Find gaps that are available between travel

Closes: #86

agenda/data.py

@@ -272,6 +272,58 @@ def find_markets_during_stay(
     return overlapping_markets
 
 
+def find_gaps(events: list[Event], min_gap_days: int = 5) -> list[tuple[date, date]]:
+    """Gaps of at least `min_gap_days` between events in a list of events."""
+    # Sort events by start date
+
+    gaps: list[tuple[date, date]] = []
+    previous_event_end = None
+
+    for event in events:
+        # Use start date for current event
+        start_date = event.as_date
+
+        # If previous event exists, calculate the gap
+        if previous_event_end:
+            gap_days = (start_date - previous_event_end).days
+            if gap_days >= (min_gap_days + 2):
+                start_end = (
+                    previous_event_end + timedelta(days=1),
+                    start_date - timedelta(days=1),
+                )
+                gaps.append(start_end)
+
+        # Update previous event end date
+        end = event.end_as_date if event.end_date else start_date
+        if not previous_event_end or end > previous_event_end:
+            previous_event_end = end
+
+    return gaps
+
+
+def busy_event(e: Event) -> bool:
+    """Busy."""
+    if e.name not in {
+        "event",
+        "accommodation",
+        "conference",
+        "dodainville",
+        "transport",
+        "meetup",
+    }:
+        return False
+
+    if e.name == "conference" and not e.going:
+        return False
+    if not e.title:
+        return True
+    if e.title == "LHG Run Club" or "Third Thursday Social" in e.title:
+        return False
+
+    lc_title = e.title.lower()
+    return "rebels" not in lc_title and "south west data social" not in lc_title
+
+
 async def get_data(
     now: datetime, config: flask.config.Config
 ) -> typing.Mapping[str, str | object]:
@@ -304,7 +356,7 @@ async def get_data(
         bristol_waste_collection_events(data_dir, today),
     )
 
-    reply = {
+    reply: dict[str, typing.Any] = {
         "now": now,
         "gbpusd": gbpusd,
         "stock_markets": stock_market.open_and_close(),
@@ -394,8 +446,25 @@ async def get_data(
         events.append(e)
 
     events += [Event(name="today", date=today)]
+
+    busy_events = [
+        e
+        for e in sorted(events, key=lambda e: e.as_date)
+        if e.as_date > today and e.as_date < next_year and busy_event(e)
+    ]
+
+    gaps = find_gaps(busy_events)
+
+    events += [Event(name="gap", date=start, end_date=end) for start, end in gaps]
+
+    # Sort events by their datetime; the "today" event is prioritised
+    # at the top of the list for today. This is achieved by sorting first by
+    # the datetime attribute, and then ensuring that events with the name
+    # "today" are ordered before others on the same date.
     events.sort(key=lambda e: (e.as_datetime, e.name != "today"))
 
+    reply["gaps"] = gaps
+
     observer = sun.bristol()
     reply["sunrise"] = sun.sunrise(observer)
     reply["sunset"] = sun.sunset(observer)