using System;
using System.Collections;
using System.Collections.Generic;

namespace DTS.DASLib.Command.SLICE.RealtimeCommands
{
    /// <summary>
    /// this class handles decoding a SPS realtime stream packet.   
    /// </summary>
    public class RealtimeStreamDecoder
    {
        /* per lp,
         * Byte index:
            0              Sync = CMD_SYNC_BYTE                              = 0xFA
            1              type = CMD_TYPE_REALTIME                     = 0x08
            2              command = CMDRT_REALTIME_STREAM_SAMPLE_PUSH = 0x08
            3              status =  STATUS_ERROR_NONE = 0x00
            4              group = 0x00;
            5              id = 0x00;
            6-7          datalength
            8-9          sequence number
            10-11     headerCRC16
            12-13     dataCRC16          (currently not used)
            …             payload
            8-byte   uint64_t Timestamp (for first sample in payload.)
            4-byte   uint32_t Channel list (bit set = channel present. Bit clear = channel not present.
            8-byte  uint64_t Sample Number (starting with first ADC scan in payload)
            Number of ADC-Scan sample = (datalength – 20) / (channellist*2)
           */
        /// <summary>
        /// this is the order of information in the packet
        /// the enum value is the offset in bytes from the start of the packet
        /// </summary>
        public enum ByteIndex
        {
            Sync = 0,
            Type = 1,
            Command = 2,
            Status = 3,
            Group = 4,
            Id = 5,
            DataLength = 6,
            SequenceNumber = 8,
            HeaderCRC = 10,
            DataCRC = 12,
            TimeStamp = 14,
            ChannelList = 22,
            SampleNumber = 26,
            DataStart = 34
        }

        public ushort SequenceNumber { get; }
        public ulong TimeStamp { get; }
        public ulong SampleNumber { get; }
        public int[] Channels { get; }

        /// <summary>
        /// this is all data, not transformed into (short) yet, which requires 2's compliment
        /// it is also for all channels in the realtime stream
        /// </summary>
        public ushort[] RtData { get; }

        /// <summary>
        /// constructors and decodes byte stream
        /// 
        /// </summary>
        /// <param name="bytes"></param>
        public RealtimeStreamDecoder(IReadOnlyList<byte> bytes)
        {
            if (bytes.Count > (int)ByteIndex.DataStart)
            {
                TimeStamp = GetULong(bytes, (int)ByteIndex.TimeStamp);
                SequenceNumber = GetUShort(bytes, (int)ByteIndex.SequenceNumber);
                SampleNumber = GetULong(bytes, (int)ByteIndex.SampleNumber);
                RtData = new ushort[(bytes.Count - (int)ByteIndex.DataStart) / 2];
                var uintChannelList = GetUINT(bytes, (int)ByteIndex.ChannelList);
                var channels = new List<int>();
                var ba = new BitArray(BitConverter.GetBytes(uintChannelList));
                for (var i = 0; i < ba.Length; i++)
                {
                    if (ba.Get(i))
                    {
                        channels.Add(i);
                    }
                }

                Channels = channels.ToArray();
                for (var idx = 0; idx < RtData.Length; idx++)
                {
                    //all the data is stored as uint16 with channel order so
                    // AABBCCDDEEFF, so split out the ushorts
                    RtData[idx] = GetRTUShort(bytes, (int)(ByteIndex.DataStart + idx * 2));
                }
            }
            else
            {
                RtData = new ushort[0];
                SampleNumber = 0;
                Channels = new int[0];
            }

        }

        /// <summary>
        /// returns a ulong given a sequence of bytes and a starting offset in bytes
        /// that the ulong starts at
        /// SLICE header bytes are sent in reverse order so we have to fix them first
        /// </summary>
        /// <param name="bytes"></param>
        /// <param name="offset"></param>
        /// <returns></returns>
        private static ulong GetULong(IReadOnlyList<byte> bytes, int offset)
        {
            return (ulong)bytes[offset + 7] << 0 |
                   (ulong)bytes[offset + 6] << 8 |
                   (ulong)bytes[offset + 5] << 16 |
                   (ulong)bytes[offset + 4] << 24 |
                   (ulong)bytes[offset + 3] << 32 |
                   (ulong)bytes[offset + 2] << 40 |
                   (ulong)bytes[offset + 1] << 48 |
                   (ulong)bytes[offset + 0] << 56;
        }

        /// <summary>
        /// gets a ushort from the response parameters
        /// SLICE header bytes are sent in reverse order, so we have to fix
        /// them first
        /// </summary>
        /// <param name="bytes"></param>
        /// <param name="offset"></param>
        /// <returns></returns>
        private static uint GetUINT(IReadOnlyList<byte> bytes, int offset)
        {
            return (uint)bytes[offset + 3] << 0 |
                   (uint)bytes[offset + 2] << 8 |
                   (uint)bytes[offset + 1] << 16 |
                   (uint)bytes[offset + 0] << 32;
        }
        /// <summary>
        /// returns a ushort given a sequence of bytes and a starting offset in bytes
        /// SLICE header bytes are sent in reverse order, so we have to fix the byte
        /// order first
        /// </summary>
        /// <param name="bytes"></param>
        /// <param name="offset"></param>
        /// <returns></returns>
        private static ushort GetUShort(IReadOnlyList<byte> bytes, int offset)
        {
            return (ushort)(bytes[offset + 1] |
                            bytes[offset + 0] << 8);
        }
        /// <summary>
        /// returns a ushort given a sequence of bytes and a starting offset in bytes
        /// note that DATA is not in reverse order unlike the header, so  we
        /// DON'T need to fix the order
        /// </summary>
        /// <param name="bytes"></param>
        /// <param name="offset"></param>
        /// <returns></returns>
        private static ushort GetRTUShort(IReadOnlyList<byte> bytes, int offset)
        {
            return (ushort)(bytes[offset + 0] |
                            bytes[offset + 1] << 8);
        }
    }
}