编写程序，完成以下功能：实现 NextDate 函数，获得当前输入日期的前一个日期。 例如：输入日期 day 变量值大于 1,则把 day 变量的值加1；如果输入日期是1–11 月份中某月的最后一天,把day 变量的值置为下个月的一个月的第一天,month 变量的值加1； 如果输入日期是12月的最后一天，则 day 变量的值置为 1，month 变量的值置为 1，year 变量的值加 1。 关于最后一天的判断：如果是有31天的月份(1,3,5,7,8,10,12)，day 变量值为 31，如果是有30天的月份(4,6,9,11)，day 变量值为 30；如果是有29天的月份(闰年的2月)，day 变量值为29；如果是有 28 天的月份(非闰年的 2 月)，day 变量值为 28。 设计测试用例进行白盒测试，使其分别满足语句覆盖、判定覆盖、条件覆盖、判定/条件覆盖、组合覆盖以及路径覆盖。

下面是一个 Python 实现的 NextDate 函数：

def next_date(year, month, day):
    if not (1 <= year <= 9999 and 1 <= month <= 12 and 1 <= day <= 31):
        return "Invalid date!"
    if month in [1, 3, 5, 7, 8, 10, 12]:
        last_day = 31
    elif month in [4, 6, 9, 11]:
        last_day = 30
    elif year % 400 == 0 or (year % 4 == 0 and year % 100 != 0):
        last_day = 29
    else:
        last_day = 28
    if day == last_day:
        if month == 12:
            return (year + 1, 1, 1)
        else:
            return (year, month + 1, 1)
    elif day < last_day:
        return (year, month, day + 1)
    else:
        return "Invalid date!"

下面是 White-box testing 设计的测试用例：

语句覆盖：

| Test Case | Input | Expected Output |
| --- | --- | --- |
| 1 | year=2021, month=6, day=18 | (2021, 6, 19) |
| 2 | year=2021, month=2, day=28 | (2021, 3, 1) |

判定覆盖：

| Test Case | Input | Expected Output |
| --- | --- | --- |
| 1 | year=2000, month=2, day=29 | (2000, 3, 1) |
| 2 | year=2100, month=2, day=28 | (2100, 3, 1) |
| 3 | year=2021, month=6, day=31 | "Invalid date!" |

条件覆盖：

| Test Case | Input | Expected Output |
| --- | --- | --- |
| 1 | year=2021, month=6, day=18 | (2021, 6, 19) |
| 2 | year=2021, month=2, day=28 | (2021, 3, 1) |
| 3 | year=2000, month=2, day=29 | (2000, 3, 1) |
| 4 | year=2100, month=2, day=28 | (2100, 3, 1) |
| 5 | year=2021, month=6, day=31 | "Invalid date!" |

判定/条件覆盖：

| Test Case | Input | Expected Output |
| --- | --- | --- |
| 1 | year=2000, month=2, day=29 | (2000, 3, 1) |
| 2 | year=2100, month=2, day=28 | "Invalid date!" |

组合覆盖：

| Test Case | Input | Expected Output |
| --- | --- | --- |
| 1 | year=2021, month=6, day=18 | (2021, 6, 19) |
| 2 | year=2021, month=2, day=28 | (2021, 3, 1) |
| 3 | year=2000, month=2, day=29 | (2000, 3, 1) |
| 4 | year=2100, month=2, day=28 | "Invalid date!" |
| 5 | year=2021, month=6, day=31 | "Invalid date!" |

路径覆盖：

| Test Case | Input | Expected Output |
| --- | --- | --- |
| 1 | year=2021, month=6, day=18 | (2021, 6, 19) |
| 2 | year=2021, month=2, day=28 | (2021, 3, 1) |
| 3 | year=2000, month=2, day=29 | (2000, 3, 1) |
| 4 | year=2100, month=2, day=28 | "Invalid date!" |
| 5 | year=2021, month=6, day=31 | "Invalid date!" |