Implement LambdaTransformer (#381)

src/Preprocessing/LambdaTransformer.php

@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Phpml\Preprocessing;
+
+final class LambdaTransformer implements Preprocessor
+{
+    /**
+     * @var callable
+     */
+    private $lambda;
+
+    public function __construct(callable $lambda)
+    {
+        $this->lambda = $lambda;
+    }
+
+    public function fit(array $samples, ?array $targets = null): void
+    {
+        // nothing to do
+    }
+
+    public function transform(array &$samples, ?array &$targets = null): void
+    {
+        foreach ($samples as &$sample) {
+            $sample = call_user_func($this->lambda, $sample);
+        }
+    }
+}

tests/Preprocessing/LambdaTransformerTest.php

@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Phpml\Tests\Preprocessing;
+
+use Phpml\Preprocessing\LambdaTransformer;
+use PHPUnit\Framework\TestCase;
+
+final class LambdaTransformerTest extends TestCase
+{
+    public function testLambdaSampleTransformation(): void
+    {
+        $transformer = new LambdaTransformer(static function ($sample): int {
+            return $sample[0] + $sample[1];
+        });
+
+        $samples = [
+            [1, 2],
+            [3, 4],
+            [5, 6],
+        ];
+
+        $transformer->transform($samples);
+
+        self::assertEquals([3, 7, 11], $samples);
+    }
+}