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.

diff --git a/src/libstrongswan/collections/hashtable.c b/src/libstrongswan/collections/hashtable.c
index 8c290ac2d556fa16394fcf3f9c15828d6bd3c78e..fa1e4a093799129cc56649a12693a31da6bf65bf 100644 (file)
--- a/src/libstrongswan/collections/hashtable.c
+++ b/src/libstrongswan/collections/hashtable.c
@@ -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);