using Microsoft.Identity.Client;
using Microsoft.SharePoint.Client;
using System;
using System.Linq;
using System.Threading.Tasks;

namespace SPOAuthMitMSAL
{
    public class AuthService
    {
        private static IPublicClientApplication _app;
        private static AuthenticationResult _authResult;

        private static readonly string[] scopes = new[] { "https://mytenant.sharepoint.com/AllSites.Read" };
        private static readonly string tenantId = "<tenantUUID>";
        private static readonly string clientId = "<EnterpriseAppUUID>";

        /// <summary>
        /// Retrieves an access token, using cache or interactive login if required.
        /// </summary>
        public static async Task<string> GetAccessTokenAsync()
        {
            // Build the MSAL app instance (once)
            if (_app == null)
            {
                Console.WriteLine($"Creating PublicClientApplicationBuilder for Tenant {tenantId} and Client {clientId}");
                _app = PublicClientApplicationBuilder.Create(clientId).WithTenantId(tenantId).WithRedirectUri($"msal{clientId}://auth").Build();
            }

            if (_authResult != null && _authResult.ExpiresOn > DateTimeOffset.UtcNow.AddMinutes(5))
            {
                Console.WriteLine("Using pre existing token");
                return _authResult.AccessToken;
            }

            try
            {
                Console.WriteLine("Enumerating cached accounts and use first if any for silent token acquisition");
                var accounts = await _app.GetAccountsAsync();
                _authResult = await _app.AcquireTokenSilent(scopes, accounts.FirstOrDefault()).ExecuteAsync();
            }
            catch (MsalUiRequiredException)
            {
                try
                {
                    Console.WriteLine("No cached accounts, starting interactive token acquisistion");
                    _authResult = Task.Run(async () => await _app.AcquireTokenInteractive(scopes).WithPrompt(Prompt.SelectAccount).ExecuteAsync()).Result;
                }
                catch (Exception oExc)
                {
                    Console.WriteLine($"AcquireTokenInteractive/WithPrompt failed: {oExc.Message}");
                    throw oExc;
                }
            }

            Console.WriteLine($"Acquired access token successfully");
            return _authResult.AccessToken;
        }

        /// <summary>
        /// Creates a ClientContext with the valid access token automatically injected.
        /// </summary>
        public static ClientContext GetContext(string siteUrl)
        {
            var ctx = new ClientContext(siteUrl);
            ctx.ExecutingWebRequest += async (sender, e) =>
            {
                string token = await GetAccessTokenAsync();
                e.WebRequestExecutor.RequestHeaders["Authorization"] = "Bearer " + token;
            };
            return ctx;
        }

        public static void Main(string[] args)
        {
            Console.WriteLine("Creating context for site");
            ClientContext oContext = GetContext("https://mytenant.sharepoint.com/sites/MySite");

            Console.WriteLine("Load file metadata");
            File oFile = oContext.Web.GetFileByServerRelativeUrl("/sites/MySite/MyList/MyDoc.pdf");
            oContext.Load(oFile);
            oContext.ExecuteQuery();

            Console.WriteLine("Getting file stream");
            ClientResult<System.IO.Stream> oSPOStream = oFile.OpenBinaryStream();
            oContext.ExecuteQuery();
            System.IO.Stream oStream = oSPOStream.Value;
            Console.WriteLine($"Got stream, transfering {oStream.Length} bytes to local file");
            byte[] oBuffer = new byte[oStream.Length];
            oStream.Read(oBuffer, 0, (int)oStream.Length);
            System.IO.File.WriteAllBytes($@"{Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments)}\MyDoc.pdf", oBuffer);
            Console.WriteLine($"Download to '{Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments)}\\MyDoc.pdf' successfull");
        }
    }
}