using System;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Microsoft.AspNetCore.Http
{
	/// <summary>
	/// Implements a basic authentication.
	/// </summary>
	public class BasicAuthMiddleware
	{
		private readonly RequestDelegate next;
		private readonly string realm;
		private readonly Func<string, string, bool> userPasswordAuth;

		/// <summary>
		/// Initializes a new instance of the <see cref="BasicAuthMiddleware"/> class.
		/// </summary>
		/// <param name="next">The following delegate in the process chain.</param>
		/// <param name="realm">The realm to display when requesting for credentials.</param>
		/// <param name="userPasswordAuth">The function <c>(user, passwd) => result</c> to validate username and password.</param>
		public BasicAuthMiddleware(RequestDelegate next, string realm, Func<string, string, bool> userPasswordAuth)
		{
			this.next = next;
			this.realm = realm;
			this.userPasswordAuth = userPasswordAuth;
		}

		/// <summary>
		/// The delegate invokation.
		/// Performs the authentication check.
		/// </summary>
		/// <param name="httpContext">The corresponding HTTP context.</param>
		/// <returns>An awaitable task.</returns>
		public async Task InvokeAsync(HttpContext httpContext)
		{
			if (httpContext.Request.Headers.TryGetValue("Authorization", out var authHeader)
				&& ((string)authHeader).StartsWith("Basic "))
			{
				string encoded = ((string)authHeader).Split(' ', StringSplitOptions.RemoveEmptyEntries).LastOrDefault() ?? "";

				string decoded = Encoding.UTF8.GetString(Convert.FromBase64String(encoded));
				string[] parts = decoded.Split(':');

				if (parts.Length >= 2)
				{
					string username = parts[0].Trim().ToLower();
					string password = parts[1].Trim();

					if (userPasswordAuth(username, password))
					{
						await next.Invoke(httpContext);
						return;
					}
				}
			}

			httpContext.Response.Headers["WWW-Authenticate"] = "Basic";
			if (!string.IsNullOrWhiteSpace(realm))
			{
				httpContext.Response.Headers["WWW-Authenticate"] += $" realm=\"{realm}\"";
			}

			httpContext.Response.StatusCode = StatusCodes.Status401Unauthorized;
		}
	}
}