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Don't include the length in the hash function if all keywords have the same

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length.
This commit is contained in:
Bruno Haible
2009-01-19 09:50:07 +00:00
parent 7fac4aa2b6
commit 57745086ac
9 changed files with 144 additions and 113 deletions
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25
src/search.cc
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@@ -1,5 +1,5 @@
/* Search algorithm.
Copyright (C) 1989-1998, 2000, 2002 Free Software Foundation, Inc.
Copyright (C) 1989-1998, 2000, 2002, 2009 Free Software Foundation, Inc.
Written by Douglas C. Schmidt <schmidt@ics.uci.edu>
and Bruno Haible <bruno@clisp.org>.
@@ -64,7 +64,7 @@
where Pos is a set of byte positions,
each alpha_inc[i] is a nonnegative integer,
each asso_values[c] is a nonnegative integer,
len (keyword) is the keyword's length if !option[NOLENGTH], or 0 otherwise.
len (keyword) is the keyword's length if _hash_includes_len, or 0 otherwise.
Theorem 1: If all keywords are different, there is a set Pos such that
all tuples (keyword[i] : i in Pos) are different.
@@ -103,7 +103,7 @@
Map (A --> B) := Hom_Set (A, B) is the set of maps from A to B, and
S(Pos) is the symmetric group over Pos.
This was the theory for option[NOLENGTH]; if !option[NOLENGTH], slight
This was the theory for !_hash_includes_len; if _hash_includes_len, slight
modifications apply:
proj1 : String --> Map (Pos --> N) x N
proj2 : Map (Pos --> N) x N --> Map (Pos --> N) / S(Pos) x N
@@ -175,6 +175,9 @@ Search::prepare ()
exit (1);
}
}
/* Determine whether the hash function shall include the length. */
_hash_includes_len = !(option[NOLENGTH] || (_min_key_len == _max_key_len));
}
/* ====================== Finding good byte positions ====================== */
@@ -238,7 +241,7 @@ Search::count_duplicates_tuple (const Positions& positions, const unsigned int *
unsigned int count = 0;
{
Hash_Table representatives (_total_keys, option[NOLENGTH]);
Hash_Table representatives (_total_keys, !_hash_includes_len);
for (KeywordExt_List *temp = _head; temp; temp = temp->rest())
{
KeywordExt *keyword = temp->first();
@@ -590,7 +593,7 @@ Search::count_duplicates_multiset (const unsigned int *alpha_inc) const
unsigned int count = 0;
{
Hash_Table representatives (_total_keys, option[NOLENGTH]);
Hash_Table representatives (_total_keys, !_hash_includes_len);
for (KeywordExt_List *temp = _head; temp; temp = temp->rest())
{
KeywordExt *keyword = temp->first();
@@ -736,7 +739,7 @@ Search::prepare_asso_values ()
_max_selchars_length = _key_positions.iterator(_max_key_len).remaining();
/* Check for duplicates, i.e. keywords with the same _selchars array
(and - if !option[NOLENGTH] - also the same length).
(and - if _hash_includes_len - also the same length).
We deal with these by building an equivalence class, so that only
1 keyword is representative of the entire collection. Only this
representative remains in the keyword list; the others are accessible
@@ -747,7 +750,7 @@ Search::prepare_asso_values ()
_list_len = _total_keys;
_total_duplicates = 0;
/* Make hash table for efficiency. */
Hash_Table representatives (_list_len, option[NOLENGTH]);
Hash_Table representatives (_list_len, !_hash_includes_len);
KeywordExt_List *prev = NULL; /* list node before temp */
for (temp = _head; temp; )
@@ -847,7 +850,7 @@ Search::prepare_asso_values ()
/* Given the bound for _asso_values[c], we have a bound for the possible
hash values, as computed in compute_hash(). */
_max_hash_value = (option[NOLENGTH] ? 0 : _max_key_len)
_max_hash_value = (_hash_includes_len ? _max_key_len : 0)
+ (_asso_value_max - 1) * _max_selchars_length;
/* Allocate a sparse bit vector for detection of collisions of hash
values. */
@@ -1307,7 +1310,7 @@ Search::find_asso_values ()
the yet undetermined asso_values[]. */
int hashcode;
{
int sum = option[NOLENGTH] ? 0 : keyword->_allchars_length;
int sum = _hash_includes_len ? keyword->_allchars_length : 0;
const unsigned int *p = keyword->_selchars;
int i = keyword->_selchars_length;
for (; i > 0; p++, i--)
@@ -1407,7 +1410,7 @@ Search::find_asso_values ()
_asso_value_max = step->_asso_value_max;
/* Reinitialize _max_hash_value. */
_max_hash_value =
(option[NOLENGTH] ? 0 : _max_key_len)
(_hash_includes_len ? _max_key_len : 0)
+ (_asso_value_max - 1) * _max_selchars_length;
/* Reinitialize _collision_detector. */
delete _collision_detector;
@@ -1470,7 +1473,7 @@ Search::find_asso_values ()
inline int
Search::compute_hash (KeywordExt *keyword) const
{
int sum = option[NOLENGTH] ? 0 : keyword->_allchars_length;
int sum = _hash_includes_len ? keyword->_allchars_length : 0;
const unsigned int *p = keyword->_selchars;
int i = keyword->_selchars_length;