Show economist double issues (weeks 25 and 51)

Closes: #62

agenda/economist.py

@@ -2,27 +2,36 @@
 
 from datetime import date, time, timedelta
 
+from dateutil.relativedelta import TH, relativedelta
+
 from . import uk_time
 from .types import Event
 
 
 def publication_dates(start_date: date, end_date: date) -> list[Event]:
     """List of Economist publication dates."""
-    # Define the publication day (Thursday) and non-publication weeks
-    publication_day = 3  # Thursday (0 - Monday, 1 - Tuesday, ..., 6 - Sunday)
     # Define the list of weeks when The Economist is not published
     non_publication_weeks = [26, 52]
+    double_issue = {
+        25: "Summer",
+        51: "Christmas",
+    }
 
-    current_date = start_date
-    publication_dates = []
     t = time(19, 0)
 
-    while current_date <= end_date:
-        if (
-            current_date.weekday() == publication_day
-            and current_date.isocalendar().week not in non_publication_weeks
-        ):
-            publication_dates.append(uk_time(current_date, t))
-        current_date += timedelta(days=1)
+    start_thursday = start_date + relativedelta(weekday=TH(+1))
+    end_thursday = end_date + relativedelta(weekday=TH(+1))
+    days = (end_thursday - start_thursday).days
 
-    return [Event(name="economist", date=pub_date) for pub_date in publication_dates]
+    events: list[Event] = []
+    for offset in range(0, days, 7):
+        pub_date = start_thursday + timedelta(days=offset)
+        week = pub_date.isocalendar().week
+        if week in non_publication_weeks:
+            continue
+        e = Event(name="economist", date=uk_time(pub_date, t))
+        if week in double_issue:
+            e.title = double_issue[week] + " double issue"
+        events.append(e)
+
+    return events