using System;
using System.Net;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.HttpOverrides;
using IPNetwork = Microsoft.AspNetCore.HttpOverrides.IPNetwork;

namespace Microsoft.AspNetCore.Builder
{
	/// <summary>
	/// Extensions for the <see cref="IApplicationBuilder"/>.
	/// </summary>
	[System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
	public static class ApplicationBuilderExtensions
	{
		/// <summary>
		/// Adds settings to run behind a reverse proxy (e.g. NginX).
		/// </summary>
		/// <remarks>
		/// A base path (e.g. running in a sub-directory /app) for the application can be defined via <c>ASPNETCORE_APPL_PATH</c> environment variable.
		/// <br/>
		/// <br/>
		/// Additionally you can specify the proxy server by using <paramref name="address"/> or a <paramref name="network"/> when there are multiple proxy servers.
		/// <br/>
		/// When neither <paramref name="address"/> nor <paramref name="network"/> is set, the default subnets are configured:
		/// <list type="bullet">
		/// <item><c>127.0.0.0/8</c></item>
		/// <item><c>::1/128</c></item>
		///
		/// <item><c>10.0.0.0/8</c></item>
		/// <item><c>172.16.0.0/12</c></item>
		/// <item><c>192.168.0.0/16</c></item>
		/// <item><c>fd00::/8</c></item>
		/// </list>
		/// </remarks>
		/// <param name="app">The application builder.</param>
		/// <param name="network">The <see cref="IPNetwork"/> where proxy requests are received from (optional).</param>
		/// <param name="address">The <see cref="IPAddress"/> where proxy requests are received from (optional).</param>
		/// <param name="basePath">A custom base path (optional, <c>ASPNETCORE_APPL_PATH</c> is prefererred).</param>
		public static IApplicationBuilder UseProxyHosting(this IApplicationBuilder app, IPNetwork network = null, IPAddress address = null, string basePath = null)
		{
			string path = Environment.GetEnvironmentVariable("ASPNETCORE_APPL_PATH");
			if (string.IsNullOrWhiteSpace(path))
				path = basePath;

			if (!string.IsNullOrWhiteSpace(path))
				app.UsePathBase(new PathString(path));

			var options = new ForwardedHeadersOptions { ForwardedHeaders = ForwardedHeaders.All };
			options.KnownProxies.Clear();
			options.KnownNetworks.Clear();

			if (network == null && address == null)
			{
				// localhost
				options.KnownNetworks.Add(new IPNetwork(IPAddress.Loopback, 8));
				options.KnownNetworks.Add(new IPNetwork(IPAddress.IPv6Loopback, 128));

				// private IPv4 networks
				// see https://en.wikipedia.org/wiki/Private_network#Private_IPv4_addresses
				options.KnownNetworks.Add(new IPNetwork(IPAddress.Parse("10.0.0.0"), 8));
				options.KnownNetworks.Add(new IPNetwork(IPAddress.Parse("172.16.0.0"), 12));
				options.KnownNetworks.Add(new IPNetwork(IPAddress.Parse("192.168.0.0"), 16));

				// private IPv6 networks
				// see https://en.wikipedia.org/wiki/Private_network#Private_IPv6_addresses
				options.KnownNetworks.Add(new IPNetwork(IPAddress.Parse("fd00::"), 8));
			}

			if (network != null)
				options.KnownNetworks.Add(network);

			if (address != null)
				options.KnownProxies.Add(address);

			app.UseForwardedHeaders(options);

			return app;
		}
	}
}