implement TfIdf transformation

README.md

@@ -6,7 +6,7 @@
 [![License](https://poser.pugx.org/php-ai/php-ml/license.svg)](https://packagist.org/packages/php-ai/php-ml)
 [![Scrutinizer Code Quality](https://scrutinizer-ci.com/g/php-ai/php-ml/badges/quality-score.png?b=develop)](https://scrutinizer-ci.com/g/php-ai/php-ml/?branch=develop)
 
-Fresh approach to Machine Learning in PHP. Note that at the moment PHP is not the best choice for machine learning but maybe this will change ...
+Fresh approach to Machine Learning in PHP. Algorithms, Cross Validation, Preprocessing, Feature Extraction and much more in one library.
 
 Simple example of classification:
 ```php

src/Phpml/FeatureExtraction/TfIdfTransformer.php

@@ -0,0 +1,54 @@
+<?php
+
+declare (strict_types = 1);
+
+namespace Phpml\FeatureExtraction;
+
+class TfIdfTransformer implements Transformer
+{
+    /**
+     * @var array
+     */
+    private $idf;
+
+    /**
+     * @param array $samples
+     * 
+     * @return array
+     */
+    public function transform(array $samples): array
+    {
+        $this->countTokensFrequency($samples);
+
+        $count = count($samples);
+        foreach ($this->idf as &$value) {
+            $value = log($count / $value, 10);
+        }
+
+        foreach ($samples as &$sample) {
+            foreach ($sample as $index => &$feature) {
+                $feature = $feature * $this->idf[$index];
+            }
+        }
+
+        return $samples;
+    }
+
+    /**
+     * @param array $samples
+     *
+     * @return array
+     */
+    private function countTokensFrequency(array $samples)
+    {
+        $this->idf = array_fill_keys(array_keys($samples[0]), 0);
+
+        foreach ($samples as $sample) {
+            foreach ($sample as $index => $count) {
+                if ($count > 0) {
+                    ++$this->idf[$index];
+                }
+            }
+        }
+    }
+}

tests/Phpml/FeatureExtraction/TfIdfTransformerTest.php

@@ -0,0 +1,29 @@
+<?php
+
+declare (strict_types = 1);
+
+namespace tests\Phpml\FeatureExtraction;
+
+use Phpml\FeatureExtraction\TfIdfTransformer;
+
+class TfIdfTransformerTest extends \PHPUnit_Framework_TestCase
+{
+    public function testTfIdfTransformation()
+    {
+        //https://en.wikipedia.org/wiki/Tf%E2%80%93idf
+
+        $samples = [
+            [0 => 1, 1 => 1, 2 => 2, 3 => 1, 4 => 0, 5 => 0],
+            [0 => 1, 1 => 1, 2 => 0, 3 => 0, 4 => 2, 5 => 3],
+        ];
+
+        $tfIdfSamples = [
+            [0 => 0, 1 => 0, 2 => 0.602, 3 => 0.301, 4 => 0, 5 => 0],
+            [0 => 0, 1 => 0, 2 => 0, 3 => 0, 4 => 0.602, 5 => 0.903],
+        ];
+
+        $transformer = new TfIdfTransformer();
+
+        $this->assertEquals($tfIdfSamples, $transformer->transform($samples), '', 0.001);
+    }
+}