using GeoVLog.Core.Models;
using System;
using System.Globalization;
using System.Text;

namespace GeoVLog.Core.Parsers;

/// <summary>
/// Parser for NMEA $GPRMC and $GPGGA sentences. Only valid messages with a
/// correct checksum are parsed.
/// </summary>
internal static class GpsParser
{
    public static bool TryParse(ReadOnlySpan<byte> msg, DateTime arrivalUtc, out GpsReading parsed)
    {
        parsed = default!;
        var line = Encoding.ASCII.GetString(msg).Trim();
        int star = line.IndexOf('*');
        if (star <= 0 || star + 3 > line.Length)
            return false;
        byte cs = 0;
        for (int i = 1; i < star; i++)
            cs ^= (byte)line[i];
        if (!byte.TryParse(line.AsSpan(star + 1, 2), NumberStyles.HexNumber, CultureInfo.InvariantCulture, out var given) || given != cs)
            return false;

        string[] parts = line[..star].Split(',');
        if (parts.Length == 0)
            return false;

        string id = parts[0];
        if (id == "$GPRMC")
            return TryParseRmc(parts, line, out parsed);
        if (id == "$GPGGA")
            return TryParseGga(parts, line, arrivalUtc.Date, out parsed);
        return false;
    }

    private static bool TryParseRmc(string[] parts, string line, out GpsReading parsed)
    {
        parsed = default!;
        if (parts.Length < 12 || parts[2] != "A")
            return false; // invalid status

        if (!TryParseDateTime(parts[1], parts[9], out DateTime fixTime))
            return false;
        if (!TryParseLatLon(parts[3], parts[4], out double lat) || !TryParseLatLon(parts[5], parts[6], out double lon))
            return false;

        double? speed = ParseDoubleOrNull(parts[7]);
        double? track = ParseDoubleOrNull(parts[8]);
        double? magVar = null;
        if (parts.Length > 10 && !string.IsNullOrEmpty(parts[10]))
        {
            magVar = ParseDoubleOrNull(parts[10]);
            if (magVar.HasValue && parts.Length > 11 && parts[11] == "W")
                magVar = -magVar;
        }
        char mode = parts.Length > 12 && !string.IsNullOrEmpty(parts[12]) ? parts[12][0] : '\0';

        parsed = new GpsReading
        {
            TimestampUtc = fixTime,
            RawLine = line,
            SentenceId = "$GPRMC",
            LatitudeDeg = lat,
            LongitudeDeg = lon,
            SpeedKnots = speed,
            HeadingDeg = track,
            Status = 'A',
            Mode = mode == '\0' ? null : mode,
            MagneticVariationDeg = magVar
        };
        return true;
    }

    private static bool TryParseGga(string[] parts, string line, DateTime date, out GpsReading parsed)
    {
        parsed = default!;
        if (parts.Length < 15)
            return false;
        if (!byte.TryParse(parts[6], NumberStyles.Integer, CultureInfo.InvariantCulture, out byte fixQuality) || fixQuality == 0)
            return false; // invalid fix

        if (!TryParseTime(parts[1], date, out DateTime fixTime))
            return false;
        if (!TryParseLatLon(parts[2], parts[3], out double lat) || !TryParseLatLon(parts[4], parts[5], out double lon))
            return false;
        if (!byte.TryParse(parts[7], NumberStyles.Integer, CultureInfo.InvariantCulture, out byte sats))
            return false;
        float? hdop = ParseFloatOrNull(parts[8]);
        if (!double.TryParse(parts[9], NumberStyles.Float, CultureInfo.InvariantCulture, out double alt))
            return false;
        char altUnit = parts[10].Length > 0 ? parts[10][0] : 'M';
        double? geoid = ParseDoubleOrNull(parts[11]);
        char geoidUnit = parts.Length > 12 && parts[12].Length > 0 ? parts[12][0] : 'M';
        double? age = parts.Length > 13 ? ParseDoubleOrNull(parts[13]) : null;
        string? station = parts.Length > 14 && parts[14].Length > 0 ? parts[14] : null;

        parsed = new GpsReading
        {
            TimestampUtc = fixTime,
            RawLine = line,
            SentenceId = "$GPGGA",
            LatitudeDeg = lat,
            LongitudeDeg = lon,
            AltitudeM = alt,
            AltitudeUnits = altUnit,
            GeoidSeparation = geoid,
            GeoidUnits = geoidUnit,
            AgeDgpsSeconds = age,
            DgpsStationId = station,
            FixQuality = fixQuality,
            NumSatellites = sats,
            Hdop = hdop
        };
        return true;
    }

    private static bool TryParseTime(string hhmmss, DateTime date, out DateTime result)
    {
        result = default;
        if (hhmmss.Length < 6)
            return false;
        if (!int.TryParse(hhmmss.Substring(0, 2), out int hh))
            return false;
        if (!int.TryParse(hhmmss.Substring(2, 2), out int mm))
            return false;
        if (!double.TryParse(hhmmss.Substring(4), NumberStyles.Float, CultureInfo.InvariantCulture, out double ss))
            return false;
        int sec = (int)ss;
        int ms = (int)Math.Round((ss - sec) * 1000.0);
        result = new DateTime(date.Year, date.Month, date.Day, hh, mm, sec, DateTimeKind.Utc).AddMilliseconds(ms);
        return true;
    }

    private static bool TryParseDateTime(string hhmmss, string ddmmyy, out DateTime result)
    {
        result = default;
        if (!TryParseTime(hhmmss, DateTime.UtcNow.Date, out DateTime temp))
            return false;
        if (ddmmyy.Length != 6 || !int.TryParse(ddmmyy.Substring(0, 2), out int dd) ||
            !int.TryParse(ddmmyy.Substring(2, 2), out int MM) ||
            !int.TryParse(ddmmyy.Substring(4, 2), out int yy))
            return false;
        int year = 2000 + yy;
        result = new DateTime(year, MM, dd, temp.Hour, temp.Minute, temp.Second, temp.Kind).AddMilliseconds(temp.Millisecond);
        return true;
    }

    private static bool TryParseLatLon(string value, string hemi, out double degrees)
    {
        degrees = 0;
        if (string.IsNullOrEmpty(value) || value.Length < 3)
            return false;
        int degLen = hemi == "N" || hemi == "S" ? 2 : 3;
        if (!double.TryParse(value.Substring(0, degLen), NumberStyles.Integer, CultureInfo.InvariantCulture, out double degPart))
            return false;
        if (!double.TryParse(value.Substring(degLen), NumberStyles.Float, CultureInfo.InvariantCulture, out double minPart))
            return false;
        double raw = degPart + (minPart / 60.0);
        const double scale = 1_000_000.0;
        degrees = Math.Truncate(raw * scale) / scale;
        if (hemi == "S" || hemi == "W")
            degrees = -degrees;
        return true;
    }

    private static double? ParseDoubleOrNull(string s) => double.TryParse(s, NumberStyles.Float, CultureInfo.InvariantCulture, out double v) ? v : null;
    private static float? ParseFloatOrNull(string s) => float.TryParse(s, NumberStyles.Float, CultureInfo.InvariantCulture, out float v) ? v : null;
}