DataStructuresCSharp/C#/Datastructures/DoublyLinkedList.cs

using System;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using C_.Datastructures.Nodes;

namespace C_.Datastructures
{
    internal class DoublyLinkedList<T>
    {
        internal DoublyLinkedListNode<T>? Head { get; set; } = default;
        internal DoublyLinkedListNode<T>? Tail { get; set; } = default;
        private int Count { get; set; } = 0;

        public static DoublyLinkedList<T> Create()
        {
            //Create a new empty list
            return new DoublyLinkedList<T>();
        }

        public static DoublyLinkedList<T> Create(T value)
        {
            //Create a new Class with a single item
            return new DoublyLinkedList<T>()
            {
                Head = DoublyLinkedListNode<T>.Create(value, default, default),
                Count = 1
            };
        }


        public static DoublyLinkedList<T> Create(DoublyLinkedList<T> list1, DoublyLinkedList<T> list2)
        {
            //Create a new list from 2 separate lists
            
            DoublyLinkedList<T> list;

            list = list1;
            if (list == default || list.Count == 0) return list2;

            //Find end of list and append fist item of next list
            if (list2 == default || list.Count == 0) return list;

            DoublyLinkedListNode<T>? end = list.Traverse();

            //Connect up pointers at ajoining section
            end!.Next = list2!.Head;
            end!.Next!.Prev = end;
            end = list2.Tail;
            list.Count += list2!.Count;

            return list;
        }

        public T? this[int i]
        {
            get
            {
                //Check Range
                if (i >= Count || i < 0) throw new System.Exception("Error! Index out of Bounds");

                //Return Value
                DoublyLinkedListNode<T>? node = Traverse(i);
                if (node != default) return node.Value;
                return default;
            }
            set
            {
                //Check Range
                if (i >= Count || i < 0) throw new System.Exception("Error! Index out of Bounds");

                //Change Value
                DoublyLinkedListNode<T>? node = Traverse(i);
                node!.Value = value;
            }
        }
        public void Append(T value)
        {
            Count++;

            //Set head to new item if list is empty
            if (Head == null)
            {//Append item to front of list (End as well)
                Head = DoublyLinkedListNode<T>.Create(value, default, default);
                Tail = Head;
                return;
            }

            //Append item to the end of the list
            Tail!.Next = DoublyLinkedListNode<T>.Create(value, default, Tail);
            Tail = Tail.Next;
        }

        public void Insert(int index, T value)
        {
            Count++;
            if (index > Count || index < 0) throw new System.Exception("Error! Index outside of Bounds");

            //Set head to new item if list is empty
            if (index == 0 || Head == null)
            {
                Head = DoublyLinkedListNode<T>.Create(value, Head, default);
                Tail = Head;
                return;
            }

            //Set tail to new item if index is the end
            if (index == Count - 1)
            {
                //Decrement count as it will be be re-incremented once appended
                Count--;
                Append(value);
                return;
            }

            //Fetch point in list and add new item
            DoublyLinkedListNode<T>? node = Traverse(index - 1);
            node!.Next = DoublyLinkedListNode<T>.Create(value, node.Next, node);

            //Create backlink in the list
            if(node.Next.Next != default)
                node.Next.Next.Prev = node.Next;
        }

        public void Delete(int index)
        {
            Count--;
            if (index > Count || index < 0) throw new System.Exception("Error! Index outside of Bounds");

            //Check if we are trying to reference the first item
            if (index == 0 && Head != default)
            {
                Head = Head!.Next;
                if(Head != default) 
                    Head.Prev = default;
                return;
            }

            //Set tail to new item if index is the end
            if (index == Count && Tail != default)
            {
                Tail = Tail!.Prev;

                if(Tail != default) 
                    Tail.Next = default;
                return;
            }

            DoublyLinkedListNode<T>? node = Traverse(index - 1);

            node!.Next = node.Next!.Next;

            //Connect item after to the the item before the node we are deleting
            if (node.Next != default)
            {
                node.Next.Prev = node;                
            }
        }


        private DoublyLinkedListNode<T>? Traverse()
        {
            //Return the final item in the list
            return Tail;
        }

        private DoublyLinkedListNode<T>? Traverse(int i)
        {
            //Determine whether to start at the start or end of the list
            int direction = 1;
            DoublyLinkedListNode<T>? node = Head;

            if (i > (Count/2))
            {
                //reverse direction of search
                direction = -1;
                node = Tail;
                //i becomes the amount of hops left to reach the item
                i = Count - i - 1;
            }

            if (node != null)
            {

                //continue to given point in the list ('i' hops)
                for (int x = 0; x < i; x++)
                {
                    if (direction == 1)
                    {//Going forwards
                        node = node!.Next;
                    }
                    else
                    {
                        node = node!.Prev;
                    }

                }
            }
            return node;
        }
        private DoublyLinkedListNode<T>? Next(DoublyLinkedListNode<T> current)
        {
            if (current != default)
                return current.Next;

            return null;
        }

        private DoublyLinkedListNode<T>? Prev(DoublyLinkedListNode<T> current)
        {
            if (current != default)
                return current.Prev;

            return null;
        }
    }
}