https://gist.github.com/kawasima/0a3953fd4d39c9c86546#file-02-md

Collatz.kt

package collatz

import kotlin.util.measureTimeMillis
import java.util.HashMap
import java.util.Arrays

val MAX = 100000

fun main(args: Array<String>) {
  println(measureTimeMillis {
    println((2..MAX).map { Pair(it, calc(it)) }.maxBy { it.second })
  })
}

val step = StepCache(MAX)

fun calc(i: Int): Int {
  val cashed = step.get(i)
  return if (cashed != 0) {
    cashed
  } else {
    val next = next(i)
    if (next == 1) {
      next
    } else {
      return step.put(i, calc(next) + 1)
    }
  }
}


class StepCache(val size: Int) {
  val cache = IntArray(size * 1000);
  {
    var count = 0;
    stream(2, { it shl 1 })
        .takeWhile { it <= size }
        .forEach { count++; cache[it] = count }
  }

  fun get(i: Int): Int {
    return if (i > size) {
      0
    } else {
      cache[i]
    }
  }

  fun put(i:Int, step:Int):Int {
    if (i > size) {
      return step
    }
    cache[i] = step
    return step
  }
}


fun next(i: Int): Int {
  return if ((i and 0x01) == 0 ) {
    i / 2
  } else {
    i * 3 + 1
  }
}

(77031, 350)
239ms

(77031, 350)
21ms