LinkedList.cs

using System.Collections.Generic;
using System.Diagnostics;
using FluentAssertions;
using Xunit;

namespace SompareWithRuby
{
    [DebuggerDisplay("{Payload}")]
    public sealed class Node<T>
    {
        public T Payload;
        public Node<T> Next;
        public Node<T> Prev;
    }

    public static class LinkedListOperations
    {
        public static Node<T> AsNode<T>(this T payload)
        {
            return new Node<T> { Payload = payload };
        }

        public static Node<T> Append<T>(this Node<T> first, T payload)
        {
            var middle = AsNode(payload);
            var last = first.Next;
            first.Next = middle;
            middle.Prev = first;
            middle.Next = last;
            if (last != null) last.Prev = middle;
            return middle;
        }
        public static IEnumerable<T> EnumerateRest<T>(this Node<T> head)
        {
            while (head != null)
            {
                yield return head.Payload;
                head = head.Next;
            }
        }
    }

    public sealed class LinkedListOperationsTests
    {
        [Fact]
        public void AsNode_Should_Set_Payload_Prev_Next_Correctly()
        {
            var node = "blah".AsNode();
            node.Payload.Should().Be("blah");
            node.Prev.Should().BeNull();
            node.Next.Should().BeNull();
        }
        [Fact]
        public void Append_Should_Crete_Two_Interlinked_Nodes()
        {
            var first = 1.AsNode();
            var second = first.Append(2);

            first.Prev.Should().BeNull();
            first.Next.Should().Be(second);

            second.Prev.Should().Be(first);
            second.Next.Should().BeNull();
        }
        [Fact]
        public void Apped_In_The_Middle_Should_Relink_Correctly()
        {
            var first = 1.AsNode();
            var last = first.Append(3);
            var middle = first.Append(2);

            first.Prev.Should().BeNull();
            first.Next.Should().Be(middle);

            middle.Prev.Should().Be(first);
            middle.Next.Should().Be(last);

            last.Prev.Should().Be(middle);
            last.Next.Should().BeNull();
        }

        [Fact]
        public void EnumerateRest_Should_Get_The_Element_And_All_The_Rest()
        {
            var first = 1.AsNode();
            first.Append(2).Append(3);

            string.Join(", ", first.EnumerateRest())
                .Should().Be("1, 2, 3");
        }
    }
}