From c815d62fdb10312c842814e94a82fc4e8f3c4e6e Mon Sep 17 00:00:00 2001
From: tp <terji78@yaho.com>
Date: Mon, 14 May 2018 01:05:30 +0100
Subject: [PATCH] improved performance of CategoricalIndex.is_monotonic*

---
 asv_bench/benchmarks/categoricals.py  | 20 +++++++++++++++++++
 doc/source/whatsnew/v0.23.1.txt       |  1 +
 pandas/core/indexes/category.py       |  4 ++--
 pandas/tests/indexes/test_category.py | 28 ++++++++++++++++-----------
 4 files changed, 40 insertions(+), 13 deletions(-)

diff --git a/asv_bench/benchmarks/categoricals.py b/asv_bench/benchmarks/categoricals.py
index ae1d7029217a4..5464e7cba22c3 100644
--- a/asv_bench/benchmarks/categoricals.py
+++ b/asv_bench/benchmarks/categoricals.py
@@ -173,3 +173,23 @@ def setup(self, dtype):
 
     def time_isin_categorical(self, dtype):
         self.series.isin(self.sample)
+
+
+class IsMonotonic(object):
+
+    def setup(self):
+        N = 1000
+        self.c = pd.CategoricalIndex(list('a' * N + 'b' * N + 'c' * N))
+        self.s = pd.Series(self.c)
+
+    def time_categorical_index_is_monotonic_increasing(self):
+        self.c.is_monotonic_increasing
+
+    def time_categorical_index_is_monotonic_decreasing(self):
+        self.c.is_monotonic_decreasing
+
+    def time_categorical_series_is_monotonic_increasing(self):
+        self.s.is_monotonic_increasing
+
+    def time_categorical_series_is_monotonic_decreasing(self):
+        self.s.is_monotonic_decreasing
diff --git a/doc/source/whatsnew/v0.23.1.txt b/doc/source/whatsnew/v0.23.1.txt
index 5c9c3e2931bd9..8c5111e712a34 100644
--- a/doc/source/whatsnew/v0.23.1.txt
+++ b/doc/source/whatsnew/v0.23.1.txt
@@ -29,6 +29,7 @@ Deprecations
 Performance Improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
+- Improved performance of :meth:`CategoricalIndex.is_monotonic_increasing`, :meth:`CategoricalIndex.is_monotonic_decreasing` and :meth:`CategoricalIndex.is_monotonic` (:issue:`21025`)
 -
 -
 
diff --git a/pandas/core/indexes/category.py b/pandas/core/indexes/category.py
index 3ffef5804acf7..78b7ae7054248 100644
--- a/pandas/core/indexes/category.py
+++ b/pandas/core/indexes/category.py
@@ -382,11 +382,11 @@ def is_unique(self):
 
     @property
     def is_monotonic_increasing(self):
-        return Index(self.codes).is_monotonic_increasing
+        return self._engine.is_monotonic_increasing
 
     @property
     def is_monotonic_decreasing(self):
-        return Index(self.codes).is_monotonic_decreasing
+        return self._engine.is_monotonic_decreasing
 
     @Appender(_index_shared_docs['index_unique'] % _index_doc_kwargs)
     def unique(self, level=None):
diff --git a/pandas/tests/indexes/test_category.py b/pandas/tests/indexes/test_category.py
index 6a1a1a5bdba4f..0e630f69b1a32 100644
--- a/pandas/tests/indexes/test_category.py
+++ b/pandas/tests/indexes/test_category.py
@@ -543,35 +543,41 @@ def test_reindex_empty_index(self):
         tm.assert_numpy_array_equal(indexer,
                                     np.array([-1, -1], dtype=np.intp))
 
-    def test_is_monotonic(self):
-        c = CategoricalIndex([1, 2, 3])
+    @pytest.mark.parametrize('data, non_lexsorted_data', [
+        [[1, 2, 3], [9, 0, 1, 2, 3]],
+        [list('abc'), list('fabcd')],
+    ])
+    def test_is_monotonic(self, data, non_lexsorted_data):
+        c = CategoricalIndex(data)
         assert c.is_monotonic_increasing
         assert not c.is_monotonic_decreasing
 
-        c = CategoricalIndex([1, 2, 3], ordered=True)
+        c = CategoricalIndex(data, ordered=True)
         assert c.is_monotonic_increasing
         assert not c.is_monotonic_decreasing
 
-        c = CategoricalIndex([1, 2, 3], categories=[3, 2, 1])
+        c = CategoricalIndex(data, categories=reversed(data))
         assert not c.is_monotonic_increasing
         assert c.is_monotonic_decreasing
 
-        c = CategoricalIndex([1, 3, 2], categories=[3, 2, 1])
+        c = CategoricalIndex(data, categories=reversed(data), ordered=True)
         assert not c.is_monotonic_increasing
-        assert not c.is_monotonic_decreasing
+        assert c.is_monotonic_decreasing
 
-        c = CategoricalIndex([1, 2, 3], categories=[3, 2, 1], ordered=True)
+        # test when data is neither monotonic increasing nor decreasing
+        reordered_data = [data[0], data[2], data[1]]
+        c = CategoricalIndex(reordered_data, categories=reversed(data))
         assert not c.is_monotonic_increasing
-        assert c.is_monotonic_decreasing
+        assert not c.is_monotonic_decreasing
 
         # non lexsorted categories
-        categories = [9, 0, 1, 2, 3]
+        categories = non_lexsorted_data
 
-        c = CategoricalIndex([9, 0], categories=categories)
+        c = CategoricalIndex(categories[:2], categories=categories)
         assert c.is_monotonic_increasing
         assert not c.is_monotonic_decreasing
 
-        c = CategoricalIndex([0, 1], categories=categories)
+        c = CategoricalIndex(categories[1:3], categories=categories)
         assert c.is_monotonic_increasing
         assert not c.is_monotonic_decreasing