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arueckauer's solution

to Collatz Conjecture in the PHP Track

Published at Jul 25 2019 · 0 comments
Instructions
Test suite
Solution

Note:

This exercise has changed since this solution was written.

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.

Running the tests

  1. Go to the root of your PHP exercise directory, which is <EXERCISM_WORKSPACE>/php. To find the Exercism workspace run

     % exercism debug | grep Workspace
    
  2. Get PHPUnit if you don't have it already.

     % wget --no-check-certificate https://phar.phpunit.de/phpunit.phar
     % chmod +x phpunit.phar
    
  3. Execute the tests:

     % ./phpunit.phar collatz-conjecture/collatz-conjecture_test.php
    

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.

collatz-conjecture_test.php

<?php

require "collatz-conjecture.php";

class CollatzConjecture extends PHPUnit\Framework\TestCase
{
    public function testZeroStepsForOne()
    {
        $this->assertEquals(0, steps(1));
    }

    public function testDivideIfEven()
    {
        $this->assertEquals(4, steps(16));
    }

    public function testEvenAndOddSteps()
    {
        $this->assertEquals(9, steps(12));
    }

    public function testLargeNumberOfEvenAndOddSteps()
    {
        $this->assertEquals(152, steps(1000000));
    }

    public function testZeroIsAnError()
    {
        $this->expectException(InvalidArgumentException::class);
        $this->expectExceptionMessage('Only positive numbers are allowed');

        steps(0);
    }

    public function testNegativeValueIsAnError()
    {
        $this->expectException(InvalidArgumentException::class);
        $this->expectExceptionMessage('Only positive numbers are allowed');

        steps(-1);
    }
}

CollatzConjecture.php

<?php

declare(strict_types = 1);

namespace Exercism\CollatzConjecture;

use InvalidArgumentException;

function steps(int $number) : int
{
    validate($number);

    $iterations = 0;
    while ($number > 1) {
        ++$iterations;
        $number = step($number);
    }

    return $iterations;
}

function validate(int $number) : void
{
    if ($number < 1) {
        throw new InvalidArgumentException('Only positive numbers are allowed');
    }
}

function step(int $number) : int
{
    if ($number & 1) {
        return $number * 3 + 1;
    }

    return $number / 2;
}

CollatzConjectureTest.php

<?php

declare(strict_types = 1);

namespace ExercismTest\CollatzConjecture;

use function Exercism\CollatzConjecture\steps;
use InvalidArgumentException;
use PHPUnit\Framework\TestCase;

class CollatzConjectureTest extends TestCase
{
    public static function setUpBeforeClass() : void
    {
        require 'CollatzConjecture.php';
    }

    public function testZeroStepsForOne() : void
    {
        $this->assertEquals(0, steps(1));
    }

    public function testDivideIfEven() : void
    {
        $this->assertEquals(4, steps(16));
    }

    public function testEvenAndOddSteps() : void
    {
        $this->assertEquals(9, steps(12));
    }

    public function testLargeNumberOfEvenAndOddSteps() : void
    {
        $this->assertEquals(152, steps(1000000));
    }

    public function testZeroIsAnError() : void
    {
        $this->expectException(InvalidArgumentException::class);
        $this->expectExceptionMessage('Only positive numbers are allowed');

        steps(0);
    }

    public function testNegativeValueIsAnError() : void
    {
        $this->expectException(InvalidArgumentException::class);
        $this->expectExceptionMessage('Only positive numbers are allowed');

        steps(-1);
    }
}

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