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4d47's solution

to Largest Series Product in the PHP Track

Instructions
Test suite
Solution

Given a string of digits, calculate the largest product for a contiguous substring of digits of length n.

For example, for the input '1027839564', the largest product for a series of 3 digits is 270 (9 * 5 * 6), and the largest product for a series of 5 digits is 7560 (7 * 8 * 3 * 9 * 5).

Note that these series are only required to occupy adjacent positions in the input; the digits need not be numerically consecutive.

For the input '73167176531330624919225119674426574742355349194934', the largest product for a series of 6 digits is 23520.

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 largest-series-product/largest-series-product_test.php
    

Source

A variation on Problem 8 at Project Euler http://projecteuler.net/problem=8

Submitting Incomplete Solutions

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

largest-series-product_test.php

<?php

include_once 'largest-series-product.php';

class SeriesTest extends PHPUnit\Framework\TestCase
{
    /**
     * Since PHP can only support Integers between +/- 9223372036854775807
     * We will deal with the series of digits as strings to avoid having them cast to floats.
     */

    public function testCanFindTheLargestProductOf2WithNumbersInOrder()
    {
        // The number starts with a 0, qualifying it to be an octal
        // So it needs to be a string so PHP doesn't complain
        $series = new Series("0123456789");
        $this->assertEquals(72, $series->largestProduct(2));
    }

    public function testCanFindTheLargestProductOf2()
    {
        $this->markTestSkipped();

        $series = new Series(576802143);
        $this->assertEquals(48, $series->largestProduct(2));
    }

    public function testFindsTheLargestProductIfSpanEqualsLength()
    {
        $this->markTestSkipped();

        $series = new Series(29);
        $this->assertEquals(18, $series->largestProduct(2));
    }

    public function testCanFindTheLargestProductOf3WithNumbersInOrder()
    {
        $this->markTestSkipped();

        $series = new Series(123456789);
        $this->assertEquals(504, $series->largestProduct(3));
    }

    public function testCanFindTheLargestProductOf3()
    {
        $this->markTestSkipped();

        $series = new Series(1027839564);
        $this->assertEquals(270, $series->largestProduct(3));
    }

    public function testCanFindTheLargestProductOf5WithNumbersInOrder()
    {
        $this->markTestSkipped();

        $series = new Series("0123456789");
        $this->assertEquals(15120, $series->largestProduct(5));
    }

    public function testCanGetTheLargestProductOfABigNumber()
    {
        $this->markTestSkipped();

        $series = new Series("73167176531330624919225119674426574742355349194934");
        $this->assertEquals(23520, $series->largestProduct(6));
    }

    public function testCanGetTheLargestProductOfABigNumberProjectEuler()
    {
        $this->markTestSkipped();

        $digits = "731671765313306249192251196744265747423553491949349698352031277450632623957831801698480186947"
            . "8851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096"
            . "3295227443043557668966489504452445231617318564030987111217223831136222989342338030813533627"
            . "6614282806444486645238749303589072962904915604407723907138105158593079608667017242712188399"
            . "8797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490"
            . "7711670556013604839586446706324415722155397536978179778461740649551492908625693219784686224"
            . "8283972241375657056057490261407972968652414535100474821663704844031998900088952434506585412"
            . "2758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828"
            . "4891288314260769004224219022671055626321111109370544217506941658960408071984038509624554443"
            . "6298123098787992724428490918884580156166097919133875499200524063689912560717606058861164671"
            . "0940507754100225698315520005593572972571636269561882670428252483600823257530420752963450";

        $series = new Series($digits);
        $this->assertEquals(23514624000, $series->largestProduct(13));
    }

    public function testReportsZeroIfTheOnlyDigitsAreZero()
    {
        $this->markTestSkipped();

        $series = new Series("0000");
        $this->assertEquals(0, $series->largestProduct(2));
    }

    public function testReportsZeroIfAllSpansIncludeZero()
    {
        $this->markTestSkipped();

        $series = new Series(99099);
        $this->assertEquals(0, $series->largestProduct(3));
    }

    /**
     * @expectedException InvalidArgumentException
     */
    public function testRejectsSpanLongerThanStringLength()
    {
        $this->markTestSkipped();

        $series = new Series(123);
        $series->largestProduct(4);
    }

    /**
     * There may be some confusion about whether this should be 1 or error.
     * The reasoning for it being 1 is this:
     * There is one 0-character string contained in the empty string.
     * That's the empty string itself.
     * The empty product is 1 (the identity for multiplication).
     * Therefore LSP('', 0) is 1.
     * It's NOT the case that LSP('', 0) takes max of an empty list.
     * So there is no error.
     * Compare against LSP('123', 4):
     * There are zero 4-character strings in '123'.
     * So LSP('123', 4) really DOES take the max of an empty list.
     * So LSP('123', 4) errors and LSP('', 0) does NOT
     *
     */
    public function testReports1ForEmptyStringAndEmptyProduct0Span()
    {
        $this->markTestSkipped();

        $series = new Series("");
        $this->assertEquals(1, $series->largestProduct(0));
    }

    /**
     * As above, there is one 0-character string in '123'.
     * So again no error. It's the empty product, 1.
     */
    public function testReports1ForNonemptyStringAndEmptyProduct0Span()
    {
        $this->markTestSkipped();

        $series = new Series("123");
        $this->assertEquals(1, $series->largestProduct(0));
    }

    /**
     * @expectedException InvalidArgumentException
     */
    public function testRejectsEmptyStringAndNonzeroSpan()
    {
        $this->markTestSkipped();

        $series = new Series("");
        $series->largestProduct(1);
    }

    /**
     * @expectedException InvalidArgumentException
     */
    public function testRejectsInvalidCharacterInDigits()
    {
        $this->markTestSkipped();

        $series = new Series("1234a5");
        $series->largestProduct(2);
    }

    /**
     * @expectedException InvalidArgumentException
     */
    public function testRejectsNegativeSpan()
    {
        $this->markTestSkipped();

        $series = new Series("12345");
        $series->largestProduct(-1);
    }
}
<?php

class Series
{
    private $digits;

    public function __construct($digits)
    {
        if (!preg_match('/^\d*$/', $digits))
            throw new InvalidArgumentException('Must only contain digits');
        $this->digits = strlen($digits) ? str_split($digits) : [];
    }

    public function largestProduct($span)
    {
        if (!preg_match('/^\d*$/', $span))
            throw new InvalidArgumentException('Must only contain digits');
        if ($span > count($this->digits))
            throw new InvalidArgumentException(sprintf('Span cannot exceed %s digits', count($this->digits)));
        if (!$span) // treat empty string and zero equally
            return 1;

        $product = 0;
        for ($i = 0; $i <= count($this->digits) - $span; $i++) {
            $product = max($this->seriesProduct($i, $span), $product);
        }
        return $product;
    }

    private function seriesProduct($start, $length)
    {
        return array_product(array_slice($this->digits, $start, $length));
    }
}

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  • What compromises have been made?
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Community comments

See what others have said about this solution
almost 2 years ago
4d47 says

Split error checking with second error message