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Add levenshtein_less_equal, optimized version for small distances.

Browse Source 
Alexander Korotkov, heavily revised by me.
This commit is contained in:
Robert Haas
2010-10-19 09:51:06 -04:00
parent 262c1a42dc
commit 604ab08145
5 changed files with 460 additions and 214 deletions
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246
contrib/fuzzystrmatch/fuzzystrmatch.c
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@ -9,15 +9,6 @@
* Copyright (c) 2001-2010, PostgreSQL Global Development Group
* ALL RIGHTS RESERVED;
*
* levenshtein()
* -------------
* Written based on a description of the algorithm by Michael Gilleland
* found at http://www.merriampark.com/ld.htm
* Also looked at levenshtein.c in the PHP 4.0.6 distribution for
* inspiration.
* Configurable penalty costs extension is introduced by Volkan
* YAZICI <volkan.yazici@gmail.com>.
*
* metaphone()
* -----------
* Modified for PostgreSQL by Joe Conway.
@ -61,6 +52,8 @@ PG_MODULE_MAGIC;
*/
extern Datum levenshtein_with_costs(PG_FUNCTION_ARGS);
extern Datum levenshtein(PG_FUNCTION_ARGS);
extern Datum levenshtein_less_equal_with_costs(PG_FUNCTION_ARGS);
extern Datum levenshtein_less_equal(PG_FUNCTION_ARGS);
extern Datum metaphone(PG_FUNCTION_ARGS);
extern Datum soundex(PG_FUNCTION_ARGS);
extern Datum difference(PG_FUNCTION_ARGS);
@ -85,16 +78,6 @@ soundex_code(char letter)
return letter;
}
/*
* Levenshtein
*/
#define MAX_LEVENSHTEIN_STRLEN 255
static int levenshtein_internal(text *s, text *t,
int ins_c, int del_c, int sub_c);
/*
* Metaphone
*/
@ -197,201 +180,9 @@ rest_of_char_same(const char *s1, const char *s2, int len)
return true;
}
/*
* levenshtein_internal - Calculates Levenshtein distance metric
* between supplied strings. Generally
* (1, 1, 1) penalty costs suffices common
* cases, but your mileage may vary.
*/
static int
levenshtein_internal(text *s, text *t,
int ins_c, int del_c, int sub_c)
{
int m,
n,
s_bytes,
t_bytes;
int *prev;
int *curr;
int *s_char_len = NULL;
int i,
j;
const char *s_data;
const char *t_data;
const char *y;
/* Extract a pointer to the actual character data. */
s_data = VARDATA_ANY(s);
t_data = VARDATA_ANY(t);
/* Determine length of each string in bytes and characters. */
s_bytes = VARSIZE_ANY_EXHDR(s);
t_bytes = VARSIZE_ANY_EXHDR(t);
m = pg_mbstrlen_with_len(s_data, s_bytes);
n = pg_mbstrlen_with_len(t_data, t_bytes);
/*
* We can transform an empty s into t with n insertions, or a non-empty t
* into an empty s with m deletions.
*/
if (!m)
return n * ins_c;
if (!n)
return m * del_c;
/*
* For security concerns, restrict excessive CPU+RAM usage. (This
* implementation uses O(m) memory and has O(mn) complexity.)
*/
if (m > MAX_LEVENSHTEIN_STRLEN ||
n > MAX_LEVENSHTEIN_STRLEN)
ereport(ERROR,
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
errmsg("argument exceeds the maximum length of %d bytes",
MAX_LEVENSHTEIN_STRLEN)));
/*
* In order to avoid calling pg_mblen() repeatedly on each character in s,
* we cache all the lengths before starting the main loop -- but if all the
* characters in both strings are single byte, then we skip this and use
* a fast-path in the main loop. If only one string contains multi-byte
* characters, we still build the array, so that the fast-path needn't
* deal with the case where the array hasn't been initialized.
*/
if (m != s_bytes || n != t_bytes)
{
int i;
const char *cp = s_data;
s_char_len = (int *) palloc((m + 1) * sizeof(int));
for (i = 0; i < m; ++i)
{
s_char_len[i] = pg_mblen(cp);
cp += s_char_len[i];
}
s_char_len[i] = 0;
}
/* One more cell for initialization column and row. */
++m;
++n;
/*
* One way to compute Levenshtein distance is to incrementally construct
* an (m+1)x(n+1) matrix where cell (i, j) represents the minimum number
* of operations required to transform the first i characters of s into
* the first j characters of t. The last column of the final row is the
* answer.
*
* We use that algorithm here with some modification. In lieu of holding
* the entire array in memory at once, we'll just use two arrays of size
* m+1 for storing accumulated values. At each step one array represents
* the "previous" row and one is the "current" row of the notional large
* array.
*/
prev = (int *) palloc(2 * m * sizeof(int));
curr = prev + m;
/*
* To transform the first i characters of s into the first 0 characters
* of t, we must perform i deletions.
*/
for (i = 0; i < m; i++)
prev[i] = i * del_c;
/* Loop through rows of the notional array */
for (y = t_data, j = 1; j < n; j++)
{
int *temp;
const char *x = s_data;
int y_char_len = n != t_bytes + 1 ? pg_mblen(y) : 1;
/*
* To transform the first 0 characters of s into the first j
* characters of t, we must perform j insertions.
*/
curr[0] = j * ins_c;
/*
* This inner loop is critical to performance, so we include a
* fast-path to handle the (fairly common) case where no multibyte
* characters are in the mix. The fast-path is entitled to assume
* that if s_char_len is not initialized then BOTH strings contain
* only single-byte characters.
*/
if (s_char_len != NULL)
{
for (i = 1; i < m; i++)
{
int ins;
int del;
int sub;
int x_char_len = s_char_len[i - 1];
/*
* Calculate costs for insertion, deletion, and substitution.
*
* When calculating cost for substitution, we compare the last
* character of each possibly-multibyte character first,
* because that's enough to rule out most mis-matches. If we
* get past that test, then we compare the lengths and the
* remaining bytes.
*/
ins = prev[i] + ins_c;
del = curr[i - 1] + del_c;
if (x[x_char_len-1] == y[y_char_len-1]
&& x_char_len == y_char_len &&
(x_char_len == 1 || rest_of_char_same(x, y, x_char_len)))
sub = prev[i - 1];
else
sub = prev[i - 1] + sub_c;
/* Take the one with minimum cost. */
curr[i] = Min(ins, del);
curr[i] = Min(curr[i], sub);
/* Point to next character. */
x += x_char_len;
}
}
else
{
for (i = 1; i < m; i++)
{
int ins;
int del;
int sub;
/* Calculate costs for insertion, deletion, and substitution. */
ins = prev[i] + ins_c;
del = curr[i - 1] + del_c;
sub = prev[i - 1] + ((*x == *y) ? 0 : sub_c);
/* Take the one with minimum cost. */
curr[i] = Min(ins, del);
curr[i] = Min(curr[i], sub);
/* Point to next character. */
x++;
}
}
/* Swap current row with previous row. */
temp = curr;
curr = prev;
prev = temp;
/* Point to next character. */
y += y_char_len;
}
/*
* Because the final value was swapped from the previous row to the
* current row, that's where we'll find it.
*/
return prev[m - 1];
}
#include "levenshtein.c"
#define LEVENSHTEIN_LESS_EQUAL
#include "levenshtein.c"
PG_FUNCTION_INFO_V1(levenshtein_with_costs);
Datum
@ -418,6 +209,33 @@ levenshtein(PG_FUNCTION_ARGS)
}
PG_FUNCTION_INFO_V1(levenshtein_less_equal_with_costs);
Datum
levenshtein_less_equal_with_costs(PG_FUNCTION_ARGS)
{
text *src = PG_GETARG_TEXT_PP(0);
text *dst = PG_GETARG_TEXT_PP(1);
int ins_c = PG_GETARG_INT32(2);
int del_c = PG_GETARG_INT32(3);
int sub_c = PG_GETARG_INT32(4);
int max_d = PG_GETARG_INT32(5);
PG_RETURN_INT32(levenshtein_less_equal_internal(src, dst, ins_c, del_c, sub_c, max_d));
}
PG_FUNCTION_INFO_V1(levenshtein_less_equal);
Datum
levenshtein_less_equal(PG_FUNCTION_ARGS)
{
text *src = PG_GETARG_TEXT_PP(0);
text *dst = PG_GETARG_TEXT_PP(1);
int max_d = PG_GETARG_INT32(2);
PG_RETURN_INT32(levenshtein_less_equal_internal(src, dst, 1, 1, 1, max_d));
}
/*
* Calculates the metaphone of an input string.
* Returns number of characters requested