Laugh & Learn - Pick Your Poison

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApplication1
{
    public class Program
    {
        public static void Main()
        {
            var ledgerKeeper = new LedgerKeeper();

            ledgerKeeper.Add(new[] { 1m, 2m, -3m, 4m });
            var ledger = ledgerKeeper.GetLedger();
            Console.WriteLine(ledger.Balance);
            
            //ledgerKeeper.Add(new[] { 1m, 2m, -3m, 4m });
            ledger.Add(1m);
            ledger.Add(2m);
            ledger.Add(-3m);
            ledger.Add(4m);
            ledgerKeeper.Commit(ledger);

            ledger = ledgerKeeper.GetLedger();
            Console.WriteLine(ledger.Balance);
            
            Console.ReadKey(true);
        }

        public class Ledger
        {
            public decimal Balance { get; protected set; }

            private List<decimal> newTransactions = new List<decimal>();

            public IEnumerable<decimal> GetNewTransactions()
            {
                return this.newTransactions.ToArray();
            }

            public Ledger(LedgerMemento memento)
            {
                this.Balance = memento.Balance;
            }

            public Ledger()
            {
                this.Balance = 0m;
            }

            public void Add(decimal transaction)
            {
                // we could optionally validate the transaction
                // i.e. - don't allow a negative balance; throw instead
                this.newTransactions.Add(transaction);

                this.Apply(transaction);
            }

            private void Apply(decimal transaction)
            {
                this.Balance += transaction;
            }

            public void Calculate(IEnumerable<decimal> history)
            {
                foreach (var d in history)
                {
                    this.Apply(d);
                }
            }

            public LedgerMemento CreateMemento()
            {
                return new LedgerMemento(this.Balance);
            }
        }

        public class LedgerKeeper
        {
            private List<decimal> history = new List<decimal>();
            private LedgerMemento memento;
            private int mementoIndex;

            public void Add(IEnumerable<decimal> newItems)
            {
                history.AddRange(newItems);
            }

            public Ledger GetLedger()
            {
                Ledger ledger;
                if (memento != null)
                {
                    ledger = new Ledger(memento);
                    ledger.Calculate(history.Skip(mementoIndex));
                }
                else
                {
                    ledger = new Ledger();
                    ledger.Calculate(history);
                }
                return ledger;
            }

            public void Commit(Ledger ledger)
            {
                this.history.AddRange(ledger.GetNewTransactions());
                this.memento = ledger.CreateMemento();
                this.mementoIndex = this.history.Count();
            }
        }

        public class LedgerMemento
        {
            public decimal Balance { get; protected set; }

            public LedgerMemento(decimal balance)
            {
                this.Balance = balance;
            }
        }
    }
}