add fit method for Transformer interface

src/Phpml/Exception/PreprocessorException.php

@@ -0,0 +1,17 @@
+<?php
+declare(strict_types = 1);
+
+namespace Phpml\Exception;
+
+class PreprocessorException extends \Exception
+{
+
+    /**
+     * @return PreprocessorException
+     */
+    public static function fitNotAllowed()
+    {
+        return new self('Fit is not allowed for this preprocessor.');
+    }
+
+}

src/Phpml/FeatureExtraction/TfIdfTransformer.php

@@ -16,15 +16,31 @@ class TfIdfTransformer implements Transformer
     /**
      * @param array $samples
      */
-    public function transform(array &$samples)
+    public function __construct(array $samples = null)
+    {
+        if($samples) {
+            $this->fit($samples);
+        }
+    }
+
+    /**
+     * @param array $samples
+     */
+    public function fit(array $samples)
     {
         $this->countTokensFrequency($samples);
 
         $count = count($samples);
         foreach ($this->idf as &$value) {
             $value = log($count / $value, 10);
         }
+    }
 
+    /**
+     * @param array $samples
+     */
+    public function transform(array &$samples)
+    {
         foreach ($samples as &$sample) {
             foreach ($sample as $index => &$feature) {
                 $feature = $feature * $this->idf[$index];

src/Phpml/FeatureExtraction/TokenCountVectorizer.php

@@ -46,6 +46,14 @@ public function __construct(Tokenizer $tokenizer, float $minDF = 0)
         $this->frequencies = [];
     }
 
+    /**
+     * @param array $samples
+     */
+    public function fit(array $samples)
+    {
+        // TODO: Implement fit() method.
+    }
+
     /**
      * @param array $samples
      */

src/Phpml/Preprocessing/Imputer.php

@@ -38,6 +38,14 @@ public function __construct($missingValue = null, Strategy $strategy, int $axis
         $this->axis = $axis;
     }
 
+    /**
+     * @param array $samples
+     */
+    public function fit(array $samples)
+    {
+        // TODO: Implement fit() method.
+    }
+
     /**
      * @param array $samples
      */

src/Phpml/Preprocessing/Normalizer.php

@@ -5,6 +5,7 @@
 namespace Phpml\Preprocessing;
 
 use Phpml\Exception\NormalizerException;
+use Phpml\Exception\PreprocessorException;
 
 class Normalizer implements Preprocessor
 {
@@ -30,6 +31,16 @@ public function __construct(int $norm = self::NORM_L2)
         $this->norm = $norm;
     }
 
+    /**
+     * @param array $samples
+     * 
+     * @throws PreprocessorException
+     */
+    public function fit(array $samples)
+    {
+        throw PreprocessorException::fitNotAllowed();
+    }
+
     /**
      * @param array $samples
      */

src/Phpml/Transformer.php

@@ -6,6 +6,12 @@
 
 interface Transformer
 {
+
+    /**
+     * @param array $samples
+     */
+    public function fit(array $samples);
+
     /**
      * @param array $samples
      */

tests/Phpml/FeatureExtraction/TfIdfTransformerTest.php

@@ -22,7 +22,7 @@ public function testTfIdfTransformation()
             [0 => 0, 1 => 0, 2 => 0, 3 => 0, 4 => 0.602, 5 => 0.903],
         ];
 
-        $transformer = new TfIdfTransformer();
+        $transformer = new TfIdfTransformer($samples);
         $transformer->transform($samples);
 
         $this->assertEquals($tfIdfSamples, $samples, '', 0.001);