from __future__ import annotations

import unittest
from collections.abc import Sequence
from typing import Protocol, TypeVar


class SupportsLessThan(Protocol):
    def __lt__(self, other: object, /) -> bool:
        ...


T = TypeVar("T", bound=SupportsLessThan)


def merge_sort(items: Sequence[T]) -> list[T]:
    """Return a sorted copy of items using merge sort."""
    if len(items) <= 1:
        return list(items)

    midpoint = len(items) // 2
    left = merge_sort(items[:midpoint])
    right = merge_sort(items[midpoint:])

    return _merge(left, right)


def _merge(left: Sequence[T], right: Sequence[T]) -> list[T]:
    """Merge two sorted sequences while preserving duplicate order."""
    merged: list[T] = []
    left_index = 0
    right_index = 0

    while left_index < len(left) and right_index < len(right):
        # Prefer the left item on ties to keep the sort stable.
        if right[right_index] < left[left_index]:
            merged.append(right[right_index])
            right_index += 1
        else:
            merged.append(left[left_index])
            left_index += 1

    merged.extend(left[left_index:])
    merged.extend(right[right_index:])
    return merged


class MergeSortTests(unittest.TestCase):
    def test_sorts_numbers(self) -> None:
        self.assertEqual(merge_sort([5, 2, 9, 1, 5, 6]), [1, 2, 5, 5, 6, 9])

    def test_handles_empty_and_single_item_lists(self) -> None:
        self.assertEqual(merge_sort([]), [])
        self.assertEqual(merge_sort([42]), [42])

    def test_does_not_mutate_input(self) -> None:
        values = [3, 1, 2]
        self.assertEqual(merge_sort(values), [1, 2, 3])
        self.assertEqual(values, [3, 1, 2])

    def test_sorts_strings(self) -> None:
        self.assertEqual(merge_sort(["pear", "apple", "banana"]), ["apple", "banana", "pear"])


if __name__ == "__main__":
    unittest.main()