// Copyright (c) 2021, Oracle and/or its affiliates.
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License, version 2.0, as
// published by the Free Software Foundation.
//
// This program is also distributed with certain software (including
// but not limited to OpenSSL) that is licensed under separate terms,
// as designated in a particular file or component or in included license
// documentation.  The authors of MySQL hereby grant you an
// additional permission to link the program and your derivative works
// with the separately licensed software that they have included with
// MySQL.
//
// Without limiting anything contained in the foregoing, this file,
// which is part of MySQL Connector/NET, is also subject to the
// Universal FOSS Exception, version 1.0, a copy of which can be found at
// http://oss.oracle.com/licenses/universal-foss-exception.
//
// This program is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
// See the GNU General Public License, version 2.0, for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software Foundation, Inc.,
// 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA

using MySql.Data.MySqlClient;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;

namespace MySql.Data.Authentication.GSSAPI.Utility
{
    /// <summary>
    /// Gets the Kerberos configuration from the "krb5.conf/krb5.ini" file
    /// </summary>
    internal class KerberosConfig
    {
        private const string KRBCONFIG_WIN = @"C:\ProgramData\MIT\Kerberos5\krb5.ini";
        private const string KRBCONFIG_LINUX = @"/etc/krb5.conf";

        private const char COMMENT_HASH = '#';
        private const char COMMENT_SEMI = ';';
        private const char SECTION_OPEN = '[';
        private const char SECTION_CLOSE = ']';
        private const char GROUP_OPEN = '{';
        private const char GROUP_CLOSE = '}';
        private static readonly char[] Equal = new char[] { '=' };

        static List<KeyValuePair<string, string>> LibDefaults = new List<KeyValuePair<string, string>>();
        static List<KeyValuePair<string, string>> Realms = new List<KeyValuePair<string, string>>();
        static List<KeyValuePair<string, string>> AppDefaults = new List<KeyValuePair<string, string>>();

        private const string SERVICE_NAME = "ldap/";
        private static string Domain;

        private static void ReadConfig()
        {
            StringReader reader;

            // Tries to get Kerberos configuration file path from environment variables, otherwise
            // set the default path.
            bool IsWin = Environment.OSVersion.Platform.ToString().StartsWith("Win");
            string krbConfigPath = Environment.GetEnvironmentVariable("KRB5_CONFIG");

            if (string.IsNullOrEmpty(krbConfigPath))
            {
                if (IsWin)
                {
                    krbConfigPath = KRBCONFIG_WIN;
                }
                else
                {
                    krbConfigPath = KRBCONFIG_LINUX;
                }
            }

            if (File.Exists(krbConfigPath))
            {
                try { reader = new StringReader(File.ReadAllText(krbConfigPath)); }
                catch { throw new MySqlException("Unable to read Kerberos configuration file."); }
            }
            else
            {
                throw new MySqlException("Kerberos configuration file is missing.");
            }

            while (TryReadLine(reader, out string currentLine))
            {
                if (CanSkip(currentLine))
                {
                    continue;
                }

                while (IsSectionLine(currentLine))
                {
                    currentLine = ReadSection(currentLine, reader);
                }
            }
        }

        internal static string GetServicePrincipalName(string username)
        {
            Domain = SplitUserNameDomain(username);
            ReadConfig();

            string kdc;

            // Try to obtain the LDAP server hostname from AppDefaults section first then on the Realms section
            try { kdc = AppDefaults.First(e => e.Key == "ldap_server_host").Value.Trim().Replace("\"", string.Empty).ToLowerInvariant(); }
            catch { kdc = Realms.First(e => e.Key == "kdc").Value.Trim().ToLowerInvariant(); }

            if (kdc.IndexOf('.') > 0)
            {
                kdc = kdc.Substring(0, kdc.IndexOf('.')).ToLowerInvariant();
            }

            return SERVICE_NAME + kdc;
        }

        private static string SplitUserNameDomain(string original)
        {
            if (string.IsNullOrWhiteSpace(original))
            {
                throw new InvalidOperationException("Username cannot be an empty string.");
            }

            var index = original.IndexOf('@');
            string domain;

            if (index == 0)
            {
                throw new InvalidOperationException("Domain cannot be an empty string.");
            }
            else
            {
                domain = original.Substring(index + 1, original.Length - index - 1);
            }

            return domain;
        }

        #region ReadConfigFile
        private static bool TryReadLine(StringReader reader, out string currentLine)
        {
            currentLine = reader.ReadLine();

            if (currentLine == null)
            {
                return false;
            }

            for (var i = 0; i < currentLine.Length; i++)
            {
                if (IsComment(currentLine[i]))
                {
                    currentLine = currentLine.Substring(0, i);
                    break;
                }
            }

            currentLine = currentLine.Trim().Trim('\uFEFF', '\u200B');

            return true;
        }

        private static bool IsComment(char ch)
        {
            return ch.Equals(COMMENT_HASH) || ch.Equals(COMMENT_SEMI);
        }

        private static bool CanSkip(string trimmed)
        {
            if (trimmed.Length == 0)
            {
                return true;
            }

            return IsComment(trimmed[0]);
        }

        private static bool IsSectionLine(string currentLine)
        {
            if (string.IsNullOrWhiteSpace(currentLine))
            {
                return false;
            }

            return currentLine[0] == SECTION_OPEN && currentLine[currentLine.Length - 1] == SECTION_CLOSE;
        }

        private static string ReadSection(string currentLine, StringReader reader)
        {
            string name = currentLine.Substring(1, currentLine.Length - 2).ToLower();

            while (TryReadLine(reader, out currentLine))
            {
                if (CanSkip(currentLine))
                {
                    continue;
                }

                if (currentLine[0] == SECTION_OPEN)
                {
                    break;
                }

                switch (name)
                {
                    case "libdefaults":
                        ReadValue(currentLine, reader, LibDefaults);
                        break;
                    case "realms":
                        ReadValue(currentLine, reader, Realms);
                        break;
                    case "appdefaults":
                        ReadValue(currentLine, reader, AppDefaults);
                        break;
                }
            }

            return currentLine;
        }

        private static void ReadValue(string currentLine, StringReader reader, List<KeyValuePair<string, string>> section)
        {
            if (currentLine.IndexOf(GROUP_OPEN) >= 0)
            {
                ReadValues(currentLine, reader, section);
            }
            else
            {
                var split = currentLine.Split(Equal, 2);

                if (split.Length == 2)
                {
                    section.Add(new KeyValuePair<string, string>(split[0].Trim(), split[1].Trim()));
                }
            }
        }

        private static void ReadValues(string currentLine, StringReader reader, List<KeyValuePair<string, string>> section)
        {
            var split = currentLine.Split(Equal, 2);

            if (split.Length != 2)
            {
                return;
            }

            while (TryReadLine(reader, out currentLine))
            {
                if (CanSkip(currentLine))
                {
                    continue;
                }

                if (currentLine[0] == GROUP_CLOSE)
                {
                    break;
                }

                if (split[0].Trim().Equals(Domain, StringComparison.OrdinalIgnoreCase)
                  || split[0].Trim().Equals("mysql", StringComparison.OrdinalIgnoreCase))
                {
                    ReadValue(currentLine, reader, section);
                }
            }
        }
        #endregion
    }
}