BUG: tighten condition for bsr eliminate_zeros fast path (scipy#9690)

* BUG: tighten condition for bsr eliminate_zeros fast path

* move test as requested by CJ

scipy/sparse/bsr.py

@@ -542,16 +542,17 @@ def transpose(self, axes=None, copy=False):
 
     def eliminate_zeros(self):
         """Remove zero elements in-place."""
+
+        if not self.nnz:
+            return  # nothing to do
+
         R,C = self.blocksize
         M,N = self.shape
 
         mask = (self.data != 0).reshape(-1,R*C).sum(axis=1)  # nonzero blocks
 
         nonzero_blocks = mask.nonzero()[0]
 
-        if len(nonzero_blocks) == 0:
-            return  # nothing to do
-
         self.data[:len(nonzero_blocks)] = self.data[nonzero_blocks]
 
         # modifies self.indptr and self.indices *in place*

scipy/sparse/tests/test_base.py

@@ -4239,6 +4239,30 @@ def test_eliminate_zeros(self):
         assert_array_equal(asp.nnz, 3*4)
         assert_array_equal(asp.todense(),bsp.todense())
 
+    # github issue #9687
+    def test_eliminate_zeros_all_zero(self):
+        np.random.seed(0)
+        m = bsr_matrix(np.random.random((12, 12)), blocksize=(2, 3))
+
+        # eliminate some blocks, but not all
+        m.data[m.data <= 0.9] = 0
+        m.eliminate_zeros()
+        assert_equal(m.nnz, 66)
+        assert_array_equal(m.data.shape, (11, 2, 3))
+
+        # eliminate all remaining blocks
+        m.data[m.data <= 1.0] = 0
+        m.eliminate_zeros()
+        assert_equal(m.nnz, 0)
+        assert_array_equal(m.data.shape, (0, 2, 3))
+        assert_array_equal(m.todense(), np.zeros((12,12)))
+
+        # test fast path
+        m.eliminate_zeros()
+        assert_equal(m.nnz, 0)
+        assert_array_equal(m.data.shape, (0, 2, 3))
+        assert_array_equal(m.todense(), np.zeros((12,12)))
+
     def test_bsr_matvec(self):
         A = bsr_matrix(arange(2*3*4*5).reshape(2*4,3*5), blocksize=(4,5))
         x = arange(A.shape[1]).reshape(-1,1)