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dugrimov's solution

to Collatz Conjecture in the Kotlin Track

Published at Nov 27 2020 · 0 comments
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.

Examples

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.

Setup

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

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

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.

Source

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.

CollatzCalculatorTest.kt

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 {
    fun computeStepCount(start: Int) = sequence {
        require(start > 0)

        yieldAll(generateSequence(start) {
            if (it % 2 == 0) {
                it / 2
            } else {
                it * 3 + 1
            }
        })
    }.takeWhile { it != 1 }.count()
}

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