tasks/pdl_swe_session_split_014/solution/gold.diff

--- a/src/aggregates/base.py
+++ b/src/aggregates/base.py
@@ -22,3 +22,8 @@ class Aggregate(ABC):
     @abstractmethod
     def value(self):
         """The current result. Pure: no state change."""
+
+    @abstractmethod
+    def combine(self, other):
+        """Fold another aggregate of the same kind into this one (used
+        when two sessions turn out to be one)."""
--- a/src/aggregates/basic.py
+++ b/src/aggregates/basic.py
@@ -30,6 +30,9 @@ class Count(Aggregate):
     def value(self):
         return self._n
 
+    def combine(self, other):
+        self._n += other._n
+
 
 class Sum(Aggregate):
     """Sum of a numeric field over contributing events (0 when none)."""
@@ -46,6 +49,9 @@ class Sum(Aggregate):
     def value(self):
         return self._total
 
+    def combine(self, other):
+        self._total += other._total
+
 
 class Min(Aggregate):
     """Minimum of a numeric field (None when no event contributed)."""
@@ -62,6 +68,10 @@ class Min(Aggregate):
     def value(self):
         return self._min
 
+    def combine(self, other):
+        if other._min is not None and (self._min is None or other._min < self._min):
+            self._min = other._min
+
 
 class Max(Aggregate):
     """Maximum of a numeric field (None when no event contributed)."""
@@ -77,3 +87,7 @@ class Max(Aggregate):
 
     def value(self):
         return self._max
+
+    def combine(self, other):
+        if other._max is not None and (self._max is None or other._max > self._max):
+            self._max = other._max
--- a/src/aggregates/distinct.py
+++ b/src/aggregates/distinct.py
@@ -21,3 +21,6 @@ class DistinctCount(Aggregate):
 
     def value(self):
         return len(self._seen)
+
+    def combine(self, other):
+        self._seen |= other._seen
new file mode 100644
--- /dev/null
+++ b/src/assign.py
@@ -0,0 +1,41 @@
+"""Event-time placement of an admissible event among a key's open sessions.
+
+Open sessions for a key are kept in start order, and any two neighbours
+are separated by more than the gap (otherwise they would be one session).
+An incoming timestamp therefore lands in exactly one of three ways: it is
+close enough to a single session to join it (in its interior, within the
+gap after its end, or within the gap before its start), it falls between
+two sessions within the gap of both — in which case those sessions were
+always one session and must be unified — or it is beyond the gap of every
+neighbour and opens a new session at its position.
+"""
+
+from __future__ import annotations
+
+from bisect import bisect_right
+
+OPEN = "open"
+ATTACH = "attach"
+MERGE = "merge"
+
+
+def place(sessions, ts, gap):
+    """Decide where ``ts`` lands among ``sessions`` (sorted by start).
+
+    Returns ``(OPEN, insert_index)``, ``(ATTACH, session_index)``, or
+    ``(MERGE, left_index)`` — merge unifies ``left_index`` with
+    ``left_index + 1``.
+    """
+    starts = [session.start for session in sessions]
+    index = bisect_right(starts, ts)
+    pred = sessions[index - 1] if index > 0 else None
+    succ = sessions[index] if index < len(sessions) else None
+    near_pred = pred is not None and ts - pred.end <= gap
+    near_succ = succ is not None and succ.start - ts <= gap
+    if near_pred and near_succ:
+        return (MERGE, index - 1)
+    if near_pred:
+        return (ATTACH, index - 1)
+    if near_succ:
+        return (ATTACH, index)
+    return (OPEN, index)
--- a/src/sessions.py
+++ b/src/sessions.py
@@ -28,9 +28,19 @@ class Session:
 
     def extend(self, event):
         """Credit a further event to this session."""
-        self.end = event.ts
+        if event.ts < self.start:
+            self.start = event.ts
+        if event.ts > self.end:
+            self.end = event.ts
         self._apply(event)
 
+    def merge_from(self, other):
+        """Unify another session's events into this one."""
+        self.start = min(self.start, other.start)
+        self.end = max(self.end, other.end)
+        for name, aggregate in self._aggregates.items():
+            aggregate.combine(other._aggregates[name])
+
     def aggregate_values(self):
         """Current aggregate results, keyed by configured name."""
         return {name: agg.value() for name, agg in self._aggregates.items()}
--- a/src/tracker.py
+++ b/src/tracker.py
@@ -15,6 +15,7 @@ downstream as it happens).
 
 from __future__ import annotations
 
+from src import assign
 from src.sessions import Session
 
 
@@ -33,11 +34,18 @@ class Sessionizer:
             self._metrics.increment("events_dropped_late")
             return False
         sessions = self._open.setdefault(event.key, [])
-        last = sessions[-1] if sessions else None
-        if last is not None and event.ts - last.end <= self._config.gap:
-            last.extend(event)
+        action, index = assign.place(sessions, event.ts, self._config.gap)
+        if action == assign.MERGE:
+            keep = sessions[index]
+            other = sessions.pop(index + 1)
+            keep.extend(event)
+            keep.merge_from(other)
+        elif action == assign.ATTACH:
+            sessions[index].extend(event)
         else:
-            sessions.append(Session(event.key, event, self._config.aggregates))
+            sessions.insert(
+                index, Session(event.key, event, self._config.aggregates)
+            )
             self._metrics.increment("sessions_opened")
         return True