From 604ab08145d0dde91279c430e08d570717c12b30 Mon Sep 17 00:00:00 2001
From: Robert Haas <rhaas@postgresql.org>
Date: Tue, 19 Oct 2010 09:51:06 -0400
Subject: [PATCH] Add levenshtein_less_equal, optimized version for small
distances.
Alexander Korotkov, heavily revised by me.
---
contrib/fuzzystrmatch/fuzzystrmatch.c | 244 ++---------
contrib/fuzzystrmatch/fuzzystrmatch.sql.in | 8 +
contrib/fuzzystrmatch/levenshtein.c | 397 ++++++++++++++++++
.../fuzzystrmatch/uninstall_fuzzystrmatch.sql | 4 +
doc/src/sgml/fuzzystrmatch.sgml | 19 +
5 files changed, 459 insertions(+), 213 deletions(-)
create mode 100644 contrib/fuzzystrmatch/levenshtein.c
diff --git a/contrib/fuzzystrmatch/fuzzystrmatch.c b/contrib/fuzzystrmatch/fuzzystrmatch.c
index 01084da4072..7265841dc5d 100644
--- a/contrib/fuzzystrmatch/fuzzystrmatch.c
+++ b/contrib/fuzzystrmatch/fuzzystrmatch.c
@@ -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
*/
@@ -197,224 +180,59 @@ 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;
+#include "levenshtein.c"
+#define LEVENSHTEIN_LESS_EQUAL
+#include "levenshtein.c"
- /* 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);
+PG_FUNCTION_INFO_V1(levenshtein_with_costs);
+Datum
+levenshtein_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);
- /* Point to next character. */
- x++;
- }
- }
+ PG_RETURN_INT32(levenshtein_internal(src, dst, ins_c, del_c, sub_c));
+}
- /* Swap current row with previous row. */
- temp = curr;
- curr = prev;
- prev = temp;
- /* Point to next character. */
- y += y_char_len;
- }
+PG_FUNCTION_INFO_V1(levenshtein);
+Datum
+levenshtein(PG_FUNCTION_ARGS)
+{
+ text *src = PG_GETARG_TEXT_PP(0);
+ text *dst = PG_GETARG_TEXT_PP(1);
- /*
- * 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];
+ PG_RETURN_INT32(levenshtein_internal(src, dst, 1, 1, 1));
}
-PG_FUNCTION_INFO_V1(levenshtein_with_costs);
+PG_FUNCTION_INFO_V1(levenshtein_less_equal_with_costs);
Datum
-levenshtein_with_costs(PG_FUNCTION_ARGS)
+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_internal(src, dst, ins_c, del_c, sub_c));
+ PG_RETURN_INT32(levenshtein_less_equal_internal(src, dst, ins_c, del_c, sub_c, max_d));
}
-PG_FUNCTION_INFO_V1(levenshtein);
+PG_FUNCTION_INFO_V1(levenshtein_less_equal);
Datum
-levenshtein(PG_FUNCTION_ARGS)
+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_internal(src, dst, 1, 1, 1));
+ PG_RETURN_INT32(levenshtein_less_equal_internal(src, dst, 1, 1, 1, max_d));
}
diff --git a/contrib/fuzzystrmatch/fuzzystrmatch.sql.in b/contrib/fuzzystrmatch/fuzzystrmatch.sql.in
index 05a347d6b8d..0e75491cbe3 100644
--- a/contrib/fuzzystrmatch/fuzzystrmatch.sql.in
+++ b/contrib/fuzzystrmatch/fuzzystrmatch.sql.in
@@ -11,6 +11,14 @@ CREATE OR REPLACE FUNCTION levenshtein (text,text,int,int,int) RETURNS int
AS 'MODULE_PATHNAME','levenshtein_with_costs'
LANGUAGE C IMMUTABLE STRICT;
+CREATE OR REPLACE FUNCTION levenshtein_less_equal (text,text,int) RETURNS int
+AS 'MODULE_PATHNAME','levenshtein_less_equal'
+LANGUAGE C IMMUTABLE STRICT;
+
+CREATE OR REPLACE FUNCTION levenshtein_less_equal (text,text,int,int,int,int) RETURNS int
+AS 'MODULE_PATHNAME','levenshtein_less_equal_with_costs'
+LANGUAGE C IMMUTABLE STRICT;
+
CREATE OR REPLACE FUNCTION metaphone (text,int) RETURNS text
AS 'MODULE_PATHNAME','metaphone'
LANGUAGE C IMMUTABLE STRICT;
diff --git a/contrib/fuzzystrmatch/levenshtein.c b/contrib/fuzzystrmatch/levenshtein.c
new file mode 100644
index 00000000000..178d0e4d754
--- /dev/null
+++ b/contrib/fuzzystrmatch/levenshtein.c
@@ -0,0 +1,397 @@
+/*
+ * levenshtein.c
+ *
+ * Functions for "fuzzy" comparison of strings
+ *
+ * Joe Conway <mail@joeconway.com>
+ *
+ * contrib/fuzzystrmatch/fuzzystrmatch.c
+ * 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>.
+ */
+
+/*
+ * External declarations for exported functions
+ */
+#ifdef LEVENSHTEIN_LESS_EQUAL
+static int levenshtein_less_equal_internal(text *s, text *t,
+ int ins_c, int del_c, int sub_c, int max_d);
+#else
+static int levenshtein_internal(text *s, text *t,
+ int ins_c, int del_c, int sub_c);
+#endif
+
+#define MAX_LEVENSHTEIN_STRLEN 255
+
+
+/*
+ * Calculates Levenshtein distance metric between supplied strings. Generally
+ * (1, 1, 1) penalty costs suffices for common cases, but your mileage may
+ * vary.
+ *
+ * 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.
+ *
+ * If max_d >= 0, we only need to provide an accurate answer when that answer
+ * is less than or equal to the bound. From any cell in the matrix, there is
+ * theoretical "minimum residual distance" from that cell to the last column
+ * of the final row. This minimum residual distance is zero when the
+ * untransformed portions of the strings are of equal length (because we might
+ * get lucky and find all the remaining characters matching) and is otherwise
+ * based on the minimum number of insertions or deletions needed to make them
+ * equal length. The residual distance grows as we move toward the upper
+ * right or lower left corners of the matrix. When the max_d bound is
+ * usefully tight, we can use this property to avoid computing the entirety
+ * of each row; instead, we maintain a start_column and stop_column that
+ * identify the portion of the matrix close to the diagonal which can still
+ * affect the final answer.
+ */
+static int
+#ifdef LEVENSHTEIN_LESS_EQUAL
+levenshtein_less_equal_internal(text *s, text *t,
+ int ins_c, int del_c, int sub_c, int max_d)
+#else
+levenshtein_internal(text *s, text *t,
+ int ins_c, int del_c, int sub_c)
+#endif
+{
+ 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;
+
+ /*
+ * For levenshtein_less_equal_internal, we have real variables called
+ * start_column and stop_column; otherwise it's just short-hand for 0
+ * and m.
+ */
+#ifdef LEVENSHTEIN_LESS_EQUAL
+ int start_column, stop_column;
+#undef START_COLUMN
+#undef STOP_COLUMN
+#define START_COLUMN start_column
+#define STOP_COLUMN stop_column
+#else
+#undef START_COLUMN
+#undef STOP_COLUMN
+#define START_COLUMN 0
+#define STOP_COLUMN m
+#endif
+
+ /* 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)));
+
+#ifdef LEVENSHTEIN_LESS_EQUAL
+ /* Initialize start and stop columns. */
+ start_column = 0;
+ stop_column = m + 1;
+
+ /*
+ * If max_d >= 0, determine whether the bound is impossibly tight. If so,
+ * return max_d + 1 immediately. Otherwise, determine whether it's tight
+ * enough to limit the computation we must perform. If so, figure out
+ * initial stop column.
+ */
+ if (max_d >= 0)
+ {
+ int min_theo_d; /* Theoretical minimum distance. */
+ int max_theo_d; /* Theoretical maximum distance. */
+ int net_inserts = n - m;
+
+ min_theo_d = net_inserts < 0 ?
+ -net_inserts * del_c : net_inserts * ins_c;
+ if (min_theo_d > max_d)
+ return max_d + 1;
+ if (ins_c + del_c < sub_c)
+ sub_c = ins_c + del_c;
+ max_theo_d = min_theo_d + sub_c * Min(m, n);
+ if (max_d >= max_theo_d)
+ max_d = -1;
+ else if (ins_c + del_c > 0)
+ {
+ /*
+ * Figure out how much of the first row of the notional matrix
+ * we need to fill in. If the string is growing, the theoretical
+ * minimum distance already incorporates the cost of deleting the
+ * number of characters necessary to make the two strings equal
+ * in length. Each additional deletion forces another insertion,
+ * so the best-case total cost increases by ins_c + del_c.
+ * If the string is shrinking, the minimum theoretical cost
+ * assumes no excess deletions; that is, we're starting no futher
+ * right than column n - m. If we do start further right, the
+ * best-case total cost increases by ins_c + del_c for each move
+ * right.
+ */
+ int slack_d = max_d - min_theo_d;
+ int best_column = net_inserts < 0 ? -net_inserts : 0;
+ stop_column = best_column + (slack_d / (ins_c + del_c)) + 1;
+ if (stop_column > m)
+ stop_column = m + 1;
+ }
+ }
+#endif
+
+ /*
+ * 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;
+
+ /* Previous and current rows of notional 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 = START_COLUMN; i < STOP_COLUMN; 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;
+
+#ifdef LEVENSHTEIN_LESS_EQUAL
+ /*
+ * In the best case, values percolate down the diagonal unchanged, so
+ * we must increment stop_column unless it's already on the right end
+ * of the array. The inner loop will read prev[stop_column], so we
+ * have to initialize it even though it shouldn't affect the result.
+ */
+ if (stop_column < m)
+ {
+ prev[stop_column] = max_d + 1;
+ ++stop_column;
+ }
+
+ /*
+ * The main loop fills in curr, but curr[0] needs a special case:
+ * to transform the first 0 characters of s into the first j
+ * characters of t, we must perform j insertions. However, if
+ * start_column > 0, this special case does not apply.
+ */
+ if (start_column == 0)
+ {
+ curr[0] = j * ins_c;
+ i = 1;
+ }
+ else
+ i = start_column;
+#else
+ curr[0] = j * ins_c;
+ i = 1;
+#endif
+
+ /*
+ * 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 < STOP_COLUMN; 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 < STOP_COLUMN; 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;
+
+#ifdef LEVENSHTEIN_LESS_EQUAL
+ /*
+ * This chunk of code represents a significant performance hit if used
+ * in the case where there is no max_d bound. This is probably not
+ * because the max_d >= 0 test itself is expensive, but rather because
+ * the possibility of needing to execute this code prevents tight
+ * optimization of the loop as a whole.
+ */
+ if (max_d >= 0)
+ {
+ /*
+ * The "zero point" is the column of the current row where the
+ * remaining portions of the strings are of equal length. There
+ * are (n - 1) characters in the target string, of which j have
+ * been transformed. There are (m - 1) characters in the source
+ * string, so we want to find the value for zp where where (n - 1)
+ * - j = (m - 1) - zp.
+ */
+ int zp = j - (n - m);
+
+ /* Check whether the stop column can slide left. */
+ while (stop_column > 0)
+ {
+ int ii = stop_column - 1;
+ int net_inserts = ii - zp;
+ if (prev[ii] + (net_inserts > 0 ? net_inserts * ins_c :
+ -net_inserts * del_c) <= max_d)
+ break;
+ stop_column--;
+ }
+
+ /* Check whether the start column can slide right. */
+ while (start_column < stop_column)
+ {
+ int net_inserts = start_column - zp;
+ if (prev[start_column] +
+ (net_inserts > 0 ? net_inserts * ins_c :
+ -net_inserts * del_c) <= max_d)
+ break;
+ /*
+ * We'll never again update these values, so we must make
+ * sure there's nothing here that could confuse any future
+ * iteration of the outer loop.
+ */
+ prev[start_column] = max_d + 1;
+ curr[start_column] = max_d + 1;
+ if (start_column != 0)
+ s_data += n != t_bytes + 1 ? pg_mblen(s_data) : 1;
+ start_column++;
+ }
+
+ /* If they cross, we're going to exceed the bound. */
+ if (start_column >= stop_column)
+ return max_d + 1;
+ }
+#endif
+ }
+
+ /*
+ * 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];
+}
diff --git a/contrib/fuzzystrmatch/uninstall_fuzzystrmatch.sql b/contrib/fuzzystrmatch/uninstall_fuzzystrmatch.sql
index 99d25485691..a39c7bfc944 100644
--- a/contrib/fuzzystrmatch/uninstall_fuzzystrmatch.sql
+++ b/contrib/fuzzystrmatch/uninstall_fuzzystrmatch.sql
@@ -18,3 +18,7 @@ DROP FUNCTION metaphone (text,int);
DROP FUNCTION levenshtein (text,text,int,int,int);
DROP FUNCTION levenshtein (text,text);
+
+DROP FUNCTION levenshtein_less_equal (text,text,int);
+
+DROP FUNCTION levenshtein_less_equal (text,text,int,int,int,int);
diff --git a/doc/src/sgml/fuzzystrmatch.sgml b/doc/src/sgml/fuzzystrmatch.sgml
index 69777e49358..01b6e127ba6 100644
--- a/doc/src/sgml/fuzzystrmatch.sgml
+++ b/doc/src/sgml/fuzzystrmatch.sgml
@@ -84,6 +84,8 @@ SELECT * FROM s WHERE difference(s.nm, 'john') > 2;
<synopsis>
levenshtein(text source, text target, int ins_cost, int del_cost, int sub_cost) returns int
levenshtein(text source, text target) returns int
+levenshtein_less_equal(text source, text target, int ins_cost, int del_cost, int sub_cost, int max_d) returns int
+levenshtein_less_equal(text source, text target, int max_d) returns int
</synopsis>
<para>
@@ -92,6 +94,11 @@ levenshtein(text source, text target) returns int
specify how much to charge for a character insertion, deletion, or
substitution, respectively. You can omit the cost parameters, as in
the second version of the function; in that case they all default to 1.
+ <literal>levenshtein_less_equal</literal> is accelerated version of
+ levenshtein functon for low values of distance. If actual distance
+ is less or equal then max_d, then <literal>levenshtein_less_equal</literal>
+ returns accurate value of it. Otherwise this function returns value
+ which is greater than max_d.
</para>
<para>
@@ -110,6 +117,18 @@ test=# SELECT levenshtein('GUMBO', 'GAMBOL', 2,1,1);
-------------
3
(1 row)
+
+test=# SELECT levenshtein_less_equal('extensive', 'exhaustive',2);
+ levenshtein_less_equal
+------------------------
+ 3
+(1 row)
+
+test=# SELECT levenshtein_less_equal('extensive', 'exhaustive',4);
+ levenshtein_less_equal
+------------------------
+ 4
+(1 row)
</screen>
</sect2>
--
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