// 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.Globalization;
using System.Net.Http;
using System.Security.Claims;
using System.Security.Cryptography;
using System.Text;
using System.Text.Encodings.Web;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Authentication.OAuth;
using Microsoft.AspNetCore.WebUtilities;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using Newtonsoft.Json.Linq;

namespace Microsoft.AspNetCore.Authentication.Facebook
{
    public class FacebookHandler : OAuthHandler<FacebookOptions>
    {
        public FacebookHandler(IOptionsMonitor<FacebookOptions> options, ILoggerFactory logger, UrlEncoder encoder, ISystemClock clock)
            : base(options, logger, encoder, clock)
        { }

        protected override async Task<AuthenticationTicket> CreateTicketAsync(ClaimsIdentity identity, AuthenticationProperties properties, OAuthTokenResponse tokens)
        {
            var endpoint = QueryHelpers.AddQueryString(Options.UserInformationEndpoint, "access_token", tokens.AccessToken);
            if (Options.SendAppSecretProof)
            {
                endpoint = QueryHelpers.AddQueryString(endpoint, "appsecret_proof", GenerateAppSecretProof(tokens.AccessToken));
            }
            if (Options.Fields.Count > 0)
            {
                endpoint = QueryHelpers.AddQueryString(endpoint, "fields", string.Join(",", Options.Fields));
            }

            var response = await Backchannel.GetAsync(endpoint, Context.RequestAborted);
            if (!response.IsSuccessStatusCode)
            {
                throw new HttpRequestException($"An error occurred when retrieving Facebook user information ({response.StatusCode}). Please check if the authentication information is correct and the corresponding Facebook Graph API is enabled.");
            }

            var payload = JObject.Parse(await response.Content.ReadAsStringAsync());

            var context = new OAuthCreatingTicketContext(new ClaimsPrincipal(identity), properties, Context, Scheme, Options, Backchannel, tokens, payload);
            context.RunClaimActions();

            await Events.CreatingTicket(context);

            return new AuthenticationTicket(context.Principal, context.Properties, Scheme.Name);

        }

        private string GenerateAppSecretProof(string accessToken)
        {
            using (var algorithm = new HMACSHA256(Encoding.ASCII.GetBytes(Options.AppSecret)))
            {
                var hash = algorithm.ComputeHash(Encoding.ASCII.GetBytes(accessToken));
                var builder = new StringBuilder();
                for (int i = 0; i < hash.Length; i++)
                {
                    builder.Append(hash[i].ToString("x2", CultureInfo.InvariantCulture));
                }
                return builder.ToString();
            }
        }

        protected override string FormatScope()
        {
            // Facebook deviates from the OAuth spec here. They require comma separated instead of space separated.
            // https://developers.facebook.com/docs/reference/dialogs/oauth
            // http://tools.ietf.org/html/rfc6749#section-3.3
            return string.Join(",", Options.Scope);
        }
    }
}