/*
 * Copyright (c) 2024 ETH Zürich, IT Services
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 */

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Timers;
using Microsoft.Win32;
using SafeExamBrowser.Logging.Contracts;
using SafeExamBrowser.SystemComponents.Contracts.Registry;
using SafeExamBrowser.SystemComponents.Contracts.Registry.Events;

namespace SafeExamBrowser.SystemComponents.Registry
{
	public class Registry : IRegistry
	{
		private const int ONE_SECOND = 1000;

		private readonly ILogger logger;
		private readonly ConcurrentDictionary<(string key, string name), object> values;

		private Timer timer;

		public event RegistryValueChangedEventHandler ValueChanged;

		public Registry(ILogger logger)
		{
			this.logger = logger;
			this.values = new ConcurrentDictionary<(string key, string name), object>();
		}

		public void StartMonitoring(string key, string name)
		{
			if (timer?.Enabled != true)
			{
				timer = new Timer(ONE_SECOND);
				timer.AutoReset = true;
				timer.Elapsed += Timer_Elapsed;
				timer.Start();
			}

			var success = TryRead(key, name, out var value);

			values.TryAdd((key, name), value);

			if (success)
			{
				logger.Debug($"Started monitoring value '{name}' from registry key '{key}'. Initial value: '{value}'.");
			}
			else
			{
				logger.Debug($"Started monitoring value '{name}' from registry key '{key}'. Value does currently not exist or initial read failed.");
			}
		}

		public void StopMonitoring()
		{
			values.Clear();

			if (timer != null)
			{
				timer.Stop();
				logger.Debug("Stopped monitoring the registry.");
			}
		}

		public void StopMonitoring(string key, string name)
		{
			values.TryRemove((key, name), out _);
		}

		public bool TryRead(string key, string name, out object value)
		{
			var defaultValue = new object();

			value = default;

			try
			{
				value = Microsoft.Win32.Registry.GetValue(key, name, defaultValue);
			}
			catch (Exception e)
			{
				logger.Error($"Failed to read value '{name}' from registry key '{key}'!", e);
			}

			return value != default && value != defaultValue;
		}

		public bool TryGetNames(string keyName, out IEnumerable<string> names)
		{
			names = default;

			if (TryOpenKey(keyName, out var key))
			{
				using (key)
				{
					try
					{
						names = key.GetValueNames();
					}
					catch (Exception e)
					{
						logger.Error($"Failed to get registry value names for '{keyName}'!", e);
					}
				}
			}
			else
			{
				logger.Warn($"Failed to get names for '{keyName}'.");
			}

			return names != default;
		}

		public bool TryGetSubKeys(string keyName, out IEnumerable<string> subKeys)
		{
			subKeys = default;

			if (TryOpenKey(keyName, out var key))
			{
				using (key)
				{
					try
					{
						subKeys = key.GetSubKeyNames();
					}
					catch (Exception e)
					{
						logger.Error($"Failed to get registry sub key names for '{keyName}'!", e);
					}
				}
			}
			else
			{
				logger.Warn($"Failed to get sub keys for '{keyName}'.");
			}

			return subKeys != default;
		}

		private bool Exists(string key, string name)
		{
			var defaultValue = new object();
			var value = default(object);

			try
			{
				value = Microsoft.Win32.Registry.GetValue(key, name, defaultValue);
			}
			catch (Exception e)
			{
				logger.Error($"Failed to read value '{name}' from registry key '{key}'!", e);
			}

			return value != default && value != defaultValue;
		}

		private void Timer_Elapsed(object sender, ElapsedEventArgs e)
		{
			foreach (var item in values)
			{
				if (Exists(item.Key.key, item.Key.name))
				{
					if (TryRead(item.Key.key, item.Key.name, out var value))
					{
						if (!Equals(item.Value, value))
						{
							logger.Debug($"Value '{item.Key.name}' from registry key '{item.Key.key}' has changed from '{item.Value}' to '{value}'!");
							ValueChanged?.Invoke(item.Key.key, item.Key.name, item.Value, value);
						}
					}
					else
					{
						logger.Error($"Failed to monitor value '{item.Key.name}' from registry key '{item.Key.key}'!");
					}
				}
			}
		}

		private bool TryOpenKey(string keyName, out RegistryKey key)
		{
			key = default;

			try
			{
				if (TryGetHiveForKey(keyName, out var hive))
				{
					if (keyName == hive.Name)
					{
						key = hive;
					}
					else
					{
						key = hive.OpenSubKey(keyName.Replace($@"{hive.Name}\", ""));
					}
				}
				else
				{
					logger.Warn($"Failed to get hive for key '{keyName}'!");
				}
			}
			catch (Exception e)
			{
				logger.Error($"Failed to open registry key '{keyName}'!", e);
			}

			return key != default;
		}

		private bool TryGetHiveForKey(string keyName, out RegistryKey hive)
		{
			var length = keyName.IndexOf('\\');
			var name = length != -1 ? keyName.Substring(0, length).ToUpperInvariant() : keyName.ToUpperInvariant();

			hive = default;

			switch (name)
			{
				case "HKEY_CLASSES_ROOT":
					hive = Microsoft.Win32.Registry.ClassesRoot;
					break;
				case "HKEY_CURRENT_CONFIG":
					hive = Microsoft.Win32.Registry.CurrentConfig;
					break;
				case "HKEY_CURRENT_USER":
					hive = Microsoft.Win32.Registry.CurrentUser;
					break;
				case "HKEY_LOCAL_MACHINE":
					hive = Microsoft.Win32.Registry.LocalMachine;
					break;
				case "HKEY_PERFORMANCE_DATA":
					hive = Microsoft.Win32.Registry.PerformanceData;
					break;
				case "HKEY_USERS":
					hive = Microsoft.Win32.Registry.Users;
					break;
			}

			return hive != default;
		}
	}
}