hashtable.cpp

生成Hash表的代码片段。注意：不是一个完整的可编译的文件。

//First let's see what data struction he used.
struct entry
{
    void *k, *v;
    unsigned int h;
    struct entry *next;
};
struct hashtable {
    unsigned int tablelength;
    struct entry **table;
    unsigned int entrycount;
    unsigned int loadlimit;
    unsigned int primeindex;
    unsigned int (*hashfn) (void *k);
    int (*eqfn) (void *k1, void *k2);
};

static const unsigned int primes[] = {
53, 97, 193, 389,
769, 1543, 3079, 6151,
12289, 24593, 49157, 98317,
196613, 393241, 786433, 1572869,
3145739, 6291469, 12582917, 25165843,
50331653, 100663319, 201326611, 402653189,
805306457, 1610612741
};
const unsigned int prime_table_length = sizeof(primes)/sizeof(primes[0]);
const float max_load_factor = 0.65;

//The import function is creat_hashtable here.
struct hashtable *create_hashtable(unsigned int minsize,
                 unsigned int (*hashf) (void*),
                 int (*eqf) (void*,void*))
{
    struct hashtable *h;
    unsigned int pindex, size = primes[0];
    /* Check requested hashtable isn't too large */
    if (minsize > (1u << 30)) return NULL;
    /* Enforce size as prime */
    for (pindex=0; pindex < prime_table_length; pindex++) {
        if (primes[pindex] > minsize) { size = primes[pindex]; break; }
    }
    h = (struct hashtable *)malloc(sizeof(struct hashtable));
    if (NULL == h) return NULL; /*oom*/
    h->table = (struct entry **)malloc(sizeof(struct entry*) * size);
    if (NULL == h->table) { free(h); return NULL; } /*oom*/
    memset(h->table, 0, size * sizeof(struct entry *));
    h->tablelength  = size;
    h->primeindex   = pindex;
    h->entrycount   = 0;
    h->hashfn       = hashf;
    h->eqfn         = eqf;
    h->loadlimit    = (unsigned int) ceil(size * max_load_factor);
    return h;
}
//We can find that "creat_hashtable" is initialing some data.
unsigned int hash(struct hashtable *h, void *k)
{
    /* Aim to protect against poor hash functions by adding logic here
     * - logic taken from java 1.4 hashtable source */
    unsigned int i = h->hashfn(k);
    i += ~(i << 9);
    i ^=  ((i >> 14) | (i << 18)); /* >>> */
    i +=  (i << 4);
    i ^=  ((i >> 10) | (i << 22)); /* >>> */
    return i;
}
//I have not understood hash(struct hashtable,void *);