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to Collatz Conjecture in the Kotlin Track

Published at Jan 30 2020 · 0 comments
Test suite

The Collatz Conjecture or 3x+1 problem can be summarized as follows:

Take any positive integer n. If n is even, divide n by 2 to get n / 2. If n is odd, multiply n by 3 and add 1 to get 3n + 1. Repeat the process indefinitely. The conjecture states that no matter which number you start with, you will always reach 1 eventually.

Given a number n, return the number of steps required to reach 1.


Starting with n = 12, the steps would be as follows:

  1. 12
  2. 6
  3. 3
  4. 10
  5. 5
  6. 16
  7. 8
  8. 4
  9. 2
  10. 1

Resulting in 9 steps. So for input n = 12, the return value would be 9.


Go through the setup instructions for Kotlin to install the necessary dependencies:


Making the test suite pass

Execute the tests with:

$ gradlew test

Use gradlew.bat if you're on Windows

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by removing the @Ignore annotation.


An unsolved problem in mathematics named after mathematician Lothar Collatz https://en.wikipedia.org/wiki/3x_%2B_1_problem

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


import org.junit.Ignore
import org.junit.Rule
import org.junit.Test
import org.junit.rules.ExpectedException
import kotlin.test.assertEquals

class CollatzCalculatorTest {

    var expectedException: ExpectedException = ExpectedException.none()

    fun `zero steps for 1`() = assertStepsEqual(1, 0)

    fun `divide if even`() = assertStepsEqual(16, 4)

    fun `even and odd steps`() = assertStepsEqual(12, 9)

    fun `large number of even and odd steps`() = assertStepsEqual(1000000, 152)

    @Test(expected = IllegalArgumentException::class)
    fun `invalid input | zero`() {

    @Test(expected = IllegalArgumentException::class)
    fun `invalid input | negative`() {


private fun assertStepsEqual(input: Int, steps: Int) = assertEquals(steps, steps(input))

private fun steps(input: Int) = CollatzCalculator.computeStepCount(input)
object CollatzCalculator {
    fun computeStepCount(start: Int): Int {
        require(start > 0)
        return calc(0, start)

    private tailrec fun calc(step: Int, value: Int): Int =
        when {
            value == 1 -> step
            value % 2 == 0 -> calc(step + 1, value / 2)
            else -> calc(step + 1, value * 3 + 1)

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