A .Net Trie in C# implementing IDictionary

Recently I decided to write a straight forward solver for a popular word game similar to Boggle. The obvious data structure for such a game is the Trie. When searching the .Net library for such a data structure, I could not find one that suited my needs. Also looking over the internet did not yield the result I desired. I decided to implement my own Trie for this application.

Before we can start implementing the Trie, we need to understand the data structure at a fundamental level. Though often thought of as a neat way to store strings in memory, the structure is much more than that. A Trie is an associative array mapping sets of keys to values. This differs from most associative arrays that map keys to values. In many uses of a Trie, the keys are strings and the values are unused. Even in this configuration a Trie's internal structure yields very useful properties such as finding all sets of keys that share the same prefix. This was the property that I needed for my word game solver, but if we are implementing the structure, we should do it correctly. I decided that a Trie can be mapped to the interface of IDictionary with at least one added method to look up entries that share the same prefix.

Seperating these Trie specific methods into a seperate interface yields:

    /// <summary>
    /// Interface for a data structure that implements a Trie.
    /// </summary>
    /// <typeparam name="TKey">Type of the Key to store. Note each entry in the Trie has multiple Keys represented by an IEnumerable.</typeparam>
    /// <typeparam name="TValue">Type of the Value to store. This type must have a public parameterless constructor.</typeparam>
    public interface ITrie<TKey, TValue> : IDictionary<IEnumerable<TKey>, TValue>
    {
        /// <summary>
        /// Find all KeyValuePairs whose keys have the prefix specified by keys
        /// </summary>
        /// <param name="keys">Keys that all results must begin with</param>
        /// <returns>Collection of KeyValuePairs whose Keys property begins with the supplied keys prefix</returns>
        ICollection<KeyValuePair<IEnumerable<TKey>, TValue>> Suffixes(IEnumerable<TKey> keys);
    }

Notice that I abstracted out the concept of sets of keys from the interface. This cleans up many methods, while still making the user aware via method signatures that entries in the associative array are represented by sets of keys and not keys. Here is what the interface becomes for a Trie implementing ITrie:

    /// <summary>
    /// Implementation of the ITrie interface, representing a collection of keys of keys and values.
    /// </summary>
    /// <typeparam name="TKey">Type of the Key to store. Note each entry in the Trie has multiple Keys represented by an IEnumerable.</typeparam>
    /// <typeparam name="TValue">Type of the Value to store. This type must have a public parameterless constructor.</typeparam>
    public class Trie<TKey, TValue> : ITrie<TKey, TValue> where TValue : new()
    {
        /// <summary>
        /// Initializes a new instance of the Trie class.
        /// </summary>
        public Trie()
        {
            this.root = new Node<TKey, TValue>();
            this.keys = new List<IEnumerable<TKey>>();
            this.values = new List<TValue>();
        }

        #region ICollectionProperties
        /// <summary>
        /// Gets the number of elements contained in the ICollection.
        /// </summary>
        public int Count;

        /// <summary>
        /// Gets a value indicating whether the ICollection is read-only.
        /// </summary>
        public bool IsReadOnly;
        #endregion

        #region IDictionaryProperties
        /// <summary>
        /// Gets an ICollection containing the keys of the IDictionary.
        /// </summary>
        public ICollection<IEnumerable<TKey>> Keys;

        /// <summary>
        /// Gets an ICollection containing the values in the IDictionary.
        /// </summary>
        public ICollection<TValue> Values;
        #endregion

        #region IDictionaryIndexers
        /// <summary>
        /// Gets or sets the element with the specified key.
        /// </summary>
        /// <param name="keys">The key of the element to get or set.</param>
        /// <returns>The element with the specified key.</returns>
        public TValue this[IEnumerable<TKey> keys];
        #endregion

        #region ICollectionMethods
        /// <summary>
        /// Adds an item to the ICollection.
        /// </summary>
        /// <param name="item">The object to add to the ICollection.</param>
        public void Add(KeyValuePair<IEnumerable<TKey>, TValue> item);

        /// <summary>
        /// Removes all items from the ICollection.
        /// </summary>
        public void Clear();

        /// <summary>
        /// Determines whether the ICollection contains a specific value.
        /// </summary>
        /// <param name="item">The object to locate in the ICollection.</param>
        /// <returns>true if item is found in the ICollection; otherwise, false.</returns>
        public bool Contains(KeyValuePair<IEnumerable<TKey>, TValue> item);

        /// <summary>
        /// Copies the elements of the ICollection to an Array, starting at a particular Array index. 
        /// </summary>
        /// <param name="array">The one-dimensional Array that is the destination of the elements copied from ICollection. The Array must have zero-based indexing.</param>
        /// <param name="arrayIndex">The zero-based index in array at which copying begins.</param>
        public void CopyTo(KeyValuePair<IEnumerable<TKey>, TValue>[] array, int arrayIndex);

        /// <summary>
        /// Removes the first occurrence of a specific object from the ICollection. 
        /// </summary>
        /// <param name="item">The object to remove from the ICollection.</param>
        /// <returns>true if item was successfully removed from the ICollection; otherwise, false. This method also returns false if item is not found in the original ICollection.</returns>
        public bool Remove(KeyValuePair<IEnumerable<TKey>, TValue> item);
        #endregion

        #region IDictionaryMethods
        /// <summary>
        /// Adds an element with the provided key and value to the IDictionary.
        /// </summary>
        /// <param name="keys">The object to use as the key of the element to add.</param>
        /// <param name="value">The object to use as the value of the element to add.</param>
        public void Add(IEnumerable<TKey> keys, TValue value);
        
        /// <summary>
        /// Determines whether the IDictionary contains an element with the specified key.
        /// </summary>
        /// <param name="keys">The key to locate in the Dictionary.</param>
        /// <returns>true if the IDictionary contains an element with the key; otherwise, false.</returns>
        public bool ContainsKey(IEnumerable<TKey> keys);

        /// <summary>
        /// Removes the element with the specified key from the IDictionary.
        /// </summary>
        /// <param name="keys">The key of the element to remove.</param>
        /// <returns>true if the element is successfully removed; otherwise, false. This method also returns false if key was not found in the original IDictionary.</returns>
        public bool Remove(IEnumerable<TKey> keys);

        /// <summary>
        /// Gets the value associated with the specified key.
        /// </summary>
        /// <param name="keys">The key whose value to get.</param>
        /// <param name="value">When this method returns, the value associated with the specified key, if the key is found; otherwise, the default value for the type of the value parameter. This parameter is passed uninitialized.</param>
        /// <returns>true if the object that implements IDictionary contains an element with the specified key; otherwise, false.</returns>
        public bool TryGetValue(IEnumerable<TKey> keys, out TValue value);
        #endregion

        #region ITrieMethods
        /// <summary>
        /// Find all KeyValuePairs whose keys have the prefix specified by keys
        /// </summary>
        /// <param name="keys">Keys that all results must begin with</param>
        /// <returns>Collection of KeyValuePairs whose Keys property begins with the supplied keys prefix</returns>
        public ICollection<KeyValuePair<IEnumerable<TKey>, TValue>> Suffixes(IEnumerable<TKey> keys);
        #endregion

        #region IEnumerableMethods
        /// <summary>
        /// Returns an enumerator that iterates through the collection. 
        /// </summary>
        /// <returns>A IEnumerator that can be used to iterate through the collection.</returns>
        public IEnumerator<KeyValuePair<IEnumerable<TKey>, TValue>> GetEnumerator();

        /// <summary>
        /// Returns an enumerator that iterates through a collection.
        /// </summary>
        /// <returns>An IEnumerator object that can be used to iterate through the collection.</returns>
        IEnumerator IEnumerable.GetEnumerator();
        #endregion
}

This makes a nice Trie that is useful not only for simple word game solvers, but also complex associate array uses. For an example use, we can use the word game. Say we are given the prefix "do" and told to determine whether for a given list of words, if there are any words that begin with this prefix. We could use linq, but this is inefficient as we have to do a string comparison on every word in the word list:

words.Where(word => word.StartsWith("do")).Any();

A Trie on the otherhand can simply navigate to a certain node of the prefix tree, and enumerate all decendents. If there are no decendents, then we know there are not any words that start with the given set of keys:

words.Suffixes("do").Count == 0;

I've zipped the library and made availble under the lenient public domain license where available. Please comment on my implementation in the comments! I definitely know we can improve the enumerator to have a smaller memory footprint.

• Trie - A C# implementation implementing to IDictionary