A splay tree in C# for .NET, fully implementing IDictionary<TKey, TValue>.

SplayTree`2.cs

/* ---- MIT LICENSE ----
Copyright (c) 2011 John Gietzen

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
------------------------*/

using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;

/// <summary>
/// Implementes a splay tree.
/// </summary>
/// <typeparam name="TKey">The type of keys associated with items in the tree.</typeparam>
/// <typeparam name="TValue">The type of values stored in the tree.</typeparam>
public class SplayTreeDictionary<TKey, TValue> : IDictionary<TKey, TValue>
{
    private int count;
    private IComparer<TKey> keyComparer;
    private SplayTreeNode root;
    private int version = 0;

    public SplayTreeDictionary()
        : this(Comparer<TKey>.Default)
    {
    }

    public SplayTreeDictionary(IComparer<TKey> keyComparer)
    {
        this.keyComparer = keyComparer;
    }

    /// <inheritdoc/>
    public int Count => this.count;

    /// <inheritdoc/>
    public bool IsReadOnly => false;

    /// <inheritdoc/>
    public ICollection<TKey> Keys => new TiedList<TKey>(this, this.version, this.AsList(node => node.Key));

    /// <inheritdoc/>
    public ICollection<TValue> Values => new TiedList<TValue>(this, this.version, this.AsList(node => node.Value));

    /// <inheritdoc/>
    public TValue this[TKey key]
    {
        get
        {
            if (this.count == 0)
            {
                throw new KeyNotFoundException("The key was not found in the tree.");
            }

            this.Splay(key);
            if (this.keyComparer.Compare(key, this.root.Key) != 0)
            {
                throw new KeyNotFoundException("The key was not found in the tree.");
            }

            return this.root.Value;
        }

        set
        {
            this.Set(key, value, throwOnExisting: false);
        }
    }

    /// <inheritdoc/>
    public void Add(TKey key, TValue value) => this.Set(key, value, throwOnExisting: true);

    /// <inheritdoc/>
    public void Add(KeyValuePair<TKey, TValue> item) => this.Set(item.Key, item.Value, throwOnExisting: true);

    /// <inheritdoc/>
    public void Clear()
    {
        this.root = null;
        this.count = 0;
        this.version++;
    }

    /// <inheritdoc/>
    public bool Contains(KeyValuePair<TKey, TValue> item)
    {
        if (this.count == 0)
        {
            return false;
        }

        this.Splay(item.Key);

        return this.keyComparer.Compare(item.Key, this.root.Key) == 0 && (object.ReferenceEquals(this.root.Value, item.Value) || (!object.ReferenceEquals(item.Value, null) && item.Value.Equals(this.root.Value)));
    }

    /// <inheritdoc/>
    public bool ContainsKey(TKey key)
    {
        if (this.count == 0)
        {
            return false;
        }

        this.Splay(key);

        return this.keyComparer.Compare(key, this.root.Key) == 0;
    }

    /// <inheritdoc/>
    public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
    {
        this.AsList(node => new KeyValuePair<TKey, TValue>(node.Key, node.Value)).CopyTo(array, arrayIndex);
    }

    /// <inheritdoc/>
    public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
    {
        return new TiedList<KeyValuePair<TKey, TValue>>(this, this.version, this.AsList(node => new KeyValuePair<TKey, TValue>(node.Key, node.Value))).GetEnumerator();
    }

    /// <inheritdoc/>
    IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();

    /// <inheritdoc/>
    public bool Remove(TKey key)
    {
        if (this.count == 0)
        {
            return false;
        }

        this.Splay(key);

        if (this.keyComparer.Compare(key, this.root.Key) != 0)
        {
            return false;
        }

        if (this.root.LeftChild == null)
        {
            this.root = this.root.RightChild;
        }
        else
        {
            var swap = this.root.RightChild;
            this.root = this.root.LeftChild;
            this.Splay(key);
            this.root.RightChild = swap;
        }

        this.version++;
        this.count--;
        return true;
    }

    /// <inheritdoc/>
    public bool Remove(KeyValuePair<TKey, TValue> item)
    {
        if (this.count == 0)
        {
            return false;
        }

        this.Splay(item.Key);

        if (this.keyComparer.Compare(item.Key, this.root.Key) == 0 && (object.ReferenceEquals(this.root.Value, item.Value) || (!object.ReferenceEquals(item.Value, null) && item.Value.Equals(this.root.Value))))
        {
            return false;
        }

        if (this.root.LeftChild == null)
        {
            this.root = this.root.RightChild;
        }
        else
        {
            var swap = this.root.RightChild;
            this.root = this.root.LeftChild;
            this.Splay(item.Key);
            this.root.RightChild = swap;
        }

        this.version++;
        this.count--;
        return true;
    }

    /// <summary>
    /// Removes items from the tree based on how deep they are in the tree.
    /// </summary>
    /// <param name="depth">The depth at which to prune the tree.</param>
    public void Trim(int depth)
    {
        if (depth < 0)
        {
            throw new ArgumentOutOfRangeException(nameof(depth), "The trim depth must not be negative.");
        }

        if (this.count == 0)
        {
            return;
        }

        if (depth == 0)
        {
            this.Clear();
        }
        else
        {
            var prevCount = this.count;
            this.count = this.Trim(this.root, depth - 1);
            if (prevCount != this.count)
            {
                this.version++;
            }
        }
    }

    /// <inheritdoc/>
    public bool TryGetValue(TKey key, out TValue value)
    {
        if (this.count == 0)
        {
            value = default(TValue);
            return false;
        }

        this.Splay(key);
        if (this.keyComparer.Compare(key, this.root.Key) != 0)
        {
            value = default(TValue);
            return false;
        }

        value = this.root.Value;
        return true;
    }

    private IList<TEnumerator> AsList<TEnumerator>(Func<SplayTreeNode, TEnumerator> selector)
    {
        if (this.root == null)
        {
            return Array.Empty<TEnumerator>();
        }

        var result = new List<TEnumerator>(this.count);
        this.PopulateList(this.root, result, selector);
        return result;
    }

    private void PopulateList<TEnumerator>(SplayTreeNode node, List<TEnumerator> list, Func<SplayTreeNode, TEnumerator> selector)
    {
        if (node.LeftChild != null)
        {
            this.PopulateList(node.LeftChild, list, selector);
        }

        list.Add(selector(node));
        if (node.RightChild != null)
        {
            this.PopulateList(node.RightChild, list, selector);
        }
    }

    private void Set(TKey key, TValue value, bool throwOnExisting)
    {
        if (this.count == 0)
        {
            this.version++;
            this.root = new SplayTreeNode(key, value);
            this.count = 1;
            return;
        }

        this.Splay(key);

        var c = this.keyComparer.Compare(key, this.root.Key);
        if (c == 0)
        {
            if (throwOnExisting)
            {
                throw new ArgumentException("An item with the same key already exists in the tree.");
            }

            this.version++;
            this.root.Value = value;
            return;
        }

        var n = new SplayTreeNode(key, value);
        if (c < 0)
        {
            n.LeftChild = this.root.LeftChild;
            n.RightChild = this.root;
            this.root.LeftChild = null;
        }
        else
        {
            n.RightChild = this.root.RightChild;
            n.LeftChild = this.root;
            this.root.RightChild = null;
        }

        this.root = n;
        this.count++;
        this.Splay(key);
        this.version++;
    }

    private void Splay(TKey key)
    {
        SplayTreeNode l, r, t, y, header;
        l = r = header = new SplayTreeNode(default(TKey), default(TValue));
        t = this.root;
        while (true)
        {
            var c = this.keyComparer.Compare(key, t.Key);
            if (c < 0)
            {
                if (t.LeftChild == null)
                {
                    break;
                }

                if (this.keyComparer.Compare(key, t.LeftChild.Key) < 0)
                {
                    y = t.LeftChild;
                    t.LeftChild = y.RightChild;
                    y.RightChild = t;
                    t = y;
                    if (t.LeftChild == null)
                    {
                        break;
                    }
                }

                r.LeftChild = t;
                r = t;
                t = t.LeftChild;
            }
            else if (c > 0)
            {
                if (t.RightChild == null)
                {
                    break;
                }

                if (this.keyComparer.Compare(key, t.RightChild.Key) > 0)
                {
                    y = t.RightChild;
                    t.RightChild = y.LeftChild;
                    y.LeftChild = t;
                    t = y;
                    if (t.RightChild == null)
                    {
                        break;
                    }
                }

                l.RightChild = t;
                l = t;
                t = t.RightChild;
            }
            else
            {
                break;
            }
        }

        l.RightChild = t.LeftChild;
        r.LeftChild = t.RightChild;
        t.LeftChild = header.RightChild;
        t.RightChild = header.LeftChild;
        this.root = t;
    }

    private int Trim(SplayTreeNode node, int depth)
    {
        if (depth == 0)
        {
            node.LeftChild = null;
            node.RightChild = null;
            return 1;
        }
        else
        {
            var count = 1;

            if (node.LeftChild != null)
            {
                count += this.Trim(node.LeftChild, depth - 1);
            }

            if (node.RightChild != null)
            {
                count += this.Trim(node.RightChild, depth - 1);
            }

            return count;
        }
    }

    private sealed class SplayTreeNode
    {
        public SplayTreeNode(TKey key, TValue value)
        {
            this.Key = key;
            this.Value = value;
        }

        public TKey Key { get; }

        public SplayTreeNode LeftChild { get; set; }

        public SplayTreeNode RightChild { get; set; }

        public TValue Value { get; set; }
    }

    [DebuggerDisplay("Count = {Count}")]
    private sealed class TiedList<T> : IList<T>
    {
        private readonly IList<T> backingList;
        private readonly SplayTreeDictionary<TKey, TValue> tree;
        private readonly int version;

        public TiedList(SplayTreeDictionary<TKey, TValue> tree, int version, IList<T> backingList)
        {
            this.tree = tree ?? throw new ArgumentNullException(nameof(tree));
            this.version = version;
            this.backingList = backingList ?? throw new ArgumentNullException(nameof(backingList));
        }

        public int Count => this.tree.count;

        public bool IsReadOnly => true;

        public T this[int index]
        {
            get
            {
                if (this.tree.version != this.version)
                {
                    throw new InvalidOperationException("The collection has been modified.");
                }

                return this.backingList[index];
            }

            set
            {
                throw new NotSupportedException();
            }
        }

        public void Add(T item) => throw new NotSupportedException();

        public void Clear() => throw new NotSupportedException();

        public bool Contains(T item)
        {
            if (this.tree.version != this.version)
            {
                throw new InvalidOperationException("The collection has been modified.");
            }

            return this.backingList.Contains(item);
        }

        public void CopyTo(T[] array, int arrayIndex)
        {
            if (this.tree.version != this.version)
            {
                throw new InvalidOperationException("The collection has been modified.");
            }

            this.backingList.CopyTo(array, arrayIndex);
        }

        public IEnumerator<T> GetEnumerator()
        {
            if (this.tree.version != this.version)
            {
                throw new InvalidOperationException("The collection has been modified.");
            }

            foreach (var item in this.backingList)
            {
                yield return item;
                if (this.tree.version != this.version)
                {
                    throw new InvalidOperationException("The collection has been modified.");
                }
            }
        }

        IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();

        public int IndexOf(T item)
        {
            if (this.tree.version != this.version)
            {
                throw new InvalidOperationException("The collection has been modified.");
            }

            return this.backingList.IndexOf(item);
        }

        public void Insert(int index, T item) => throw new NotSupportedException();

        public bool Remove(T item) => throw new NotSupportedException();

        public void RemoveAt(int index) => throw new NotSupportedException();
    }
}