一聚教程网:一个值得你收藏的教程网站

热门教程

利用PHP实现Hash表功能

时间:2022-06-24 15:16:40 编辑:袖梨 来源:一聚教程网

  Hash表作为最重要的数据结构之一,也叫做散列表。使用PHP实现Hash表的功能。PHP可以模拟实现Hash表的增删改查。通过对key的映射到数组中的一个位置来访问。映射函数叫做Hash函数,存放记录的数组称为Hash表。
Hash函数把任意长度的和类型的key转换成固定长度输出。不同的key可能拥有相同的hash。
Hash表的时间复杂度为O(1)

 代码如下 复制代码

class HashTable{
    private $arr = array();
    private $size = 10;
    public function __construct(){
        //SplFixedArray创建的数组比一般的Array()效率更高,因为更接近C的数组。创建时需要指定尺寸
        $this->arr = new SplFixedArray($this->size);
    }

    /**
     * Description: 简单hash算法。输入key,输出hash后的整数
     * @param $key
     * @return int
     */
    private function simpleHash($key){
        $len = strlen($key);
        //key中每个字符所对应的ASCII的值
        $asciiTotal = 0;
        for($i=0; $i<$len; $i++){
            $asciiTotal += ord($key[$i]);
        }
        return $asciiTotal % $this->size;
    }

    /**
     * Description: 赋值
     * @param $key
     * @param $value
     * @return bool
     */
    public function set($key, $value){
        $hash = $this->simpleHash($key);
        $this->arr[$hash] = $value;
        return true;
    }

    /**
     * Description: 取值
     * @param $key
     * @return mixed
     */
    public function get($key){
        $hash = $this->simpleHash($key);
        return $this->arr[$hash];
    }

    public function getList(){
        return $this->arr;
    }

    public function editSize($size){
        $this->size = $size;
        $this->arr->setSize($size);
    }
}
?>

下面对我们的HashTable进行测试。

 代码如下 复制代码

//测试1
$arr = new HashTable();
for($i=0; $i<15; $i++){
    $arr->set('key'.$i, 'value'.$i);
}
print_r($arr->getList());
//SplFixedArray Object
//(
//    [0] => value14
//    [1] => value4
//    [2] => value5
//    [3] => value6
//    [4] => value7
//    [5] => value8
//    [6] => value10
//    [7] => value11
//    [8] => value12
//    [9] => value13
//)
//不同的key可能产生相同的hash值,那么赋值的时候后操作会覆盖前操作。

//测试2
$arr->editSize(15);
for($i=0; $i<15; $i++){
    $arr->set('key'.$i, 'value'.$i);
}
print_r($arr->getList());
//SplFixedArray Object
//(
//    [0] => value14
//    [1] => value4
//    [2] => value0
//    [3] => value1
//    [4] => value2
//    [5] => value3
//    [6] => value10
//    [7] => value11
//    [8] => value12
//    [9] => value13
//    [10] => value14
//    [11] => value9
//    [12] =>
//    [13] =>
//    [14] =>
//)
?>

   

改变了值之后可以存放更多的元素。但是仍然存在不同的key可能产生相同的hash值,那么赋值的时候后操作会覆盖前操作的问题。这种冲突的问题我们来用拉链法解决。

    拉链法解决冲突。拉链法解决冲突的做法是将所有的相同Hash值的key放在一个链表中,比如key3和key14在hash之后都是0,那么在数组的键为0的地方存储这两个值,形式是链表。如果不能理解我的文字,请看下面的示例,看一下打印信息就明白了。拉链法是什么,就是链表。
    创建一个HashNode类,用来存储key和value的值,并且存储相同hash的另一个元素。在同一条链上,查找越后的元素越费时。时间复杂度为O(n).

 代码如下 复制代码

class HashNode{
    public $key;
    public $value;
    public $nextNode;
    public function __construct($key, $value, $nextNode=Null){
        $this->key = $key;
        $this->value = $value;
        $this->nextNode = $nextNode;
    }
}
class NewHashTable{
    private $arr;
    private $size = 10;
    public function __construct(){
        $this->arr = new SplFixedArray($this->size);
    }
    private function simpleHash($key){
        $asciiTotal = 0;
        $len = strlen($key);
        for($i=0; $i<$len; $i++){
            $asciiTotal += ord($key[$i]);
        }
        return $asciiTotal % $this->size;
    }
    public function set($key, $value){
        $hash = $this->simpleHash($key);
        if(isset($this->arr[$hash])){
            $newNode = new HashNode($key, $value, $this->arr[$hash]);
        }else{
            $newNode = new HashNode($key, $value, null);
        }
        $this->arr[$hash] = $newNode;
        return true;
    }
    public function get($key){
        $hash = $this->simpleHash($key);
        $current = $this->arr[$hash];
        while(!empty($current)){
            if($current->key == $key){
                return $current->value;
            }
            $current = $current->nextNode;
        }
        return NULL;
    }
    public function getList(){
        return $this->arr;
    }
}
?>

   

对我们新的HashTable进行测试

 代码如下 复制代码
//测试1
$newArr = new NewHashTable();
for($i=0; $i<30; $i++){
    $newArr->set('key'.$i, 'value'.$i);
}
print_r($newArr->getList());
var_dump($newArr->get('key3'));
//SplFixedArray Object
//(
//    [0] => HashNode Object
//(
//    [key] => key23
//            [value] => value23
//            [nextNode] => HashNode Object
//(
//    [key] => key14
//                    [value] => value14
//                    [nextNode] => HashNode Object
//(
//    [key] => key3
//                            [value] => value3
//                            [nextNode] =>
//                        )
//
//                )
//
//        )
//
//    [1] => HashNode Object
//(
//    [key] => key24
//            [value] => value24
//            [nextNode] => HashNode Object
//(
//    [key] => key15
//                    [value] => value15
//                    [nextNode] => HashNode Object
//(
//    [key] => key4
//                            [value] => value4
//                            [nextNode] =>
//                        )
//
//                )
//
//        )
//
//    [2] => HashNode Object
//(
//    [key] => key25
//            [value] => value25
//            [nextNode] => HashNode Object
//(
//    [key] => key16
//                    [value] => value16
//                    [nextNode] => HashNode Object
//(
//    [key] => key5
//                            [value] => value5
//                            [nextNode] =>
//                        )
//
//                )
//
//        )
//
//    [3] => HashNode Object
//(
//    [key] => key26
//            [value] => value26
//            [nextNode] => HashNode Object
//(
//    [key] => key17
//                    [value] => value17
//                    [nextNode] => HashNode Object
//(
//    [key] => key6
//                            [value] => value6
//                            [nextNode] =>
//                        )
//
//                )
//
//        )
//
//    [4] => HashNode Object
//(
//    [key] => key27
//            [value] => value27
//            [nextNode] => HashNode Object
//(
//    [key] => key18
//                    [value] => value18
//                    [nextNode] => HashNode Object
//(
//    [key] => key7
//                            [value] => value7
//                            [nextNode] =>
//                        )
//
//                )
//
//        )
//
//    [5] => HashNode Object
//(
//    [key] => key28
//            [value] => value28
//            [nextNode] => HashNode Object
//(
//    [key] => key19
//                    [value] => value19
//                    [nextNode] => HashNode Object
//(
//    [key] => key8
//                            [value] => value8
//                            [nextNode] =>
//                        )
//
//                )
//
//        )
//
//    [6] => HashNode Object
//(
//    [key] => key29
//            [value] => value29
//            [nextNode] => HashNode Object
//(
//    [key] => key10
//                    [value] => value10
//                    [nextNode] => HashNode Object
//(
//    [key] => key9
//                            [value] => value9
//                            [nextNode] =>
//                        )
//
//                )
//
//        )
//
//    [7] => HashNode Object
//(
//    [key] => key20
//            [value] => value20
//            [nextNode] => HashNode Object
//(
//    [key] => key11
//                    [value] => value11
//                    [nextNode] => HashNode Object
//(
//    [key] => key0
//                            [value] => value0
//                            [nextNode] =>
//                        )
//
//                )
//
//        )
//
//    [8] => HashNode Object
//(
//    [key] => key21
//            [value] => value21
//            [nextNode] => HashNode Object
//(
//    [key] => key12
//                    [value] => value12
//                    [nextNode] => HashNode Object
//(
//    [key] => key1
//                            [value] => value1
//                            [nextNode] =>
//                        )
//
//                )
//
//        )
//
//    [9] => HashNode Object
//(
//    [key] => key22
//            [value] => value22
//            [nextNode] => HashNode Object
//(
//    [key] => key13
//                    [value] => value13
//                    [nextNode] => HashNode Object
//(
//    [key] => key2
//                            [value] => value2
//                            [nextNode] =>
//                        )
//
//                )
//
//        )
//
//)
//string(6) "value3"
?>

热门栏目