fix Windows build: stub dense_matrix symbols and guard dense bindings

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Transurgeon

setup.py

@@ -45,6 +45,9 @@ def finalize_options(self) -> None:
     if not any(ex in s for ex in _exclude)
 ] + ["sparsediffpy/_bindings/bindings.c"]
 
+if system == "windows":
+    diff_engine_sources.append("sparsediffpy/_bindings/dense_matrix_stub.c")
+
 # Define _POSIX_C_SOURCE on Linux for clock_gettime and struct timespec
 defines = []
 if system == "linux":

sparsediffpy/_bindings/bindings.c

@@ -11,7 +11,9 @@
 #include "atoms/const_vector_mult.h"
 #include "atoms/constant.h"
 #include "atoms/cos.h"
+#ifndef _MSC_VER
 #include "atoms/dense_matmul.h"
+#endif
 #include "atoms/diag_vec.h"
 #include "atoms/entr.h"
 #include "atoms/exp.h"
@@ -112,12 +114,16 @@ static PyMethodDef DNLPMethods[] = {
     {"make_xexp", py_make_xexp, METH_VARARGS, "Create xexp node"},
     {"make_sparse_left_matmul", py_make_sparse_left_matmul, METH_VARARGS,
      "Create sparse left matmul node (A @ f(x))"},
+#ifndef _MSC_VER
     {"make_dense_left_matmul", py_make_dense_left_matmul, METH_VARARGS,
      "Create dense left matmul node (A @ f(x)) where A is dense"},
+#endif
     {"make_sparse_right_matmul", py_make_sparse_right_matmul, METH_VARARGS,
      "Create sparse right matmul node (f(x) @ A)"},
+#ifndef _MSC_VER
     {"make_dense_right_matmul", py_make_dense_right_matmul, METH_VARARGS,
      "Create dense right matmul node (f(x) @ A) where A is dense"},
+#endif
     {"make_quad_form", py_make_quad_form, METH_VARARGS,
      "Create quadratic form node (x' * Q * x)"},
     {"make_quad_over_lin", py_make_quad_over_lin, METH_VARARGS,

sparsediffpy/_bindings/dense_matrix_stub.c

@@ -0,0 +1,25 @@
+/*
+ * Stub implementations for dense matrix functions on Windows,
+ * where BLAS is not available. These satisfy the linker for
+ * left_matmul.c which references them, but the dense matmul
+ * bindings are excluded via #ifndef _MSC_VER so they are
+ * never actually called.
+ */
+#ifdef _MSC_VER
+#include "utils/matrix.h"
+#include <stdlib.h>
+
+Matrix *new_dense_matrix(int m, int n, const double *data)
+{
+    (void) m;
+    (void) n;
+    (void) data;
+    return NULL;
+}
+
+Matrix *dense_matrix_trans(const Dense_Matrix *self)
+{
+    (void) self;
+    return NULL;
+}
+#endif