Optimize Quick Sort with in-place partitioning and hybrid approach

sorts/quick_sort.py

@@ -1,5 +1,5 @@
 """
-A pure Python implementation of the quick sort algorithm
+A pure Python implementation of the quick sort algorithm with optimizations
 
 For doctests run following command:
 python3 -m doctest -v quick_sort.py
@@ -13,9 +13,117 @@
 from random import randrange
 
 
+def insertion_sort(collection: list, left: int, right: int) -> None:
+    """Insertion sort for small arrays (optimization for quicksort).
+
+    Args:
+        collection: List to sort
+        left: Starting index
+        right: Ending index (inclusive)
+    """
+    for i in range(left + 1, right + 1):
+        key = collection[i]
+        j = i - 1
+        while j >= left and collection[j] > key:
+            collection[j + 1] = collection[j]
+            j -= 1
+        collection[j + 1] = key
+
+
+def median_of_three(collection: list, left: int, mid: int, right: int) -> int:
+    """Return the index of the median of three elements.
+
+    Args:
+        collection: List to find median in
+        left: Left index
+        mid: Middle index
+        right: Right index
+
+    Returns:
+        Index of the median element
+    """
+    a, b, c = collection[left], collection[mid], collection[right]
+    if (a <= b <= c) or (c <= b <= a):
+        return mid
+    elif (b <= a <= c) or (c <= a <= b):
+        return left
+    else:
+        return right
+
+
+def partition_hoare(collection: list, left: int, right: int) -> int:
+    """Hoare partition scheme for quicksort.
+
+    Args:
+        collection: List to partition
+        left: Left boundary
+        right: Right boundary
+
+    Returns:
+        Final position of pivot
+    """
+    # Use median of three for better pivot selection
+    mid = left + (right - left) // 2
+    pivot_idx = median_of_three(collection, left, mid, right)
+
+    # Move pivot to the beginning
+    collection[left], collection[pivot_idx] = collection[pivot_idx], collection[left]
+    pivot = collection[left]
+
+    i, j = left - 1, right + 1
+
+    while True:
+        i += 1
+        while collection[i] < pivot:
+            i += 1
+
+        j -= 1
+        while collection[j] > pivot:
+            j -= 1
+
+        if i >= j:
+            return j
+
+        collection[i], collection[j] = collection[j], collection[i]
+
+
+def quick_sort_inplace(collection: list, left: int = 0, right: int = None) -> None:
+    """Optimized in-place quicksort with multiple optimizations.
+
+    Optimizations:
+    - In-place partitioning (O(1) extra space vs O(n))
+    - Median-of-three pivot selection
+    - Hybrid with insertion sort for small arrays
+    - Hoare partition scheme (better for duplicates)
+
+    Args:
+        collection: List to sort (modified in-place)
+        left: Left boundary
+        right: Right boundary
+    """
+    if right is None:
+        right = len(collection) - 1
+
+    if left < right:
+        # Use insertion sort for small arrays (optimization)
+        if right - left < 10:
+            insertion_sort(collection, left, right)
+            return
+
+        # Partition and get pivot position
+        pivot_pos = partition_hoare(collection, left, right)
+
+        # Recursively sort left and right partitions
+        quick_sort_inplace(collection, left, pivot_pos)
+        quick_sort_inplace(collection, pivot_pos + 1, right)
+
+
 def quick_sort(collection: list) -> list:
     """A pure Python implementation of quicksort algorithm.
 
+    This is the original implementation kept for compatibility.
+    For better performance, use quick_sort_optimized().
+
     :param collection: a mutable collection of comparable items
     :return: the same collection ordered in ascending order
 
@@ -43,6 +151,40 @@
     return [*quick_sort(lesser), pivot, *quick_sort(greater)]
 
 
+def quick_sort_optimized(collection: list) -> list:
+    """Optimized quicksort with in-place partitioning and multiple optimizations.
+
+    Performance improvements:
+    - O(1) extra space vs O(n) in original
+    - Better pivot selection (median of three)
+    - Hybrid with insertion sort for small arrays
+    - More efficient partitioning
+
+    Args:
+        collection: List to sort
+
+    Returns:
+        Sorted list
+
+    Examples:
+    >>> quick_sort_optimized([0, 5, 3, 2, 2])
+    [0, 2, 2, 3, 5]
+    >>> quick_sort_optimized([])
+    []
+    >>> quick_sort_optimized([-2, 5, 0, -45])
+    [-45, -2, 0, 5]
+    >>> quick_sort_optimized([3, 3, 3, 3, 3])
+    [3, 3, 3, 3, 3]
+    """
+    if len(collection) < 2:
+        return collection
+
+    # Create a copy to avoid modifying the original
+    result = collection.copy()
+    quick_sort_inplace(result)
+    return result
+
+
 if __name__ == "__main__":
     # Get user input and convert it into a list of integers
     user_input = input("Enter numbers separated by a comma:\n").strip()