Optimization shell sort

[realDuYuanChao]

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
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[poyea]

It doesn't quite seem to me that this is an optimization. May I know the reasoning behind this?

[realDuYuanChao]

It doesn't quite seem to me that this is an optimization. May I know the reasoning behind this?

If array is sorted already. It's no need to execute assignment statement. See this

[poyea]

@shellhub I've done a simple benchmark for this on random and sorted lists. I'd say they're similar in performance... Can you illustrate with an example in the test & comment: like

# consider test case [xxx]

[cclauss]

@shellhub Please create a timeit (or similar) benchmark that proves that the proposed change delivers measurable performance improvement.

[realDuYuanChao]

def shell_sort_slow(collection):
    gaps = [701, 301, 132, 57, 23, 10, 4, 1]
    for gap in gaps:
        for i in range(gap, len(collection)):
            insert_value = collection[i]
            j = i
            while j >= gap and collection[j - gap] > insert_value:
                collection[j] = collection[j - gap]
                j -= gap
            collection[j] = insert_value
    return collection


def shell_sort_faster(collection):
    gaps = [701, 301, 132, 57, 23, 10, 4, 1]
    for gap in gaps:
        for i in range(gap, len(collection)):
            insert_value = collection[i]
            j = i
            while j >= gap and collection[j - gap] > insert_value:
                collection[j] = collection[j - gap]
                j -= gap
            if j != i:
                collection[j] = insert_value
    return collection


def benchmark():
    from timeit import timeit
    print("shell_sort_slow", timeit("z.shell_sort_slow(z.nums)", setup="import __main__ as z"))
    print("shell_sort_faster", timeit("z.shell_sort_faster(z.nums)", setup="import __main__ as z"))


if __name__ == "__main__":
    nums = [i for i in range(1, 1000)]
    benchmark()

output:

('shell_sort_slow', 16.11249089241028)
('shell_sort_faster', 13.815173864364624)

@poyea @cclauss

[realDuYuanChao]

Please review this PR. Thanks @poyea @cclauss @dhruvmanila @mateuszz0000