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Published at Mar 09 2021
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Instructions

Test suite

Solution

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:

- 12
- 6
- 3
- 10
- 5
- 16
- 8
- 4
- 2
- 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:

https://exercism.io/tracks/kotlin/installation

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

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 {
@Rule
@JvmField
var expectedException: ExpectedException = ExpectedException.none()
@Test
fun `zero steps for 1`() = assertStepsEqual(1, 0)
@Ignore
@Test
fun `divide if even`() = assertStepsEqual(16, 4)
@Ignore
@Test
fun `even and odd steps`() = assertStepsEqual(12, 9)
@Ignore
@Test
fun `large number of even and odd steps`() = assertStepsEqual(1000000, 152)
@Ignore
@Test(expected = IllegalArgumentException::class)
fun `invalid input | zero`() {
steps(0)
}
@Ignore
@Test(expected = IllegalArgumentException::class)
fun `invalid input | negative`() {
steps(-15)
}
}
private fun assertStepsEqual(input: Int, steps: Int) = assertEquals(steps, steps(input))
private fun steps(input: Int) = CollatzCalculator.computeStepCount(input)
```

```
object CollatzCalculator {
private fun next(n: Int, steps: Int): Int =
when (n) {
1 -> steps
else -> next(if (n % 2 == 0) n / 2 else 3 * n + 1, steps + 1)
}
fun computeStepCount(start: Int): Int =
if (start < 1) throw IllegalArgumentException() else next(start, 0)
}
```

A huge amount can be learned from reading other peopleâ€™s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

- What compromises have been made?
- Are there new concepts here that you could read more about to improve your understanding?

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