From 9010d417e2b541b1080358dc2991b06a927b49e0 Mon Sep 17 00:00:00 2001
From: local <local@local>
Date: Thu, 5 Nov 2026 10:50:31 +0000
Subject: [PATCH] simple: validate exact expected coverage sequence in step()

---
 simple/ovl.py            | 13 +++++++------
 tests/simple/test_ovl.py | 13 +++++++++++++
 2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/simple/ovl.py b/simple/ovl.py
index 94f9eb28..4a19d1dc 100644
--- a/simple/ovl.py
+++ b/simple/ovl.py
@@ -304,13 +304,14 @@ def next_target(cursor, ranges):
 def step(model, opt, b, ctl):
     ids = b["target_ids"][b["mask"]]
     ranges = excluded(ctl)
-    first = next_target(ctl["cursor"], ranges)
-    last = first + ids.numel() - 1
-    for s, n in ranges:  # an excluded document inside the batch's span widens it by exactly its size
-        if first < s <= last:
-            last += n
-    if ids.numel() == 0 or ids[0].item() != first or ids[-1].item() != last:
+    expected, cursor = [], ctl["cursor"]
+    for _ in range(ids.numel()):  # the exact id sequence: consecutive, skipping each excluded range
+        cursor = next_target(cursor, ranges)
+        expected.append(cursor)
+        cursor += 1
+    if ids.numel() == 0 or ids.tolist() != expected:
         raise SystemExit(f"coverage error at cursor {ctl['cursor']}")  # gap, repeat or reorder
+    last = expected[-1]
     for g in opt.param_groups:
         g["lr"] = learning_rate(ctl)
     x, y, m = (b[k].to(DEV, non_blocking=True) for k in ("inputs", "targets", "mask"))
diff --git a/tests/simple/test_ovl.py b/tests/simple/test_ovl.py
index 90d851f0..cb09c483 100644
--- a/tests/simple/test_ovl.py
+++ b/tests/simple/test_ovl.py
@@ -185,6 +185,19 @@ def test_gap_of_wrong_size_or_place_fails(self):
             with self.assertRaisesRegex(SystemExit, "coverage error"):
                 ovl.step(self.model, self.opt, self.batch(ids), self.ctl(10, exclude))
 
+    def test_middle_duplicate_fails(self):
+        with self.assertRaisesRegex(SystemExit, "coverage error"):
+            ovl.step(self.model, self.opt, self.batch([10, 11, 11, 13]), self.ctl(10))
+
+    def test_middle_reorder_fails(self):
+        with self.assertRaisesRegex(SystemExit, "coverage error"):
+            ovl.step(self.model, self.opt, self.batch([10, 12, 11, 13]), self.ctl(10))
+
+    def test_wrong_excluded_middle_range_fails(self):
+        # 12 is excluded but trained, while 11 (not excluded) is skipped; endpoints still match.
+        with self.assertRaisesRegex(SystemExit, "coverage error"):
+            ovl.step(self.model, self.opt, self.batch([10, 12, 13, 14]), self.ctl(10, [[12, 1]]))
+
     def test_exclusion_only_applies_to_its_phase(self):
         c = {"phase": "conversation", "global_step": 0, "cursor": 10, "exclude": {"wikipedia": [[12, 3]]}}
         with self.assertRaisesRegex(SystemExit, "coverage error"):