fix trip_schedulin_choice comment

Author: janzill

activitysim/abm/models/trip_scheduling_choice.py

@@ -279,14 +279,28 @@ def run_trip_scheduling_choice(
         ) in chunk.adaptive_chunked_choosers(state, indirect_tours, trace_label):
             # Sort the choosers and get the schedule alternatives
             choosers = choosers.sort_index()
-            # FIXME-EET: For explicit error term choices, we need a stable alternative ID. Currently, we use
-            # SCHEDULE_ID, which justs enumerates all schedule alternatives, of which there are choosers times
-            # alternative, in the order they are processed, which depends on if there stops on outward/return leg.
-            # We might want to change SCHEDULE_ID to a fixed pattern of all possible combinations of
-            # (outbound, main, inbound) duration for the maximum possible tour duration (max time window). For
-            # 30min intervals, this leads to 1225 alternatives and therefore reasonable memory-wise for random numbers.
-            # It looks like all that would need to change for this is the generation of the schedule alternatives and
-            # the lookup of choices as elements in schedule after simulation because choosers are indexed by tour_id.
+            # FIXME-EET: under use_explicit_error_terms, error terms here are aligned positionally, not keyed
+            # to a stable alternative ID: no alts_context is passed to _interaction_sample_simulate (this direct
+            # private call also bypasses the public wrapper's warning about that), so each tour draws one EV1
+            # error per alternative slot from its own tour_id-keyed channel, and the j-th draw attaches to the
+            # j-th row of the tour's block in `schedules`. SCHEDULE_ID plays no role in the alignment; it is a
+            # per-call running enumeration used only to look up the chosen row after the simulation.
+            #
+            # The per-tour row order is canonical: get_pattern_index_and_arrays sorts each tour's feasible
+            # windows lexicographically by (main, outbound, inbound) duration via np.unique, independent of
+            # chunk composition and processing order. Error terms are therefore stable across scenarios for
+            # every tour whose stop pattern (outbound/inbound) and duration are unchanged. They are NOT aligned
+            # for a tour whose duration or stop pattern changes: the lexicographic enumeration shifts, so
+            # position j maps to a different duration triple.
+            #
+            # To keep draws aligned across such changes too, key them to a canonical universe of schedule
+            # patterns -- e.g. all (outbound, inbound) duration pairs up to the maximum time window, with the
+            # main leg implied by the tour duration; for 30min intervals that is 1225 stable IDs, reasonable
+            # memory-wise for random numbers, and a duration change then keeps the error terms of unchanged
+            # (outbound, inbound) allocations. This would mean making SCHEDULE_ID that stable pattern ID and
+            # passing an alts_context (see the alt_nrs_df machinery in _interaction_sample_simulate); the
+            # post-simulation lookup below would then need to match on (tour_id, SCHEDULE_ID) pairs since IDs
+            # would repeat across tours.
 
             schedules = generate_schedule_alternatives(choosers).sort_index()
 

activitysim/core/interaction_sample_simulate.py

@@ -565,10 +565,11 @@ def interaction_sample_simulate(
     # are NOT guaranteed to be consistent across scenarios that differ in alternative
     # availability. We cannot make this a hard error today because two production callers
     # rely on the warning-only fallback:
-    #   - trip_scheduling_choice: SCHEDULE_ID is a per-call enumeration that depends on
-    #     chunk composition and tour duration distribution (see FIXME in
-    #     trip_scheduling_choice.py:282-289 for the proposed redesign that would key
-    #     SCHEDULE_ID to a fixed (OB, MAIN, IB) duration tuple).
+    #   - trip_scheduling_choice: draws align positionally to each tour's canonical
+    #     schedule enumeration, which is stable while a tour's stop pattern and duration
+    #     are unchanged but shifts when either changes (see the FIXME-EET in
+    #     trip_scheduling_choice.py for the proposed stable-id redesign; note it calls
+    #     _interaction_sample_simulate directly and so does not pass through this warning).
     #   - tour_od_choice: OD id is a string concatenation `f"{orig}_{dest}"`; a stable
     #     integer universe would be O(n_zones^2) error terms per chooser, which is too
     #     large to allocate.