Fix comparison ordering to use absolute rank gaps

app/services/comparison.py

@@ -1,7 +1,5 @@
 """Viewer-safe, four-domain comparison calculations (#281)."""
 
-from math import sqrt
-
 from sqlalchemy.orm import Session
 
 from app.services.shelves import Shelf
@@ -10,16 +8,13 @@
 MIN_SHARED_FOR_SCORE = 5
 RESULT_LIMIT = 5
 METHOD = (
-    'We adjust for different list sizes and give extra weight to favorites '
-    'near the top.'
+    'We compare rank positions directly. Smaller differences mean closer agreement.'
 )
 
 
-def _position(rank: int, count: int) -> float:
-    """Top-weighted 0..1 position: zero is best, one is last."""
-    if count <= 1:
-        return 0.0
-    return sqrt((rank - 1) / (count - 1))
+def _rank_gap(your_rank: int, their_rank: int) -> int:
+    """Return the absolute number of places between two rankings."""
+    return abs(your_rank - their_rank)
 
 
 def _item(catalog, **extra) -> dict:
@@ -77,14 +72,6 @@ def compare_shelf(  # pylint: disable=too-many-locals
         getattr(row, shelf.join_col): row
         for row in db.query(tracker).filter(tracker.user_id == viewer.pk).all()
     }
-    viewer_placed = [
-        row
-        for row in viewer_trackers.values()
-        if row.on_rankings and row.rank is not None
-    ]
-    target_count = len(target_rows)
-    viewer_count = len(viewer_placed)
-
     base['recommendations'] = [
         _item(
             item,
@@ -102,14 +89,12 @@ def compare_shelf(  # pylint: disable=too-many-locals
         mine = viewer_trackers.get(item.pk)
         if mine is None or not mine.on_rankings or mine.rank is None:
             continue
-        my_position = _position(mine.rank, viewer_count)
-        their_position = _position(target_tracker.rank, target_count)
         shared.append(
             _item(
                 item,
                 your_rank=mine.rank,
                 their_rank=target_tracker.rank,
-                gap=round(abs(my_position - their_position), 4),
+                gap=_rank_gap(mine.rank, target_tracker.rank),
             )
         )
 
@@ -122,7 +107,7 @@ def compare_shelf(  # pylint: disable=too-many-locals
     )[:RESULT_LIMIT]
     if len(shared) >= MIN_SHARED_FOR_SCORE:
         mean_gap = sum(item['gap'] for item in shared) / len(shared)
-        base['alignment_score'] = round(max(0.0, 1 - mean_gap) * 100)
+        base['alignment_score'] = round(max(0.0, 100 - mean_gap))
         base['alignment_status'] = 'ready'
 
     if watchlist_visible:

tests/integration/router_comparison_test.py

@@ -112,8 +112,9 @@ def test_comparison_scores_visible_rankings_and_marks_watchlist(
     ]
     assert movies['recommendations'][0]['on_your_watchlist'] is True
     assert movies['recommendations'][1]['on_your_watchlist'] is False
-    assert movies['biggest_gaps'][0]['gap'] >= movies['biggest_gaps'][-1]['gap']
-    assert movies['most_aligned'][0]['gap'] <= movies['most_aligned'][-1]['gap']
+    assert [item['gap'] for item in movies['biggest_gaps']] == [4, 4, 2, 2, 0]
+    assert [item['gap'] for item in movies['most_aligned']] == [0, 2, 2, 4, 4]
+    assert movies['most_aligned'][0]['title'] == 'Movie 3'
 
 
 def test_hidden_watchlist_does_not_block_ranked_comparison(test_client: TestClient):

tests/unit/services/comparison_test.py

@@ -1,16 +1,13 @@
 # pylint: disable=missing-function-docstring
-"""The ranking normalization behind comparison scores."""
+"""The direct rank-distance calculation behind comparisons."""
 
-from app.services.comparison import _position
+from app.services.comparison import _rank_gap
 
 
-def test_top_weighting_expands_differences_near_the_favorites():
-    # Both spans are ten places long, but the top-of-list span deliberately
-    # counts for more than the same raw distance near the bottom.
-    top_gap = _position(11, 100) - _position(1, 100)
-    bottom_gap = _position(100, 100) - _position(90, 100)
-    assert top_gap > bottom_gap
+def test_equal_ranks_have_no_gap():
+    assert _rank_gap(14, 14) == 0
 
 
-def test_one_item_list_has_a_stable_best_position():
-    assert _position(1, 1) == 0.0
+def test_gap_is_the_absolute_rank_difference():
+    assert _rank_gap(88, 10) == 78
+    assert _rank_gap(10, 88) == 78