using System;
using System.Collections.Generic;
using System.Text;
using log4net;

namespace bsmd.AIS2Service
{
    internal abstract class NMEA
    {
        protected string type = "";
        protected string _data;
        protected string[] elements = null;

        protected static ILog _log = LogManager.GetLogger(typeof(NMEA));

        public enum Status
        {
            OK,
            UNKNOWN_TYPE,
            CHECKSUM,
            ILLEGAL_ARGUMENT
        }

        protected abstract void Decode();

        /// <summary>
        /// public method to decode input data strings into respective NMEA sentences
        /// </summary>
        /// <param name="data"></param>
        /// <param name="status"></param>
        /// <returns></returns>
        public static NMEA Decode(string data, ref Status status)
        {
            try
            {
                if (data == null)
                {
                    status = Status.ILLEGAL_ARGUMENT;
                    return null;
                }

                if (data[0] != '$' && data[0] != '!')
                {
                    status = Status.ILLEGAL_ARGUMENT;
                    return null; // no NMEA sentence
                }

                string[] elements = data.Trim().Substring(1).Split(',');

                NMEA sentence = NMEA.CreateNMEAElement(elements[0]);
                if (sentence == null)
                {
                    status = Status.UNKNOWN_TYPE;
                    return null;
                }

                sentence.elements = elements;
                sentence._data = data.Trim(); ;

                if (!sentence.IsChecksumOK)
                {
                    status = Status.CHECKSUM;
                    return null;
                }

                sentence.Decode();

                return sentence;
            }
            catch (Exception ex)
            {
                _log.ErrorFormat("Error decoding sentence: {0}, {1}", ex.Message, ex.StackTrace);
                return null;
            }                
        }

        /// <summary>
        /// Factory method for nmea types
        /// </summary>
        protected static NMEA CreateNMEAElement(string type)
        {
            NMEA result = null;

            switch (type.ToUpper())
            {
                case "AIVDM":
                case "AIVDO":
                    result = new NMEA_AIS_Sentence();
                    break;

                case "PNMLS":
                    result = new NMEA_PNMLS_Sentence();
                    break;

                default:
                    _log.WarnFormat("ignoring unsupported NMEA type {0}", type);
                    break;
            }

            if (result != null)
                result.type = type.ToUpper();

            return result;
        }

        #region NMEA checksum

        protected bool IsChecksumOK
        {
            get
            {
                return _data.Substring(_data.IndexOf('*') + 1) == this.CalculateChecksum();
            }
        }

        private string CalculateChecksum()
        {
            int checksum = Convert.ToByte(_data[1]);
            for (int i = 2; i < _data.IndexOf('*'); i++)
            {
                checksum ^= Convert.ToByte(_data[i]);
            }
            return checksum.ToString("X2");
        }

        #endregion

    }
}