//-----------------------------------------------------------------------
// <copyright file="MemoryNonceStore.cs" company="Outercurve Foundation">
//     Copyright (c) Outercurve Foundation. All rights reserved.
// </copyright>
//-----------------------------------------------------------------------

namespace DotNetOpenAuth.Messaging.Bindings {
	using System;
	using System.Collections.Generic;
	using System.Linq;

	/// <summary>
	/// An in-memory nonce store.  Useful for single-server web applications.
	/// NOT for web farms.
	/// </summary>
	internal class MemoryNonceStore : INonceStore {
		/// <summary>
		/// How frequently we should take time to clear out old nonces.
		/// </summary>
		private const int AutoCleaningFrequency = 10;

		/// <summary>
		/// The maximum age a message can be before it is discarded.
		/// </summary>
		/// <remarks>
		/// This is useful for knowing how long used nonces must be retained.
		/// </remarks>
		private readonly TimeSpan maximumMessageAge;

		/// <summary>
		/// A list of the consumed nonces.
		/// </summary>
		private readonly SortedDictionary<DateTime, List<string>> usedNonces = new SortedDictionary<DateTime, List<string>>();

		/// <summary>
		/// A lock object used around accesses to the <see cref="usedNonces"/> field.
		/// </summary>
		private object nonceLock = new object();

		/// <summary>
		/// Where we're currently at in our periodic nonce cleaning cycle.
		/// </summary>
		private int nonceClearingCounter;

		/// <summary>
		/// Initializes a new instance of the <see cref="MemoryNonceStore"/> class.
		/// </summary>
		internal MemoryNonceStore()
			: this(StandardExpirationBindingElement.MaximumMessageAge) {
		}

		/// <summary>
		/// Initializes a new instance of the <see cref="MemoryNonceStore"/> class.
		/// </summary>
		/// <param name="maximumMessageAge">The maximum age a message can be before it is discarded.</param>
		internal MemoryNonceStore(TimeSpan maximumMessageAge) {
			this.maximumMessageAge = maximumMessageAge;
		}

		#region INonceStore Members

		/// <summary>
		/// Stores a given nonce and timestamp.
		/// </summary>
		/// <param name="context">The context, or namespace, within which the <paramref name="nonce"/> must be unique.</param>
		/// <param name="nonce">A series of random characters.</param>
		/// <param name="timestamp">The timestamp that together with the nonce string make it unique.
		/// The timestamp may also be used by the data store to clear out old nonces.</param>
		/// <returns>
		/// True if the nonce+timestamp (combination) was not previously in the database.
		/// False if the nonce was stored previously with the same timestamp.
		/// </returns>
		/// <remarks>
		/// The nonce must be stored for no less than the maximum time window a message may
		/// be processed within before being discarded as an expired message.
		/// If the binding element is applicable to your channel, this expiration window
		/// is retrieved or set using the
		/// <see cref="StandardExpirationBindingElement.MaximumMessageAge"/> property.
		/// </remarks>
		public bool StoreNonce(string context, string nonce, DateTime timestamp) {
			if (timestamp.ToUniversalTimeSafe() + this.maximumMessageAge < DateTime.UtcNow) {
				// The expiration binding element should have taken care of this, but perhaps
				// it's at the boundary case.  We should fail just to be safe.
				return false;
			}

			// We just concatenate the context with the nonce to form a complete, namespace-protected nonce.
			string completeNonce = context + "\0" + nonce;

			lock (this.nonceLock) {
				List<string> nonces;
				if (!this.usedNonces.TryGetValue(timestamp, out nonces)) {
					this.usedNonces[timestamp] = nonces = new List<string>(4);
				}

				if (nonces.Contains(completeNonce)) {
					return false;
				}

				nonces.Add(completeNonce);

				// Clear expired nonces if it's time to take a moment to do that.
				// Unchecked so that this can int overflow without an exception.
				unchecked {
					this.nonceClearingCounter++;
				}
				if (this.nonceClearingCounter % AutoCleaningFrequency == 0) {
					this.ClearExpiredNonces();
				}

				return true;
			}
		}

		#endregion

		/// <summary>
		/// Clears consumed nonces from the cache that are so old they would be
		/// rejected if replayed because it is expired.
		/// </summary>
		public void ClearExpiredNonces() {
			lock (this.nonceLock) {
				var oldNonceLists = this.usedNonces.Keys.Where(time => time.ToUniversalTimeSafe() + this.maximumMessageAge < DateTime.UtcNow).ToList();
				foreach (DateTime time in oldNonceLists) {
					this.usedNonces.Remove(time);
				}

				// Reset the auto-clean counter so that if this method was called externally
				// we don't auto-clean right away.
				this.nonceClearingCounter = 0;
			}
		}
	}
}