app/api/includes/CnbRates.php

<?php

/**
 * Devizove kurzy CNB s file cache per datum.
 *
 * Pouziti:
 *   $cnb = CnbRates::getInstance();
 *   $czk = $cnb->toCzk(1000.0, 'EUR', '2026-03-01');
 */

declare(strict_types=1);

class CnbRates
{
    private const API_URL = 'https://api.cnb.cz/cnbapi/exrates/daily';
    private const CACHE_FILE = 'cnb_rates_cache.json';

    // Kurzy starsi nez dnesek se nemeni, cachujem navzdy.
    // Dnesni kurz cachujeme na 6 hodin (muze se behem dne aktualizovat).
    private const TODAY_CACHE_TTL = 21600;

    /**
     * In-memory cache: date => currency => {rate, amount}
     * @var array<string, array<string, array{rate: float, amount: int}>>
     */
    private array $ratesByDate = [];

    private static ?CnbRates $instance = null;

    public static function getInstance(): self
    {
        if (self::$instance === null) {
            self::$instance = new self();
        }
        return self::$instance;
    }

    private function __construct()
    {
        $this->loadCache();
    }

    /**
     * Prevede castku na CZK dle kurzu platneho k danemu datu.
     */
    public function toCzk(float $amount, string $currency, string $date = ''): float
    {
        if ($currency === 'CZK') {
            return $amount;
        }

        $rates = $this->getRatesForDate($date ?: date('Y-m-d'));
        if (!isset($rates[$currency])) {
            return $amount;
        }

        $info = $rates[$currency];
        return $amount * ($info['rate'] / $info['amount']);
    }

    /**
     * Secte pole [{amount, currency, date?}] do jedne CZK castky.
     * Kazda polozka muze mit vlastni datum pro kurz.
     *
     * @param array<int, array{amount: float, currency: string, date?: string}> $items
     */
    public function sumToCzk(array $items): float
    {
        $total = 0.0;
        foreach ($items as $item) {
            $total += $this->toCzk(
                (float) $item['amount'],
                (string) $item['currency'],
                (string) ($item['date'] ?? '')
            );
        }
        return round($total, 2);
    }

    /**
     * @return array<string, array{rate: float, amount: int}>
     */
    private function getRatesForDate(string $date): array
    {
        if (isset($this->ratesByDate[$date])) {
            return $this->ratesByDate[$date];
        }

        $rates = $this->fetchFromApi($date);
        if ($rates !== []) {
            $this->ratesByDate[$date] = $rates;
            $this->saveCache();
        }

        return $rates;
    }

    // --- Cache ---

    private function getCachePath(): string
    {
        return sys_get_temp_dir() . DIRECTORY_SEPARATOR . self::CACHE_FILE;
    }

    private function loadCache(): void
    {
        $path = $this->getCachePath();
        if (!file_exists($path)) {
            return;
        }

        $content = file_get_contents($path);
        if ($content === false) {
            return;
        }

        $data = json_decode($content, true);
        if (!is_array($data)) {
            return;
        }

        $today = date('Y-m-d');
        foreach ($data as $date => $entry) {
            if (!is_array($entry) || !isset($entry['rates'], $entry['fetched_at'])) {
                continue;
            }
            // Dnesni kurz expiruje po TTL, starsi zustavaji navzdy
            if ($date === $today) {
                $age = time() - (int) $entry['fetched_at'];
                if ($age > self::TODAY_CACHE_TTL) {
                    continue;
                }
            }
            $this->ratesByDate[$date] = $entry['rates'];
        }
    }

    private function saveCache(): void
    {
        $path = $this->getCachePath();

        // Nacist existujici cache a mergovat
        $existing = [];
        if (file_exists($path)) {
            $content = file_get_contents($path);
            if ($content !== false) {
                $decoded = json_decode($content, true);
                if (is_array($decoded)) {
                    $existing = $decoded;
                }
            }
        }

        $now = time();
        foreach ($this->ratesByDate as $date => $rates) {
            // Neprepisuj existujici pokud uz tam je (zachovej fetched_at)
            if (!isset($existing[$date])) {
                $existing[$date] = [
                    'rates' => $rates,
                    'fetched_at' => $now,
                ];
            }
        }

        $json = json_encode($existing, JSON_THROW_ON_ERROR);
        file_put_contents($path, $json, LOCK_EX);
    }

    // --- API ---

    /**
     * @return array<string, array{rate: float, amount: int}>
     */
    private function fetchFromApi(string $date): array
    {
        $url = self::API_URL . '?lang=EN&date=' . urlencode($date);

        $context = stream_context_create([
            'http' => ['timeout' => 5],
        ]);

        $response = @file_get_contents($url, false, $context);
        if ($response === false) {
            return [];
        }

        $data = json_decode($response, true);
        if (!is_array($data) || !isset($data['rates'])) {
            return [];
        }

        $rates = [];
        foreach ($data['rates'] as $entry) {
            if (!isset($entry['currencyCode'], $entry['rate'], $entry['amount'])) {
                continue;
            }
            $rates[$entry['currencyCode']] = [
                'rate' => (float) $entry['rate'],
                'amount' => (int) $entry['amount'],
            ];
        }

        return $rates;
    }
}