Implement ColumnFilter preprocessor (#378)

src/Preprocessing/ColumnFilter.php

@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Phpml\Preprocessing;
+
+final class ColumnFilter implements Preprocessor
+{
+    /**
+     * @var string[]
+     */
+    private $datasetColumns = [];
+
+    /**
+     * @var string[]
+     */
+    private $filterColumns = [];
+
+    public function __construct(array $datasetColumns, array $filterColumns)
+    {
+        $this->datasetColumns = array_map(static function (string $column): string {
+            return $column;
+        }, $datasetColumns);
+        $this->filterColumns = array_map(static function (string $column): string {
+            return $column;
+        }, $filterColumns);
+    }
+
+    public function fit(array $samples, ?array $targets = null): void
+    {
+        //nothing to do
+    }
+
+    public function transform(array &$samples, ?array &$targets = null): void
+    {
+        $keys = array_intersect($this->datasetColumns, $this->filterColumns);
+
+        foreach ($samples as &$sample) {
+            $sample = array_values(array_intersect_key($sample, $keys));
+        }
+    }
+}

tests/Preprocessing/ColumnFilterTest.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Phpml\Tests\Preprocessing;
+
+use Phpml\Preprocessing\ColumnFilter;
+use PHPUnit\Framework\TestCase;
+
+final class ColumnFilterTest extends TestCase
+{
+    public function testFilterColumns(): void
+    {
+        $datasetColumns = ['age', 'income', 'kids', 'beersPerWeek'];
+        $filterColumns = ['income', 'beersPerWeek'];
+        $samples = [
+            [21, 100000, 1, 4],
+            [35, 120000, 0, 12],
+            [33, 200000, 4, 0],
+        ];
+
+        $filter = new ColumnFilter($datasetColumns, $filterColumns);
+        $filter->transform($samples);
+
+        self::assertEquals([[100000, 4], [120000, 12], [200000, 0]], $samples);
+    }
+}