From 08ceab4f166046c4a036db975cc77cff76fef40d Mon Sep 17 00:00:00 2001
From: Bharat Kaurav <117659673+TheNetherWatcher@users.noreply.github.com>
Date: Sat, 31 Oct 2026 21:24:42 +0530
Subject: [PATCH 1/2] Document and test what the XQRS backsearch skip relies on

The skip added in #596 is only correct while _update_noise leaves qrs_thr
alone and _update_qrs moves last_qrs_peak_num to the current peak. Name
both in the comment and add a test that fails if either changes.
---
 src/wfdb/processing/qrs.py |  7 ++++---
 tests/test_processing.py   | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/src/wfdb/processing/qrs.py b/src/wfdb/processing/qrs.py
index 022ac219..38c7f255 100644
--- a/src/wfdb/processing/qrs.py
+++ b/src/wfdb/processing/qrs.py
@@ -610,9 +610,10 @@ def _backsearch(self):
 
         """
         if self.last_qrs_peak_num is not None:
-            # Skip peaks a previous backsearch already rejected. Their result
-            # can only change after a QRS update, which moves
-            # last_qrs_peak_num past them.
+            # Skip peaks a previous backsearch already rejected. A backsearch result
+            # depends only on qrs_thr and last_qrs_ind, which change only in
+            # _update_qrs, and _update_qrs moves last_qrs_peak_num to the current
+            # peak. If either of these stops being true, remove this skip.
             start = max(self.last_qrs_peak_num, self._backsearch_peak_num) + 1
             for peak_num in range(start, self.peak_num + 1):
                 if self._is_qrs(peak_num=peak_num, backsearch=True):
diff --git a/tests/test_processing.py b/tests/test_processing.py
index fe1ca4c9..141d24b6 100644
--- a/tests/test_processing.py
+++ b/tests/test_processing.py
@@ -221,6 +221,39 @@ def spy(peak_num, backsearch=False):
         n_after = np.sum(xqrs.qrs_inds > 150 * fs)
         assert n_before > 30 and n_after > 30
 
+    def test_xqrs_backsearch_skip_assumptions(self):
+        """The backsearch skip relies on how XQRS updates its state (#597)."""
+        sig, fields = wfdb.rdsamp("sample-data/100", channels=[0])
+        fs = fields["fs"]
+        x = sig[:, 0]
+        rng = np.random.default_rng(0)
+        gap = 0.01 * rng.standard_normal(120 * fs)
+        # Scaled-down beats after the gap are only found by backsearch
+        y = np.concatenate([x[: 30 * fs], gap, 0.4 * x[30 * fs : 60 * fs]])
+
+        xqrs = processing.XQRS(sig=y, fs=fs)
+        update_noise = xqrs._update_noise
+        update_qrs = xqrs._update_qrs
+
+        def noise_spy(peak_num):
+            qrs_thr = xqrs.qrs_thr
+            update_noise(peak_num)
+            # A noise update must not change the result of a backsearch
+            assert xqrs.qrs_thr == qrs_thr
+
+        def qrs_spy(peak_num, backsearch=False):
+            update_qrs(peak_num, backsearch=backsearch)
+            # Peaks up to the current one count as already searched
+            assert xqrs.last_qrs_peak_num == xqrs.peak_num
+
+        with (
+            mock.patch.object(xqrs, "_update_noise", side_effect=noise_spy),
+            mock.patch.object(xqrs, "_update_qrs", side_effect=qrs_spy),
+        ):
+            xqrs.detect(verbose=False)
+
+        assert len(xqrs.backsearch_qrs_inds) > 0
+
 
 class TestXQRSDetect:
     """Tests for the xqrs_detect convenience wrapper."""

From e531bffc718cab6809b19f4afb122735c2e548f7 Mon Sep 17 00:00:00 2001
From: Bharat Kaurav <117659673+TheNetherWatcher@users.noreply.github.com>
Date: Sat, 31 Oct 2026 21:50:21 +0530
Subject: [PATCH 2/2] Also check last_qrs_ind in the noise update test

---
 tests/test_processing.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tests/test_processing.py b/tests/test_processing.py
index 141d24b6..2641b20b 100644
--- a/tests/test_processing.py
+++ b/tests/test_processing.py
@@ -236,10 +236,10 @@ def test_xqrs_backsearch_skip_assumptions(self):
         update_qrs = xqrs._update_qrs
 
         def noise_spy(peak_num):
-            qrs_thr = xqrs.qrs_thr
+            qrs_thr, last_qrs_ind = xqrs.qrs_thr, xqrs.last_qrs_ind
             update_noise(peak_num)
             # A noise update must not change the result of a backsearch
-            assert xqrs.qrs_thr == qrs_thr
+            assert (xqrs.qrs_thr, xqrs.last_qrs_ind) == (qrs_thr, last_qrs_ind)
 
         def qrs_spy(peak_num, backsearch=False):
             update_qrs(peak_num, backsearch=backsearch)