From 1f2ad8050d517d799db174fa213693830e145333 Mon Sep 17 00:00:00 2001
From: Transurgeon <peter.zijie@gmail.com>
Date: Sun, 28 Jun 2026 18:56:07 -0400
Subject: [PATCH] Add make_kron binding for the native kron atom

Expose new_kron via make_kron(param_capsule, child_capsule, const_is_left,
p, q, r, s): the variable-free operand is the parameter side, const_is_left
selects which operand it is, and (p, q)/(r, s) are A's/B's dims. Replaces the
orphaned kron.h stub (which referenced a nonexistent new_kron_identity and was
never wired into bindings.c). Bumps the SparseDiffEngine submodule to the
kron-native-atom commit.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 SparseDiffEngine                    |  2 +-
 sparsediffpy/_bindings/atoms/kron.h | 54 +++++++++++++++++++++++++++++
 sparsediffpy/_bindings/bindings.c   |  3 ++
 3 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 sparsediffpy/_bindings/atoms/kron.h

diff --git a/SparseDiffEngine b/SparseDiffEngine
index 0a3070d..339a4cf 160000
--- a/SparseDiffEngine
+++ b/SparseDiffEngine
@@ -1 +1 @@
-Subproject commit 0a3070d329cafd9d9fe14970ec045a674fbe797c
+Subproject commit 339a4cf9c4b7a1e6a57273a388f357391bd0f86e
diff --git a/sparsediffpy/_bindings/atoms/kron.h b/sparsediffpy/_bindings/atoms/kron.h
new file mode 100644
index 0000000..40eabc8
--- /dev/null
+++ b/sparsediffpy/_bindings/atoms/kron.h
@@ -0,0 +1,54 @@
+#ifndef ATOM_KRON_H
+#define ATOM_KRON_H
+
+#include "common.h"
+
+/* Kronecker product Z = kron(A, B). Exactly one operand is variable-free and is
+ * passed as the parameter capsule (wrap constants with make_parameter and
+ * param_id=-1 / PARAM_FIXED); the other carries the variables and is passed as
+ * the child capsule.
+ *
+ * Python signature:
+ *   make_kron(param_capsule, child_capsule, const_is_left, p, q, r, s)
+ * - const_is_left: 1 -> A=param, B=child; 0 -> A=child, B=param.
+ * - (p, q): A's dims; (r, s): B's dims. */
+static PyObject *py_make_kron(PyObject *self, PyObject *args)
+{
+    PyObject *param_capsule;
+    PyObject *child_capsule;
+    int const_is_left, p, q, r, s;
+
+    if (!PyArg_ParseTuple(args, "OOiiiii", &param_capsule, &child_capsule,
+                          &const_is_left, &p, &q, &r, &s))
+    {
+        return NULL;
+    }
+
+    expr *param_node =
+        (expr *) PyCapsule_GetPointer(param_capsule, EXPR_CAPSULE_NAME);
+    if (!param_node)
+    {
+        PyErr_SetString(PyExc_ValueError, "invalid parameter capsule");
+        return NULL;
+    }
+
+    expr *child =
+        (expr *) PyCapsule_GetPointer(child_capsule, EXPR_CAPSULE_NAME);
+    if (!child)
+    {
+        PyErr_SetString(PyExc_ValueError, "invalid child capsule");
+        return NULL;
+    }
+
+    expr *node = new_kron(param_node, child, const_is_left, p, q, r, s);
+    if (!node)
+    {
+        PyErr_SetString(PyExc_RuntimeError, "failed to create kron node");
+        return NULL;
+    }
+
+    expr_retain(node); /* Capsule owns a reference */
+    return PyCapsule_New(node, EXPR_CAPSULE_NAME, expr_capsule_destructor);
+}
+
+#endif /* ATOM_KRON_H */
diff --git a/sparsediffpy/_bindings/bindings.c b/sparsediffpy/_bindings/bindings.c
index d394fa4..6fd9c07 100644
--- a/sparsediffpy/_bindings/bindings.c
+++ b/sparsediffpy/_bindings/bindings.c
@@ -16,6 +16,7 @@
 #include "atoms/getters.h"
 #include "atoms/hstack.h"
 #include "atoms/index.h"
+#include "atoms/kron.h"
 #include "atoms/left_matmul.h"
 #include "atoms/log.h"
 #include "atoms/logistic.h"
@@ -94,6 +95,8 @@ static PyMethodDef DNLPMethods[] = {
      "Create elementwise multiply node"},
     {"make_convolve", py_make_convolve, METH_VARARGS,
      "Create 1D full convolution node: y = conv(kernel_param, child)"},
+    {"make_kron", py_make_kron, METH_VARARGS,
+     "Create Kronecker product node: Z = kron(A, B)"},
     {"make_matmul", py_make_matmul, METH_VARARGS,
      "Create matrix multiplication node (Z = X @ Y)"},
     {"make_param_scalar_mult", py_make_param_scalar_mult, METH_VARARGS,