using System;
using System.Data;
using System.Data.SqlClient;
using Microsoft.AspNetCore.Cryptography.KeyDerivation;

namespace SecureCore.Authentication
{
    public static class PasswordManager
    {
        private static string Pepper { get; set; }
        private static KeyDerivationPrf KeyType { get; } = KeyDerivationPrf.HMACSHA512;
        private static int KeySize { get; } = 512 / 8;
        private static int SaltSize { get; } = 128 / 8; //128 bit salt
        public static string SectionName { get; } = "PasswordSettings";
        private static int MinPasswordLength { get; set; }
        private static int MaxPasswordLength { get; set; }
        private static int Iterations { get; set; }

        //Application enforced absolutes that can not fall short of or be exceeded by the configuartion settings of this application.
        private static int AbsoluteMinPasswordLength { get; } = 16;
        private static int AbsoluteMaxPasswordLength { get; } = 128; 
        private static int AbsoluteMinPepperLength { get; } = 32;
        private static int AbsoluteMinWorkFactor { get; set; } = 10000;

        public static void InitializeSettings()
        {
            if (!string.IsNullOrEmpty(Pepper)) throw new InvalidOperationException("The PasswordManager's settings have already been initialized. Operation aborted.");

            if (AppSettingsManager.TryGetSettingInt($"{SectionName}.MaxLength", out int maxPasswordLength))
            {
                //As noted in this article https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html#maximum-password-lengths
                //allowing passwords that are too long can result in a denial-of-service attack. So we must enforce an upper bound
                //on the length of passwords. The recommended length is between 64 and 128, so if the length is set long than
                //128 characters, just default to 128 as that's a safe upper limit.
                if (maxPasswordLength > AbsoluteMaxPasswordLength || maxPasswordLength < AbsoluteMinPasswordLength) maxPasswordLength = AbsoluteMaxPasswordLength;
            }
            else maxPasswordLength = AbsoluteMaxPasswordLength;

            if (AppSettingsManager.TryGetSettingInt($"{SectionName}.MinLength", out int minPasswordLength))
            {
                //There was no mention of a min password length in the above article, so I've chosen on a whim that 16 should
                //be a safe enough min on a password's length. So as usual, just ignore settings that are out of bounds and 
                //instead stick to safe values that are in bounds.
                if (minPasswordLength < AbsoluteMinPasswordLength || minPasswordLength > AbsoluteMaxPasswordLength) minPasswordLength = AbsoluteMinPasswordLength;
            }
            else minPasswordLength = AbsoluteMinPasswordLength;
            // Validate the settings further.
            if (minPasswordLength == maxPasswordLength || minPasswordLength > maxPasswordLength)
            {
                minPasswordLength = AbsoluteMinPasswordLength;
                maxPasswordLength = AbsoluteMaxPasswordLength;
            }

            MinPasswordLength = minPasswordLength;
            MaxPasswordLength = maxPasswordLength;
            //Now read in the pepper. A pepper being a string of characters at least 32 characters long that is NOT stored in the database and is used in conjunction with hashing sensitive user data.
            if (AppSettingsManager.TryGetSettingString($"{SectionName}.Pepper", out string pepper))
            {
                //As noted here https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html a pepper should be at least 32 bytes in size.
                if (pepper.Length < AbsoluteMinPepperLength) throw new Exception("A pepper must be at least 32 characters long for security reasons.");
            }
            else
                throw new Exception($"A pepper, generated by a cryptographically secure random number generator, that is at least 32 characters long, must be supplied in the appsettings.json file (Key path: {SectionName}.Pepper).");

            Pepper = pepper;
            //Finally, the work factor (A.K.A. iterations) for the hashing algorithm.
            if (AppSettingsManager.TryGetSettingInt($"{SectionName}.Iterations", out int iterations))
            {
                //The work factor must be of a certain strength and if it fails this check then we will be forced to ignore it and use the recommended work factor
                //as stated here: https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html#pbkdf2
                //As stated in the above link, 10,000 iterations is the lowest we should ever go. So for security's sake, that's the lower bounds that will be allowed.
                if (iterations < AbsoluteMinWorkFactor) iterations = AbsoluteMinWorkFactor;
            }
            //By default, we'll go with the highest security setting if one isn't provided by an admin.
            //To quote the above link:
            //"The work factor for PBKDF2 is implemented through the iteration count, which should be at least 10,000 
            //(although values of up to 100,000 may be appropriate in higher security environments)."
            else iterations = 100000;

            Iterations = iterations;
        }

        public static (string Hash, string Salt) HashPassword(string password)
        {
            var salt = new byte[SaltSize];

            ByteGenerator.GetRandomBytes(ref salt);

            return (GetHash(password, salt), Convert.ToBase64String(salt));
        }

        public static (bool IsValid, string Message) IsPasswordValid(string password)
        {
            if (string.IsNullOrEmpty(password)) return (false, "No password has been supplied, and thus is not valid.");
            if (password.Length < MinPasswordLength) return (false, $"Your password is too short, it must be at least {MinPasswordLength} characters long and not exceed {MaxPasswordLength} characters.");
            if (password.Length > MaxPasswordLength) return (false, $"Your password is too long, it must not exceed {MaxPasswordLength} characters and must contain at least {MinPasswordLength} characters.");

            return (true, string.Empty);
        }

        public static bool IsPasswordAMatch(string password, string salt, string passwordHash)
        {
            var saltBytes = Convert.FromBase64String(salt);

            return passwordHash == GetHash(password, saltBytes);
        }

        public static void InsertPasswordResetRequest(string email, string sessionToken, DateTime expirationDate, string userAgent, string ipAddress, string connectionString)
        {
            using var connection = new SqlConnection(connectionString);
            using var command = new SqlCommand("LogPasswordResetRequest", connection) { CommandType = CommandType.StoredProcedure };

            command.Parameters.AddWithValue("SessionToken", sessionToken);
            command.Parameters.AddWithValue("ExpirationDate", expirationDate);
            command.Parameters.AddWithValue("Email", email);
            command.Parameters.AddWithValue("UserAgent", userAgent);
            command.Parameters.AddWithValue("IpAddress", ipAddress);

            connection.Open();

            command.ExecuteNonQuery();
        }

        public static void ResetPassword(string passwordHash, string salt, string sessionToken, string connectionString)
        {
            using var connection = new SqlConnection(connectionString);
            using var command = new SqlCommand("ResetPassword", connection) { CommandType = CommandType.StoredProcedure };

            command.Parameters.AddWithValue("PasswordHash", passwordHash);
            command.Parameters.AddWithValue("Salt", salt);
            command.Parameters.AddWithValue("SessionToken", sessionToken);

            connection.Open();

            command.ExecuteNonQuery();
        }

        public static (string PasswordHash, string Salt) GetPasswordHashAndSalt(string username, string connectionString)
        {
            using var connection = new SqlConnection(connectionString);
            using var command = new SqlCommand("SELECT [Password Hash], [Salt] FROM dbo.GetUserPasswordHashAndSalt(@Username)", connection);

            command.Parameters.AddWithValue("Username", username);

            connection.Open();

            var reader = command.ExecuteReader();

            reader.Read();

            if (!reader.HasRows) return (string.Empty, string.Empty);

            return (reader["Password Hash"].ToString(), reader["Salt"].ToString());
        }

        public static string HashStringData(string data, string salt = "")
        {
            _ = Array.Empty<byte>();
            byte[] saltBytes;

            if (!string.IsNullOrEmpty(salt))
                saltBytes = Convert.FromBase64String(salt);
            else
            {
                saltBytes = new byte[SaltSize];
                ByteGenerator.GetRandomBytes(ref saltBytes);
            }

            return GetHash(data, saltBytes);
        }

        private static string GetHash(string password, byte[] salt)
        {
            return Convert.ToBase64String(KeyDerivation.Pbkdf2($"{Pepper}{password}", salt, KeyType, Iterations, KeySize));
        }
    }
}