如何在 Python 自定义字典中实现整数类型键的一致性哈希？

Question

我正在尝试在 Python 中从头开始构建字典。我已经完成了大部分工作，但遇到了一个小问题。首先，我首先要说我正在使用内置的 Python hash() 来获取键的 hash_result （键可以是 int 或 str）然后索引由 hash_result 字典容量的百分比。如果密钥是一串字符，则一切正常。一旦键是整数，我的自定义字典就会崩溃。有时一切正常，其他时候，键在添加到字典时得到哈希值 0（例如），但在字典中搜索键 returns 一个 KeyError，因为键映射到索引 0 而不是 4。我相信起初，索引是通过 hash(key) % capacity(例如 4) 计算的，但是一旦容量增加 x2，函数 hash(key) % capacity(now 8 because x2) 返回的索引不同，这导致了问题。我在维基百科上看到了这个公式（hash(key) % capacity）。我有兴趣了解这是否是我面临的问题，如果不是，究竟是什么导致了这种不良行为以及如何解决它。下面是我的代码：

    class MyDictionary:
    
    __LOAD_FACTOR_LIMIT = 0.75
    __DEFAULT_CAPACITY = 4
    
    def __init__(self):
        self.__capacity = self.__DEFAULT_CAPACITY
        self.__keys = [[] for i in range(self.__capacity)]
        self.__values = [[] for i in range(self.__capacity)]

    @property
    def keys(self):
        return [item for current_list in self.__keys for item in current_list]

    @property
    def values(self):
        return [value for value_list in self.__values for value in value_list]

    def __setitem__(self, key, value):
        while self.__compute_load_factor() >= self.__LOAD_FACTOR_LIMIT:
            self.__extend_dict()

        index_hash = self.__hash_function(key)

        if self.__is_key_in_dict(index_hash, key):
            self.__set_value_to_an_existing_key(index_hash, key, value)
            return

        self.__set_value_to_a_new_key(index_hash, key, value)

    def __getitem__(self, key):
        index_hash = self.__hash_function(key)
        if self.__is_key_in_dict(index_hash, key):
            index_bucket = self.__get_index_bucket(index_hash, key)
            return self.__values[index_hash][index_bucket]
        raise KeyError('Key is not in dictionary!')

    def __str__(self):
        key_values = zip(self.keys, self.values)
        result = '{' + ", ".join([f"{key}: {value}"
                                  if isinstance(key, int) else f"'{key}': {value}"
                                  for key, value in key_values]) + '}'
        return result

    def __hash_function(self, key):
        index_hash = hash(key) % self.__capacity
        return index_hash

    def __is_key_in_dict(self, index_hash, key):
        if key in self.__keys[index_hash]:
            return True
        return False

    def __get_index_bucket(self, index_hash, key):
        index_bucket = self.__keys[index_hash].index(key)
        return index_bucket

    def __extend_dict(self):
        self.__keys += [[] for i in range(self.__capacity)]
        self.__values += [[] for i in range(self.__capacity)]
        self.__capacity *= 2

    def __set_value_to_an_existing_key(self, index_hash, key, value):
        index_bucket = self.__get_index_bucket(index_hash, key)
        self.__values[index_hash][index_bucket] = value

    def __set_value_to_a_new_key(self, index_hash, key, value):
        self.__keys[index_hash].append(key)
        self.__values[index_hash].append(value)

    def __compute_load_factor(self):
        k = len(self.__keys)
        n = len([bucket for bucket in self.__keys if bucket])
        return n / k

    def get(self, key, return_value=None):
        try:
            index_hash = self.__hash_function(key)
            index_bucket = self.__get_index_bucket(index_hash, key)
            if self.__is_key_in_dict(index_hash, key):
                return self.__keys[index_hash][index_bucket]
            raise KeyError('Key is not in dictionary!')
        except KeyError:
            return return_value

    def add(self):
        pass

    def pop(self):
        pass

    def clear(self):
        self.__capacity = self.__DEFAULT_CAPACITY
        self.__keys = [[] for i in range(self.__capacity)]
        self.__values = [[] for i in range(self.__capacity)]

    def items(self):
        zipped_key_value = zip(self.keys, self.values)
        return [item for item in zipped_key_value]



dictionary = MyDictionary()
dictionary.add()
dictionary[4] = 'hey'
dictionary['2'] = 'cya'
dictionary['4'] = 'welcome'
dictionary['5'] = 'welcome'
dictionary['32'] = 'heya'
dictionary['31'] = 'heya'
dictionary['36'] = 'heya'
dictionary['34'] = 'heya'
print(dictionary[4])

Answer 1

这是因为当负载超过阈值时，您通过调用 __extend_dict 方法来增加容量（存储在 __capacity 属性中），这使得存储桶的索引存储的现有值不再有效，因为您总是通过对容量取模来导出索引。

因此，每次增加字典的容量时，您都应该在新索引处重新插入现有的键和值：

def __extend_dict(self):
    self.__capacity *= 2
    new_keys = [[] for _ in range(self.__capacity)]
    new_values = [[] for _ in range(self.__capacity)]
    for keys, values in zip(self.__keys, self.__values):
        for key, value in zip(keys, values):
            index_hash = self.__hash_function(key)
            new_keys[index_hash].append(key)
            new_values[index_hash].append(value)
    self.__keys = new_keys
    self.__values = new_values

演示：https://replit.com/@blhsing/NewEnchantingPerimeter

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