1.3 The shop

namespace Shopfront.Inventory;
 
// The catalogue and its stock, in memory for now.
public class Catalogue
{
    private readonly Dictionary<string, (decimal Price, int Stock)> items = new()
    {
        ["MUG"] = (12.00m, 10),
        ["TEE"] = (25.00m, 3),
        ["CAP"] = (18.50m, 0),
    };
 
    public decimal? PriceOf(string sku) => items.TryGetValue(sku, out var item) ? item.Price : null;
 
    // Reserves every line or none of them. Returns the first SKU that is short, or null.
    public string? TryReserve(IReadOnlyList<(string Sku, int Quantity)> lines)
    {
        lock (items)
        {
            var shortSku = lines
                .Where(l => items[l.Sku].Stock < l.Quantity)
                .Select(l => l.Sku)
                .FirstOrDefault();
            if (shortSku is not null)
                return shortSku;
 
            foreach (var (sku, quantity) in lines)
                items[sku] = items[sku] with { Stock = items[sku].Stock - quantity };
            return null;
        }
    }
}

namespace Shopfront.Payments;
 
public interface IPaymentProvider
{
    Task<string> Charge(string customerId, decimal amount, CancellationToken cancellationToken);
}

namespace Shopfront.Payments;
 
// Always succeeds. Chapter 2 makes it fail.
public class FakePaymentProvider : IPaymentProvider
{
    public Task<string> Charge(
        string customerId,
        decimal amount,
        CancellationToken cancellationToken
    ) => Task.FromResult($"tx_{Guid.NewGuid():N}"[..11]);
}