4 files changed, 414 insertions, 1 deletions

diff --git a/src/backend/utils/adt/Makefile b/src/backend/utils/adt/Makefile
index 7b4391bba17..3ea9bf435a3 100644
--- a/src/backend/utils/adt/Makefile
+++ b/src/backend/utils/adt/Makefile
@@ -38,4 +38,6 @@ OBJS = acl.o arrayfuncs.o array_selfuncs.o array_typanalyze.o \
 
 like.o: like.c like_match.c
 
+varlena.o: varlena.c levenshtein.c
+
 include $(top_srcdir)/src/backend/common.mk
diff --git a/src/backend/utils/adt/levenshtein.c b/src/backend/utils/adt/levenshtein.c
new file mode 100644
index 00000000000..a8670e9a85b
--- /dev/null
+++ b/src/backend/utils/adt/levenshtein.c
@@ -0,0 +1,386 @@
+/*-------------------------------------------------------------------------
+ *
+ * levenshtein.c
+ *	  Levenshtein distance implementation.
+ *
+ * Original author:  Joe Conway <mail@joeconway.com>
+ *
+ * This file is included by varlena.c twice, to provide matching code for (1)
+ * Levenshtein distance with custom costings, and (2) Levenshtein distance with
+ * custom costings and a "max" value above which exact distances are not
+ * interesting.  Before the inclusion, we rely on the presence of the inline
+ * function rest_of_char_same().
+ *
+ * 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.
+ *
+ * Copyright (c) 2001-2014, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	src/backend/utils/adt/levenshtein.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#define MAX_LEVENSHTEIN_STRLEN		255
+
+/*
+ * Calculates Levenshtein distance metric between supplied csrings, which are
+ * not necessarily null-terminated.  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.
+ */
+int
+#ifdef LEVENSHTEIN_LESS_EQUAL
+varstr_levenshtein_less_equal(const char *source, int slen, const char *target,
+							  int tlen, int ins_c, int del_c, int sub_c,
+							  int max_d)
+#else
+varstr_levenshtein(const char *source, int slen, const char *target, int tlen,
+				   int ins_c, int del_c, int sub_c)
+#endif
+{
+	int			m,
+				n;
+	int		   *prev;
+	int		   *curr;
+	int		   *s_char_len = NULL;
+	int			i,
+				j;
+	const char *y;
+
+	/*
+	 * For varstr_levenshtein_less_equal, 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
+
+	m = pg_mbstrlen_with_len(source, slen);
+	n = pg_mbstrlen_with_len(target, tlen);
+
+	/*
+	 * 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 further 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 != slen || n != tlen)
+	{
+		int			i;
+		const char *cp = source;
+
+		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 = target, j = 1; j < n; j++)
+	{
+		int		   *temp;
+		const char *x = source;
+		int			y_char_len = n != tlen + 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 (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)
+					source += (s_char_len != NULL) ? s_char_len[start_column - 1] : 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/src/backend/utils/adt/varlena.c b/src/backend/utils/adt/varlena.c
index c3171b549a6..b3f397e9595 100644
--- a/src/backend/utils/adt/varlena.c
+++ b/src/backend/utils/adt/varlena.c
@@ -1546,7 +1546,6 @@ varstr_cmp(char *arg1, int len1, char *arg2, int len2, Oid collid)
 	return result;
 }
 
-
 /* text_cmp()
  * Internal comparison function for text strings.
  * Returns -1, 0 or 1
@@ -4747,3 +4746,24 @@ text_format_nv(PG_FUNCTION_ARGS)
 {
 	return text_format(fcinfo);
 }
+
+/*
+ * Helper function for Levenshtein distance functions. Faster than memcmp(),
+ * for this use case.
+ */
+static inline bool
+rest_of_char_same(const char *s1, const char *s2, int len)
+{
+	while (len > 0)
+	{
+		len--;
+		if (s1[len] != s2[len])
+			return false;
+	}
+	return true;
+}
+
+/* Expand each Levenshtein distance variant */
+#include "levenshtein.c"
+#define LEVENSHTEIN_LESS_EQUAL
+#include "levenshtein.c"
diff --git a/src/include/utils/builtins.h b/src/include/utils/builtins.h
index 3ba34f88eec..417fd1771a8 100644
--- a/src/include/utils/builtins.h
+++ b/src/include/utils/builtins.h
@@ -786,6 +786,11 @@ extern Datum textoverlay_no_len(PG_FUNCTION_ARGS);
 extern Datum name_text(PG_FUNCTION_ARGS);
 extern Datum text_name(PG_FUNCTION_ARGS);
 extern int	varstr_cmp(char *arg1, int len1, char *arg2, int len2, Oid collid);
+extern int	varstr_levenshtein(const char *source, int slen, const char *target,
+				   int tlen, int ins_c, int del_c, int sub_c);
+extern int	varstr_levenshtein_less_equal(const char *source, int slen,
+							  const char *target, int tlen, int ins_c,
+							  int del_c, int sub_c, int max_d);
 extern List *textToQualifiedNameList(text *textval);
 extern bool SplitIdentifierString(char *rawstring, char separator,
 					  List **namelist);