libihash uses open addressing. Previously, quadratic probing in both
directions was used to resolve collisions. Quadratic probing might
result in a less efficient use of caches.
Also, prime numbers of the form 4 * i + 3 were used as array sizes.
This was used in combination with the integer modulo operation for
hashing. It has been known for some time that libihash suffers from
collisions, so a integer hash function is now applied to the keys to
derive the index.
Use linear probing instead. Also, use powers of two for the array
sizes, so a bit mask can be used for the modulo operation.
* libihash/ihash.c (ihash_sizes, ihash_nsizes): Remove.
(find_index): Use linear probing and fast modulo operation.
(add_one): Likewise.
* libihash/ihash.h (HURD_IHASH_MIN_SIZE): New macro.
---
libihash/ihash.c | 121 ++++++-------------------------------------------------
libihash/ihash.h | 4 ++
2 files changed, 17 insertions(+), 108 deletions(-)
diff --git a/libihash/ihash.c b/libihash/ihash.c
index 1de4c35..e74a2c5 100644
--- a/libihash/ihash.c
+++ b/libihash/ihash.c
@@ -31,55 +31,6 @@
#include <assert.h>
#include "ihash.h"
-
-
-/* The prime numbers of the form 4 * i + 3 for some i, all greater
- than twice the previous one and smaller than 2^40 (for now). */
-static const uint64_t ihash_sizes[] =
-{
- 3,
- 7,
- 19,
- 43,
- 103,
- 211,
- 431,
- 863,
- 1747,
- 3499,
- 7019,
- 14051,
- 28111,
- 56239,
- 112507,
- 225023,
- 450067,
- 900139,
- 1800311,
- 3600659,
- 7201351,
- 14402743,
- 28805519,
- 57611039,
- 115222091,
- 230444239,
- 460888499,
- 921777067,
- 1843554151,
- UINT64_C (3687108307),
- UINT64_C (7374216631),
- UINT64_C (14748433279),
- UINT64_C (29496866579),
- UINT64_C (58993733159),
- UINT64_C (117987466379),
- UINT64_C (235974932759),
- UINT64_C (471949865531),
- UINT64_C (943899731087)
-};
-
-static const unsigned int ihash_nsizes = (sizeof ihash_sizes
- / sizeof ihash_sizes[0]);
-
/* Integer hashing follows Thomas Wang's paper about his 32/64-bits
mix functions :
@@ -126,40 +77,24 @@ static inline int
find_index (hurd_ihash_t ht, hurd_ihash_key_t key)
{
unsigned int idx;
- unsigned int i;
unsigned int up_idx;
- unsigned int down_idx;
+ unsigned int mask = ht->size - 1;
- idx = hash_int32 (key, 32) % ht->size;
+ idx = hash_int32 (key, 32) & mask;
if (ht->items[idx].value == _HURD_IHASH_EMPTY || ht->items[idx].key == key)
return idx;
- /* Instead of calculating idx + 1, idx + 4, idx + 9, ..., idx + i^2,
- we add 1, 3, 5, 7, etc to the previous index. We do this in both
- directions separately. */
- i = 1;
up_idx = idx;
- down_idx = idx;
do
{
- up_idx = (up_idx + i) % ht->size;
+ up_idx = (up_idx + 1) & mask;
if (ht->items[up_idx].value == _HURD_IHASH_EMPTY
|| ht->items[up_idx].key == key)
return up_idx;
-
- if (down_idx < i)
- down_idx += ht->size;
- down_idx = (down_idx - i) % ht->size;
- if (ht->items[down_idx].value == _HURD_IHASH_EMPTY
- || ht->items[down_idx].key == key)
- return down_idx;
-
- /* After (ht->size - 1) / 2 iterations, this will be 0. */
- i = (i + 2) % ht->size;
}
- while (i);
+ while (up_idx != idx);
/* If we end up here, the item could not be found. Return any
invalid index. */
@@ -282,53 +217,26 @@ add_one (hurd_ihash_t ht, hurd_ihash_key_t key,
hurd_ihash_value_t value)
{
unsigned int idx;
unsigned int first_free;
+ unsigned int mask = ht->size - 1;
- idx = hash_int32 (key, 32) % ht->size;
+ idx = hash_int32 (key, 32) & mask;
first_free = idx;
if (ht->items[idx].value != _HURD_IHASH_EMPTY && ht->items[idx].key != key)
{
- /* Instead of calculating idx + 1, idx + 4, idx + 9, ..., idx +
- i^2, we add 1, 3, 5, 7, ... 2 * i - 1 to the previous index.
- We do this in both directions separately. */
- unsigned int i = 1;
unsigned int up_idx = idx;
- unsigned int down_idx = idx;
do
{
- up_idx = (up_idx + i) % ht->size;
+ up_idx = (up_idx + 1) & mask;
if (ht->items[up_idx].value == _HURD_IHASH_EMPTY
|| ht->items[up_idx].key == key)
{
idx = up_idx;
break;
}
- if (first_free == idx
- && ht->items[up_idx].value == _HURD_IHASH_DELETED)
- first_free = up_idx;
-
- if (down_idx < i)
- down_idx += ht->size;
- down_idx = (down_idx - i) % ht->size;
- if (down_idx < 0)
- down_idx += ht->size;
- else
- down_idx %= ht->size;
- if (ht->items[down_idx].value == _HURD_IHASH_EMPTY
- || ht->items[down_idx].key == key)
- {
- idx = down_idx;
- break;
- }
- if (first_free == idx
- && ht->items[down_idx].value == _HURD_IHASH_DELETED)
- first_free = down_idx;
-
- /* After (ht->size - 1) / 2 iterations, this will be 0. */
- i = (i + 2) % ht->size;
}
- while (i);
+ while (up_idx != idx);
}
/* Remove the old entry for this key if necessary. */
@@ -377,15 +285,12 @@ hurd_ihash_add (hurd_ihash_t ht, hurd_ihash_key_t key,
hurd_ihash_value_t item)
}
/* The hash table is too small, and we have to increase it. */
- for (i = 0; i < ihash_nsizes; i++)
- if (ihash_sizes[i] > old_ht.size)
- break;
- if (i == ihash_nsizes
- || ihash_sizes[i] > SIZE_MAX / sizeof (struct _hurd_ihash_item))
- return ENOMEM; /* Surely will be true momentarily. */
-
ht->nr_items = 0;
- ht->size = ihash_sizes[i];
+ if (ht->size == 0)
+ ht->size = HURD_IHASH_MIN_SIZE;
+ else
+ ht->size <<= 1;
+
/* calloc() will initialize all values to _HURD_IHASH_EMPTY implicitely. */
ht->items = calloc (ht->size, sizeof (struct _hurd_ihash_item));
diff --git a/libihash/ihash.h b/libihash/ihash.h
index 6bdc925..057babc 100644
--- a/libihash/ihash.h
+++ b/libihash/ihash.h
@@ -93,6 +93,10 @@ typedef struct hurd_ihash *hurd_ihash_t;
/* Construction and destruction of hash tables. */
+/* The size of the initial allocation in number of items. This must
+ be a power of two. */
+#define HURD_IHASH_MIN_SIZE 32
+
/* The default value for the maximum load factor in percent. */
#define HURD_IHASH_MAX_LOAD_DEFAULT 75
--
2.0.0.rc0
