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php的扩展与嵌入--php扩展中的数组和哈希表1

Jun 13, 2016 am 09:46 AM
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在php中,数组的底层实现就是哈希表,都是以key-value的形式出现的。在php的Zend引擎中,针对不同的哈希表操作,都有着专门的对哈希表进行操作的api。


Creation

对于哈希表而言,每次初始化的方式都是一样的,都由下面这个函数zend_hash_init来完成:

int zend_hash_init(HashTable *ht, uint nSize,
    hash_func_t pHashFunction,
    dtor_func_t pDestructor, zend_bool persistent)
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其中ht是指向哈希表的指针,既可以对一个已存在的hashtable变量取引用。也可以为新的hashtable申请内存。一般的方法就是:

ALLOC_HASHTABLE(ht),相当于ht = emalloc(sizeof(HashTable));。

nSize是哈希表的最大元素数,是为了提前申请好内存考虑的。如果它不是2的指数倍,会根据下式增长nSize = pow(2, ceil(log(nSize, 2)));,比如如果给了5,那么会增长到8.这个应该是为了内存管理比较方便所采用的机制。

pHashFunction属于以前版本的zend eigine函数,在新版本中一直设为NULL即可。

pDestructor指向当哈希表中的元素被删掉的时候(zend_hash_del() zend_hash_update())所调用的方法的入口,也就是一个相应的回调函数。假如说给定了method_name函数,那么在函数实现的时候:

void method_name(void *pElement)
pElement指向被删掉的元素

persistent这个是一个标志位,表示是否是持久型的哈希表,持久型的数据是独立于请求之外的,不会在RSHUTDOWN的时候被注销掉。但是如果设1的话,那么ht在申请内存的时候一定要使用pemalloc().

举个例子:在每个php请求生命周期中对symbol_table初始化的时候都会看到zend_hash_init(&EG(symbol_table), 50, NULL, ZVAL_PTR_DTOR, 0);
每当unset的时候,相应的存储在哈希表中的zval*都被发送给zval_ptr_dtor()进行销毁。


Population:

有四种主要的插入和更新哈希表中数据的函数:

int zend_hash_add(HashTable *ht, char *arKey, uint nKeyLen,
                void *pData, uint nDataSize, void **pDest);
int zend_hash_update(HashTable *ht, char *arKey, uint nKeyLen,
                void *pData, uint nDataSize, void **pDest);
int zend_hash_index_update(HashTable *ht, ulong h,
                void *pData, uint nDataSize, void **pDest);
int zend_hash_next_index_insert(HashTable *ht,
                void *pData, uint nDataSize, void **pDest);
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前两个函数添加带字符串索引的数据到hashtable中,比如php中$foo['bar'] = 'barvalue',那么在扩展中:

zend_hash_add(fooHashTbl, "bar", sizeof("bar"), &barZval, sizeof(zval*), NULL);

就把相应key值和对应的表值加入到了hashtable中去了。

add和update唯一的区别是如果key已经存在的话,add会失败的。

后两个函数是向ht中添加数字索引的数据。

zend_hash_next_index_insert()函数不需要索引值参数,而是自己直接计算出下一个数字索引值。

而如果想自己获得下一个元素的数字索引值也可以通过zend_hash_next_free_element()来获得索引。
ulong nextid = zend_hash_next_free_element(ht);
zend_hash_index_update(ht, nextid, &data, sizeof(data), NULL);
上面这段代码就相当于:

zend_hash_next_index_insert(HashTable *ht, &data,sizeof(data),NULL).

其中pDest参数可以用来存储新加入的元素的地址值。



Recall:查找

一般来说,有两种获得哈希表中数据的方法:

int zend_hash_find(HashTable *ht, char *arKey, uint nKeyLength,
                                        void **pData);
int zend_hash_index_find(HashTable *ht, ulong h, void **pData);
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在下面的这个例子中可以更清楚的看到:

void hash_sample(HashTable *ht, sample_data *data1)
{
   sample_data *data2;
   ulong targetID = zend_hash_next_free_element(ht);//获取下一个索引的位置
   if (zend_hash_index_update(ht, targetID,
           data1, sizeof(sample_data), NULL) == FAILURE) {//把数据data1插入到哈希表的下一个索引的位置中去
       /* Should never happen */
       return;
   }
   if(zend_hash_index_find(ht, targetID, (void **)&data2) == FAILURE) {//利用id去寻找哈希表中的值,如果找到的话把值放在data2中。
       /* Very unlikely since we just added this element */
       return;
   }
   /* data1 != data2, however *data1 == *data2 */
}
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除了获得哈希表中的值之外,有的时候更重要的是知道一些元素的存在:

int zend_hash_exists(HashTable *ht, char *arKey, uint nKeyLen);
int zend_hash_index_exists(HashTable *ht, ulong h);
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分别针对字符串索引和数字的索引。返回的是1和0.
if (zend_hash_exists(EG(active_symbol_table),
                                "foo", sizeof("foo"))) {//确定活动的符号表中是否存在foo变量
    /* $foo is set */
} else {
    /* $foo does not exist */
}
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Quick Population and Recall 当需要对同一个字符串的key进行许多操作的时候比如先检测有没有,然后插入再修改之类的,可以使用zend_get_hash_value来进行提速。这个函数的返回值可以和quick系列的函数使用,从而达到加速的目的。因为不需要再重复计算字符串的散列值,而是直接使用已有的散列值
ulong zend_get_hash_value(char *arKey, uint nKeyLen);
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用这个返回值传给下面的quick系列函数就可以达到加速的目的:
int zend_hash_quick_add(HashTable *ht,
    char *arKey, uint nKeyLen, ulong hashval,
    void *pData, uint nDataSize, void **pDest);
int zend_hash_quick_update(HashTable *ht,
    char *arKey, uint nKeyLen, ulong hashval,
    void *pData, uint nDataSize, void **pDest);
int zend_hash_quick_find(HashTable *ht,
    char *arKey, uint nKeyLen, ulong hashval, void **pData);
int zend_hash_quick_exists(HashTable *ht,
    char *arKey, uint nKeyLen, ulong hashval);
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下面给出了一个在两个哈希表之间进行数据拷贝的例子:
void php_sample_hash_copy(HashTable *hta, HashTable *htb,
                    char *arKey, uint nKeyLen TSRMLS_DC)
{
    ulong hashval = zend_get_hash_value(arKey, nKeyLen);//获得用来加速的散列值hashval
    zval **copyval;
    if (zend_hash_quick_find(hta, arKey, nKeyLen,
                hashval, (void**)©val) == FAILURE) {//首先要在hta table里面找到相应的元素,并且存储在copyval中。
        /* arKey doesn't actually exist */
        return;
    }
    /* The zval* is about to be owned by another hash table */
    (*copyval)->refcount__gc++;//相应zval*变量的引用次数+1
    zend_hash_quick_update(htb, arKey, nKeyLen, hashval,
                copyval, sizeof(zval*), NULL);//把从hta中拿来的copyval放在htb里面。
}
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注意并没有zend_hash_del函数。

Copy and Merging 有三个方法可以进行数据的拷贝,先来看第一个:
typedef void (*copy_ctor_func_t)(void *pElement);
void zend_hash_copy(HashTable *target, HashTable *source,
            copy_ctor_func_t pCopyConstructor,
            void *tmp, uint size);
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在source中的每个元素都会被拷贝到target中.通过pCopyConstructor的处理可以使得在拷贝变量的时候对这些变量的ref_count进行加一的操作。target中原有的与source中索引位置相同的元素会被替换掉,而其他的元素则会被保留。
tmp这里放NULL,低版本才会用到。
size的话代表每个元素的大小,一般是sizeof(zval *)。
void zend_hash_merge(HashTable *target, HashTable *source,
            copy_ctor_func_t pCopyConstructor,
            void *tmp, uint size, int overwrite);
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主要是多了一个overwrite的参数,如果非0,那就跟copy一样,如果是0,那就对于已经存在的元素就不会进行复制了。

下面的这一组函数允许使用一个归并的检查进行选择性的复制:
typedef zend_bool (*merge_checker_func_t)(HashTable *target_ht,
    void *source_data, zend_hash_key *hash_key, void *pParam);
void zend_hash_merge_ex(HashTable *target, HashTable *source,
            copy_ctor_func_t pCopyConstructor, uint size,
            merge_checker_func_t pMergeSource, void *pParam);
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pMergeSource回调函数使得可以选择性的进行合并,而不是全部合并,这个给人的感觉有点像c语言里面快速排序函数所留的函数入口,可以决定排序的方式。
下面给出了一个应用的例子:
zend_bool associative_only(HashTable *ht, void *pData,
            zend_hash_key *hash_key, void *pParam)
{
    /* True if there's a key, false if there's not */
    return (hash_key->arKey && hash_key->nKeyLength);//字符串类型的key,因为存在nKeyLength
}
void merge_associative(HashTable *target, HashTable *source)
{
    zend_hash_merge_ex(target, source, zval_add_ref,
                sizeof(zval*), associative_only, NULL);
}
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