2022/9/14---Python实现抽象数据类型：映射

# 映射数据类型在查找这一操作时的复杂度为O（1），要实现这样的快速查找，必须有一个结可以快速查找到key的结构，这可以用哈希表实现
class Map:
    """映射数据类型"""

    def __init__(self, size):
        """初始化映射"""
        # 维护一个hash表和一个与之对应的列表
        self.size = size  # size是hash表的大小，一般取素数
        self.slots = [None] * self.size
        self.values = [None] * self.size
        self._step = 1  # 再散列时探测空槽的步长。不能设置为size的倍数，否者形成周期，永远无法探测到空槽

    def hash_function(self, key):
        """散列函数，将key转换为hash值"""
        return key % self.size

    def rehash_function(self, hash_value):
        """再散列函数，防止散列冲突"""
        return (hash_value + self._step) % self._step

    def __getitem__(self, item):
        """此特殊方法实现用[]对键值的访问"""
        return self.get(item)

    def __setitem__(self, key, value):
        return self.put(key,value)

    def put(self, key, value):
        """添加键值到映射中"""
        hash_value = self.hash_function(key)  # 理论上应该存放key的槽号
        if self.slots[hash_value] is None:  # 检查槽是不是空
            self.slots[hash_value] = key
            self.values[hash_value] = value
        elif self.slots[hash_value] == key:
            self.values[hash_value] = value
        else:
            new_hash_value = self.rehash_function(hash_value)
            while self.slots[new_hash_value] is not None and self.slots[new_hash_value] != key:  # 如果槽不空且槽不是存的key
                # ，那么都还要继续rehash
                new_hash_value = self.rehash_function(new_hash_value)
            if self.slots[new_hash_value] is None:
                self.slots[new_hash_value] = key
                self.values[new_hash_value] = value
            else:
                self.values[hash_value] = value

    def get(self, key):  # 实现逻辑是，由于两个表是并行的，找到key的位置也就找到value的位置了，再返回value就好
        """通过键查找键值"""
        theoretic_position = self.hash_function(key)  # key的理论槽号
        position = theoretic_position
        if self.slots[position] == key:
            return self.values[position]
        else:
            new_position = self.rehash_function(position)
            while self.slots[new_position] != key and self.slots[new_position]  is not None:  # 没找到继续找，找了一轮没找到就不用找了
                new_position = self.rehash_function(position)
            if new_position == position:
                return None
            else:
                return self.values[new_position]

#创建空映射
map=Map(4)
#添加键值对到映射中
map[1]=12
map[3]=14
map[21]=7
#查看映射
print(map[1],map[3],map[21])