// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Reflection;
using System.Runtime.ExceptionServices;
using Microsoft.AspNetCore.Hosting.Builder;
using Microsoft.AspNetCore.Hosting.Internal;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.DependencyInjection.Extensions;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.ObjectPool;

namespace Microsoft.AspNetCore.Hosting
{
    /// <summary>
    /// A builder for <see cref="IWebHost"/>
    /// </summary>
    public class WebHostBuilder : IWebHostBuilder
    {
        private readonly IHostingEnvironment _hostingEnvironment;
        private readonly List<Action<WebHostBuilderContext, IServiceCollection>> _configureServicesDelegates;

        private IConfiguration _config;
        private WebHostOptions _options;
        private WebHostBuilderContext _context;
        private bool _webHostBuilt;
        private List<Action<WebHostBuilderContext, IConfigurationBuilder>> _configureAppConfigurationBuilderDelegates;

        /// <summary>
        /// Initializes a new instance of the <see cref="WebHostBuilder"/> class.
        /// </summary>
        public WebHostBuilder()
        {
            _hostingEnvironment = new HostingEnvironment();
            _configureServicesDelegates = new List<Action<WebHostBuilderContext, IServiceCollection>>();
            _configureAppConfigurationBuilderDelegates = new List<Action<WebHostBuilderContext, IConfigurationBuilder>>();

            _config = new ConfigurationBuilder()
                .AddEnvironmentVariables(prefix: "ASPNETCORE_")
                .Build();

            if (string.IsNullOrEmpty(GetSetting(WebHostDefaults.EnvironmentKey)))
            {
                // Try adding legacy environment keys, never remove these.
                UseSetting(WebHostDefaults.EnvironmentKey, Environment.GetEnvironmentVariable("Hosting:Environment")
                    ?? Environment.GetEnvironmentVariable("ASPNET_ENV"));
            }

            if (string.IsNullOrEmpty(GetSetting(WebHostDefaults.ServerUrlsKey)))
            {
                // Try adding legacy url key, never remove this.
                UseSetting(WebHostDefaults.ServerUrlsKey, Environment.GetEnvironmentVariable("ASPNETCORE_SERVER.URLS"));
            }

            _context = new WebHostBuilderContext
            {
                Configuration = _config
            };
        }

        /// <summary>
        /// Get the setting value from the configuration.
        /// </summary>
        /// <param name="key">The key of the setting to look up.</param>
        /// <returns>The value the setting currently contains.</returns>
        public string GetSetting(string key)
        {
            return _config[key];
        }

        /// <summary>
        /// Add or replace a setting in the configuration.
        /// </summary>
        /// <param name="key">The key of the setting to add or replace.</param>
        /// <param name="value">The value of the setting to add or replace.</param>
        /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
        public IWebHostBuilder UseSetting(string key, string value)
        {
            _config[key] = value;
            return this;
        }

        /// <summary>
        /// Adds a delegate for configuring additional services for the host or web application. This may be called
        /// multiple times.
        /// </summary>
        /// <param name="configureServices">A delegate for configuring the <see cref="IServiceCollection"/>.</param>
        /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
        public IWebHostBuilder ConfigureServices(Action<IServiceCollection> configureServices)
        {
            if (configureServices == null)
            {
                throw new ArgumentNullException(nameof(configureServices));
            }

            return ConfigureServices((_ , services) => configureServices(services));
        }

        /// <summary>
        /// Adds a delegate for configuring additional services for the host or web application. This may be called
        /// multiple times.
        /// </summary>
        /// <param name="configureServices">A delegate for configuring the <see cref="IServiceCollection"/>.</param>
        /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
        public IWebHostBuilder ConfigureServices(Action<WebHostBuilderContext, IServiceCollection> configureServices)
        {
            if (configureServices == null)
            {
                throw new ArgumentNullException(nameof(configureServices));
            }

            _configureServicesDelegates.Add(configureServices);
            return this;
        }

        /// <summary>
        /// Adds a delegate for configuring the <see cref="IConfigurationBuilder"/> that will construct an <see cref="IConfiguration"/>.
        /// </summary>
        /// <param name="configureDelegate">The delegate for configuring the <see cref="IConfigurationBuilder" /> that will be used to construct an <see cref="IConfiguration" />.</param>
        /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
        /// <remarks>
        /// The <see cref="IConfiguration"/> and <see cref="ILoggerFactory"/> on the <see cref="WebHostBuilderContext"/> are uninitialized at this stage.
        /// The <see cref="IConfigurationBuilder"/> is pre-populated with the settings of the <see cref="IWebHostBuilder"/>.
        /// </remarks>
        public IWebHostBuilder ConfigureAppConfiguration(Action<WebHostBuilderContext, IConfigurationBuilder> configureDelegate)
        {
            if (configureDelegate == null)
            {
                throw new ArgumentNullException(nameof(configureDelegate));
            }

            _configureAppConfigurationBuilderDelegates.Add(configureDelegate);
            return this;
        }

        /// <summary>
        /// Builds the required services and an <see cref="IWebHost"/> which hosts a web application.
        /// </summary>
        public IWebHost Build()
        {
            if (_webHostBuilt)
            {
                throw new InvalidOperationException(Resources.WebHostBuilder_SingleInstance);
            }
            _webHostBuilt = true;

            // Warn about deprecated environment variables
            if (Environment.GetEnvironmentVariable("Hosting:Environment") != null)
            {
                Console.WriteLine("The environment variable 'Hosting:Environment' is obsolete and has been replaced with 'ASPNETCORE_ENVIRONMENT'");
            }

            if (Environment.GetEnvironmentVariable("ASPNET_ENV") != null)
            {
                Console.WriteLine("The environment variable 'ASPNET_ENV' is obsolete and has been replaced with 'ASPNETCORE_ENVIRONMENT'");
            }

            if (Environment.GetEnvironmentVariable("ASPNETCORE_SERVER.URLS") != null)
            {
                Console.WriteLine("The environment variable 'ASPNETCORE_SERVER.URLS' is obsolete and has been replaced with 'ASPNETCORE_URLS'");
            }

            var hostingServices = BuildCommonServices(out var hostingStartupErrors);
            var applicationServices = hostingServices.Clone();
            var hostingServiceProvider = hostingServices.BuildServiceProvider();

            AddApplicationServices(applicationServices, hostingServiceProvider);

            var host = new WebHost(
                applicationServices,
                hostingServiceProvider,
                _options,
                _config,
                hostingStartupErrors);

            host.Initialize();

            return host;
        }

        private IServiceCollection BuildCommonServices(out AggregateException hostingStartupErrors)
        {
            hostingStartupErrors = null;

            _options = new WebHostOptions(_config, Assembly.GetEntryAssembly()?.GetName().Name);

            if (!_options.PreventHostingStartup)
            {
                var exceptions = new List<Exception>();

                // Execute the hosting startup assemblies
                foreach (var assemblyName in _options.HostingStartupAssemblies)
                {
                    try
                    {
                        var assembly = Assembly.Load(new AssemblyName(assemblyName));

                        foreach (var attribute in assembly.GetCustomAttributes<HostingStartupAttribute>())
                        {
                            var hostingStartup = (IHostingStartup)Activator.CreateInstance(attribute.HostingStartupType);
                            hostingStartup.Configure(this);
                        }
                    }
                    catch (Exception ex)
                    {
                        // Capture any errors that happen during startup
                        exceptions.Add(new InvalidOperationException($"Startup assembly {assemblyName} failed to execute. See the inner exception for more details.", ex));
                    }
                }

                if (exceptions.Count > 0)
                {
                    hostingStartupErrors = new AggregateException(exceptions);
                }
            }

            var contentRootPath = ResolveContentRootPath(_options.ContentRootPath, AppContext.BaseDirectory);

            // Initialize the hosting environment
            _hostingEnvironment.Initialize(contentRootPath, _options);
            _context.HostingEnvironment = _hostingEnvironment;

            var services = new ServiceCollection();
            services.AddSingleton(_hostingEnvironment);
            services.AddSingleton(_context);

            var builder = new ConfigurationBuilder()
                .SetBasePath(_hostingEnvironment.ContentRootPath)
                .AddInMemoryCollection(_config.AsEnumerable());

            foreach (var configureAppConfiguration in _configureAppConfigurationBuilderDelegates)
            {
                configureAppConfiguration(_context, builder);
            }

            var configuration = builder.Build();
            services.AddSingleton<IConfiguration>(configuration);
            _context.Configuration = configuration;

            var listener = new DiagnosticListener("Microsoft.AspNetCore");
            services.AddSingleton<DiagnosticListener>(listener);
            services.AddSingleton<DiagnosticSource>(listener);

            services.AddTransient<IApplicationBuilderFactory, ApplicationBuilderFactory>();
            services.AddTransient<IHttpContextFactory, HttpContextFactory>();
            services.AddScoped<IMiddlewareFactory, MiddlewareFactory>();
            services.AddOptions();
            services.AddLogging();

            // Conjure up a RequestServices
            services.AddTransient<IStartupFilter, AutoRequestServicesStartupFilter>();
            services.AddTransient<IServiceProviderFactory<IServiceCollection>, DefaultServiceProviderFactory>();

            // Ensure object pooling is available everywhere.
            services.AddSingleton<ObjectPoolProvider, DefaultObjectPoolProvider>();

            if (!string.IsNullOrEmpty(_options.StartupAssembly))
            {
                try
                {
                    var startupType = StartupLoader.FindStartupType(_options.StartupAssembly, _hostingEnvironment.EnvironmentName);

                    if (typeof(IStartup).GetTypeInfo().IsAssignableFrom(startupType.GetTypeInfo()))
                    {
                        services.AddSingleton(typeof(IStartup), startupType);
                    }
                    else
                    {
                        services.AddSingleton(typeof(IStartup), sp =>
                        {
                            var hostingEnvironment = sp.GetRequiredService<IHostingEnvironment>();
                            var methods = StartupLoader.LoadMethods(sp, startupType, hostingEnvironment.EnvironmentName);
                            return new ConventionBasedStartup(methods);
                        });
                    }
                }
                catch (Exception ex)
                {
                    var capture = ExceptionDispatchInfo.Capture(ex);
                    services.AddSingleton<IStartup>(_ =>
                    {
                        capture.Throw();
                        return null;
                    });
                }
            }

            foreach (var configureServices in _configureServicesDelegates)
            {
                configureServices(_context, services);
            }

            return services;
        }

        private void AddApplicationServices(IServiceCollection services, IServiceProvider hostingServiceProvider)
        {
            // We are forwarding services from hosting contrainer so hosting container
            // can still manage their lifetime (disposal) shared instances with application services.
            // NOTE: This code overrides original services lifetime. Instances would always be singleton in
            // application container.
            var listener = hostingServiceProvider.GetService<DiagnosticListener>();
            services.Replace(ServiceDescriptor.Singleton(typeof(DiagnosticListener), listener));
            services.Replace(ServiceDescriptor.Singleton(typeof(DiagnosticSource), listener));
        }

        private string ResolveContentRootPath(string contentRootPath, string basePath)
        {
            if (string.IsNullOrEmpty(contentRootPath))
            {
                return basePath;
            }
            if (Path.IsPathRooted(contentRootPath))
            {
                return contentRootPath;
            }
            return Path.Combine(Path.GetFullPath(basePath), contentRootPath);
        }
    }
}