using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.IO;
using System.Threading;
using System.Threading.Tasks;
using Xamarin.Forms.Internals;

namespace Xamarin.Forms
{
	public sealed class UriImageSource : ImageSource
	{
		internal const string CacheName = "ImageLoaderCache";

		public static readonly BindableProperty UriProperty = BindableProperty.Create("Uri", typeof(Uri), typeof(UriImageSource), default(Uri),
			propertyChanged: (bindable, oldvalue, newvalue) => ((UriImageSource)bindable).OnUriChanged(), validateValue: (bindable, value) => value == null || ((Uri)value).IsAbsoluteUri);

		static readonly IIsolatedStorageFile Store = Device.PlatformServices.GetUserStoreForApplication();

		static readonly object s_syncHandle = new object();
		static readonly Dictionary<string, LockingSemaphore> s_semaphores = new Dictionary<string, LockingSemaphore>();

		TimeSpan _cacheValidity = TimeSpan.FromDays(1);

		bool _cachingEnabled = true;

		static UriImageSource()
		{
			if (!Store.GetDirectoryExistsAsync(CacheName).Result)
				Store.CreateDirectoryAsync(CacheName).Wait();
		}

		public TimeSpan CacheValidity
		{
			get { return _cacheValidity; }
			set
			{
				if (_cacheValidity == value)
					return;

				OnPropertyChanging();
				_cacheValidity = value;
				OnPropertyChanged();
			}
		}

		public bool CachingEnabled
		{
			get { return _cachingEnabled; }
			set
			{
				if (_cachingEnabled == value)
					return;

				OnPropertyChanging();
				_cachingEnabled = value;
				OnPropertyChanged();
			}
		}

		[TypeConverter(typeof(UriTypeConverter))]
		public Uri Uri
		{
			get { return (Uri)GetValue(UriProperty); }
			set { SetValue(UriProperty, value); }
		}

		[EditorBrowsable(EditorBrowsableState.Never)]
		public async Task<Stream> GetStreamAsync(CancellationToken userToken = default(CancellationToken))
		{
			OnLoadingStarted();
			userToken.Register(CancellationTokenSource.Cancel);
			Stream stream;

			try
			{
				stream = await GetStreamAsync(Uri, CancellationTokenSource.Token);
				OnLoadingCompleted(false);
			}
			catch (OperationCanceledException)
			{
				OnLoadingCompleted(true);
				throw;
			}
			catch (Exception ex)
			{
				Log.Warning("Image Loading", $"Error getting stream for {Uri}: {ex}");
				throw;
			}

			return stream;
		}

		static string GetCacheKey(Uri uri)
		{
			return Device.PlatformServices.GetMD5Hash(uri.AbsoluteUri);
		}

		async Task<bool> GetHasLocallyCachedCopyAsync(string key, bool checkValidity = true)
		{
			DateTime now = DateTime.UtcNow;
			DateTime? lastWriteTime = await GetLastWriteTimeUtcAsync(key).ConfigureAwait(false);
			return lastWriteTime.HasValue && now - lastWriteTime.Value < CacheValidity;
		}

		static async Task<DateTime?> GetLastWriteTimeUtcAsync(string key)
		{
			string path = Path.Combine(CacheName, key);
			if (!await Store.GetFileExistsAsync(path).ConfigureAwait(false))
				return null;

			return (await Store.GetLastWriteTimeAsync(path).ConfigureAwait(false)).UtcDateTime;
		}

		async Task<Stream> GetStreamAsync(Uri uri, CancellationToken cancellationToken = default(CancellationToken))
		{
			cancellationToken.ThrowIfCancellationRequested();

			Stream stream;
			if (!CachingEnabled)
			{
				try
				{
					stream = await Device.GetStreamAsync(uri, cancellationToken).ConfigureAwait(false);
				}
				catch (Exception ex) 
				{
					Log.Warning("Image Loading", $"Error getting stream for {Uri}: {ex}");
					stream = null;
				}
			}
			else
				stream = await GetStreamFromCacheAsync(uri, cancellationToken).ConfigureAwait(false);
			return stream;
		}

		async Task<Stream> GetStreamAsyncUnchecked(string key, Uri uri, CancellationToken cancellationToken)
		{
			if (await GetHasLocallyCachedCopyAsync(key).ConfigureAwait(false))
			{
				var retry = 5;
				while (retry >= 0)
				{
					int backoff;
					try
					{
						Stream result = await Store.OpenFileAsync(Path.Combine(CacheName, key), FileMode.Open, FileAccess.Read).ConfigureAwait(false);
						return result;
					}
					catch (IOException)
					{
						// iOS seems to not like 2 readers opening the file at the exact same time, back off for random amount of time
						backoff = new Random().Next(1, 5);
						retry--;
					}

					if (backoff > 0)
					{
						await Task.Delay(backoff);
					}
				}
				return null;
			}

			Stream stream;
			try
			{
				stream = await Device.GetStreamAsync(uri, cancellationToken).ConfigureAwait(false);
				if (stream == null)
					return null;
			}
			catch (Exception ex)
			{
				Log.Warning("Image Loading", $"Error getting stream for {Uri}: {ex}");
				return null;
			}

			Stream writeStream = await Store.OpenFileAsync(Path.Combine(CacheName, key), FileMode.Create, FileAccess.Write).ConfigureAwait(false);
			await stream.CopyToAsync(writeStream, 16384, cancellationToken).ConfigureAwait(false);
			if (writeStream != null)
				writeStream.Dispose();

			stream.Dispose();

			return await Store.OpenFileAsync(Path.Combine(CacheName, key), FileMode.Open, FileAccess.Read).ConfigureAwait(false);
		}

		async Task<Stream> GetStreamFromCacheAsync(Uri uri, CancellationToken cancellationToken)
		{
			string key = GetCacheKey(uri);
			LockingSemaphore sem;
			lock (s_syncHandle)
			{
				if (s_semaphores.ContainsKey(key))
					sem = s_semaphores[key];
				else
					s_semaphores.Add(key, sem = new LockingSemaphore(1));
			}

			try
			{
				await sem.WaitAsync(cancellationToken);
				Stream stream = await GetStreamAsyncUnchecked(key, uri, cancellationToken);
				if (stream == null)
				{
					sem.Release();
					return null;
				}
				var wrapped = new StreamWrapper(stream);
				wrapped.Disposed += (o, e) => sem.Release();
				return wrapped;
			}
			catch (OperationCanceledException)
			{
				sem.Release();
				throw;
			}
		}

		void OnUriChanged()
		{
			if (CancellationTokenSource != null)
				CancellationTokenSource.Cancel();
			OnSourceChanged();
		}
	}
}