Sieve of Eratosthenes algorithm (#159)

Co-authored-by: Michal Zarnecki <[email protected]>

DIRECTORY.md

@@ -57,6 +57,7 @@
     * [Problem7](./Maths/ProjectEuler/Problem7.php)
     * [Problem8](./Maths/ProjectEuler/Problem8.php)
     * [Problem9](./Maths/ProjectEuler/Problem9.php)
+  * [Eratosthenessieve](./Maths/EratosthenesSieve.php)
 
 ## Searches
   * [Binarysearch](./Searches/BinarySearch.php)
@@ -119,6 +120,7 @@
   * Maths
     * [Mathstest](./tests/Maths/MathsTest.php)
     * [Projecteulertest](./tests/Maths/ProjectEulerTest.php)
+    * [Eratosthenessievetest](./tests/Maths/EratosthenesSieveTest.php)
   * Searches
     * [Searchestest](./tests/Searches/SearchesTest.php)
   * Sorting

Maths/EratosthenesSieve.php

@@ -0,0 +1,30 @@
+<?php
+
+/**
+ * The Sieve of Eratosthenes is an algorithm is an ancient algorithm for finding all prime numbers up to any given limit
+ * https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
+ *
+ * @author Michał Żarnecki https://github.com/rzarno
+ * @param int $number the limit of prime numbers to generate
+ * @return string[] prime numbers up to the given limit
+ */
+function eratosthenesSieve(int $number): array
+{
+    $primes = range(1, $number);
+    $primes = array_combine($primes, $primes);
+    $limit = sqrt($number);
+    $current = 2;
+    while ($current < $limit) {
+        $multiplied = $current;
+        $factor = 1;
+        while ($multiplied < $number) {
+            $factor++;
+            $multiplied = $current * $factor;
+            if (isset($primes[$multiplied])) {
+                unset($primes[$multiplied]);
+            }
+        }
+        $current += 1;
+    }
+    return array_values($primes);
+}

tests/Maths/EratosthenesSieveTest.php

@@ -0,0 +1,16 @@
+<?php
+
+require_once __DIR__ . '/../../vendor/autoload.php';
+require_once __DIR__ . '/../../Maths/EratosthenesSieve.php';
+
+use PHPUnit\Framework\TestCase;
+
+class EratosthenesSieveTest extends TestCase
+{
+    public function testEratosthenesSieve()
+    {
+        $result = eratosthenesSieve(30);
+
+        $this->assertEquals($result, [1, 2, 3, 5, 7, 11, 13, 17, 19, 23, 29]);
+    }
+}