pike实现

稍微读了一下源码，记录一些我认为有必要的东西，只是为了以防老年痴呆！由于整体的源码还没有看得很清楚，所以大量的错误是允许的，此篇笔记也会一直处于 查询-修改 的迭代过程。

首先拿mapping开刀

　　mapping其实就是C++中的multimap，但是支持更多。

　　array values(mapping)。这个方法可以返回所有mapping中的value，那么values()究竟作了什么呢？源码中是这样定义的：

 1 PMOD_EXPORT struct array *mapping_values(struct mapping *m)
 2 {
 3   INT32 e;
 4   struct keypair *k;
 5   struct array *a;
 6   struct svalue *s;
 7
 8 #ifdef PIKE_DEBUG
 9   if(m->data->refs <=0)
10     Pike_fatal("Zero refs in mapping->data\n");
11 #endif
12
13   check_mapping_for_destruct(m);
14
15   a=allocate_array(m->data->size);
16   s=ITEM(a);
17
18   /* no locking required */
19   NEW_MAPPING_LOOP(m->data) assign_svalue(s++, & k->val);
20
21   a->type_field = m->data->val_types;
22
23 #ifdef PIKE_DEBUG
24   if(d_flag > 1) check_mapping_type_fields(m);
25 #endif
26
27   return a;
28 }

　　可以看到，在第15行直接申请了足够存储所有value的内存大小，第19行是一个宏，其循环了所有的可能的value，然后assign_svalue就会将每个k->value给拷贝到返回值array中，并且在27行返回a。

　　mapping也可以将字符串作为key的，但是它是如何依靠一个ke=string来迅速地找到对应的value的呢？其实mapping使用的siphash（维基百科， 原著论文）来哈希字符串的，也就是先将字符串哈希成一个值，再跟其他的key的哈希值来匹配，这样就容易找到了。如果有兴趣可以看pike中的实现：

 1 static size_t low_hashmem_siphash24( const void *s, size_t len, size_t nbytes, size_t key )
 2 {
 3   const unsigned char * in = (const unsigned char*)s;
 4   unsigned long long inlen = MINIMUM(len, nbytes);
 5
 6   /* "some pseudo randomly generated bytes" 伪随机产生的字节 */
 7   unsigned INT64 v0 = 0x736f6d6570736575ULL;
 8   unsigned INT64 v1 = 0x646f72616e646f6dULL;
 9   unsigned INT64 v2 = 0x6c7967656e657261ULL;
10   unsigned INT64 v3 = 0x7465646279746573ULL;
11   unsigned INT64 b;
12   unsigned INT64 k0 = (unsigned INT64)key;
13   unsigned INT64 k1 = (unsigned INT64)key;
14   unsigned INT64 m;
15   const unsigned char *end = in + inlen - ( inlen % sizeof( unsigned INT64 ) );
16   const int left = inlen & 7;
17   b = ( ( unsigned INT64 )inlen ) << 56;
18   v3 ^= k1;
19   v2 ^= k0;
20   v1 ^= k1;
21   v0 ^= k0;
22
23   for ( ; in != end; in += 8 )
24   {
25     m = U8TO64_LE( in );
26     v3 ^= m;
27     SIPROUND;
28     SIPROUND;
29     v0 ^= m;
30   }
31
32   switch( left )
33   {
34   case 7: b |= ( ( unsigned INT64 )in[ 6] )  << 48;
35
36   case 6: b |= ( ( unsigned INT64 )in[ 5] )  << 40;
37
38   case 5: b |= ( ( unsigned INT64 )in[ 4] )  << 32;
39
40   case 4: b |= ( ( unsigned INT64 )in[ 3] )  << 24;
41
42   case 3: b |= ( ( unsigned INT64 )in[ 2] )  << 16;
43
44   case 2: b |= ( ( unsigned INT64 )in[ 1] )  <<  8;
45
46   case 1: b |= ( ( unsigned INT64 )in[ 0] ); break;
47
48   case 0: break;
49   }
50
51   v3 ^= b;
52   SIPROUND;
53   SIPROUND;
54   v0 ^= b;
55   v2 ^= 0xff;
56   SIPROUND;
57   SIPROUND;
58   SIPROUND;
59   SIPROUND;
60   b = v0 ^ v1 ^ v2  ^ v3;
61   return (size_t)b;
62 }

low_hashmen_siphash24