From 1424ca2cc6d1cbec961e6e55358b81b1f4c1c457 Mon Sep 17 00:00:00 2001
From: ebonnal <bonnal.enzo.dev@gmail.com>
Date: Sat, 4 Jan 2025 21:35:19 +0000
Subject: [PATCH] functools: add `running`

Co-authored-by: pierrenodet <nodet.pierre@gmail.com>
---
 README.md                        | 15 +++++++++++++++
 streamable/__init__.py           |  2 +-
 streamable/util/functiontools.py | 14 ++++++++++++++
 tests/test_readme.py             | 10 ++++++++++
 4 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 4810bb9c..5b5fe242 100644
--- a/README.md
+++ b/README.md
@@ -172,6 +172,21 @@ zeros: Stream[int] = (
 assert list(zeros) == [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
 ```
 
+### "running map"
+
+> {TODO: add description}
+
+```python
+from streamable import running
+
+cumulative_sum: Stream[int] = (
+    integers
+    .map(running(lambda cumsum, i: cumsum + i, initial=0))
+)
+
+assert list(cumulative_sum) == [0, 1, 3, 6, 10, 15, 21, 28, 36, 45]
+```
+
 
 
 ## `.foreach`
diff --git a/streamable/__init__.py b/streamable/__init__.py
index a51ad250..9266bca0 100644
--- a/streamable/__init__.py
+++ b/streamable/__init__.py
@@ -1,2 +1,2 @@
 from streamable.stream import Stream
-from streamable.util.functiontools import star
+from streamable.util.functiontools import running, star
diff --git a/streamable/util/functiontools.py b/streamable/util/functiontools.py
index fdabf049..5c64d1f2 100644
--- a/streamable/util/functiontools.py
+++ b/streamable/util/functiontools.py
@@ -114,3 +114,17 @@ def add(a: int, b: int) -> int:
     ```
     """
     return _Star(func)
+
+
+def running(func: Callable[[R, T], R], initial: R) -> Callable[[T], R]:
+    """
+    TODO
+    """
+    acc = initial
+
+    def _(elem: T) -> R:
+        nonlocal acc
+        acc = func(acc, elem)
+        return acc
+
+    return _
diff --git a/tests/test_readme.py b/tests/test_readme.py
index 225401e8..afe281a3 100644
--- a/tests/test_readme.py
+++ b/tests/test_readme.py
@@ -90,6 +90,16 @@ def test_starmap_example(self) -> None:
 
         assert list(zeros) == [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
 
+    def test_running_map_example(self) -> None:
+        from streamable import running
+
+        cumulative_sum: Stream[int] = (
+            integers
+            .map(running(lambda cumsum, i: cumsum + i, initial=0))
+        )
+
+        assert list(cumulative_sum) == [0, 1, 3, 6, 10, 15, 21, 28, 36, 45]
+
     def test_foreach_example(self) -> None:
         state: List[int] = []
         appending_integers: Stream[int] = integers.foreach(state.append)