Project Euler の問題44より。 http://projecteuler.net/problem=44 Scala版は遅くて使い物にならないのだが何故？

Prob44_2.d

import std.stdio;
import std.math;

class Penta
{
private:
    pure uint pentagonal(uint n) {
        return n*(3*n - 1)/2;
    }

    pure uint[] pentagonals(uint n) {
        uint[] result;
        foreach(i; 0 .. (n + 1)) {
            result ~= pentagonal(i);
        }
        return result;
    }

    pure bool isPenta(uint n) {
        double dbl = (1.0 + sqrt(1.0 + 24*n))/6.0;
        return pentas[cast(uint)dbl] == n ||
            pentas[cast(uint)dbl + 1] == n;
    }

public:
    uint[] pentas;

    this() {
        pentas = pentagonals(20000);
    }

    bool exec(uint j, uint k) {
        uint pj = pentas[j];
        uint pk = pentas[k];
        return isPenta(pj + pk) && isPenta(abs(pk - pj));
    }
}

void main()
{
    auto penta = new Penta;
    for(int i=1; i!=10000; ++i) {
        for(int j=i + 1; j!=10000; ++j) {
            if(penta.exec(i, j)) {
                uint diff = penta.pentas[j] - penta.pentas[i];
                writeln(diff);
                return;
            }
        }
    }
}

unittest
{
    assert(pentagonal(3) == 12);
    assert(pentagonals(10) == [0, 1, 5, 12, 22, 35, 51, 70, 92, 117, 145]);
}

Prob44_2.scala

object Prob44_2 {
  def pentagonal(n: Int) = n*(3*n - 1)/2
  val pentagonals = Stream.from(0).map(pentagonal).takeWhile(_ < (Int.MaxValue >> 2)).toList

  def isPenta(n: Int) = {
    val dbl = (1.0 + math.sqrt(1 + 24*n))/6.0
    val int = dbl.toInt
    pentagonal(int) == n || pentagonal(int + 1) == n
  }

  def check(j: Int, k: Int): Boolean = {
    val pj = pentagonals(j)
    val pk = pentagonals(k)
    isPenta(pj + pk) && isPenta((pj - pk).abs)
  }

  def main(args: Array[String]) {
    val result = for {
      i <- 1 to 10000
      j <- (i + 1) to 10001
      if check(i, j)
    } yield (i, j)
    println(result.head)
  }
}