From 2d302469d58224ffa3b0ef7ff5983fc240acddeb Mon Sep 17 00:00:00 2001
From: Krzysztof Wojtas <kwojtas@gmail.com>
Date: Tue, 1 Oct 2024 23:58:24 +0200
Subject: [PATCH] #67 min_heapify

---
 src/solutions/chapter6/__init__.py           |  0
 src/solutions/chapter6/section2/__init__.py  |  0
 src/solutions/chapter6/section2/exercise3.py | 25 ++++++++++++
 test/test_solutions/test_chapter6.py         | 40 ++++++++++++++++++++
 4 files changed, 65 insertions(+)
 create mode 100644 src/solutions/chapter6/__init__.py
 create mode 100644 src/solutions/chapter6/section2/__init__.py
 create mode 100644 src/solutions/chapter6/section2/exercise3.py
 create mode 100644 test/test_solutions/test_chapter6.py

diff --git a/src/solutions/chapter6/__init__.py b/src/solutions/chapter6/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/src/solutions/chapter6/section2/__init__.py b/src/solutions/chapter6/section2/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/src/solutions/chapter6/section2/exercise3.py b/src/solutions/chapter6/section2/exercise3.py
new file mode 100644
index 0000000..ae5f0d2
--- /dev/null
+++ b/src/solutions/chapter6/section2/exercise3.py
@@ -0,0 +1,25 @@
+from book.chapter6.section1 import left
+from book.chapter6.section1 import right
+
+
+def min_heapify(A, i: int) -> None:
+    """Restores the min-heap property violated by a single node.
+
+    Implements:
+        Min-Heapify
+
+    Args:
+        A: the array representing a min-heap in which the min-heap property is violated by a single node
+        i: the index of the node in A that is not smaller than either of its children
+    """
+    l = left(i)
+    r = right(i)
+    if l <= A.heap_size and A[l] < A[i]:
+        smallest = l
+    else:
+        smallest = i
+    if r <= A.heap_size and A[r] < A[smallest]:
+        smallest = r
+    if smallest != i:
+        A[i], A[smallest] = A[smallest], A[i]
+        min_heapify(A, smallest)
diff --git a/test/test_solutions/test_chapter6.py b/test/test_solutions/test_chapter6.py
new file mode 100644
index 0000000..37fbb59
--- /dev/null
+++ b/test/test_solutions/test_chapter6.py
@@ -0,0 +1,40 @@
+from hypothesis import given
+from hypothesis import strategies as st
+from hypothesis.strategies import integers
+from hypothesis.strategies import lists
+
+from book.chapter6.section1 import build_max_heap
+from book.data_structures import Array
+from solutions.chapter6.section2.exercise3 import min_heapify
+from test_case import ClrsTestCase
+from test_util import create_array
+from util import range_of
+
+
+def build_min_heap_by_inversion(A: Array, n: int) -> None:
+    """Relies on the fact that when we replace each element of a max heap by the element's opposite, the resulting array
+    is a min heap."""
+    build_max_heap(A, n)
+    for i in range_of(1, to=n):
+        A[i] = -A[i]
+
+
+class TestChapter6(ClrsTestCase):
+
+    @given(st.data())
+    def test_min_heapify(self, data):
+        elements = data.draw(lists(integers(), min_size=1))
+        n = len(elements)
+        A = create_array([-x for x in elements])
+        build_min_heap_by_inversion(A, n)
+        self.assertMinHeap(A)
+        new_root = data.draw(integers(max_value=A[1] - 1))
+        elements.remove(A[1])
+        elements.append(new_root)
+        A[1] = new_root  # possibly violate the min-heap property at the root
+
+        min_heapify(A, 1)
+
+        self.assertEqual(A.heap_size, n)
+        self.assertMinHeap(A)
+        self.assertArrayPermuted(A, elements, end=n)