php 数组学习

php内部的数组使用hash table(拉链hash)来实现的。PHP的Hash采用的是目前最为普遍的DJBX33A (Daniel J. Bernstein, Times 33 with Addition), 这个算法被广泛运用与多个软件项目,Apache, Perl和Berkeley DB等. 对于字符串而言这是目前所知道的最好的哈希算法，原因在于该算法的速度非常快，而且分类非常好(冲突小,分布均匀).

算法核心：hash(i) = hash(i-1) * 33 + str[i]

但是php 没有简单的用

sub perlhash { $hash = 0; foreach (split //, shift) { $hash = $hash*33 + ord($_); } return $hash; }

而是使用了如下的方法计算一个string的hash.

static inline ulong zend_inline_hash_func(char *arKey, uint nKeyLength)
{
	register ulong hash = 5381;

	/* variant with the hash unrolled eight times */
	for (; nKeyLength >= 8; nKeyLength -= 8) {
		hash = ((hash << 5) + hash) + *arKey++;
		hash = ((hash << 5) + hash) + *arKey++;
		hash = ((hash << 5) + hash) + *arKey++;
		hash = ((hash << 5) + hash) + *arKey++;
		hash = ((hash << 5) + hash) + *arKey++;
		hash = ((hash << 5) + hash) + *arKey++;
		hash = ((hash << 5) + hash) + *arKey++;
		hash = ((hash << 5) + hash) + *arKey++;
	}
	switch (nKeyLength) {
		case 7: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
		case 6: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
		case 5: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
		case 4: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
		case 3: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
		case 2: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
		case 1: hash = ((hash << 5) + hash) + *arKey++; break;
		case 0: break;
EMPTY_SWITCH_DEFAULT_CASE()
	}
	return hash;
}

初看可能看不懂，NB的地方有三个：

1，使用了(hash << 5) + hash，而不是 hash*33,自然快了

2，使用的unrolled。PHP鼓励8位一下的字符索引, 他以8为单位使用unrolled来提高效率, 这不得不说也是个很细节的,很细致的地方.似乎傻乎乎的写了重复的8条语句，其实不然…

3, hash用到了register。而且不是0开始，是从5381开始。为啥用5381？

为啥要用33这个倍数？

源码中的注释是这样写的。

/*
 * DJBX33A (Daniel J. Bernstein, Times 33 with Addition)
 *
 * This is Daniel J. Bernstein's popular `times 33' hash function as
 * posted by him years ago on comp.lang.c. It basically uses a function
 * like ``hash(i) = hash(i-1) * 33 + str[i]''. This is one of the best
 * known hash functions for strings. Because it is both computed very
 * fast and distributes very well.
 *
 * The magic of number 33, i.e. why it works better than many other
 * constants, prime or not, has never been adequately explained by
 * anyone. So I try an explanation: if one experimentally tests all
 * multipliers between 1 and 256 (as RSE did now) one detects that even
 * numbers are not useable at all. The remaining 128 odd numbers
 * (except for the number 1) work more or less all equally well. They
 * all distribute in an acceptable way and this way fill a hash table
 * with an average percent of approx. 86%. 
 *
 * If one compares the Chi^2 values of the variants, the number 33 not
 * even has the best value. But the number 33 and a few other equally
 * good numbers like 17, 31, 63, 127 and 129 have nevertheless a great
 * advantage to the remaining numbers in the large set of possible
 * multipliers: their multiply operation can be replaced by a faster
 * operation based on just one shift plus either a single addition
 * or subtraction operation. And because a hash function has to both
 * distribute good _and_ has to be very fast to compute, those few
 * numbers should be preferred and seems to be the reason why Daniel J.
 * Bernstein also preferred it.
 *
 *
 *                  -- Ralf S. Engelschall 
 */

说白了，就是33即很好避免冲撞，又容易通过移位快速计算得到。两相权衡的选择。

下面说说php数组内部的结构

这个是php hashtable的定义

typedef struct _hashtable {
uint nTableSize;        /* 散列表大小, Hash值的区间 */
uint nTableMask;        /* 等于nTableSize -1, 用于快速定位 */
uint nNumOfElements;    /* HashTable中实际元素的个数 */
ulong nNextFreeElement; /* 下个空闲可用位置的数字索引 */
Bucket *pInternalPointer;   /* 内部位置指针, 会被reset, current这些遍历函数使用 */
Bucket *pListHead;      /* 头元素, 用于线性遍历 */
Bucket *pListTail;      /* 尾元素, 用于线性遍历 */
Bucket **arBuckets;     /* 实际的存储容器 */
dtor_func_t pDestructor;/* 元素的析构函数(指针) */
zend_bool persistent;
unsigned char nApplyCount; /* 循环遍历保护 */
zend_bool bApplyProtection;
#if ZEND_DEBUG
int inconsistent;
#endif
} HashTable;

这个是Bucket 的定义

typedef struct bucket {
ulong h;                        /* 数字索引/hash值 */
uint nKeyLength;                /* 字符索引的长度 */
void *pData;                    /* 数据 */
void *pDataPtr;                 /* 数据指针 */
struct bucket *pListNext;               /* 下一个元素, 用于线性遍历 */
struct bucket *pListLast;       /* 上一个元素, 用于线性遍历 */
struct bucket *pNext;                   /* 处于同一个拉链中的下一个元素 */
struct bucket *pLast;                   /* 处于同一拉链中的上一个元素 */
char arKey[1]; /* 节省内存,方便初始化的技巧 */
} Bucket;

 

 

1，php数组的随机访问：通过bucket数组来实现随即访问，计算hash值，来找到。如果冲撞找pNext指针，来继续找。

2，php数组的顺序访问：通过pListHead来找到第一个元素，然后pListNext找到下一个，pInternalPointer表示当前访问到的。

3，pInternalPointer在每次foreach的时候，会被reset成pListHead。而list(xx,xx,xxx) = each($arr)是用的时候，要手动去reset这个数组。

4，PHP中遍历数组的顺序, 是和元素的添加先后相关的。而不是key的大小（当key是数字的时候）。

5，Bucket 定义中char arKey[1] 用到了类似c++中的结构体中的灵活数组域（flexible array member）概念。