diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
new file mode 100644
index 0000000..a00cee1
--- /dev/null
+++ b/.pre-commit-config.yaml
@@ -0,0 +1,18 @@
+repos:
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v4.5.0
+    hooks:
+      - id: trailing-whitespace
+      - id: end-of-file-fixer
+      - id: check-yaml
+      - id: check-added-large-files
+
+  - repo: https://github.com/astral-sh/ruff-pre-commit
+    rev: v0.3.0
+    hooks:
+      - id: ruff   # Linting
+
+  - repo: https://github.com/psf/black
+    rev: stable
+    hooks:
+      - id: black  # formatting
diff --git a/src/__init__.py b/src/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/src/caesar_cipher_helper b/src/caesar_cipher_helper
new file mode 100644
index 0000000..2ba5dfb
--- /dev/null
+++ b/src/caesar_cipher_helper
@@ -0,0 +1,39 @@
+"""
+Lösung des Katas: Caesar Cipher Helper
+Link: https://www.codewars.com/kata/526d42b6526963598d0004db
+"""
+
+
+beginning_alphabet = ord("A")
+ending_alphabet = ord("Z")
+
+class CaesarCipher(object):
+    def __init__(self, offset):
+        self.offset = offset
+
+    def encode(self, st):
+        upper = st.upper()
+        encoded_st = ""
+        for char in upper:
+            if beginning_alphabet <= ord(char) <= ending_alphabet:
+                encoded_char = (ord(char) + self.offset)
+                if encoded_char > ending_alphabet:
+                    encoded_char -= (ending_alphabet - (beginning_alphabet - 1))
+                encoded_st += chr(encoded_char)
+            else:
+                encoded_st += char
+
+        return encoded_st
+
+    def decode(self, st):
+        decoded_st = ""
+        for char in st:
+            if beginning_alphabet <= ord(char) <= ending_alphabet:
+                decoded_char = (ord(char) - self.offset)
+                if decoded_char < beginning_alphabet:
+                    decoded_char += (ending_alphabet - (beginning_alphabet - 1))
+                decoded_st += chr(decoded_char)
+            else:
+                decoded_st += char
+
+        return decoded_st
diff --git a/src/kata_mean_square_error.py b/src/kata_mean_square_error.py
new file mode 100644
index 0000000..fa179f6
--- /dev/null
+++ b/src/kata_mean_square_error.py
@@ -0,0 +1,13 @@
+"""
+Lösung des Katas: Mean Square Error von kylehill
+Link zum Kata: https://www.codewars.com/kata/51edd51599a189fe7f000015
+
+"""
+
+
+def solution(array_a, array_b):
+    numbers_list = list(zip(array_a, array_b))
+    differences = [x[0] - x[1] for x in numbers_list]
+    mean_square_error = sum([x * x for x in differences]) / len(differences)
+
+    return mean_square_error
diff --git a/src/moduleA.py b/src/moduleA.py
new file mode 100644
index 0000000..b7f0906
--- /dev/null
+++ b/src/moduleA.py
@@ -0,0 +1,10 @@
+def addition(a=1, b=2):
+    return a + b
+
+
+def f(x: int, y: int) -> int:
+    return x + y
+
+
+if __name__ == "__main__":
+    print(addition(5, 5))
diff --git a/tests/test_moduleA.py b/tests/test_moduleA.py
new file mode 100644
index 0000000..42fe6cc
--- /dev/null
+++ b/tests/test_moduleA.py
@@ -0,0 +1,6 @@
+from src.moduleA import addition
+
+
+def test_a():
+    assert addition() == 3
+    assert addition(5, 5) == 10