这个哈希表构造函数为什么要通过获得一个质数表来获取哈希表的大小?

/* Find a prime near, but greather than or equal to SIZE. The primes
are looked up from a table with a selection of primes convenient
for this purpose.

PRIME_OFFSET is a minor optimization: it specifies start position
for the search for the large enough prime. The final offset is
stored in the same variable. That way the list of primes does not
have to be scanned from the beginning each time around. */

static int
prime_size (int size, int *prime_offset)
{
static const int primes[] = {
13, 19, 29, 41, 59, 79, 107, 149, 197, 263, 347, 457, 599, 787, 1031,
1361, 1777, 2333, 3037, 3967, 5167, 6719, 8737, 11369, 14783,
19219, 24989, 32491, 42257, 54941, 71429, 92861, 120721, 156941,
204047, 265271, 344857, 448321, 582821, 757693, 985003, 1280519,
1664681, 2164111, 2813353, 3657361, 4754591, 6180989, 8035301,
10445899, 13579681, 17653589, 22949669, 29834603, 38784989,
50420551, 65546729, 85210757, 110774011, 144006217, 187208107,
243370577, 316381771, 411296309, 534685237, 695090819, 903618083,
1174703521, 1527114613, 1837299131, 2147483647
};
int i;

for (i = *prime_offset; i < countof (primes); i++)
if (primes[i] >= size)
{
/* Set the offset to the next prime. That is safe because,
next time we are called, it will be with a larger SIZE,
which means we could never return the same prime anyway.
(If that is not the case, the caller can simply reset
*prime_offset.) */
*prime_offset = i + 1;
return primes[i];
}

abort ();
}


struct hash_table *
hash_table_new (int items,
unsigned long (*hash_function) (const void *),
int (*test_function) (const void *, const void *))
{
int size;
struct hash_table *ht = xnew (struct hash_table);

ht->hash_function = hash_function ? hash_function : hash_pointer;
ht->test_function = test_function ? test_function : cmp_pointer;

/* If the size of struct hash_table ever becomes a concern, this
field can go. (Wget doesn't create many hashes.) */
ht->prime_offset = 0;

/* Calculate the size that ensures that the table will store at
least ITEMS keys without the need to resize. */
size = 1 + items / HASH_MAX_FULLNESS;
size = prime_size (size, &ht->prime_offset);
ht->size = size;
ht->resize_threshold = size * HASH_MAX_FULLNESS;
/*assert (ht->resize_threshold >= items);*/

ht->mappings = xnew_array (struct mapping, ht->size); /*allocate space for hash-table*/

/* Mark mappings as empty. We use 0xff rather than 0 to mark empty
keys because it allows us to use NULL/0 as keys. 给哈希表置成全1表示空*/
memset (ht->mappings, INVALID_PTR_BYTE, size * sizeof (struct mapping));

ht->count = 0; /*刚初始化的哈希表的非空条目个数当然为0*/

return ht;
}