diff --git a/.gitignore b/.gitignore
index f2a1ce6..e214b97 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,7 @@
 index.php
 nbproject/
+.idea/
+.php_cs.cache
 
 # Created by http://www.gitignore.io
 
diff --git a/.php_cs.dist b/.php_cs.dist
new file mode 100644
index 0000000..a70eec1
--- /dev/null
+++ b/.php_cs.dist
@@ -0,0 +1,14 @@
+<?php
+
+return PhpCsFixer\Config::create()
+    ->setRules([
+        '@PSR2' => true,
+        'ordered_imports' => true,
+        'phpdoc_order' => true,
+        'simplified_null_return' => false,
+        'no_unused_imports' => true,
+    ])->setFinder(
+        PhpCsFixer\Finder::create()
+            ->in(__DIR__.'/src')
+            ->in(__DIR__.'/tests')
+    );
diff --git a/.travis.yml b/.travis.yml
index 678b85e..30db937 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,5 @@
 language: php
 php:
-  - '7.0'
   - '7.1'
   - '7.2'
 before_script: composer install
diff --git a/README.md b/README.md
index 974d75a..0edc3af 100644
--- a/README.md
+++ b/README.md
@@ -3,12 +3,13 @@
 [![Build Status](https://travis-ci.org/Ekman/Luhn-Algorithm.svg?branch=master)](https://travis-ci.org/Ekman/Luhn-Algorithm)
 
 This is an implementation of the Luhn Algorithm in PHP. The Luhn Algorithm is
-used to validate things like credit cards and national identifcation numbers.
-More information on the algorithm can be found at [Wikipedia](http://en.wikipedia.org/wiki/Luhn_algorithm)
+used to validate things like credit cards and national identification numbers.
+More information on the algorithm can be found at [Wikipedia](http://en.wikipedia.org/wiki/Luhn_algorithm).
 
 ## Installation
 
-Can be installed using composer:
+Install with [Composer](https://getcomposer.org/):
+
 ```bash
 composer require nekman/luhn-algorithm
 ```
@@ -19,22 +20,21 @@ Use the class like this:
 
 ```php
 use Nekman\LuhnAlgorithm\LuhnAlgorithmFactory;
+use Nekman\LuhnAlgorithm\Number;
 
 $luhn = LuhnAlgorithmFactory::create();
 
-if ($luhn->isValid(123456789)) {
-	// Number is valid.
+// Validate a credit card number entered in a form.
+$ccNumber = Number::fromString($creditCard);
+if ($luhn->isValid($ccNumber)) {
+	// Credit card number is valid.
 }
 
-$checkSum = $luhn->calcCheckSum(123456789);
-
-$checkDigit = $luhn->calcCheckDigit(123456789);
-```
+// These methods are used internally by the library. You're free
+// to make use of them as well.
+$number = new Number(12345);
 
-## Changelog
+$checksum = $luhn->calcChecksum($number);
 
-* 4.0.0 - Rewrite of the implementation.
-* 3.0.0 - Completely restructured the interface of the library. 
-* 2.0.1 - Fixed typos in interface.
-* 2.0.0 - Added namespace.
-* 1.0.0 - Initial release.
+$checkDigit = $luhn->calcCheckDigit($number);
+```
\ No newline at end of file
diff --git a/composer.json b/composer.json
index 239e734..dc867e3 100644
--- a/composer.json
+++ b/composer.json
@@ -31,9 +31,10 @@
 		}
 	},
 	"require": {
-		"php": "^7.0"
+		"php": "^7.1"
 	},
 	"require-dev": {
-		"phpunit/phpunit": "^6.5"
+		"phpunit/phpunit": "^6.5",
+		"friendsofphp/php-cs-fixer": "^2.10"
 	}
 }
diff --git a/src/Contract/LuhnAlgorithmInterface.php b/src/Contract/LuhnAlgorithmInterface.php
index 83388d4..e80cacd 100644
--- a/src/Contract/LuhnAlgorithmInterface.php
+++ b/src/Contract/LuhnAlgorithmInterface.php
@@ -1,66 +1,63 @@
-<?php
-
-/*
- * The MIT License (MIT)
- *
- * Copyright (c) 2014 Niklas Ekman
- *
- * Permission is hereby granted, free of charge, to any person obtaining a copy of
- * this software and associated documentation files (the "Software"), to deal in
- * the Software without restriction, including without limitation the rights to
- * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
- * the Software, and to permit persons to whom the Software is furnished to do so,
- * subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in all
- * copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
- * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
- * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
- * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
- * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
- */
-
-namespace Nekman\LuhnAlgorithm\Contract;
-
-/**
- * Handles the Luhn Algorithm.
- * 
- * @link http://en.wikipedia.org/wiki/Luhn_algorithm 
- */
-interface LuhnAlgorithmInterface {
-    /**
-     * Determine if a number is valid according to the Luhn Algorithm.
-     * 
-     * @param string $input The number to validate.
-     * 
-     * @return bool true if number is valid, false otherwise.
-     * 
-     * @throws \InvalidArgumentException If the input is invalid.
-     */
-    public function isValid(string $input): bool;
-
-    /**
-     * Calculate the check digit for an input.
-     * 
-     * @param int $input The input to calculate the check digit for.
-     * 
-     * @return int The check digit.
-     * 
-     * @throws \InvalidArgumentException If the input is invalid.
-     */
-    public function calcCheckDigit(int $input): int;
-
-    /**
-     * Calulates the checksum for number.
-     * 
-     * @param int $input The number to calculate the checksum for.
-     * 
-     * @return int The checksum.
-     * 
-     * @throws \InvalidArgumentException If the input is invalid.
-     */
-    public function calcChecksum(int $input): int;
-}
+<?php
+
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2014 Niklas Ekman
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+namespace Nekman\LuhnAlgorithm\Contract;
+
+/**
+ * Handles the Luhn Algorithm.
+ *
+ * @link http://en.wikipedia.org/wiki/Luhn_algorithm
+ */
+interface LuhnAlgorithmInterface
+{
+    /**
+     * Determine if a number is valid according to the Luhn Algorithm.
+     *
+     * @param NumberInterface $number The number to validate.
+     *
+     * @return bool true if number is valid, false otherwise.
+     */
+    public function isValid(NumberInterface $number): bool;
+
+    /**
+     * Calculate the check digit for an input.
+     *
+     * @param NumberInterface $number The number to calculate the check digit for.
+     *
+     * @return int The check digit.
+     *
+     */
+    public function calcCheckDigit(NumberInterface $number): int;
+
+    /**
+     * Calulates the checksum for number.
+     *
+     * @param NumberInterface $number The number to calculate the checksum for.
+     *
+     * @return int The checksum.
+     *
+     */
+    public function calcChecksum(NumberInterface $number): int;
+}
diff --git a/src/Contract/NumberInterface.php b/src/Contract/NumberInterface.php
new file mode 100644
index 0000000..771b803
--- /dev/null
+++ b/src/Contract/NumberInterface.php
@@ -0,0 +1,46 @@
+<?php
+
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2014 Niklas Ekman
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+namespace Nekman\LuhnAlgorithm\Contract;
+
+/**
+ * Describes the structure of a number in the Luhn Algorithm.
+ */
+interface NumberInterface
+{
+    /**
+     * Get the number, without check digit.
+     *
+     * @return int
+     */
+    public function getNumber(): int;
+
+    /**
+     * Get the check digit for the number.
+     *
+     * @return int|null The check digit or null if it has not been calculated yet.
+     */
+    public function getCheckDigit(): ?int;
+}
diff --git a/src/LuhnAlgorithm.php b/src/LuhnAlgorithm.php
index 10e7766..5563245 100644
--- a/src/LuhnAlgorithm.php
+++ b/src/LuhnAlgorithm.php
@@ -26,68 +26,71 @@
 namespace Nekman\LuhnAlgorithm;
 
 use Nekman\LuhnAlgorithm\Contract\LuhnAlgorithmInterface;
+use Nekman\LuhnAlgorithm\Contract\NumberInterface;
 
 /**
- * Handles the Luhn Algorithm.
- * 
- * @link http://en.wikipedia.org/wiki/Luhn_algorithm 
+ * {@inheritdoc}
  */
-class LuhnAlgorithm implements LuhnAlgorithmInterface {
-	/**
-	 * {@inheritDoc}
-	 */
-	public function isValid(string $input): bool {
-		// Remove everything except digits from the input.
-		$number = (int) preg_replace("/[^\d]/", "", $input);
-		
-		$checksum = $this->calcChecksum($number);
+class LuhnAlgorithm implements LuhnAlgorithmInterface
+{
+    /**
+     * {@inheritDoc}
+     */
+    public function isValid(NumberInterface $number): bool
+    {
+        if ($number->getCheckDigit() === null) {
+            throw new \InvalidArgumentException("Check digit cannot be null.");
+        }
 
-		// If the checksum  is divisible by 10 it is valid
-		return ($checksum % 10) === 0;
-	}
+        $checksum = $this->calcChecksum($number);
+        $sum = $checksum + $number->getCheckDigit();
 
-	/**
-	 * {@inheritDoc}
-	 */
-	public function calcCheckDigit(int $input): int {
-		$checkSum = (string) $this->calcChecksum($input . 0);
-		
-		// Get the last digit of the checksum
-		$checkDigit = (int) $checkSum[strlen($checkSum) - 1];
+        // If the checksum is divisible by 10 it is valid.
+        return ($sum % 10) === 0;
+    }
 
-		// If the checkdigit is not 0, then subtract the  value from 10
-		return $checkDigit === 0 ? $checkDigit : 10 - $checkDigit;
-	}
+    /**
+     * {@inheritDoc}
+     */
+    public function calcCheckDigit(NumberInterface $number): int
+    {
+        $checksum = $this->calcChecksum($number);
+        
+        // Get the last digit of the checksum.
+        $checkDigit = $checksum % 10;
 
-	/**
-	 * {@inheritDoc}
-	 */
-	public function calcChecksum(int $input): int {
-		$input = (string) $input;
-		$length = strlen($input);
+        // If the check digit is not 0, then subtract the value from 10.
+        return $checkDigit === 0
+            ? $checkDigit
+            : 10 - $checkDigit;
+    }
 
-		$checkSum = 0;
+    /**
+     * {@inheritDoc}
+     */
+    public function calcChecksum(NumberInterface $number): int
+    {
+        $number = (string) $number->getNumber();
+        $nDigits = strlen($number);
+        $checksum = 0;
+        $parity = $nDigits % 2;
 
-		// Start at the next last digit
-		for ($i = $length - 2; $i >= 0; $i -= 2) {
-			// Multiply number with 2
-			$tmp = (int) ($input[$i]) * 2;
+        for ($i = 0; $i < $nDigits; $i++) {
+            $digit = (int) $number[$i];
 
-			// If a 2 digit number, split and add togheter
-			if ($tmp > 9) {
-				$tmp = ($tmp / 10) + ($tmp % 10);
-			}
+            // Every other digit, starting from the leftmost,
+            // shall be doubled.
+            if (($i % 2) !== $parity) {
+                $digit *= 2;
 
-			// Sum it upp
-			$checkSum += $tmp;
-		}
+                if ($digit > 9) {
+                    $digit -= 9;
+                }
+            }
 
-		// Start at the next last digit
-		for ($i = $length - 1; $i >= 0; $i -= 2) {
-			// Sum it upp
-			$checkSum += (int) $input[$i];
-		}
+            $checksum += $digit;
+        }
 
-		return $checkSum;
-	}
+        return $checksum;
+    }
 }
diff --git a/src/LuhnAlgorithmFactory.php b/src/LuhnAlgorithmFactory.php
index 17eeb13..6a95952 100644
--- a/src/LuhnAlgorithmFactory.php
+++ b/src/LuhnAlgorithmFactory.php
@@ -1,48 +1,49 @@
-<?php
-
-/*
- * The MIT License (MIT)
- *
- * Copyright (c) 2014 Niklas Ekman
- *
- * Permission is hereby granted, free of charge, to any person obtaining a copy of
- * this software and associated documentation files (the "Software"), to deal in
- * the Software without restriction, including without limitation the rights to
- * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
- * the Software, and to permit persons to whom the Software is furnished to do so,
- * subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in all
- * copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
- * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
- * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
- * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
- * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
- */
-
-namespace Nekman\LuhnAlgorithm;
-
-use Nekman\LuhnAlgorithm\Contract\LuhnAlgorithmInterface;
-use Nekman\LuhnAlgorithm\LuhmAlgorithm;
-
-/**
- * Factory for creating implementations of the Luhn Algorithm.
- */
-class LuhnAlgorithmFactory
-{
-    private function __construct() {
-        // Only static methods.
-    }
-
-    /**
-     * Create a new instance of an implementation of the Luhn Algorithm.
-     * 
-     * @return LuhnAlgorithmInterface Implementation of the Luhn Algorithm.
-     */
-    public static function create(): LuhnAlgorithmInterface {
-        return new LuhnAlgorithm();
-    }
-}
+<?php
+
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2014 Niklas Ekman
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+namespace Nekman\LuhnAlgorithm;
+
+use Nekman\LuhnAlgorithm\Contract\LuhnAlgorithmInterface;
+
+/**
+ * Factory for creating implementations of the Luhn Algorithm.
+ */
+class LuhnAlgorithmFactory
+{
+    private function __construct()
+    {
+        // Only static methods.
+    }
+
+    /**
+     * Create a new instance of an implementation of the Luhn Algorithm.
+     *
+     * @return LuhnAlgorithmInterface Implementation of the Luhn Algorithm.
+     */
+    public static function create(): LuhnAlgorithmInterface
+    {
+        return new LuhnAlgorithm();
+    }
+}
diff --git a/src/Number.php b/src/Number.php
new file mode 100644
index 0000000..822b9ef
--- /dev/null
+++ b/src/Number.php
@@ -0,0 +1,100 @@
+<?php
+
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2014 Niklas Ekman
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+namespace Nekman\LuhnAlgorithm;
+
+use Nekman\LuhnAlgorithm\Contract\NumberInterface;
+
+/**
+ * Input for the Luhn Algorithm contains a number and a check digit.
+ */
+class Number implements NumberInterface
+{
+    /**
+     * @var int
+     */
+    private $number;
+
+    /**
+     * @var int|null
+     */
+    private $checkDigit;
+
+    /**
+     * Input constructor.
+     * @param int $number The number.
+     * @param int|null $checkDigit [Optional] The check digit for the number.
+     */
+    public function __construct(int $number, ?int $checkDigit = null)
+    {
+        $this->number = $number;
+        $this->checkDigit = $checkDigit;
+    }
+
+    /**
+     * Create a new number from an input that contains the check digit
+     * already.
+     *
+     * @param string $input The input that contains the check digit already.
+     *
+     * @return self
+     */
+    public static function fromString(string $input): self
+    {
+        $input = (int) preg_replace("/[^\d]/", "", $input);
+
+        // Get the last digit.
+        $checkDigit = $input % 10;
+
+        // Remove the last digit.
+        $number = (int) ($input / 10);
+
+        return new self($number, $checkDigit);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getNumber(): int
+    {
+        return $this->number;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getCheckDigit(): ?int
+    {
+        return $this->checkDigit;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function __toString()
+    {
+        return (string) $this->number . (string) $this->checkDigit;
+    }
+}
diff --git a/tests/LuhnAlgorithmFactoryTest.php b/tests/LuhnAlgorithmFactoryTest.php
index 865607a..f887f3e 100644
--- a/tests/LuhnAlgorithmFactoryTest.php
+++ b/tests/LuhnAlgorithmFactoryTest.php
@@ -1,36 +1,38 @@
-<?php
-
-/*
- * The MIT License (MIT)
- *
- * Copyright (c) 2014 Niklas Ekman
- *
- * Permission is hereby granted, free of charge, to any person obtaining a copy of
- * this software and associated documentation files (the "Software"), to deal in
- * the Software without restriction, including without limitation the rights to
- * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
- * the Software, and to permit persons to whom the Software is furnished to do so,
- * subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in all
- * copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
- * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
- * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
- * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
- * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
- */
-
-namespace Nekman\LuhnAlgorithm\Test;
-
-use PHPUnit\Framework\TestCase;
-use Nekman\LuhnAlgorithm\LuhnAlgorithm;
-use Nekman\LuhnAlgorithm\LuhnAlgorithmFactory;
-
-class LuhmAlgorithmFactoryTest extends TestCase {
-    public function testCreate() {
-        $this->assertInstanceOf(LuhnAlgorithm::class, LuhnAlgorithmFactory::create());
-    }
-}
+<?php
+
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2014 Niklas Ekman
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+namespace Nekman\LuhnAlgorithm\Test;
+
+use Nekman\LuhnAlgorithm\LuhnAlgorithm;
+use Nekman\LuhnAlgorithm\LuhnAlgorithmFactory;
+use PHPUnit\Framework\TestCase;
+
+class LuhmAlgorithmFactoryTest extends TestCase
+{
+    public function testCreate()
+    {
+        $this->assertInstanceOf(LuhnAlgorithm::class, LuhnAlgorithmFactory::create());
+    }
+}
diff --git a/tests/LuhnAlgorithmTest.php b/tests/LuhnAlgorithmTest.php
index b32dd50..de6a036 100644
--- a/tests/LuhnAlgorithmTest.php
+++ b/tests/LuhnAlgorithmTest.php
@@ -25,60 +25,70 @@
 
 namespace Nekman\LuhnAlgorithm\Test;
 
-use PHPUnit\Framework\TestCase;
 use Nekman\LuhnAlgorithm\LuhnAlgorithm;
+use Nekman\LuhnAlgorithm\Number;
+use PHPUnit\Framework\TestCase;
 
-class LuhnAlgorithmTest extends TestCase {
-	/**
-	 * @var LuhnAlgorithm
-	 */
-	private $object;
+class LuhnAlgorithmTest extends TestCase
+{
+    /**
+     * @var LuhnAlgorithm
+     */
+    private $luhn;
 
-	public function setUp() {
-		parent::setUp();
-		
-		$this->object = new LuhnAlgorithm();
-	}
+    public function setUp()
+    {
+        parent::setUp();
+        
+        $this->luhn = new LuhnAlgorithm();
+    }
 
-	/**
-	 * @dataProvider provideIsValid_success
-	 */
-	public function testIsValid_success($number, $expected) {
-		$this->assertEquals($expected, $this->object->isValid($number));
-	}
+    /**
+     * @dataProvider provideIsValid_success
+     */
+    public function testIsValid_success($number, $expected)
+    {
+        $this->assertEquals($expected, $this->luhn->isValid($number));
+    }
 
-	public function provideIsValid_success() {
-		return [
-			["410321-9202", true],
-			["410321 - 9202", true],
-			[4103219202, true],
-		];
-	}
+    public function provideIsValid_success()
+    {
+        return [
+            [new Number(12345, 5), true],
+            [new Number(410321920, 2), true],
+            [new Number(3199723370002, 0), true],
+        ];
+    }
 
-	/**
-	 * @dataProvider provideCalcChecksum_success
-	 */
-	public function testCalcChecksum_success($number, $expected) {
-		$this->assertEquals($expected, $this->object->calcChecksum($number));
-	}
+    /**
+     * @dataProvider provideCalcChecksum_success
+     */
+    public function testCalcChecksum_success($number, $expected)
+    {
+        $this->assertEquals($expected, $this->luhn->calcChecksum($number));
+    }
 
-	public function provideCalcChecksum_success() {
-		return [
-			[4103219202, 30],
-		];
-	}
+    public function provideCalcChecksum_success()
+    {
+        return [
+            [new Number(3199723370002), 50],
+        ];
+    }
 
-	/**
-	 * @dataProvider provideCalcCheckDigit_success
-	 */
-	public function testCalcCheckDigit_success($number, $expected) {
-		$this->assertEquals($expected, $this->object->calcCheckDigit($number));
-	}
+    /**
+     * @dataProvider provideCalcCheckDigit_success
+     */
+    public function testCalcCheckDigit_success($number, $expected)
+    {
+        $this->assertEquals($expected, $this->luhn->calcCheckDigit($number));
+    }
 
-	public function provideCalcCheckDigit_success() {
-		return [
-			[12345, 5],
-			[410321920, 2],
-		];
-	}
+    public function provideCalcCheckDigit_success()
+    {
+        return [
+            [new Number(12345), 5],
+            [new Number(410321920), 2],
+            [new Number(3199723370002), 0],
+        ];
+    }
 }
diff --git a/tests/NumberTest.php b/tests/NumberTest.php
new file mode 100644
index 0000000..443ab3f
--- /dev/null
+++ b/tests/NumberTest.php
@@ -0,0 +1,63 @@
+<?php
+
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2014 Niklas Ekman
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+namespace Nekman\LuhnAlgorithm\Test;
+
+use Nekman\LuhnAlgorithm\Number;
+use PHPUnit\Framework\TestCase;
+
+class NumberTest extends TestCase
+{
+    /**
+     * @dataProvider provideWithCheckDigit_success
+     */
+    public function testWithCheckDigit_success($number, $expected)
+    {
+        $this->assertEquals($expected, Number::fromString($number));
+    }
+
+    public function provideWithCheckDigit_success()
+    {
+        return [
+            ["410321-9202", new Number(410321920, 2)],
+            [4103219202, new Number(410321920, 2)]
+        ];
+    }
+
+    /**
+     * @dataProvider provideToString_success
+     */
+    public function testToString_success($number, $expected)
+    {
+        $this->assertEquals($expected, (string) $number);
+    }
+
+    public function provideToString_success()
+    {
+        return [
+            [new Number(12345, 5), "123455"]
+        ];
+    }
+}