session_start();
if (!isset($_SESSION['cart'])) {
    $_SESSION['cart'] = [];
}

$productId = (int)$_POST['product_id'];
$quantity  = (int)$_POST['quantity'] ?? 1;

// 查询商品库存（模拟数据库查询）
$stock = getProductStock($productId); // 自定义函数获取库存

if ($quantity > $stock) {
    die("库存不足");
}

if (isset($_SESSION['cart'][$productId])) {
    $_SESSION['cart'][$productId] += $quantity;
} else {
    $_SESSION['cart'][$productId] = $quantity;
}

$action = $_GET['action'] ?? '';
$productId = (int)$_GET['id'];

if (!isset($_SESSION['cart'][$productId])) {
    die("商品不在购物车中");
}

switch ($action) {
    case 'add':
        $newQty = $_SESSION['cart'][$productId] + 1;
        if ($newQty > getProductStock($productId)) {
            echo "增加后超出库存";
        } else {
            $_SESSION['cart'][$productId] = $newQty;
        }
        break;

    case 'reduce':
        if ($_SESSION['cart'][$productId] > 1) {
            $_SESSION['cart'][$productId]--;
        } else {
            unset($_SESSION['cart'][$productId]); // 数量为1时点减少则移除
        }
        break;

    case 'remove':
        unset($_SESSION['cart'][$productId]);
        break;
}

function getProductStock($productId) {
    global $pdo;
    $stmt = $pdo->prepare("SELECT stock FROM products WHERE id = ? FOR UPDATE");
    $stmt->execute([$productId]);
    return $stmt->fetchColumn() ?: 0;
}

function updateCart(action, id) {
    fetch(`cart.php?action=${action}&id=${id}`)
      .then(res => res.json())
      .then(data => {
          if (data.success) {
              document.getElementById(`qty-${id}`).textContent = data.qty;
          } else {
              alert(data.message);
          }
      });
}

echo json_encode([
    'success' => true,
    'qty'     => $_SESSION['cart'][$productId],
    'message' => '更新成功'
]);