key值最多膨胀64倍,查找时间复杂度为len(key)*4的hash_map

[haoruixiang]

常用的字符串hash算法是根据字符串算出一个整数，再根据这个整数进行存储(http://blog.csdn.net/djinglan/article/details/8812934)，而整数的hash一般是定义一个很大的空间，而进行一次性hash，浪费空间很严重，而且计算出来的整数不能保证唯一性，有可能是重复的。

根据key，value的特性，我设计了一种新的方式去定位字符串，这种方法所需要的空间不是很大，每个key都有唯一，查找复杂度也是固定不变的，就是key的长度。

首先：
一个字节的范围0~255，对字节进行编码，最简单的是256进制编码，一个256的数组，字节就可以直接定位了，例如：

struct node{
        void * node[256] 
    };
    node head;
    char * key = "sdada";
    char * p = key;
    node * pnode = &head;
    void * data = 0;
    do{
        unsigned char c = p[0];
        data = pnode->node[c];
        p++;
        if (!p[0]){ break;}
        pnode = (node*)data;
    }while(false);

当然实际中，会比这段代码复杂，需要将键值和数据分开。
key = "sdada"只要查找5次就能找到值了，这里有个缺点一个字节膨胀了256*4 = 1024倍。即膨胀系数为1024

256是16的平方倍数，我们可以用16进制编码：

struct node{
        void * node[16] 
    };
    node head;
    char * key = "sdada";
    char * p = key;
    node * pnode = &head;
    void * data = 0;
    do{
        unsigned char c = p[0];
        int index1 = c/16;
        int index2 = c%16;
        pnode = (node*)pnode->node[index1];
        data =  (node*)pnode->node[index2];
        p++;
        if (!p[0]){ break;}
        pnode = (node*)data;
    }while(false);

进行16进制编码后，发现字节膨胀系数变成 （16+16）*4 = 128，膨胀系数128比1024少了许多，只是查找次数是256编码的2倍，但是这是可以接受的。
后面还实验了对字节进行4进制编码：
结果：膨胀系数为 （4+4+4+4）*4 = 64 ，查找次数为16进制的2倍，256进制的4倍
2进制编码：
结果：膨胀系数为 （2+2+2+2+2+2+2+2）*4 = 64 与4进制相同，查找次数为4进制的2倍，256进制的8倍。
这种几种编码还有一种因素，那就是节点复用率，经测试发现，节点复用率 2进制编码 》4进制编码 =》16进制编码 =》256进制编码。

测试例子：

time_t t = time(0);
    hdc_hash_map map;
    for (int i = 0; i <1000000; i++)
    {
        int sas = i;
        map.set((const char*)(&sas),sizeof(int),0);
    }
   printf("%ld\n",time(0)-t);

100万个key, 每个key长度为4字节，用4进制编码进行hash，计算需要（1000000*64*4）= 244M的空间，实际占用108M，花费时间2s时间。

我个人建议：对字符进行4进制编码，进行hash最划算，膨胀系数：64，查找次数为key长度的4倍，还有节点复用率也不错，具体需要做详细测试。

[赵4老师]

搜“暴雪哈希算法”

[haoruixiang]

在网上查了资料，与我介绍的这种不同吧， 我设计的是一种树形的存储结构： ，每棵字节树含有256个节点，data节点后面再接入下一个字节树，数据存储的时候按照规则建树，查询的时候只要按照规则查询就行了。

[haoruixiang]

static int hdc_pe4[256][4] = {
    {0,0,0,0},{0,0,0,1},{0,0,0,2},{0,0,0,3},
    {0,0,1,0},{0,0,1,1},{0,0,1,2},{0,0,1,3},
    {0,0,2,0},{0,0,2,1},{0,0,2,2},{0,0,2,3},
    {0,0,3,0},{0,0,3,1},{0,0,3,2},{0,0,3,3},
    {0,1,0,0},{0,1,0,1},{0,1,0,2},{0,1,0,3},
    {0,1,1,0},{0,1,1,1},{0,1,1,2},{0,1,1,3},
    {0,1,2,0},{0,1,2,1},{0,1,2,2},{0,1,2,3},
    {0,1,3,0},{0,1,3,1},{0,1,3,2},{0,1,3,3},
    {0,2,0,0},{0,2,0,1},{0,2,0,2},{0,2,0,3},
    {0,2,1,0},{0,2,1,1},{0,2,1,2},{0,2,1,3},
    {0,2,2,0},{0,2,2,1},{0,2,2,2},{0,2,2,3},
    {0,2,3,0},{0,2,3,1},{0,2,3,2},{0,2,3,3},
    {0,3,0,0},{0,3,0,1},{0,3,0,2},{0,3,0,3},
    {0,3,1,0},{0,3,1,1},{0,3,1,2},{0,3,1,3},
    {0,3,2,0},{0,3,2,1},{0,3,2,2},{0,3,2,3},
    {0,3,3,0},{0,3,3,1},{0,3,3,2},{0,3,3,3},
    {1,0,0,0},{1,0,0,1},{1,0,0,2},{1,0,0,3},
    {1,0,1,0},{1,0,1,1},{1,0,1,2},{1,0,1,3},
    {1,0,2,0},{1,0,2,1},{1,0,2,2},{1,0,2,3},
    {1,0,3,0},{1,0,3,1},{1,0,3,2},{1,0,3,3},
    {1,1,0,0},{1,1,0,1},{1,1,0,2},{1,1,0,3},
    {1,1,1,0},{1,1,1,1},{1,1,1,2},{1,1,1,3},
    {1,1,2,0},{1,1,2,1},{1,1,2,2},{1,1,2,3},
    {1,1,3,0},{1,1,3,1},{1,1,3,2},{1,1,3,3},
    {1,2,0,0},{1,2,0,1},{1,2,0,2},{1,2,0,3},
    {1,2,1,0},{1,2,1,1},{1,2,1,2},{1,2,1,3},
    {1,2,2,0},{1,2,2,1},{1,2,2,2},{1,2,2,3},
    {1,2,3,0},{1,2,3,1},{1,2,3,2},{1,2,3,3},
    {1,3,0,0},{1,3,0,1},{1,3,0,2},{1,3,0,3},
    {1,3,1,0},{1,3,1,1},{1,3,1,2},{1,3,1,3},
    {1,3,2,0},{1,3,2,1},{1,3,2,2},{1,3,2,3},
    {1,3,3,0},{1,3,3,1},{1,3,3,2},{1,3,3,3},
    {2,0,0,0},{2,0,0,1},{2,0,0,2},{2,0,0,3},
    {2,0,1,0},{2,0,1,1},{2,0,1,2},{2,0,1,3},
    {2,0,2,0},{2,0,2,1},{2,0,2,2},{2,0,2,3},
    {2,0,3,0},{2,0,3,1},{2,0,3,2},{2,0,3,3},
    {2,1,0,0},{2,1,0,1},{2,1,0,2},{2,1,0,3},
    {2,1,1,0},{2,1,1,1},{2,1,1,2},{2,1,1,3},
    {2,1,2,0},{2,1,2,1},{2,1,2,2},{2,1,2,3},
    {2,1,3,0},{2,1,3,1},{2,1,3,2},{2,1,3,3},
    {2,2,0,0},{2,2,0,1},{2,2,0,2},{2,2,0,3},
    {2,2,1,0},{2,2,1,1},{2,2,1,2},{2,2,1,3},
    {2,2,2,0},{2,2,2,1},{2,2,2,2},{2,2,2,3},
    {2,2,3,0},{2,2,3,1},{2,2,3,2},{2,2,3,3},
    {2,3,0,0},{2,3,0,1},{2,3,0,2},{2,3,0,3},
    {2,3,1,0},{2,3,1,1},{2,3,1,2},{2,3,1,3},
    {2,3,2,0},{2,3,2,1},{2,3,2,2},{2,3,2,3},
    {2,3,3,0},{2,3,3,1},{2,3,3,2},{2,3,3,3},
    {3,0,0,0},{3,0,0,1},{3,0,0,2},{3,0,0,3},
    {3,0,1,0},{3,0,1,1},{3,0,1,2},{3,0,1,3},
    {3,0,2,0},{3,0,2,1},{3,0,2,2},{3,0,2,3},
    {3,0,3,0},{3,0,3,1},{3,0,3,2},{3,0,3,3},
    {3,1,0,0},{3,1,0,1},{3,1,0,2},{3,1,0,3},
    {3,1,1,0},{3,1,1,1},{3,1,1,2},{3,1,1,3},
    {3,1,2,0},{3,1,2,1},{3,1,2,2},{3,1,2,3},
    {3,1,3,0},{3,1,3,1},{3,1,3,2},{3,1,3,3},
    {3,2,0,0},{3,2,0,1},{3,2,0,2},{3,2,0,3},
    {3,2,1,0},{3,2,1,1},{3,2,1,2},{3,2,1,3},
    {3,2,2,0},{3,2,2,1},{3,2,2,2},{3,2,2,3},
    {3,2,3,0},{3,2,3,1},{3,2,3,2},{3,2,3,3},
    {3,3,0,0},{3,3,0,1},{3,3,0,2},{3,3,0,3},
    {3,3,1,0},{3,3,1,1},{3,3,1,2},{3,3,1,3},
    {3,3,2,0},{3,3,2,1},{3,3,2,2},{3,3,2,3},
    {3,3,3,0},{3,3,3,1},{3,3,3,2},{3,3,3,3}
};

这是每个字节固定的查找或者建树的规则。