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4d47's solution

to Run Length Encoding in the PHP Track

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Instructions
Test suite
Solution

Implement run-length encoding and decoding.

Run-length encoding (RLE) is a simple form of data compression, where runs (consecutive data elements) are replaced by just one data value and count.

For example we can represent the original 53 characters with only 13.

"WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWB"  ->  "12WB12W3B24WB"

RLE allows the original data to be perfectly reconstructed from the compressed data, which makes it a lossless data compression.

"AABCCCDEEEE"  ->  "2AB3CD4E"  ->  "AABCCCDEEEE"

For simplicity, you can assume that the unencoded string will only contain the letters A through Z (either lower or upper case) and whitespace. This way data to be encoded will never contain any numbers and numbers inside data to be decoded always represent the count for the following character.

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 run-length-encoding/run-length-encoding_test.php
    

Source

Wikipedia https://en.wikipedia.org/wiki/Run-length_encoding

Submitting Incomplete Solutions

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

run-length-encoding_test.php

<?php

require "run-length-encoding.php";

/**
 * Class RunLengthEncodingTest
 */
class RunLengthEncodingTest extends PHPUnit\Framework\TestCase
{
    public function testEncodeEmptyString()
    {
        $this->assertEquals('', encode(''));
    }

    public function testEncodeSingleCharactersOnly()
    {
        $this->assertEquals('XYZ', encode('XYZ'));
    }

    public function testDecodeEmptyString()
    {
        $this->assertEquals('', decode(''));
    }

    public function testDecodeSingleCharactersOnly()
    {
        $this->assertEquals('XYZ', decode('XYZ'));
    }

    public function testEncodeSimple()
    {
        $this->assertEquals('2A3B4C', encode('AABBBCCCC'));
    }

    public function testDecodeSimple()
    {
        $this->assertEquals('AABBBCCCC', decode('2A3B4C'));
    }

    public function testEncodeWithSingleValues()
    {
        $this->assertEquals(
            '12WB12W3B24WB',
            encode('WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWB')
        );
    }

    public function testDecodeWithSingleValues()
    {
        $this->assertEquals(
            'WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWB',
            decode('12WB12W3B24WB')
        );
    }

    public function testDecodeMultipleWhitespaceMixedInString()
    {
        $this->assertEquals('  hsqq qww  ', decode('2 hs2q q2w2 '));
    }

    public function testEncodeDecodeCombination()
    {
        $this->assertEquals('zzz ZZ  zZ', decode(encode('zzz ZZ  zZ')));
    }
}
<?php

function encode(string $input): string
{
    return preg_replace_callback('/(.)\1*/', 'encode_match', $input);
}

function decode(string $input): string
{
    return preg_replace_callback('/(\d+)?(.)/', 'decode_match', $input);
}

function encode_match(array $match): string
{
    $n = strlen($match[0]);
    return $n > 1 ? $n . $match[1] : $match[1];
}

function decode_match(array $match): string
{
    return str_repeat($match[2], intval($match[1]) ?: 1);
};

What can you learn from this solution?

A huge amount can be learnt from reading other people’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 I could read more about to develop my understanding?