BUG: return different function if sparse multiplication is empty

gyral_structure/cuda.py

@@ -137,10 +137,15 @@ def sparse_mult_prepare(sparse):
     """Returns a function that does the GPU multiplication for the provided sparse matrix.
     """
     gpu_sparse = _sparse_mult_to_gpu(sparse)
+    output = gpuarray.zeros((M, ), dtype='f4' if dtype == 'float' else 'f8')
+    if len(gpu_sparse[-1]) == 0:
+        def empty_sparse_mult(vec_in):
+            return output
+        return empty_sparse_mult
+
     M = sparse.shape[0]
     gpu_func = cached_func(_sparse_mult_mod_vector(gpu_sparse[0].size - 1), 'sparse_matrix_mult')
     input = gpuarray.zeros((sparse.shape[1], ), dtype='f4' if dtype == 'float' else 'f8')
-    output = gpuarray.zeros((M, ), dtype='f4' if dtype == 'float' else 'f8')
     stream = drv.Stream()
     def sparse_mult(vec_in):
         """Does a sparse matrix multiplication.