Add levenshtein_less_equal, optimized version for small distances.

Alexander Korotkov, heavily revised by me.

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));
 }
 
 

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;

contrib/fuzzystrmatch/levenshtein.c

+/*
+ * 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];
+}

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);

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>