BUG: check for monotonic bin arrays in digitize

The check for monotonic bin arrays of digitize doesn't properly handle
inputs with repeated entries at the beginning of the array:

```
>>> np.__version__
'1.8.0'
>>> np.digitize([1], [0, 0 , 2])
array([2], dtype=int64)
>>> np.digitize([1], [2, 2, 0])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: The bins must be monotonically increasing or decreasing
```

Modified `check_array_monotonic` in `_compiled_base.c` to skip over repeating
entries before deciding to check for increasing or decreasing monotonicity
and added relevant tests to `test_function_base.py`.

1 files changed, 13 insertions, 0 deletions

diff --git a/numpy/lib/tests/test_function_base.py b/numpy/lib/tests/test_function_base.py
index 494b512f7..0d2a66f53 100644
--- a/numpy/lib/tests/test_function_base.py
+++ b/numpy/lib/tests/test_function_base.py
@@ -760,6 +760,19 @@ class TestDigitize(TestCase):
         x = rand(10)
         bins = np.linspace(x.min(), x.max(), 10)
         assert_(np.all(digitize(x, bins, True) != 10))
+    
+    def test_monotonic(self):
+        x = [0]
+        bins = [0, 0, 1]
+        digitize(x, bins)
+        bins = [1, 1, 0]
+        digitize(x, bins)
+        bins = [1, 1, 1, 1]
+        digitize(x, bins)
+        bins = [0, 0, 1, 0]
+        assert_raises(ValueError, digitize, x, bins)
+        bins = [1, 1, 0, 1]
+        assert_raises(ValueError, digitize, x, bins)
 
 
 class TestUnwrap(TestCase):