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from itertools import accumulate

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def cp932_substr(source, max_length):
    """CP932での最大バイト数(max_length) を超えない文字数で丸める"""

    if not source:
        return ""
    data = map(lambda i: len(i.encode("cp932")), source)
    target = filter(lambda i: i <= max_length, accumulate(data))
    count = len(list(target))
    return source[:count]