From fcb2ce3cf731a5a1e2a9d8a86d8a1cd4fb8a2533 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 26 Jun 2026 18:19:56 +0000
Subject: [PATCH 1/5] Initial plan


From 731daa2eebd3d98ed16464a2e454a35307a6bd1d Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 26 Jun 2026 18:25:30 +0000
Subject: [PATCH 2/5] Add slice_to_unit C API and tests

---
 src/__init__.py   |  2 +-
 src/__init__.pyi  |  1 +
 src/_arraykit.c   |  2 +-
 src/methods.c     | 42 ++++++++++++++++++++++++++++++++++++++++++
 src/methods.h     |  4 ++++
 test/test_util.py | 17 +++++++++++++++++
 6 files changed, 66 insertions(+), 2 deletions(-)

diff --git a/src/__init__.py b/src/__init__.py
index 9c8bfb8d..1a0648c2 100644
--- a/src/__init__.py
+++ b/src/__init__.py
@@ -28,6 +28,7 @@
 from ._arraykit import first_true_1d as first_true_1d
 from ._arraykit import first_true_2d as first_true_2d
 from ._arraykit import slice_to_ascending_slice as slice_to_ascending_slice
+from ._arraykit import slice_to_unit as slice_to_unit
 from ._arraykit import array_to_tuple_array as array_to_tuple_array
 from ._arraykit import array_to_tuple_iter as array_to_tuple_iter
 from ._arraykit import nonzero_1d as nonzero_1d
@@ -37,4 +38,3 @@
 from ._arraykit import AutoMap as AutoMap
 from ._arraykit import FrozenAutoMap as FrozenAutoMap
 from ._arraykit import NonUniqueError as NonUniqueError
-
diff --git a/src/__init__.pyi b/src/__init__.pyi
index 888bb58c..8f8beb53 100644
--- a/src/__init__.pyi
+++ b/src/__init__.pyi
@@ -207,5 +207,6 @@ def is_objectable_dt64(__array: np.ndarray, /) -> bool: ...
 def is_objectable(__array: np.ndarray, /) -> bool: ...
 def astype_array(__array: np.ndarray, __dtype: np.dtype | None, /) -> np.ndarray: ...
 def slice_to_ascending_slice(__slice: slice, __size: int) -> slice: ...
+def slice_to_unit(__slice: slice, /) -> tp.Optional[int]: ...
 def array_to_tuple_array(__array: np.ndarray) -> np.ndarray: ...
 def array_to_tuple_iter(__array: np.ndarray) -> tp.Iterator[tp.Tuple[tp.Any, ...]]: ...
\ No newline at end of file
diff --git a/src/_arraykit.c b/src/_arraykit.c
index 49b30298..7ce6eb20 100644
--- a/src/_arraykit.c
+++ b/src/_arraykit.c
@@ -22,6 +22,7 @@ static PyMethodDef arraykit_methods[] =  {
     {"column_1d_filter", column_1d_filter, METH_O, NULL},
     {"row_1d_filter", row_1d_filter, METH_O, NULL},
     {"slice_to_ascending_slice", slice_to_ascending_slice, METH_VARARGS, NULL},
+    {"slice_to_unit", slice_to_unit, METH_O, NULL},
     {"array_deepcopy",
             (PyCFunction)array_deepcopy,
             METH_VARARGS | METH_KEYWORDS,
@@ -157,4 +158,3 @@ PyInit__arraykit(void)
 #endif
     return m;
 }
-
diff --git a/src/methods.c b/src/methods.c
index d98f75cd..b82138e8 100644
--- a/src/methods.c
+++ b/src/methods.c
@@ -64,6 +64,48 @@ slice_to_ascending_slice(PyObject *Py_UNUSED(m), PyObject *args) {
     return AK_slice_to_ascending_slice(slice, PyLong_AsSsize_t(size));
 }
 
+PyObject *
+slice_to_unit(PyObject *Py_UNUSED(m), PyObject *a)
+{
+    if (!PySlice_Check(a)) {
+        return PyErr_Format(PyExc_TypeError,
+                "Expected a slice, not %s",
+                Py_TYPE(a)->tp_name);
+    }
+    PyObject* py_start = ((PySliceObject*)a)->start;
+    PyObject* py_stop = ((PySliceObject*)a)->stop;
+    PyObject* py_step = ((PySliceObject*)a)->step;
+
+    if (py_start == Py_None || py_stop == Py_None) {
+        Py_RETURN_NONE;
+    }
+
+    Py_ssize_t step = 1;
+    if (py_step != Py_None) {
+        step = PyLong_AsSsize_t(py_step);
+        if (step == -1 && PyErr_Occurred()) {
+            return NULL;
+        }
+    }
+    if (step != 1) {
+        Py_RETURN_NONE;
+    }
+
+    Py_ssize_t start = PyLong_AsSsize_t(py_start);
+    if (start == -1 && PyErr_Occurred()) {
+        return NULL;
+    }
+    Py_ssize_t stop = PyLong_AsSsize_t(py_stop);
+    if (stop == -1 && PyErr_Occurred()) {
+        return NULL;
+    }
+
+    if (start < 0 || stop < 0 || stop - start != 1) {
+        Py_RETURN_NONE;
+    }
+    return PyLong_FromSsize_t(start);
+}
+
 PyObject *
 column_2d_filter(PyObject *Py_UNUSED(m), PyObject *a)
 {
diff --git a/src/methods.h b/src/methods.h
index 1d33a558..63a1f817 100644
--- a/src/methods.h
+++ b/src/methods.h
@@ -14,6 +14,10 @@ row_1d_filter(PyObject *Py_UNUSED(m), PyObject *a);
 PyObject *
 slice_to_ascending_slice(PyObject *Py_UNUSED(m), PyObject *args);
 
+// Return an integer when a slice is exactly a single positive-position unit.
+PyObject *
+slice_to_unit(PyObject *Py_UNUSED(m), PyObject *a);
+
 // Reshape if necessary a flat ndim 1 array into a 2D array with one columns and rows of length.
 // related example: https://github.com/RhysU/ar/blob/master/ar-python.cpp
 PyObject *
diff --git a/test/test_util.py b/test/test_util.py
index 7cabbb32..b5efd4e8 100644
--- a/test/test_util.py
+++ b/test/test_util.py
@@ -23,6 +23,7 @@
 from arraykit import first_true_1d
 from arraykit import first_true_2d
 from arraykit import slice_to_ascending_slice
+from arraykit import slice_to_unit
 from arraykit import array_to_tuple_array
 from arraykit import array_to_tuple_iter
 
@@ -953,6 +954,22 @@ def test_slice_to_ascending_slice_i(self) -> None:
             slice(1, 2, None)
             )
 
+    def test_slice_to_unit_a(self) -> None:
+        self.assertEqual(slice_to_unit(slice(3, 4)), 3)
+        self.assertEqual(slice_to_unit(slice(0, 1)), 0)
+
+    def test_slice_to_unit_b(self) -> None:
+        self.assertIsNone(slice_to_unit(slice(0, 2)))
+        self.assertIsNone(slice_to_unit(slice(0, 1, 2)))
+        self.assertIsNone(slice_to_unit(slice(0, 1, -1)))
+        self.assertIsNone(slice_to_unit(slice(None, 1)))
+        self.assertIsNone(slice_to_unit(slice(0, None)))
+        self.assertIsNone(slice_to_unit(slice(-1, 0)))
+
+    def test_slice_to_unit_c(self) -> None:
+        with self.assertRaises(TypeError):
+            _ = slice_to_unit(3)
+
 
 if __name__ == '__main__':
     unittest.main()

From fa270ae90f7a8a8c985726865d60f7a446844e44 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 26 Jun 2026 18:28:13 +0000
Subject: [PATCH 3/5] Refine slice_to_unit tests and cleanup

---
 src/methods.c     | 2 +-
 test/test_util.py | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/methods.c b/src/methods.c
index b82138e8..aec33e64 100644
--- a/src/methods.c
+++ b/src/methods.c
@@ -60,7 +60,7 @@ slice_to_ascending_slice(PyObject *Py_UNUSED(m), PyObject *args) {
             &PyLong_Type, &size)) {
         return NULL;
     }
-    // will delegate NULL on eroror
+    // will delegate NULL on error
     return AK_slice_to_ascending_slice(slice, PyLong_AsSsize_t(size));
 }
 
diff --git a/test/test_util.py b/test/test_util.py
index b5efd4e8..322862b0 100644
--- a/test/test_util.py
+++ b/test/test_util.py
@@ -960,6 +960,7 @@ def test_slice_to_unit_a(self) -> None:
 
     def test_slice_to_unit_b(self) -> None:
         self.assertIsNone(slice_to_unit(slice(0, 2)))
+        self.assertIsNone(slice_to_unit(slice(5, 5)))
         self.assertIsNone(slice_to_unit(slice(0, 1, 2)))
         self.assertIsNone(slice_to_unit(slice(0, 1, -1)))
         self.assertIsNone(slice_to_unit(slice(None, 1)))

From f6e051cdb3d7b38b344ac85d99a61105f195e7b5 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 26 Jun 2026 18:40:42 +0000
Subject: [PATCH 4/5] Handle None start in slice_to_unit

---
 src/methods.c     | 11 +++++++----
 test/test_util.py |  3 ++-
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/src/methods.c b/src/methods.c
index aec33e64..3bbce711 100644
--- a/src/methods.c
+++ b/src/methods.c
@@ -76,7 +76,7 @@ slice_to_unit(PyObject *Py_UNUSED(m), PyObject *a)
     PyObject* py_stop = ((PySliceObject*)a)->stop;
     PyObject* py_step = ((PySliceObject*)a)->step;
 
-    if (py_start == Py_None || py_stop == Py_None) {
+    if (py_stop == Py_None) {
         Py_RETURN_NONE;
     }
 
@@ -91,9 +91,12 @@ slice_to_unit(PyObject *Py_UNUSED(m), PyObject *a)
         Py_RETURN_NONE;
     }
 
-    Py_ssize_t start = PyLong_AsSsize_t(py_start);
-    if (start == -1 && PyErr_Occurred()) {
-        return NULL;
+    Py_ssize_t start = 0;
+    if (py_start != Py_None) {
+        start = PyLong_AsSsize_t(py_start);
+        if (start == -1 && PyErr_Occurred()) {
+            return NULL;
+        }
     }
     Py_ssize_t stop = PyLong_AsSsize_t(py_stop);
     if (stop == -1 && PyErr_Occurred()) {
diff --git a/test/test_util.py b/test/test_util.py
index 322862b0..d27d5f2e 100644
--- a/test/test_util.py
+++ b/test/test_util.py
@@ -957,14 +957,15 @@ def test_slice_to_ascending_slice_i(self) -> None:
     def test_slice_to_unit_a(self) -> None:
         self.assertEqual(slice_to_unit(slice(3, 4)), 3)
         self.assertEqual(slice_to_unit(slice(0, 1)), 0)
+        self.assertEqual(slice_to_unit(slice(None, 1)), 0)
 
     def test_slice_to_unit_b(self) -> None:
         self.assertIsNone(slice_to_unit(slice(0, 2)))
         self.assertIsNone(slice_to_unit(slice(5, 5)))
         self.assertIsNone(slice_to_unit(slice(0, 1, 2)))
         self.assertIsNone(slice_to_unit(slice(0, 1, -1)))
-        self.assertIsNone(slice_to_unit(slice(None, 1)))
         self.assertIsNone(slice_to_unit(slice(0, None)))
+        self.assertIsNone(slice_to_unit(slice(None, 2)))
         self.assertIsNone(slice_to_unit(slice(-1, 0)))
 
     def test_slice_to_unit_c(self) -> None:

From 674dfeea460e9c34fb983e60435f3d05174a9eda Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Sat, 27 Jun 2026 00:49:02 +0000
Subject: [PATCH 5/5] Apply remaining changes

---
 src/__init__.pyi  |  2 +-
 src/methods.c     |  6 +++---
 src/methods.h     |  2 +-
 test/test_util.py | 14 +++++++-------
 4 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/src/__init__.pyi b/src/__init__.pyi
index 8f8beb53..1484f9c0 100644
--- a/src/__init__.pyi
+++ b/src/__init__.pyi
@@ -207,6 +207,6 @@ def is_objectable_dt64(__array: np.ndarray, /) -> bool: ...
 def is_objectable(__array: np.ndarray, /) -> bool: ...
 def astype_array(__array: np.ndarray, __dtype: np.dtype | None, /) -> np.ndarray: ...
 def slice_to_ascending_slice(__slice: slice, __size: int) -> slice: ...
-def slice_to_unit(__slice: slice, /) -> tp.Optional[int]: ...
+def slice_to_unit(__slice: slice, /) -> int: ...
 def array_to_tuple_array(__array: np.ndarray) -> np.ndarray: ...
 def array_to_tuple_iter(__array: np.ndarray) -> tp.Iterator[tp.Tuple[tp.Any, ...]]: ...
\ No newline at end of file
diff --git a/src/methods.c b/src/methods.c
index 3bbce711..d2a2543a 100644
--- a/src/methods.c
+++ b/src/methods.c
@@ -77,7 +77,7 @@ slice_to_unit(PyObject *Py_UNUSED(m), PyObject *a)
     PyObject* py_step = ((PySliceObject*)a)->step;
 
     if (py_stop == Py_None) {
-        Py_RETURN_NONE;
+        return PyLong_FromLong(-1);
     }
 
     Py_ssize_t step = 1;
@@ -88,7 +88,7 @@ slice_to_unit(PyObject *Py_UNUSED(m), PyObject *a)
         }
     }
     if (step != 1) {
-        Py_RETURN_NONE;
+        return PyLong_FromLong(-1);
     }
 
     Py_ssize_t start = 0;
@@ -104,7 +104,7 @@ slice_to_unit(PyObject *Py_UNUSED(m), PyObject *a)
     }
 
     if (start < 0 || stop < 0 || stop - start != 1) {
-        Py_RETURN_NONE;
+        return PyLong_FromLong(-1);
     }
     return PyLong_FromSsize_t(start);
 }
diff --git a/src/methods.h b/src/methods.h
index 63a1f817..5d2a80c4 100644
--- a/src/methods.h
+++ b/src/methods.h
@@ -14,7 +14,7 @@ row_1d_filter(PyObject *Py_UNUSED(m), PyObject *a);
 PyObject *
 slice_to_ascending_slice(PyObject *Py_UNUSED(m), PyObject *args);
 
-// Return an integer when a slice is exactly a single positive-position unit.
+// Return an integer when a slice is exactly a single positive-position unit, else -1.
 PyObject *
 slice_to_unit(PyObject *Py_UNUSED(m), PyObject *a);
 
diff --git a/test/test_util.py b/test/test_util.py
index d27d5f2e..4a58cb61 100644
--- a/test/test_util.py
+++ b/test/test_util.py
@@ -960,13 +960,13 @@ def test_slice_to_unit_a(self) -> None:
         self.assertEqual(slice_to_unit(slice(None, 1)), 0)
 
     def test_slice_to_unit_b(self) -> None:
-        self.assertIsNone(slice_to_unit(slice(0, 2)))
-        self.assertIsNone(slice_to_unit(slice(5, 5)))
-        self.assertIsNone(slice_to_unit(slice(0, 1, 2)))
-        self.assertIsNone(slice_to_unit(slice(0, 1, -1)))
-        self.assertIsNone(slice_to_unit(slice(0, None)))
-        self.assertIsNone(slice_to_unit(slice(None, 2)))
-        self.assertIsNone(slice_to_unit(slice(-1, 0)))
+        self.assertEqual(slice_to_unit(slice(0, 2)), -1)
+        self.assertEqual(slice_to_unit(slice(5, 5)), -1)
+        self.assertEqual(slice_to_unit(slice(0, 1, 2)), -1)
+        self.assertEqual(slice_to_unit(slice(0, 1, -1)), -1)
+        self.assertEqual(slice_to_unit(slice(0, None)), -1)
+        self.assertEqual(slice_to_unit(slice(None, 2)), -1)
+        self.assertEqual(slice_to_unit(slice(-1, 0)), -1)
 
     def test_slice_to_unit_c(self) -> None:
         with self.assertRaises(TypeError):