using System;

namespace CSharpVitamins
{
	/// <summary>
	/// Represents a globally unique identifier (GUID) with a 
	/// shorter string value. Sguid
	/// </summary>
	public struct ShortGuid
	{
		#region Static

		/// <summary>
		/// A read-only instance of the ShortGuid class whose value 
		/// is guaranteed to be all zeroes. 
		/// </summary>
		public static readonly ShortGuid Empty = new ShortGuid(Guid.Empty);

		#endregion

		#region Fields

		Guid _guid;
		string _value;

		#endregion

		#region Contructors

		/// <summary>
		/// Creates a ShortGuid from a base64 encoded string
		/// </summary>
		/// <param name="value">The encoded guid as a 
		/// base64 string</param>
		public ShortGuid(string value)
		{
			_value = value;
			_guid = Decode(value);
		}

		/// <summary>
		/// Creates a ShortGuid from a Guid
		/// </summary>
		/// <param name="guid">The Guid to encode</param>
		public ShortGuid(Guid guid)
		{
			_value = Encode(guid);
			_guid = guid;
		}

		#endregion

		#region Properties

		/// <summary>
		/// Gets/sets the underlying Guid
		/// </summary>
		public Guid Guid
		{
			get { return _guid; }
			set
			{
				if (value != _guid)
				{
					_guid = value;
					_value = Encode(value);
				}
			}
		}

		/// <summary>
		/// Gets/sets the underlying base64 encoded string
		/// </summary>
		public string Value
		{
			get { return _value; }
			set
			{
				if (value != _value)
				{
					_value = value;
					_guid = Decode(value);
				}
			}
		}

		#endregion

		#region ToString

		/// <summary>
		/// Returns the base64 encoded guid as a string
		/// </summary>
		/// <returns></returns>
		public override string ToString()
		{
			return _value;
		}

		#endregion

		#region Equals

		/// <summary>
		/// Returns a value indicating whether this instance and a 
		/// specified Object represent the same type and value.
		/// </summary>
		/// <param name="obj">The object to compare</param>
		/// <returns></returns>
		public override bool Equals(object obj)
		{
			if (obj is ShortGuid)
				return _guid.Equals(((ShortGuid)obj)._guid);
			if (obj is Guid)
				return _guid.Equals((Guid)obj);
			if (obj is string)
				return _guid.Equals(((ShortGuid)obj)._guid);
			return false;
		}

		#endregion

		#region GetHashCode

		/// <summary>
		/// Returns the HashCode for underlying Guid.
		/// </summary>
		/// <returns></returns>
		public override int GetHashCode()
		{
			return _guid.GetHashCode();
		}

		#endregion

		#region NewGuid

		/// <summary>
		/// Initialises a new instance of the ShortGuid class
		/// </summary>
		/// <returns></returns>
		public static ShortGuid NewGuid()
		{
			return new ShortGuid(Guid.NewGuid());
		}

		#endregion

		#region Encode

		/// <summary>
		/// Creates a new instance of a Guid using the string value, 
		/// then returns the base64 encoded version of the Guid.
		/// </summary>
		/// <param name="value">An actual Guid string (i.e. not a ShortGuid)</param>
		/// <returns></returns>
		public static string Encode(string value)
		{
			Guid guid = new Guid(value);
			return Encode(guid);
		}

		/// <summary>
		/// Encodes the given Guid as a base64 string that is 22 
		/// characters long.
		/// </summary>
		/// <param name="guid">The Guid to encode</param>
		/// <returns></returns>
		public static string Encode(Guid guid)
		{
			string encoded = Convert.ToBase64String(guid.ToByteArray());
			encoded = encoded
				.Replace("/", "_")
				.Replace("+", "-");
			return encoded.Substring(0, 22);
		}

		#endregion

		#region Decode

		/// <summary>
		/// Decodes the given base64 string
		/// </summary>
		/// <param name="value">The base64 encoded string of a Guid</param>
		/// <returns>A new Guid</returns>
		public static Guid Decode(string value)
		{
			value = value
				.Replace("_", "/")
				.Replace("-", "+");
			byte[] buffer = Convert.FromBase64String(value + "==");
			return new Guid(buffer);
		}

		#endregion

		#region Operators

		/// <summary>
		/// Determines if both ShortGuids have the same underlying 
		/// Guid value.
		/// </summary>
		/// <param name="x"></param>
		/// <param name="y"></param>
		/// <returns></returns>
		public static bool operator ==(ShortGuid x, ShortGuid y)
		{
			if ((object)x == null) return (object)y == null;
			return x._guid == y._guid;
		}

		/// <summary>
		/// Determines if both ShortGuids do not have the 
		/// same underlying Guid value.
		/// </summary>
		/// <param name="x"></param>
		/// <param name="y"></param>
		/// <returns></returns>
		public static bool operator !=(ShortGuid x, ShortGuid y)
		{
			return !(x == y);
		}

		/// <summary>
		/// Implicitly converts the ShortGuid to it's string equivilent
		/// </summary>
		/// <param name="shortGuid"></param>
		/// <returns></returns>
		public static implicit operator string(ShortGuid shortGuid)
		{
			return shortGuid._value;
		}

		/// <summary>
		/// Implicitly converts the ShortGuid to it's Guid equivilent
		/// </summary>
		/// <param name="shortGuid"></param>
		/// <returns></returns>
		public static implicit operator Guid(ShortGuid shortGuid)
		{
			return shortGuid._guid;
		}

		/// <summary>
		/// Implicitly converts the string to a ShortGuid
		/// </summary>
		/// <param name="shortGuid"></param>
		/// <returns></returns>
		public static implicit operator ShortGuid(string shortGuid)
		{
			return new ShortGuid(shortGuid);
		}

		/// <summary>
		/// Implicitly converts the Guid to a ShortGuid 
		/// </summary>
		/// <param name="guid"></param>
		/// <returns></returns>
		public static implicit operator ShortGuid(Guid guid)
		{
			return new ShortGuid(guid);
		}

		#endregion
	}
}