using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.Encodings.Web;
using System.Threading.Tasks;

using AuthStudy.Authentication.Basic.Events;

using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using Microsoft.Net.Http.Headers;

namespace AuthStudy.Authentication.Basic
{
    public class BasicAuthenticationHandler : AuthenticationHandler<BasicAuthenticationOptions>
    {
        private const string _Scheme = "BasicScheme";

        private readonly UTF8Encoding _utf8ValidatingEncoding = new UTF8Encoding(false, true);

        public BasicAuthenticationHandler
        (
            IOptionsMonitor<BasicAuthenticationOptions> options,
            ILoggerFactory logger,
            UrlEncoder encoder,
            ISystemClock clock
        )
         : base(options, logger, encoder, clock)
        {
        }

        protected new BasicAuthenticationEvents? Events
        {
            get { return (BasicAuthenticationEvents?)base.Events; }
            set { base.Events = value; }
        }

        protected override Task<object> CreateEventsAsync() => Task.FromResult<object>(new BasicAuthenticationEvents());

        protected override async Task<AuthenticateResult> HandleAuthenticateAsync()
        {
            string? authorizationHeader = Request.Headers["Authorization"];
            if (string.IsNullOrEmpty(authorizationHeader))
            {
                return AuthenticateResult.NoResult();
            }

            // Exact match on purpose, rather than using string compare
            // asp.net request parsing will always trim the header and remove trailing spaces
            if (_Scheme == authorizationHeader)
            {
                const string noCredentialsMessage = "Authorization scheme was Basic but the header had no credentials.";
                Logger.LogInformation(noCredentialsMessage);
                return AuthenticateResult.Fail(noCredentialsMessage);
            }

            if (!authorizationHeader.StartsWith(_Scheme + ' ', StringComparison.OrdinalIgnoreCase))
            {
                return AuthenticateResult.NoResult();
            }

            string encodedCredentials = authorizationHeader.Substring(_Scheme.Length).Trim();

            try
            {
                string decodedCredentials = string.Empty;
                byte[] base64DecodedCredentials;
                try
                {
                    base64DecodedCredentials = Convert.FromBase64String(encodedCredentials);
                }
                catch (FormatException)
                {
                    const string failedToDecodeCredentials = "Cannot convert credentials from Base64.";
                    Logger.LogInformation(failedToDecodeCredentials);
                    return AuthenticateResult.Fail(failedToDecodeCredentials);
                }

                try
                {
                    decodedCredentials = _utf8ValidatingEncoding.GetString(base64DecodedCredentials);
                }
                catch (Exception ex)
                {
                    const string failedToDecodeCredentials = "Cannot build credentials from decoded base64 value, exception {ex.Message} encountered.";
                    Logger.LogInformation(failedToDecodeCredentials, ex.Message);
                    return AuthenticateResult.Fail(ex.Message);
                }

                var delimiterIndex = decodedCredentials.IndexOf(":", StringComparison.OrdinalIgnoreCase);
                if (delimiterIndex == -1)
                {
                    const string missingDelimiterMessage = "Invalid credentials, missing delimiter.";
                    Logger.LogInformation(missingDelimiterMessage);
                    return AuthenticateResult.Fail(missingDelimiterMessage);
                }

                var username = decodedCredentials.Substring(0, delimiterIndex);
                var password = decodedCredentials.Substring(delimiterIndex + 1);

                var validateCredentialsContext = new ValidateCredentialsContext(Context, Scheme, Options)
                {
                    Username = username,
                    Password = password
                };

                if (Events != null)
                {
                    await Events.ValidateCredentials(validateCredentialsContext);
                }


                if (validateCredentialsContext.Result != null &&
                    validateCredentialsContext.Result.Succeeded)
                {
                    var ticket = new AuthenticationTicket(validateCredentialsContext.Principal, Scheme.Name);
                    return AuthenticateResult.Success(ticket);
                }

                if (validateCredentialsContext.Result != null &&
                    validateCredentialsContext.Result.Failure != null)
                {
                    return AuthenticateResult.Fail(validateCredentialsContext.Result.Failure);
                }

                return AuthenticateResult.NoResult();
            }
            catch (Exception ex)
            {
                var authenticationFailedContext = new BasicAuthenticationFailedContext(Context, Scheme, Options)
                {
                    Exception = ex
                };

                if (Events != null)
                {
                    await Events.AuthenticationFailed(authenticationFailedContext).ConfigureAwait(true);
                }

                if (authenticationFailedContext.Result != null)
                {
                    return authenticationFailedContext.Result;
                }

                throw;
            }
        }

        protected override Task HandleChallengeAsync(AuthenticationProperties properties)
        {
            if (!Request.IsHttps && !Options.AllowInsecureProtocol)
            {
                const string insecureProtocolMessage = "Request is HTTP, Basic Authentication will not respond.";
                Logger.LogInformation(insecureProtocolMessage);
                Response.StatusCode = StatusCodes.Status421MisdirectedRequest;
            }
            else
            {
                Response.StatusCode = 401;
                if (!Options.SuppressWWWAuthenticateHeader)
                {
                    var headerValue = _Scheme + $" realm=\"{Options.Realm}\"";
                    Response.Headers.Append(HeaderNames.WWWAuthenticate, headerValue);
                }
            }

            return Task.CompletedTask;
        }
    }
}