John基于整数哈希函数设计的字符串哈希函数是否优良？是否适用于姓氏？

Nice question! Let's break this down thoroughly to evaluate if this hash function is "good" and whether it works for last name use cases.

First: Is this hash function "good"? #

First, let's recap the implementation we're looking at:

def h(s):
    if s == '':
        return 0
    else:
        return g(g(ord(s[0]) + h(s[1:])))

A "good" hash function typically needs three key traits:

• Low collision rate: Different inputs should rarely produce the same hash value
• Avalanche effect: Tiny changes to the input (like reordering characters or changing one character) should result in a drastically different hash
• Efficient computation: Fast to calculate, with minimal overhead

This function fails hard on the first two, and has a minor issue with the third:

1. No resistance to order-based collisions: The core of the calculation is the sum of all characters' ASCII values (since addition is commutative—order doesn't matter). Any string with the same total ASCII sum will produce the exact same hash, regardless of character order. For example:
   • "abc" and "cba" (97+98+99 = 99+98+97 = 294) will have identical hashes
   • "aab" and "baa" will also collide
2. No avalanche effect: Reordering characters is a significant input change, but it doesn't alter the hash at all. Even changing a character to another with the same ASCII value (though rare for letters) would leave the hash unchanged.
3. Recursion overhead: For long strings, this recursive implementation will hit Python's default recursion depth limit, causing a stack overflow error. Iterative implementations are far safer for this kind of calculation.

Bottom line: This is not a good hash function by any standard metric.

Second: Is it suitable for last name scenarios? #

Last name use cases (like database indexing, user identity matching, or deduplication) rely heavily on the hash function's ability to distinguish between distinct last names—even those that are similar or have typos.

This hash function is a terrible fit here for two big reasons:

• High collision risk for real-world names: Even though reversed last names are rare, there are plenty of scenarios where different last names could have the same ASCII sum. For example, "Lee" (76+101+101=278) and "Eel" (69+101+108=278) would collide. Typos that reorder characters (like "Smith" vs "Smtih") would also produce identical hashes, making it impossible to distinguish between correct and incorrect entries.
• Fails to capture name uniqueness: Last names are defined by their character order as much as their characters. A hash function that ignores order can't represent that uniqueness, which is critical for any system handling names.

Verdict: This hash function is completely unsuitable for last name scenarios.

内容的提问来源于stack exchange，提问作者Раджаб Эльдар оглы Агамов