum of even elements of fibonacci numbers" should "10" in {
    sum(fib(10), _ % 2 == 0) should equal (10)
  }  
}