diff --git a/src/__init__.py b/src/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/src/heapsort.py b/src/heapsort.py
new file mode 100644
index 0000000..cfce9f1
--- /dev/null
+++ b/src/heapsort.py
@@ -0,0 +1,41 @@
+def swap(arr, i, j):
+    arr[i], arr[j] = arr[j], arr[i]
+
+
+def sift_down(arr, i, upper):
+    while True:
+        lef, rig = i * 2 + 1, i * 2 + 2
+
+        if rig < upper:
+            if arr[i] >= max(arr[lef], arr[rig]):
+                return
+            if arr[lef] > arr[rig]:
+                swap(arr, i, lef)
+                i = lef
+            else:
+                swap(arr, i, rig)
+                i = rig
+
+        elif lef < upper:
+            if arr[lef] > arr[i]:
+                swap(arr, i, lef)
+                i = lef
+            else:
+                return
+
+        else:
+            return
+
+
+def heapsort(arr):
+    for j in range((len(arr) - 2) // 2, -1, -1):
+        sift_down(arr, j, len(arr))
+
+    for end in range(len(arr) - 1, 0, -1):
+        swap(arr, 0, end)
+        sift_down(arr, 0, end)
+
+
+arr = [4, 90, 6, 14, 35, 7]
+heapsort(arr)
+print(arr)
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..3bae518
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,31 @@
+import pytest
+import random
+
+
+@pytest.fixture
+def sample_list():
+    return [4, 90, 6, 14, 35, 7]
+
+
+@pytest.fixture
+def many_duplicates():
+    return [i % 6 for i in range(200)]
+
+
+@pytest.fixture(
+    params=[
+        [],
+        [1],
+        [2, 1],
+        [5, 5, 5, 5],
+        [10, -1, 3, 5],
+    ]
+)
+def case_list(request):
+    return request.param
+
+
+@pytest.fixture
+def random_list():
+    random.seed(52)
+    return [random.randint(-1000, 1000) for i in range(100)]
diff --git a/tests/test_heapsort.py b/tests/test_heapsort.py
new file mode 100644
index 0000000..5950eb3
--- /dev/null
+++ b/tests/test_heapsort.py
@@ -0,0 +1,61 @@
+import pytest
+from src.heapsort import heapsort
+
+
+def test_heapsort():
+    a = [3, 1, 2]
+    heapsort(a)
+    assert a == [1, 2, 3]
+
+
+def test_negative_heapsort():
+    a = [-3, -1, -2]
+    heapsort(a)
+    assert a == [-3, -2, -1]
+
+
+@pytest.mark.parametrize(
+    "input_list, expected",
+    [
+        ([], []),
+        ([1], [1]),
+        ([2, 2, 2], [2, 2, 2]),
+        ([4, 3, 2, 1], [1, 2, 3, 4]),
+    ],
+)
+def test_parametrized(input_list, expected):
+    arr = list(input_list)
+    heapsort(arr)
+    assert arr == expected
+
+
+def test_duplicates_and_negatives():
+    arr = [0, -1, 5, -1, 3, 0, 5]
+    expected = sorted(arr)
+    heapsort(arr)
+    assert arr == expected
+
+
+def test_heapsort_sample_list(sample_list):
+    arr = sample_list.copy()
+    heapsort(arr)
+    assert arr == sorted(sample_list)
+
+
+def test_heapsort_many_duplicates(many_duplicates):
+    arr = many_duplicates.copy()
+    heapsort(arr)
+    assert arr == sorted(many_duplicates)
+
+
+def test_heapsort_cases(case_list):
+    arr = list(case_list)
+    heapsort(arr)
+    assert arr == sorted(case_list)
+
+
+def test_heapsort_random(random_list):
+    a = random_list.copy()
+    b = list(a)
+    heapsort(b)
+    assert b == sorted(a)