protected IEnumerable<(string email, double totalCost)> CalculateTotalCostByCustomer(IEnumerable<Order> orders)
{
return orders
.GroupBy(o => o.Email, StringComparer.CurrentCultureIgnoreCase)
.OrderBy(g => g.Key, StringComparer.CurrentCultureIgnoreCase)
.Select(orderGroup =>
{
var totalCostInCents = orderGroup
.SelectMany(o => o.Items)
.Select(i => i.Quantity * ProductCatalog.FindProductByCode(i.ProductCode).PriceInCents)
.Sum();
return (orderGroup.Key, (double) (totalCostInCents / 100));
});
}