Integrate diff-engine as git submodule

.github/workflows/build.yml

@@ -54,6 +54,8 @@ jobs:
 
     steps:
       - uses: actions/checkout@v5
+        with:
+          submodules: recursive
       - name: Set Additional Envs
         run: |
           echo "PYTHON_SUBVERSION=$(echo $PYTHON_VERSION | cut -c 3-)" >> $GITHUB_ENV
@@ -114,6 +116,8 @@ jobs:
 
     steps:
       - uses: actions/checkout@v5
+        with:
+          submodules: recursive
       - uses: actions/setup-python@v6
         with:
           python-version: ${{ matrix.python-version }}

.github/workflows/test_nlp_solvers.yml

@@ -22,6 +22,8 @@ jobs:
         with:
           python-version: "3.12"
       - uses: actions/checkout@v5
+        with:
+          submodules: recursive
       - name: Set up Conda
         uses: conda-incubator/setup-miniconda@v3
         with:

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "diff_engine_core"]
+	path = diff_engine_core
+	url = https://github.com/dance858/DNLP-diff-engine

cvxpy/reductions/solvers/nlp_solvers/diff_engine/c_problem.py

@@ -22,11 +22,10 @@
 
 # Import the low-level C bindings
 try:
-    import dnlp_diff_engine as _diffengine
+    import _diffengine
 except ImportError as e:
     raise ImportError(
-        "dnlp-diff-engine is required for NLP solving. "
-        "Install with: pip install dnlp-diff-engine"
+        "NLP support requires diff-engine. Rebuild with: pip install -e ."
     ) from e
 
 from cvxpy.reductions.solvers.nlp_solvers.diff_engine.converters import (

cvxpy/reductions/solvers/nlp_solvers/diff_engine/converters.py

@@ -27,11 +27,10 @@
 
 # Import the low-level C bindings
 try:
-    import dnlp_diff_engine as _diffengine
+    import _diffengine
 except ImportError as e:
     raise ImportError(
-        "dnlp-diff-engine is required for NLP solving. "
-        "Install with: pip install dnlp-diff-engine"
+        "NLP support requires diff-engine. Rebuild with: pip install -e ."
     ) from e
 
 

setup.py

@@ -52,7 +52,7 @@
 
 setup_versioning.write_version_py()
 VERSION = setup_versioning.VERSION
-extensions = [setup_extensions.cvxcore, setup_extensions.sparsecholesky]
+extensions = [setup_extensions.cvxcore, setup_extensions.sparsecholesky, setup_extensions.diffengine]
 
 setup(
     name="cvxpy",

setup/extensions.py

@@ -13,6 +13,7 @@
 See the License for the specific language governing permissions and
 limitations under the License.
 """
+import glob
 import platform
 
 from pybind11.setup_helpers import Pybind11Extension
@@ -59,3 +60,35 @@ def not_on_windows(s: str) -> str:
     extra_compile_args=compiler_args,
     extra_link_args=['-O3'],
 )
+
+# Diff engine C extension for NLP support
+# Gather all C source files from diff_engine_core (submodule)
+diff_engine_sources = (
+    glob.glob('diff_engine_core/src/**/*.c', recursive=True) +
+    ['diff_engine_core/python/bindings.c']
+)
+
+# Filter out any Python wrapper files (we only want pure C)
+diff_engine_sources = [
+    s for s in diff_engine_sources
+    if 'dnlp_diff_engine' not in s
+]
+
+diffengine_compiler_args = [
+    '-O3',
+    '-std=c99',
+    '-Wall',
+    not_on_windows('-Wextra'),
+]
+
+diffengine = Extension(
+    '_diffengine',
+    sources=diff_engine_sources,
+    include_dirs=[
+        'diff_engine_core/include/',
+        'diff_engine_core/src/',
+        'diff_engine_core/python/',
+    ],
+    extra_compile_args=diffengine_compiler_args,
+    extra_link_args=['-lm'] if platform.system().lower() != 'windows' else [],
+)