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Reimplement text_position and related functions to use Boyer-Moore-Horspool 

searching instead of naive matching.  In the worst case this has the same
O(M*N) complexity as the naive method, but the worst case is hard to hit,
and the average case is very fast, especially with longer patterns.

David Rowley
This commit is contained in:
Tom Lane 2008-09-07 04:20:00 +00:00
parent 2cf3f6694f
commit e6a310b281
1 changed files with 187 additions and 32 deletions
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219
src/backend/utils/adt/varlena.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/utils/adt/varlena.c,v 1.167 2008/05/27 00:13:09 tgl Exp $
* $PostgreSQL: pgsql/src/backend/utils/adt/varlena.c,v 1.168 2008/09/07 04:20:00 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -39,6 +39,9 @@ typedef struct
pg_wchar *wstr2; /* note: these are palloc'd */
int len1; /* string lengths in logical characters */
int len2;
/* Skip table for Boyer-Moore-Horspool search algorithm: */
int skiptablemask; /* mask for ANDing with skiptable subscripts */
int skiptable[256]; /* skip distance for given mismatched char */
} TextPositionState;
#define DatumGetUnknownP(X) ((unknown *) PG_DETOAST_DATUM(X))
@ -753,7 +756,7 @@ text_substring(Datum str, int32 start, int32 length, bool length_not_specified)
* If we're working with an untoasted source, no need to do an extra
* copying step.
*/
if (VARATT_IS_COMPRESSED(DatumGetPointer(str)) ||
if (VARATT_IS_COMPRESSED(DatumGetPointer(str)) ||
VARATT_IS_EXTERNAL(DatumGetPointer(str)))
slice = DatumGetTextPSlice(str, slice_start, slice_size);
else
@ -866,6 +869,7 @@ text_position(text *t1, text *t2)
return result;
}
/*
* text_position_setup, text_position_next, text_position_cleanup -
* Component steps of text_position()
@ -909,64 +913,215 @@ text_position_setup(text *t1, text *t2, TextPositionState *state)
state->len1 = len1;
state->len2 = len2;
}
/*
* Prepare the skip table for Boyer-Moore-Horspool searching. In these
* notes we use the terminology that the "haystack" is the string to be
* searched (t1) and the "needle" is the pattern being sought (t2).
*
* If the needle is empty or bigger than the haystack then there is no
* point in wasting cycles initializing the table. We also choose not
* to use B-M-H for needles of length 1, since the skip table can't
* possibly save anything in that case.
*/
if (len1 >= len2 && len2 > 1)
{
int searchlength = len1 - len2;
int skiptablemask;
int last;
int i;
/*
* First we must determine how much of the skip table to use. The
* declaration of TextPositionState allows up to 256 elements, but for
* short search problems we don't really want to have to initialize so
* many elements --- it would take too long in comparison to the
* actual search time. So we choose a useful skip table size based on
* the haystack length minus the needle length. The closer the needle
* length is to the haystack length the less useful skipping becomes.
*
* Note: since we use bit-masking to select table elements, the skip
* table size MUST be a power of 2, and so the mask must be 2^N-1.
*/
if (searchlength < 16)
skiptablemask = 3;
else if (searchlength < 64)
skiptablemask = 7;
else if (searchlength < 128)
skiptablemask = 15;
else if (searchlength < 512)
skiptablemask = 31;
else if (searchlength < 2048)
skiptablemask = 63;
else if (searchlength < 4096)
skiptablemask = 127;
else
skiptablemask = 255;
state->skiptablemask = skiptablemask;
/*
* Initialize the skip table. We set all elements to the needle
* length, since this is the correct skip distance for any character
* not found in the needle.
*/
for (i = 0; i <= skiptablemask; i++)
state->skiptable[i] = len2;
/*
* Now examine the needle. For each character except the last one,
* set the corresponding table element to the appropriate skip
* distance. Note that when two characters share the same skip table
* entry, the one later in the needle must determine the skip distance.
*/
last = len2 - 1;
if (!state->use_wchar)
{
const char *str2 = state->str2;
for (i = 0; i < last; i++)
state->skiptable[(unsigned char) str2[i] & skiptablemask] = last - i;
}
else
{
const pg_wchar *wstr2 = state->wstr2;
for (i = 0; i < last; i++)
state->skiptable[wstr2[i] & skiptablemask] = last - i;
}
}
}
static int
text_position_next(int start_pos, TextPositionState *state)
{
int pos = 0,
p,
px;
int haystack_len = state->len1;
int needle_len = state->len2;
int skiptablemask = state->skiptablemask;
Assert(start_pos > 0); /* else caller error */
if (state->len2 <= 0)
if (needle_len <= 0)
return start_pos; /* result for empty pattern */
start_pos--; /* adjust for zero based arrays */
/* Done if the needle can't possibly fit */
if (haystack_len < start_pos + needle_len)
return 0;
if (!state->use_wchar)
{
/* simple case - single byte encoding */
char *p1 = state->str1;
char *p2 = state->str2;
const char *haystack = state->str1;
const char *needle = state->str2;
const char *haystack_end = &haystack[haystack_len];
const char *hptr;
/* no use in searching str past point where search_str will fit */
px = (state->len1 - state->len2);
p1 += start_pos - 1;
for (p = start_pos - 1; p <= px; p++)
if (needle_len == 1)
{
if ((*p1 == *p2) && (strncmp(p1, p2, state->len2) == 0))
/* No point in using B-M-H for a one-character needle */
char nchar = *needle;
hptr = &haystack[start_pos];
while (hptr < haystack_end)
{
pos = p + 1;
break;
if (*hptr == nchar)
return hptr - haystack + 1;
hptr++;
}
}
else
{
const char *needle_last = &needle[needle_len - 1];
/* Start at startpos plus the length of the needle */
hptr = &haystack[start_pos + needle_len - 1];
while (hptr < haystack_end)
{
/* Match the needle scanning *backward* */
const char *nptr;
const char *p;
nptr = needle_last;
p = hptr;
while (*nptr == *p)
{
/* Matched it all? If so, return 1-based position */
if (nptr == needle)
return p - haystack + 1;
nptr--, p--;
}
/*
* No match, so use the haystack char at hptr to decide how
* far to advance. If the needle had any occurrence of that
* character (or more precisely, one sharing the same
* skiptable entry) before its last character, then we advance
* far enough to align the last such needle character with
* that haystack position. Otherwise we can advance by the
* whole needle length.
*/
hptr += state->skiptable[(unsigned char) *hptr & skiptablemask];
}
p1++;
}
}
else
{
/* not as simple - multibyte encoding */
pg_wchar *p1 = state->wstr1;
pg_wchar *p2 = state->wstr2;
/* The multibyte char version. This works exactly the same way. */
const pg_wchar *haystack = state->wstr1;
const pg_wchar *needle = state->wstr2;
const pg_wchar *haystack_end = &haystack[haystack_len];
const pg_wchar *hptr;
/* no use in searching str past point where search_str will fit */
px = (state->len1 - state->len2);
p1 += start_pos - 1;
for (p = start_pos - 1; p <= px; p++)
if (needle_len == 1)
{
if ((*p1 == *p2) && (pg_wchar_strncmp(p1, p2, state->len2) == 0))
/* No point in using B-M-H for a one-character needle */
pg_wchar nchar = *needle;
hptr = &haystack[start_pos];
while (hptr < haystack_end)
{
pos = p + 1;
break;
if (*hptr == nchar)
return hptr - haystack + 1;
hptr++;
}
}
else
{
const pg_wchar *needle_last = &needle[needle_len - 1];
/* Start at startpos plus the length of the needle */
hptr = &haystack[start_pos + needle_len - 1];
while (hptr < haystack_end)
{
/* Match the needle scanning *backward* */
const pg_wchar *nptr;
const pg_wchar *p;
nptr = needle_last;
p = hptr;
while (*nptr == *p)
{
/* Matched it all? If so, return 1-based position */
if (nptr == needle)
return p - haystack + 1;
nptr--, p--;
}
/*
* No match, so use the haystack char at hptr to decide how
* far to advance. If the needle had any occurrence of that
* character (or more precisely, one sharing the same
* skiptable entry) before its last character, then we advance
* far enough to align the last such needle character with
* that haystack position. Otherwise we can advance by the
* whole needle length.
*/
hptr += state->skiptable[*hptr & skiptablemask];
}
p1++;
}
}
return pos;
return 0; /* not found */
}
static void