Add the isPrimeNumber method (#4)

* add required extension: ext-gmp
* update the PHP version
* remove checking PHP 5.6
* add allow_failure for hhvm and 5.6
* remove hhvm and php 5.6
* add the isPrimeNumber method.
* modify the test prime number method

src/BigInteger.php

@@ -221,6 +221,23 @@ public function subtract(string $value): BigInteger
         return $this->assignValue($calculatedValue);
     }
 
+    /**
+     * Checks if the big integr is the prime number.
+     *
+     * @param float $probabilityFactor A normalized factor between 0 and 1 used for checking the probability.
+     * @return bool Returns true if the number is a prime number false if not.
+     */
+    public function isPrimeNumber(float $probabilityFactor = 1.0): bool
+    {
+        $reps = (int)floor(($probabilityFactor * 5.0) + 5.0);
+
+        if ($reps < 5 || $reps > 10) {
+            throw new InvalidArgumentException('The provided probability number should be 5 to 10.');
+        }
+
+        return gmp_prob_prime($this->value, $reps) !== 0;
+    }
+
     /**
      * Checks if this object is mutable.
      *

tests/BigIntegerPrimeNumberTest.php

@@ -0,0 +1,75 @@
+<?php
+
+namespace PHP\Math\BigIntegerTest;
+
+use PHP\Math\BigInteger\BigInteger;
+use PHPUnit_Framework_TestCase;
+
+class BigIntegerPrimeNumberTest extends PHPUnit_Framework_TestCase
+{
+    public function testIsPrimeNumberWithouttPrimeNumber()
+    {
+        // Arrange
+        $bigInteger = new BigInteger('6');
+
+        // Act
+        $withoutPrimeNumber = $bigInteger->isPrimeNumber();
+
+        // Assert
+        $this::assertFalse($withoutPrimeNumber);
+    }
+
+    public function testIsPrimeNumberWithPrimeNumber()
+    {
+        // Arrange
+        $bigInteger = new BigInteger('11');
+
+        // Act
+        $withPrimeNumber = $bigInteger->isPrimeNumber();
+
+        // Assert
+        $this::assertTrue($withPrimeNumber);
+    }
+
+    public function testIsPrimeNumberWithoutProbabilePrimeNumber()
+    {
+        // Arrange
+        $bigInteger = new BigInteger('1111111111111111111');
+
+        // Act
+        $probabilePrimeNumber = $bigInteger->isPrimeNumber();
+
+        // Assert
+        $this::assertTrue($probabilePrimeNumber);
+    }
+
+    /**
+     * @expectedException InvalidArgumentException
+     */
+    public function testIsPrimeNumberProvidedProbabilityNumberIsLessThan5()
+    {
+        // Arrange
+        $bigInteger = new BigInteger('1111111111111111111');
+
+        // Act
+        $probabilePrimeNumber = $bigInteger->isPrimeNumber(-1.0);
+
+        // Assert
+        // ...
+    }
+
+    /**
+     * @expectedException InvalidArgumentException
+     */
+    public function testIsPrimeNumberProvidedProbabilityNumberIsGreaterThan10()
+    {
+        // Arrange
+        $bigInteger = new BigInteger('1111111111111111111');
+
+        // Act
+        $probabilePrimeNumber = $bigInteger->isPrimeNumber(2.0);
+
+        // Assert
+        // ...
+    }
+}