merge sort

.gitignore

@@ -3,4 +3,5 @@ venv
 .vscode
 .coverage
 htmlcov
-__*
+__*
+.venv

src/sort/merge_sort.py

@@ -0,0 +1,38 @@
+def sort(array):
+    """Sorts array in-place using merge sort."""
+
+    def merge_sort(start_idx, end_idx, buffer):
+        if start_idx >= end_idx:
+            return
+
+        mid = (start_idx + end_idx) // 2
+        merge_sort(start_idx, mid, buffer)
+        merge_sort(mid + 1, end_idx, buffer)
+        merge(start_idx, mid, end_idx, buffer)
+
+    def merge(start_idx, mid, end_idx, buffer):
+        buffer[start_idx : end_idx + 1] = array[start_idx : end_idx + 1]
+        left = start_idx
+        right = mid + 1
+        current = start_idx
+
+        while left <= mid and right <= end_idx:
+            if buffer[left] <= buffer[right]:
+                array[current] = buffer[left]
+                left += 1
+            else:
+                array[current] = buffer[right]
+                right += 1
+            current += 1
+
+        while left <= mid:
+            array[current] = buffer[left]
+            left += 1
+            current += 1
+
+        # elements from right half already in correct place
+
+    if len(array) <= 1:
+        return
+
+    merge_sort(0, len(array) - 1, array[:])

tst/sort/test_merge_sort.py

@@ -0,0 +1,31 @@
+from src.sort.merge_sort import sort
+
+
+class TestMergeSort:
+    def test_empty_array(self):
+        data = []
+        sort(data)
+        assert data == []
+
+    def test_single_element(self):
+        data = [42]
+        sort(data)
+        assert data == [42]
+
+    def test_sorted_and_reverse_inputs(self):
+        sorted_array = [1, 2, 3, 4, 5]
+        reverse_array = list(reversed(sorted_array))
+        sort(sorted_array)
+        sort(reverse_array)
+        assert sorted_array == [1, 2, 3, 4, 5]
+        assert reverse_array == [1, 2, 3, 4, 5]
+
+    def test_mixed_values(self):
+        data = [5, -1, 3, 0, 3, 2, -5]
+        sort(data)
+        assert data == [-5, -1, 0, 2, 3, 3, 5]
+
+    def test_duplicates_preserved(self):
+        data = ["b", "a", "c", "a", "b", "a"]
+        sort(data)
+        assert data == ["a", "a", "a", "b", "b", "c"]