Count down the band change timer in the status bar

SOBandChangeTimerDuration and MOBandChangeTimerDuration say how long a
station has to stay on a band it has just moved to. Both were read by
nothing: N1MM counts them down in its info window, which this program
does not have.

The countdown now stands in the entry window's status bar, next to the
contacts and multipliers, ticking with the clock that is already there.
BandChangeRules.StayLeft works it out from the log, starting the stay at
the minute after the contact that changed band, which is where N1MM
starts it.

Which contests show it is decided at runtime rather than by a list: the
rules come from the contest for the entry's own category, so a file that
asks for no stay, or asks for one only of single-op entries while the
station is entered multi-op, shows nothing at all.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RoGtneMQaz4M9w7Kk49AVD

src/Nonemm.Contests/BandChangeRules.cs

@@ -8,6 +8,10 @@ namespace Nonemm.Contests;
 /// `Max` is how many changes the entry is allowed. The window the count is
 /// taken over is the clock hour, unless `PerContest` counts the whole contest
 /// or `PerPeriod` counts the hour up to now.
+///
+/// `MinimumStay` is how long the station has to stay on a band it has just
+/// moved to. Both the count and the stay are worked out from the log, as
+/// N1MM's are: the contact that lands on another band is the change.
 public sealed record BandChangeRules(
     int Max,
     bool PerContest = false,
@@ -20,6 +24,9 @@ public sealed record BandChangeRules(
 
     public bool IsCounted => Max > 0;
 
+    /// True for a contest that says how long a station has to stay on a band.
+    public bool HasStayTimer => MinimumStay > TimeSpan.Zero;
+
     /// How many band changes fall in the window that ends at `now`. The first
     /// contact of the log is not a change.
     public int CountedIn(IEnumerable<Qso> qsos, DateTime now)
@@ -36,6 +43,42 @@ public sealed record BandChangeRules(
         return changes;
     }
 
+    /// How much of the stay on this band is left. Zero once the time is up,
+    /// and zero for a log that has not changed band yet.
+    ///
+    /// The stay is counted from the minute after the contact that changed
+    /// band, which is where N1MM starts it: a contact at 12:00:40 puts the
+    /// station on the band from 12:01.
+    public TimeSpan StayLeft(IEnumerable<Qso> qsos, DateTime now)
+    {
+        if (!HasStayTimer || LastChange(qsos) is not { } changed)
+        {
+            return TimeSpan.Zero;
+        }
+        TimeSpan left = MinimumStay - (now - NextMinute(changed));
+        return left > TimeSpan.Zero ? left : TimeSpan.Zero;
+    }
+
+    /// When the last contact that moved the station to another band was, or
+    /// null while every contact has been on one band.
+    private DateTime? LastChange(IEnumerable<Qso> qsos)
+    {
+        List<Qso> ordered = qsos.OrderBy(q => q.TimestampUtc).ToList();
+        DateTime? found = null;
+        for (int at = 1; at < ordered.Count; at++)
+        {
+            if (IsChange(ordered[at - 1], ordered[at]))
+            {
+                found = ordered[at].TimestampUtc;
+            }
+        }
+        return found;
+    }
+
+    private static DateTime NextMinute(DateTime when) =>
+        new DateTime(when.Year, when.Month, when.Day, when.Hour, when.Minute, 0, when.Kind)
+            .AddMinutes(1);
+
     private bool IsChange(Qso before, Qso after) =>
         before.Band != after.Band
         || (CountsModeChanges && before.Mode.Category != after.Mode.Category);