ulapp/FieldLogger/Models/GnssFix.cs

using System.Globalization;

namespace FieldLogger.Models;

public enum GnssFixStatus { NoFix = 0, Single = 1, Dgps = 2, RtkFixed = 4, RtkFloat = 5 }

/// <summary>
/// A parsed $GNPOS sentence from the Maglink (H11) RTK receiver.
/// </summary>
public sealed class GnssFix
{
    public double Latitude { get; init; }
    public double Longitude { get; init; }
    public double Altitude { get; init; }
    public double AltitudeCorrected { get; init; }
    public GnssFixStatus Status { get; init; }
    public double Hdop { get; init; }
    public double Hrms { get; init; }
    public double Vrms { get; init; }
    public int SatellitesUsed { get; init; }
    public int SatellitesVisible { get; init; }
    public double SpeedKmh { get; init; }
    public double Heading { get; init; }
    public double BatteryVoltage { get; init; }
    public int BatteryPercent { get; init; }
    public bool NtripConnected { get; init; }
    public int RtcmSize { get; init; }
    public double CorrectionAgeSeconds { get; init; }
    public long UnixTimestamp { get; init; }
    public double TiltAngle { get; init; }
    public string RawSentence { get; init; } = "";

    /// <summary>Local UTC time this fix was received by the app.</summary>
    public DateTime ReceivedUtc { get; init; } = DateTime.UtcNow;

    public bool HasPosition => Status != GnssFixStatus.NoFix;

    public string StatusLabel => Status switch
    {
        GnssFixStatus.NoFix => "No Fix",
        GnssFixStatus.Single => "Single",
        GnssFixStatus.Dgps => "DGPS",
        GnssFixStatus.RtkFixed => "RTK Fixed",
        GnssFixStatus.RtkFloat => "RTK Float",
        _ => Status.ToString(),
    };

    /// <summary>Parses a "$GNPOS,..." sentence (checksum must already be validated).</summary>
    public static GnssFix? TryParse(string sentence)
    {
        var body = NmeaSentence.Body(sentence);
        var f = body.Split(',');
        if (f.Length < 20 || f[0] != "GNPOS")
            return null;

        try
        {
            return new GnssFix
            {
                Latitude = D(f[1]),
                Longitude = D(f[2]),
                Altitude = D(f[3]),
                AltitudeCorrected = D(f[4]),
                Status = (GnssFixStatus)I(f[5]),
                Hdop = D(f[6]),
                Hrms = D(f[7]),
                Vrms = D(f[8]),
                SatellitesUsed = I(f[9]),
                SatellitesVisible = I(f[10]),
                SpeedKmh = D(f[11]),
                Heading = D(f[12]),
                BatteryVoltage = D(f[13]),
                BatteryPercent = I(f[14]),
                NtripConnected = I(f[15]) != 0,
                RtcmSize = I(f[16]),
                CorrectionAgeSeconds = D(f[17]),
                UnixTimestamp = long.Parse(f[18], CultureInfo.InvariantCulture),
                TiltAngle = D(f[19]),
                RawSentence = sentence.Trim(),
            };
        }
        catch (FormatException)
        {
            return null;
        }
    }

    private static double D(string s) => double.Parse(s, NumberStyles.Float, CultureInfo.InvariantCulture);
    private static int I(string s) => int.Parse(s, CultureInfo.InvariantCulture);
}

/// <summary>
/// A parsed $GNDEV device-information sentence from the Maglink receiver.
/// </summary>
public sealed class GnssDeviceInfo
{
    public string SerialNumber { get; init; } = "";
    public string PcbVersion { get; init; } = "";
    public string FirmwareVersion { get; init; } = "";
    public string Imei { get; init; } = "";
    public string Imsi { get; init; } = "";
    public string Iccid { get; init; } = "";

    public static GnssDeviceInfo? TryParse(string sentence)
    {
        var f = NmeaSentence.Body(sentence).Split(',');
        if (f.Length < 7 || f[0] != "GNDEV")
            return null;

        return new GnssDeviceInfo
        {
            SerialNumber = f[1],
            PcbVersion = f[2],
            FirmwareVersion = f[3],
            Imei = f[4],
            Imsi = f[5],
            Iccid = f[6],
        };
    }
}

/// <summary>NMEA framing helpers ($...*hh checksum).</summary>
public static class NmeaSentence
{
    /// <summary>Returns the sentence body between '$' and '*' (or end of string).</summary>
    public static string Body(string sentence)
    {
        var s = sentence.Trim().TrimStart('$');
        var star = s.IndexOf('*');
        return star >= 0 ? s[..star] : s;
    }

    public static byte Checksum(string sentence)
    {
        byte checksum = 0;
        foreach (var c in Body(sentence))
            checksum ^= (byte)c;
        return checksum;
    }

    /// <summary>Validates the trailing *hh checksum. Sentences without a checksum fail validation.</summary>
    public static bool VerifyChecksum(string sentence)
    {
        var star = sentence.LastIndexOf('*');
        if (star < 0 || star + 3 > sentence.TrimEnd().Length)
            return false;

        var hex = sentence.Substring(star + 1, 2);
        if (!byte.TryParse(hex, System.Globalization.NumberStyles.HexNumber,
                System.Globalization.CultureInfo.InvariantCulture, out var expected))
            return false;

        return Checksum(sentence) == expected;
    }
}