Add slowsort algorithm

slowsort.py

@@ -0,0 +1,76 @@
+"""
+Slowsort
+
+Slowsort is a humorous, deliberately inefficient sorting algorithm based on the
+"multiply and surrender" paradigm — the opposite of divide and conquer. It was
+invented by Andrei Broder and Jorge Stolfi and published in their 1986 paper
+"Pessimal Algorithms and Simplexity Analysis".
+
+The algorithm works recursively:
+1. Find the maximum of the first half and second half of the array separately.
+2. Compare those two maximums and place the larger one at the end.
+3. Recursively sort everything except the last element.
+
+Slowsort is provably non-optimal and runs in superpolynomial time even on
+average, making it slower than bogosort for small inputs but guaranteed to
+terminate.
+
+Time Complexity:  O(n^(log n / 2)) — superpolynomial, worse than any polynomial
+Space Complexity: O(log n) due to recursion stack
+
+Reference:
+    https://en.wikipedia.org/wiki/Slowsort
+"""
+
+from __future__ import annotations
+
+
+def slowsort(arr: list[int], low: int, high: int) -> None:
+    """
+    Recursively sort arr[low..high] in place using the slowsort algorithm.
+
+    Args:
+        arr:  The list of integers to sort (modified in place).
+        low:  The starting index of the subarray to sort.
+        high: The ending index of the subarray to sort (inclusive).
+
+    >>> a = [5, 3, 8, 1, 9, 2]
+    >>> slowsort(a, 0, len(a) - 1)
+    >>> a
+    [1, 2, 3, 5, 8, 9]
+
+    >>> b = [1]
+    >>> slowsort(b, 0, 0)
+    >>> b
+    [1]
+
+    >>> c = [4, 4, 4]
+    >>> slowsort(c, 0, len(c) - 1)
+    >>> c
+    [4, 4, 4]
+
+    >>> d = [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
+    >>> slowsort(d, 0, len(d) - 1)
+    >>> d
+    [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+    """
+    if low >= high:
+        return
+    mid = (low + high) // 2
+    slowsort(arr, low, mid)
+    slowsort(arr, mid + 1, high)
+    if arr[mid] > arr[high]:
+        arr[mid], arr[high] = arr[high], arr[mid]
+    slowsort(arr, low, high - 1)
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+
+    user_input = input("Enter numbers separated by commas: ").strip()
+    unsorted = [int(x) for x in user_input.split(",")]
+    print(f"Unsorted: {unsorted}")
+    slowsort(unsorted, 0, len(unsorted) - 1)
+    print(f"Sorted:   {unsorted}")