add chapter about data repositories, and start work on perfomance chapter

implementation/16-data-repository/src/Repository/FileSystemMarkdownPageRepo.php

@@ -0,0 +1,61 @@
+<?php declare(strict_types=1);
+
+namespace Lubian\NoFramework\Repository;
+
+use Lubian\NoFramework\Exception\InternalServerError;
+use Lubian\NoFramework\Exception\NotFound;
+use Lubian\NoFramework\Model\MarkdownPage;
+
+use function array_filter;
+use function array_map;
+use function array_values;
+use function count;
+use function file_get_contents;
+use function glob;
+use function str_replace;
+use function substr;
+
+final class FileSystemMarkdownPageRepo implements MarkdownPageRepo
+{
+    public function __construct(
+        private readonly string $dataPath
+    ) {
+    }
+
+    /** @inheritDoc  */
+    public function all(): array
+    {
+        $files = glob($this->dataPath . '*.md');
+        if ($files === false) {
+            throw new InternalServerError('cannot read pages');
+        }
+        return array_map(function (string $filename) {
+            $content = file_get_contents($filename);
+            if ($content === false) {
+                throw new InternalServerError('cannot read pages');
+            }
+            $idAndTitle = str_replace([$this->dataPath, '.md'], ['', ''], $filename);
+            return new MarkdownPage(
+                (int) substr($idAndTitle, 0, 2),
+                substr($idAndTitle, 3),
+                $content
+            );
+        }, $files);
+    }
+
+    public function byName(string $name): MarkdownPage
+    {
+        $pages = array_values(
+            array_filter(
+                $this->all(),
+                fn (MarkdownPage $p) => $p->title === $name,
+            )
+        );
+
+        if (count($pages) !== 1) {
+            throw new NotFound;
+        }
+
+        return $pages[0];
+    }
+}