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hashtable: Use quadratic probing 

This reduces the clustering problem (primary clustering) but is not
completely free of it (secondary clustering) it still reduces the maximum
and average probing lengths.
This commit is contained in:
Tobias Brunner 2020-04-24 19:11:25 +02:00
parent 736fae4e6c
commit 93b2c2066f
1 changed files with 10 additions and 8 deletions
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18
src/libstrongswan/collections/hashtable.c
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@ -272,11 +272,13 @@ static void init_hashtable(private_hashtable_t *this, u_int size)
}
/**
* Calculate the next bucket using simple linear probing for now.
* Calculate the next bucket using quadratic probing (the sequence is h(k) + 1,
* h(k) + 3, h(k) + 6, h(k) + 10, ...).
*/
static inline u_int get_next(private_hashtable_t *this, u_int row)
static inline u_int get_next(private_hashtable_t *this, u_int row, u_int *p)
{
return (row + 1) & this->mask;
*p += 1;
return (row + *p) & this->mask;
}
/**
@ -287,7 +289,7 @@ static inline pair_t *find_key(private_hashtable_t *this, const void *key,
u_int *out_hash, u_int *out_row)
{
pair_t *pair;
u_int hash, row, removed, index;
u_int hash, row, p = 0, removed, index;
bool found_removed = FALSE;
if (!this->count && !out_hash && !out_row)
@ -318,7 +320,7 @@ static inline pair_t *find_key(private_hashtable_t *this, const void *key,
lookup_success(&this->profile);
return pair;
}
row = get_next(this, row);
row = get_next(this, row, &p);
index = get_index(this, row);
}
if (out_hash)
@ -354,7 +356,7 @@ static inline u_int insert_item(private_hashtable_t *this, u_int row)
static bool rehash(private_hashtable_t *this, u_int size)
{
pair_t *old_items, *pair;
u_int old_count, i, row, index;
u_int old_count, i, p, row, index;
if (size > MAX_SIZE)
{
@ -378,9 +380,9 @@ static bool rehash(private_hashtable_t *this, u_int size)
{
row = pair->hash & this->mask;
index = get_index(this, row);
while (index)
for (p = 0; index;)
{
row = get_next(this, row);
row = get_next(this, row, &p);
index = get_index(this, row);
}
index = insert_item(this, row);