/// <summary>
///     Represents an L7 PSEM service request with its payload.
/// </summary>
/// <param name = "Code">The service request code.</param>
/// <param name = "Payload">The service-specific payload bytes.</param>
public readonly record struct ServiceRequest(ServiceCode Code, ReadOnlyMemory<byte> Payload)
{
#region Factory Methods
    /// <summary>Creates an Identify request.</summary>
    public static ServiceRequest Identify() => new(ServiceCode.Identify, ReadOnlyMemory<byte>.Empty);
    /// <summary>Creates a Terminate request.</summary>
    public static ServiceRequest Terminate() => new(ServiceCode.Terminate, ReadOnlyMemory<byte>.Empty);
    /// <summary>Creates a Disconnect request.</summary>
    public static ServiceRequest Disconnect() => new(ServiceCode.Disconnect, ReadOnlyMemory<byte>.Empty);
    /// <summary>Creates a Logoff request.</summary>
    public static ServiceRequest Logoff() => new(ServiceCode.Logoff, ReadOnlyMemory<byte>.Empty);
    /// <summary>Creates a Wait (keep-alive) request.</summary>
    public static ServiceRequest Wait() => new(ServiceCode.Wait, ReadOnlyMemory<byte>.Empty);
    /// <summary>Creates a Logon request.</summary>
    /// <param name = "userId">The user ID (2 bytes).</param>
    /// <param name = "userName">The username (max 10 bytes, null-padded).</param>
    public static ServiceRequest Logon(ushort userId, string userName)
    {
        ArgumentNullException.ThrowIfNull(userName);
        var payload = new byte[12];
        payload[0] = (byte)(userId >> 8);
        payload[1] = (byte)(userId & 0xFF);
        var nameBytes = Encoding.ASCII.GetBytes(userName);
        Array.Copy(nameBytes, 0, payload, 2, Math.Min(nameBytes.Length, 10));
        return new ServiceRequest(ServiceCode.Logon, payload);
    }

    /// <summary>Creates a Security (password) request.</summary>
    /// <param name = "password">The password bytes (max 20 bytes, null-padded).</param>
    public static ServiceRequest Security(ReadOnlySpan<byte> password)
    {
        var payload = new byte[20];
        password[..Math.Min(password.Length, 20)].CopyTo(payload);
        return new ServiceRequest(ServiceCode.Security, payload);
    }

    /// <summary>Creates a Negotiate request.</summary>
    /// <param name = "maxPacketSize">Maximum packet size.</param>
    /// <param name = "maxPackets">Maximum number of packets.</param>
    /// <param name = "baudRates">Supported baud rates.</param>
    public static ServiceRequest Negotiate(ushort maxPacketSize, byte maxPackets, ReadOnlySpan<BaudRate> baudRates)
    {
        var payload = new byte[3 + baudRates.Length];
        payload[0] = (byte)(maxPacketSize >> 8);
        payload[1] = (byte)(maxPacketSize & 0xFF);
        payload[2] = maxPackets;
        for (var i = 0; i < baudRates.Length; i++)
            payload[3 + i] = (byte)baudRates[i];
        return new ServiceRequest((ServiceCode)((byte)ServiceCode.NegotiateBase + baudRates.Length), payload);
    }

    /// <summary>Creates a full table read request.</summary>
    /// <param name = "tableId">The table ID to read.</param>
    public static ServiceRequest ReadFull(ushort tableId)
    {
        var payload = new byte[2];
        payload[0] = (byte)(tableId >> 8);
        payload[1] = (byte)(tableId & 0xFF);
        return new ServiceRequest(ServiceCode.ReadFull, payload);
    }

    /// <summary>Creates a partial (offset) table read request.</summary>
    /// <param name = "tableId">The table ID to read.</param>
    /// <param name = "offset">Byte offset within the table (0 to 0xFFFFFF).</param>
    /// <param name = "count">Number of bytes to read.</param>
    public static ServiceRequest ReadOffset(ushort tableId, int offset, ushort count)
    {
        ArgumentOutOfRangeException.ThrowIfNegative(offset);
        ArgumentOutOfRangeException.ThrowIfGreaterThan(offset, 0xFFFFFF);
        var payload = new byte[7];
        payload[0] = (byte)(tableId >> 8);
        payload[1] = (byte)(tableId & 0xFF);
        payload[2] = (byte)((offset >> 16) & 0xFF);
        payload[3] = (byte)((offset >> 8) & 0xFF);
        payload[4] = (byte)(offset & 0xFF);
        payload[5] = (byte)(count >> 8);
        payload[6] = (byte)(count & 0xFF);
        return new ServiceRequest(ServiceCode.ReadOffset, payload);
    }

    /// <summary>Creates a full table write request.</summary>
    /// <param name = "tableId">The table ID to write.</param>
    /// <param name = "data">The data to write.</param>
    public static ServiceRequest WriteFull(ushort tableId, ReadOnlySpan<byte> data)
    {
        var payload = new byte[2 + data.Length];
        payload[0] = (byte)(tableId >> 8);
        payload[1] = (byte)(tableId & 0xFF);
        data.CopyTo(payload.AsSpan(2));
        return new ServiceRequest(ServiceCode.WriteFull, payload);
    }

    /// <summary>Creates a write-with-offset request.</summary>
    /// <param name = "tableId">The table ID to write.</param>
    /// <param name = "offset">Byte offset within the table (0 to 0xFFFFFF).</param>
    /// <param name = "data">The data to write.</param>
    public static ServiceRequest WriteOffset(ushort tableId, int offset, ReadOnlySpan<byte> data)
    {
        ArgumentOutOfRangeException.ThrowIfNegative(offset);
        ArgumentOutOfRangeException.ThrowIfGreaterThan(offset, 0xFFFFFF);
        var payload = new byte[5 + data.Length];
        payload[0] = (byte)(tableId >> 8);
        payload[1] = (byte)(tableId & 0xFF);
        payload[2] = (byte)((offset >> 16) & 0xFF);
        payload[3] = (byte)((offset >> 8) & 0xFF);
        payload[4] = (byte)(offset & 0xFF);
        data.CopyTo(payload.AsSpan(5));
        return new ServiceRequest(ServiceCode.WriteOffset, payload);
    }

#endregion
#region Methods
    /// <summary>
    ///     Encodes this service request into a byte buffer for inclusion in a PSEM frame.
    /// </summary>
    /// <param name = "destination">The destination span.</param>
    /// <returns>Number of bytes written.</returns>
    public int Encode(Span<byte> destination)
    {
        destination[0] = (byte)Code;
        Payload.Span.CopyTo(destination[1..]);
        return 1 + Payload.Length;
    }

    /// <summary>Gets the total encoded length of this request.</summary>
    public int EncodedLength => 1 + Payload.Length;
#endregion
}