namespace LaaProduction.SQL
{
    using LaaProduction.SQL.Interfaces;

    using System;
    using System.Collections.Generic;
    using System.Data.SqlClient;
    using System.Linq;

    /// <summary>
    /// A wrapper class around the <see cref="System.Data.SqlClient"/>. 
    /// Exposes <see cref="System.Data.SqlClient.SqlConnection"/>, <see cref="System.Data.SqlClient.SqlCommand"/> and <see cref="System.Data.SqlClient.SqlDataReader"/> classes 
    /// in a fluent manner, all exposed methods can be chained in a single C# query. 
    /// </summary>
    public class SQLConnection : ISQLConnection
    {
        private readonly string connectionString;
        private readonly Action<string> errorCallback;

        /// <summary>
        /// When <paramref name="connectionString"/> is null, white space or is not a valid connection string exception is thrown. 
        /// </summary>
        /// <param name="connectionString"><see cref="System.String"/> - SQL connection string.</param>
        public SQLConnection(string connectionString)
        {
            connectionString.ThrowIfNullOrWhiteSpace(nameof(connectionString));

            var connectionStringBuilder = new SqlConnectionStringBuilder(connectionString);

            this.connectionString = connectionStringBuilder.ToString();
        }

        /// <summary>
        /// When <paramref name="connectionString"/> is null, white space or is not a valid connection string exception is thrown. 
        /// Additionaly accepts <see cref="System.Action{System.String}"/> for error handling. 
        /// Whenever the sql query throws error on execution, this will be passesd within the callback.
        /// </summary>
        /// <param name="connectionString"><see cref="System.String"/> - SQL connection string.</param>
        /// <param name="errorCallback"><see cref="System.Action{System.String}"/> - error handler for SQL execution errors.</param>
        public SQLConnection(string connectionString, Action<string> errorCallback = null) 
            : this(connectionString)
            => this.errorCallback = errorCallback;

        /// <summary>
        /// Creates a <see cref="LaaProduction.SQL.Interfaces.ISQLCommand"/> wrapper that accepts a SQL query string. 
        /// </summary>
        /// <param name="commandText"><see cref="System.String"/> - SQL query string</param>
        /// <returns><see cref="LaaProduction.SQL.Interfaces.ISQLCommand"/> - that exposes basic functionalities from the <see cref="System.Data.SqlClient.SqlCommand" />.</returns>
        public ISQLCommand CreateCommand(string commandText)
            => new SQLCommand(this, commandText);

        /// <summary>
        /// Static method that creates a <see cref="LaaProduction.SQL.Interfaces.ISQLConnection"/> client with given connection string.
        /// </summary>
        /// <param name="connectionString"><see cref="System.String"/> - SQL connection string.</param>
        /// <returns><see cref="LaaProduction.SQL.Interfaces.ISQLConnection"/></returns>
        public static ISQLConnection CreateSQLConnection(string connectionString)
            => new SQLConnection(connectionString);

        /// <summary>
        /// Static method that creates a <see cref="LaaProduction.SQL.Interfaces.ISQLConnection"/> client with given connection string and error callback that receives all error messages.
        /// </summary>
        /// <param name="connectionString"><see cref="System.String"/> - SQL connection string.</param>
        /// <param name="errorCallback"><see cref="System.Action{System.String}"/> - error handler for SQL execution errors.</param>
        /// <returns><see cref="LaaProduction.SQL.Interfaces.ISQLConnection"/></returns>
        public static ISQLConnection CreateSQLConnection(string connectionString, Action<string> errorCallback = null)
            => new SQLConnection(connectionString, errorCallback);

        /// <summary>
        /// Internal method called from <see cref="LaaProduction.SQL.Interfaces.ISQLCommand"/>. 
        /// Like in a <see cref="System.Data.SqlClient.SqlCommand"/> performs INSERT, UPDATE, DELETE queries and returns the number of rows affected.
        /// </summary>
        /// <param name="command"><see cref="System.String"/> - SQL query string</param>
        /// <returns><see cref="System.Int32"/> - the number of rows affected.</returns>
        internal int ExecuteNonQuery(SQLCommand command)
        {
            command.ThrowIfNull(nameof(command));

            var rowsAffected = 0;

            using (var sqlConnection = new SqlConnection(this.connectionString))
            {
                sqlConnection.OpenWithErrorHandling(this.errorCallback);

                using (var sqlCommand = sqlConnection.CreateCommand())
                {
                    sqlCommand.CommandText = command.CommandText;

                    sqlCommand.Parameters.AddRange(command.Parameters);

                    rowsAffected = sqlCommand.ExecuteNonQuery();

                    sqlCommand.Parameters.Clear();
                }
            }

            return rowsAffected;
        }

        /// <summary>
        /// Internal method called from <see cref="LaaProduction.SQL.Interfaces.ISQLCommand"/>. 
        /// Like in a <see cref="System.Data.SqlClient.SqlCommand"/> performs SELECT queries and returns a enumerable of the results readed.
        /// </summary>
        /// <typeparam name="T">Is the type that returns the <see cref="System.Func{IFluentSQLReader, T} expression"/></typeparam>
        /// <param name="command"><see cref="System.String"/> - SQL query string</param>
        /// <param name="expression">Expression that reads specified data from the <see cref="LaaProduction.SQL.Interfaces.ISQLReader"/>.</param>
        /// <returns><see cref="System.Collections.Generic.IEnumerable{T}"/></returns>
        internal IEnumerable<T> ExecuteReader<T>(SQLCommand command, Func<ISQLReader, T> expression)
        {
            command.ThrowIfNull(nameof(command));
            expression.ThrowIfNull(nameof(expression));

            using (var sqlConnection = new SqlConnection(this.connectionString))
            {
                sqlConnection.OpenWithErrorHandling(this.errorCallback);

                using (var sqlCommand = sqlConnection.CreateCommand())
                {
                    sqlCommand.CommandText = command.CommandText;

                    sqlCommand.Parameters.AddRange(command.Parameters);

                    using (SQLReader sqlReader = sqlCommand.ExecuteReader())
                    {
                        sqlCommand.Parameters.Clear();

                        while (sqlReader.Read())
                        {
                            yield return expression(sqlReader);
                        }
                    }
                }
            }
        }

        /// <summary>
        /// Internal method called from <see cref="LaaProduction.SQL.Interfaces.ISQLCommand"/>. 
        /// Like in a <see cref="System.Data.SqlClient.SqlCommand"/> performs SELECT, READ, UPDATE, DELETE queries and returns first column from the first row from the result.
        /// </summary>
        /// <typeparam name="T">From user specified.</typeparam>
        /// <param name="command"><see cref="System.String"/> - SQL query string</param>
        /// <param name="expression">Expression that reads specified data from the <see cref="LaaProduction.SQL.Interfaces.ISQLReader"/>.</param>
        /// <returns><see cref="System.Collections.Generic.IEnumerable{T}"/></returns>
        internal T ExecuteScalar<T>(SQLCommand command, Func<ISQLReader, T> expression)
        {
            command.ThrowIfNull(nameof(command));
            expression.ThrowIfNull(nameof(expression));

            var scalarResult = default(T);

            using (var sqlConnection = new SqlConnection(this.connectionString))
            {
                sqlConnection.OpenWithErrorHandling(this.errorCallback);

                using (var sqlCommand = sqlConnection.CreateCommand())
                {
                    sqlCommand.CommandText = command.CommandText;

                    sqlCommand.Parameters.AddRange(command.Parameters);

                    var scalarObject = sqlCommand.ExecuteScalar();

                    sqlCommand.Parameters.Clear();

                    if (scalarObject is T scalarValue)
                    {
                        scalarResult = scalarValue;
                    }
                }
            }

            return scalarResult;
        }

        /// <summary>
        /// Is same like <see cref="LaaProduction.SQL.SQLConnection.ExecuteReader{T}(SQLCommand, Func{ISQLReader, T})"/> but returns only the first data row.
        /// </summary>
        /// <typeparam name="T">From user specified.</typeparam>
        /// <param name="command"><see cref="System.String"/> - SQL query string</param>
        /// <param name="expression">Expression that reads specified data from the <see cref="LaaProduction.SQL.Interfaces.ISQLReader"/>.</param>
        /// <returns><see cref="{T}"/></returns>
        internal T FirstOrDefault<T>(SQLCommand command, Func<ISQLReader, T> expression) 
            => this
                .ExecuteReader(command, expression)
                .FirstOrDefault();
    }
}