Initial commit

solution/Buyers/Buyers.cs

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+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}.");
+        }
+    }
+}