Nonemm/src/Nonemm.Network/LogChecksum.cs

using System.Globalization;
using System.Text;
using Nonemm.Core;

namespace Nonemm.Network;

/// The checksum of one block of contacts. Two stations compare these to find
/// out whether they hold the same log without sending the log.
///
/// It is N1MM's: a CRC32 over the `DXLOG` columns of every contact in the
/// block, in the order the columns are read out of the database, oldest first
/// and by callsign within a timestamp. The columns this program works out from
/// the rules — points, the three multiplier flags, the country prefix, the
/// continent, the WPX prefix and `IsOriginal` — are left out, because the two
/// stations are allowed to disagree about them. `ContestNR` counts only when it
/// is -1.
///
/// The checksum is over what a station stored, so a field that one program
/// writes as empty and the other as a space is a difference. N1MM writes a
/// single space into `GridSquare`, `MiscText`, `ContactType` and
/// `RoverLocation` when it has nothing for them, so an empty value in those
/// four is hashed as a space.
///
/// A checksum that disagrees is not a fault: the block is sent again, which is
/// what a resync is for. It only costs traffic.
public static class LogChecksum
{
    public static int Of(IEnumerable<Qso> contacts)
    {
        StringBuilder text = new();
        IEnumerable<Qso> ordered = contacts
            .OrderBy(q => q.TimestampUtc)
            .ThenBy(q => q.Call.Text, StringComparer.Ordinal);
        foreach (Qso qso in ordered)
        {
            Append(text, qso);
        }
        return Crc32.Of(text.ToString());
    }

    /// The contacts of one station inside one block of time. Both ends of a
    /// resync pick the same rows this way.
    public static IReadOnlyList<Qso> Block(
        IEnumerable<Qso> contacts,
        string computerName,
        DateTime fromUtc,
        TimeSpan length) =>
        contacts
            .Where(q =>
                string.Equals(q.StationName, computerName, StringComparison.OrdinalIgnoreCase)
                && q.TimestampUtc >= fromUtc
                && q.TimestampUtc < fromUtc + length)
            .OrderBy(q => q.TimestampUtc)
            .ThenBy(q => q.Call.Text, StringComparer.Ordinal)
            .ToList();

    private static void Append(StringBuilder text, Qso qso)
    {
        text.Append(qso.TimestampUtc.ToString("yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture));
        text.Append(qso.Call.Text);
        Number(text, qso.Frequency.Kilohertz);
        Number(text, qso.QsxFrequency.Kilohertz);
        text.Append(qso.Mode.Name);
        text.Append(qso.ContestName);
        text.Append(qso.SentReport);
        text.Append(qso.ReceivedReport);
        text.Append(qso.StationPrefix);
        text.Append(qso.Qth);
        text.Append(qso.Name);
        text.Append(qso.Comment);
        Number(text, qso.ReceivedNumber);
        text.Append(qso.Section);
        text.Append(qso.Precedence);
        Number(text, qso.Check);
        Number(text, qso.Zone);
        Number(text, qso.SentNumber);
        text.Append(qso.Power);
        Number(text, qso.Band?.MegahertzLabel ?? 0);
        text.Append(qso.Exchange1);
        Number(text, qso.RadioNumber);
        text.Append(qso.Operator);
        text.Append(Filled(qso.GridSquare));
        // the contest number counts only when it says the contact belongs to no
        // contest
        if (qso.ContestNumber == -1)
        {
            text.Append("-1");
        }
        text.Append(Filled(qso.MiscText));
        text.Append(Filled(qso.ContactType));
        Number(text, qso.RunPosition);
        text.Append(Filled(qso.RoverLocation));
        Flag(text, qso.IsRadioInterfaced);
        Number(text, qso.NetworkedComputerNumber);
        text.Append(qso.StationName);
        Flag(text, qso.IsRunQso);
        text.Append(qso.Id);
        Flag(text, qso.IsClaimed);
    }

    /// A column N1MM stores as a single space when it has nothing for it.
    private static string Filled(string value) => value.Length == 0 ? " " : value;

    private static void Number(StringBuilder text, double value) =>
        text.Append(value.ToString(CultureInfo.InvariantCulture));

    private static void Number(StringBuilder text, int value) =>
        text.Append(value.ToString(CultureInfo.InvariantCulture));

    private static void Flag(StringBuilder text, bool value) => text.Append(value ? '1' : '0');
}