Add entropy calculator

Author: sebastianfeldmann

composer.json

@@ -30,7 +30,8 @@
         }
     },
     "require": {
-        "php": ">=8.0"
+        "php": ">=8.0",
+        "ext-mbstring": "*"
     },
     "scripts": {
         "post-install-cmd": "tools/phive install --force-accept-unsigned",

src/Entropy/Shannon.php

@@ -0,0 +1,51 @@
+<?php
+
+/**
+ * This file is part of CaptainHook Secrets.
+ *
+ * (c) Sebastian Feldmann <sf@sebastian-feldmann.info>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace CaptainHook\Secrets\Entropy;
+
+/**
+ * Secret Detector
+ *
+ * @package CaptainHook-Secrets
+ * @author  Sebastian Feldmann <sf@sebastian-feldmann.info>
+ * @link    https://github.com/captainhookphp/secrets
+ * @since   Class available since Release 0.9.1
+ */
+class Shannon
+{
+    /**
+     * Return the entropy of a given string
+     *
+     * @param string $text
+     * @return float
+     */
+    public static function entropy(string $text): float
+    {
+        if (empty($text)) {
+            return 0;
+        }
+
+        $charMap = [];
+        foreach (mb_str_split($text) as $char) {
+            $charMap[$char] = isset($charMap[$char]) ? $charMap[$char] + 1 : 1;
+        }
+
+        $entropy = 0;
+        $length  = strlen($text);
+        foreach ($charMap as $char => $amount) {
+            $p       = $amount / $length;
+            $entropy = $entropy - ($p * log($p, 2));
+        }
+        return $entropy;
+    }
+}

tests/Entropy/ShannonTest.php

@@ -0,0 +1,56 @@
+<?php
+
+/**
+ * This file is part of CaptainHook Secrets.
+ *
+ * (c) Sebastian Feldmann <sf@sebastian-feldmann.info>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace CaptainHook\Secrets\Entropy;
+
+use CaptainHook\Secrets\Detector;
+use CaptainHook\Secrets\Regex\Supplier\Aws;
+use PHPUnit\Framework\TestCase;
+
+class ShannonTest extends TestCase
+{
+    public function testEntropyWikiExample(): void
+    {
+        $this->assertEquals(1.85, round(Shannon::entropy('1223334444'), 2));
+    }
+
+    public function testEntropyEmptyString(): void
+    {
+        $this->assertEquals(0, Shannon::entropy(''));
+    }
+
+    public function testEntropyOneCharacterString(): void
+    {
+        $this->assertEquals(0, Shannon::entropy('A'));
+    }
+
+    public function testEntropyMultipleSimilarCharacters(): void
+    {
+        $this->assertEquals(0, Shannon::entropy('111111'));
+    }
+
+    public function testEntropyTwoCharacters(): void
+    {
+        $this->assertEquals(1.0, Shannon::entropy('111222'));
+    }
+
+    public function testEntropyRegularToken(): void
+    {
+        $this->assertTrue(Shannon::entropy('lg45ve1mz1yf21pb30qd') > 4);
+    }
+
+    public function testEntropyLongClassName(): void
+    {
+        $this->assertTrue(Shannon::entropy('EntropyDetectionManagerProvider') < 4);
+    }
+}