learning-hour-maker/exercises/slice-with-lambda/solution/Buyers/Buyers.cs

using System;
using System.Collections.Generic;

namespace IceCreamBuyers
{
    /// <summary>
    /// Reference end state of exercise 3 (extract lambda, slide, extract method
    /// with a function parameter — the pure rep of Slice with Lambda).
    /// </summary>
    public class Buyers
    {
        private readonly PriceList priceList = new();
        private readonly List<Buyer> buyers = new()
        {
            new("Anke", new List<IceCream> { IceCream.Strawberry, IceCream.Vanilla }),
            new("Bram", new List<IceCream> { IceCream.Chocolate }),
            new("Celine", new List<IceCream> { IceCream.Vanilla, IceCream.Chocolate }),
        };

        /// <summary>
        /// No-arg overload: callers unchanged, the "real" pricing function baked in.
        /// </summary>
        public int ShoppingListTotal(string buyerName)
        {
            return ShoppingListTotal(ByName(buyerName), priceList.PriceFor);
        }

        /// <summary>
        /// The seam: the pricing source is injected as a function.
        /// Testable: pass any pure function instead of calling the pricing service.
        /// </summary>
        public int ShoppingListTotal(Buyer buyer, Func<IceCream, int> priceFor)
        {
            var total = 0;
            foreach (var flavour in buyer.Favourites)
            {
                total += buyer.ScoopsOf(flavour) * priceFor(flavour);
            }
            return total;
        }

        private Buyer ByName(string name)
        {
            return buyers.Find(b => b.Name == name)
                ?? throw new InvalidOperationException($"Unknown buyer {name}.");
        }
    }
}