From c41e0e59446412511c5737cf5b6ba8f289e75e7e Mon Sep 17 00:00:00 2001
From: Scott Gasch <scott@gasch.org>
Date: Sat, 18 Sep 2021 18:12:26 -0700
Subject: [PATCH] Most/least common item in a list.

---
 list_utils.py | 41 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/list_utils.py b/list_utils.py
index 993ca8a..533317e 100644
--- a/list_utils.py
+++ b/list_utils.py
@@ -1,7 +1,8 @@
 #!/usr/bin/env python3
 
+from collections import Counter
 from itertools import islice
-from typing import Any, Iterator, List
+from typing import Any, Iterator, List, Mapping
 
 
 def shard(lst: List[Any], size: int) -> Iterator[Any]:
@@ -47,6 +48,44 @@ def prepend(item: Any, lst: List[Any]) -> List[Any]:
     return lst
 
 
+def population_counts(lst: List[Any]) -> Mapping[Any, int]:
+    """
+    Return a population count mapping for the list (i.e. the keys are
+    list items and the values are the number of occurrances of that
+    list item in the original list.
+
+    >>> population_counts([1, 1, 1, 2, 2, 3, 3, 3, 4])
+    Counter({1: 3, 3: 3, 2: 2, 4: 1})
+
+    """
+    return Counter(lst)
+
+
+def most_common_item(lst: List[Any]) -> Any:
+
+    """
+    Return the most common item in the list.  In the case of ties,
+    which most common item is returned will be random.
+
+    >>> most_common_item([1, 1, 1, 2, 2, 3, 3, 3, 3, 4, 4])
+    3
+
+    """
+    return population_counts(lst).most_common(1)[0][0]
+
+
+def least_common_item(lst: List[Any]) -> Any:
+    """
+    Return the least common item in the list.  In the case of
+    ties, which least common item is returned will be random.
+
+    >>> least_common_item([1, 1, 1, 2, 2, 3, 3, 3, 4])
+    4
+
+    """
+    return population_counts(lst).most_common()[-1][0]
+
+
 if __name__ == '__main__':
     import doctest
     doctest.testmod()
-- 
2.47.1