Challenge - Performance optimization

get(string $key, callable $loader) calls the loader when the key is missing and stores the value. The benchmark(callable $fn): array function measures time with hrtime(true) and memory with memory_get_usage(true); it returns an array keyed by result, ms, bytes. ReportService takes a PDO, creates products and categories tables, inserts two products; eagerReport() runs a single JOIN query to avoid the N+1 problem.
Example solution:
<?php
// ETL + Reflection + Performance: write Cache + benchmark + ReportService

function readRows(array $rows): Generator {
    foreach ($rows as $row) {
        yield $row;
    }
}

function transform(iterable $rows, callable $fn): Generator {
    foreach ($rows as $row) {
        yield $fn($row);
    }
}

function pipeline(array $rows, callable $fn): Generator {
    yield from transform(readRows($rows), $fn);
}

function loadAll(iterable $rows): array {
    $out = [];
    foreach ($rows as $row) {
        $out[] = $row;
    }
    return $out;
}

#[Attribute]
class Field {
    public function __construct(public string $name = "") {}
}

class Product {
    public function __construct(
        #[Field("id")] public int $id,
        #[Field("name")] public string $name,
    ) {}
}

class ReflectionSerializer {
    public function serialize(object $obj): array {
        $rc = new ReflectionClass($obj);
        $out = [];
        foreach ($rc->getProperties() as $prop) {
            $attrs = $prop->getAttributes(Field::class);
            if (empty($attrs)) continue;
            $field = $attrs[0]->newInstance();
            $key = $field->name !== "" ? $field->name : $prop->getName();
            $out[$key] = $prop->getValue($obj);
        }
        return $out;
    }
}

class Cache {
    private array $store = [];
    public function get(string $key, callable $loader) {
        if (!isset($this->store[$key])) {
            $this->store[$key] = $loader();
        }
        return $this->store[$key];
    }
}

function benchmark(callable $fn): array {
    $start = hrtime(true);
    $result = $fn();
    $elapsed = (int)((hrtime(true) - $start) / 1000000);
    $mem = memory_get_usage(true);
    return ["result" => $result, "ms" => $elapsed, "bytes" => $mem];
}

class ReportService {
    private PDO $pdo;
    public function __construct(PDO $pdo) {
        $this->pdo = $pdo;
        $this->pdo->exec("CREATE TABLE products (id INTEGER PRIMARY KEY, name TEXT, category_id INTEGER)");
        $this->pdo->exec("CREATE TABLE categories (id INTEGER PRIMARY KEY, name TEXT)");
        $this->pdo->prepare("INSERT INTO categories (name) VALUES (?)")->execute(["kirtasiye"]);
        $this->pdo->prepare("INSERT INTO products (name, category_id) VALUES (?, ?)")->execute(["kale", 1]);
        $this->pdo->prepare("INSERT INTO products (name, category_id) VALUES (?, ?)")->execute(["defter", 1]);
    }
    public function eagerReport(): array {
        $sql = "SELECT p.name AS product, c.name AS category FROM products p JOIN categories c ON p.category_id = c.id ORDER BY p.id";
        return $this->pdo->query($sql)->fetchAll(PDO::FETCH_ASSOC);
    }
}

// Demo:
$pdo = new PDO("sqlite::memory:");
$s = new ReportService($pdo);
$cache = new Cache();
$rows = $cache->get("report", fn() => $s->eagerReport());
$second = $cache->get("report", fn() => $s->eagerReport());
echo "rows: " . count($rows) . "\n";
echo "cached: " . ($rows === $second ? "1" : "0") . "\n";
$r = benchmark(fn() => $s->eagerReport());
echo "ms_int: " . (is_int($r["ms"]) ? "1" : "0") . "\n";
echo "bytes_int: " . (is_int($r["bytes"]) ? "1" : "0") . "\n";