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* [Tarantool-patches] [PATCH] sql: Replace naive position alg with KMP
@ 2021-11-14  9:13 Lord via Tarantool-patches
  0 siblings, 0 replies; only message in thread
From: Lord via Tarantool-patches @ 2021-11-14  9:13 UTC (permalink / raw)
  To: tarantool-patches; +Cc: Gleb

From: Gleb <kashkin.nsaa@gmail.com>

Replaces positioning logic in POSITION() and REPLACE() SQL functions
with Knuth-Morris-Pratt algorithm by adding two different versions
of the latter for STRING and VARBINARY arguments and corresponding
prefix table generators.
    
Improved asymptotics of positioning logic to:
time O(haystack_len + needle_len), mem O(needle_len)
As one of collating types is a Unicode string,
ICU UCharIterators had to be used and thus Unicode
letters are compared as UChar32 values and normalization
is not done ('a' and 'ä' are treated as different)


Closes tarantool/tarantool#6492
---
 src/box/sql/func.c             | 402 ++++++++++++++++++++++++++-------
 1 file changed, 321 insertions(+), 90 deletions(-)

diff --git a/src/box/sql/func.c b/src/box/sql/func.c
index 29d713fd0..badfbc176 100644
--- a/src/box/sql/func.c
+++ b/src/box/sql/func.c
@@ -291,6 +291,144 @@ absFunc(sql_context * context, int argc, sql_value ** argv)
 	}
 }
 
+/**
+ * Implementation of prefix table generator 
+ * prefix_table_gen() is a part of Knuth-Morris-Pratt algorithm
+ * that is used in position() and replaceFunc() to find 
+ * instances of `needle` in `haystack` bin data
+ */
+inline static void
+prefix_table_gen_bin(const unsigned char *needle,
+                     const size_t         n_needle_bytes,
+                     size_t              *table)
+{
+	size_t i = 1, j = 0;
+	table[0] = 0;
+	while (i < n_needle_bytes) {
+		if (needle[i] != needle[j]) {
+			if (j == 0) {
+				table[i] = 0;
+				++i;
+			} else {
+				j = table[j - 1];
+			}
+		} else {
+			table[i] = j + 1;
+			++i;
+			++j;
+		}
+	}
+}
+
+/**
+ * Implementation of prefix table generator using UCharIterators 
+ * prefix_table_gen() is a part of Knuth-Morris-Pratt algorithm
+ * that is used in position() to find instances of `needle`  
+ * in `haystack` unicode string
+ */
+inline static void 
+prefix_table_gen_ustr(const UCharIterator *needle_begin,
+                      const unsigned char *needle_str,
+                      const size_t         n_needle_bytes, 
+                      const size_t         n_needle_chars,
+                      size_t               table[]) 
+{
+	table[0] = 0;
+	UCharIterator needle_iter;
+	UCharIterator second_iter;
+	uiter_setUTF8(&needle_iter, needle_str, n_needle_bytes);
+	uiter_setUTF8(&second_iter, needle_str, n_needle_bytes);
+	needle_iter.next(&needle_iter);
+	while (needle_iter.getIndex(&needle_iter, UITER_CURRENT) < n_needle_chars) {
+		if (needle_iter.current(&needle_iter) != second_iter.current(&second_iter)) {
+			if (second_iter.getIndex(&second_iter, UITER_CURRENT) == 0) {
+				table[needle_iter.getIndex(&needle_iter, UITER_CURRENT)] = 0;
+				needle_iter.next(&needle_iter);   
+			} else {
+				second_iter.move(&second_iter, table[second_iter.getIndex(&second_iter, UITER_CURRENT) - 1], UITER_START);
+			}
+		} else {
+			table[needle_iter.getIndex(&needle_iter, UITER_CURRENT)] = second_iter.getIndex(&second_iter, UITER_CURRENT) + 1;
+			needle_iter.next(&needle_iter);
+			second_iter.next(&second_iter);
+		}
+	}
+}
+
+/**
+ * Implementation of the KMP algorithm to find substring 
+ * in a bin array starting from given position.
+ *
+ * Requires index for starting position in `haystack`.
+ *
+ * Returns found substring position + 1 or 0 if there is no such substring.
+ */
+static int 
+kmp_bin(const unsigned char *haystack_str, 
+        const unsigned char *needle_str, 
+        const size_t         n_haystack_bytes, 
+        const size_t         n_needle_bytes,
+              size_t	     i,
+        const size_t        *prefix_table)	  
+{
+	size_t j = 0;
+	int position = 0;
+	while (i < n_haystack_bytes) {
+		if (needle_str[j] == haystack_str[i]) {
+			++i;
+			++j;
+		} else {
+			if (j == 0)
+				++i;
+			else
+				j = prefix_table[j - 1];
+		}
+		if (j >= n_needle_bytes) {
+			position = i - j + 1;
+			return position;
+		}
+	}
+	return position;
+}
+
+/**
+ * Implementation of the KMP algorithm to find substring
+ * in an array of unicode chars starting from given iterator.
+ *
+ * Requires UCharIterator [haystack_iter] of starting position 
+ * in `haystack`, iterator gets set to the first item after found
+ * `needle` or reaches the end of haystack if there is no such substring.
+ *
+ * Returns found substring position + 1 or 0 if no such substring.
+ */
+inline static int 
+kmp_ustr(struct sql_context *context, 
+         UCharIterator	    *haystack_iter, 
+         UCharIterator      *needle_iter, 
+         const size_t        n_haystack_chars, 
+         const size_t       *prefix_table)
+{
+	struct coll *coll = sqlGetFuncCollSeq(context);
+	int position = 0;
+	while (haystack_iter->getIndex(haystack_iter, UITER_CURRENT) < n_haystack_chars) {
+		if (needle_iter->current(needle_iter) == haystack_iter->current(haystack_iter)) {
+			needle_iter->next(needle_iter);
+			haystack_iter->next(haystack_iter);
+		} else {
+			if (needle_iter->getIndex(needle_iter, UITER_CURRENT) == 0) 
+				haystack_iter->next(haystack_iter);
+			else 
+				needle_iter->move(needle_iter, prefix_table[needle_iter->getIndex(needle_iter, UITER_CURRENT) - 1], UITER_START);
+		}
+		if (!needle_iter->hasNext(needle_iter)) {
+			position = haystack_iter->getIndex(haystack_iter, UITER_CURRENT) - needle_iter->getIndex(needle_iter, UITER_CURRENT) + 1;
+			haystack_iter->next(haystack_iter);
+			return position;
+		}
+	}
+	return position;
+}
+
 /**
  * Implementation of the position() function.
  *
@@ -339,9 +477,10 @@ position_func(struct sql_context *context, int argc, struct Mem **argv)
 		return;
 	}
 
-	int n_needle_bytes = mem_len_unsafe(needle);
-	int n_haystack_bytes = mem_len_unsafe(haystack);
+	size_t n_needle_bytes	= mem_len_unsafe(needle);
+	size_t n_haystack_bytes = mem_len_unsafe(haystack);
 	int position = 1;
+	size_t *prefix_table = NULL;
 	if (n_needle_bytes > 0) {
 		const unsigned char *haystack_str;
 		const unsigned char *needle_str;
@@ -351,18 +490,24 @@ position_func(struct sql_context *context, int argc, struct Mem **argv)
 			assert(needle_str != NULL);
 			assert(haystack_str != NULL || n_haystack_bytes == 0);
 			/*
-			 * Naive implementation of substring
-			 * searching: matching time O(n * m).
-			 * Can be improved.
+			 * Basic version of the KMP alg
+			 * searching: matching time O(n + m), requires mem O(m).
 			 */
-			while (n_needle_bytes <= n_haystack_bytes &&
-			       memcmp(haystack_str, needle_str, n_needle_bytes) != 0) {
-				position++;
-				n_haystack_bytes--;
-				haystack_str++;
+			prefix_table = calloc(n_needle_bytes, sizeof(size_t));
+
+			if (prefix_table != NULL) {									
+				prefix_table_gen_bin(needle_str, n_needle_bytes, prefix_table);
+				position = kmp_bin(haystack_str, needle_str, n_haystack_bytes, n_needle_bytes, 0, prefix_table);
+			} else {
+				while (n_needle_bytes <= n_haystack_bytes &&
+					   memcmp(haystack_str, needle_str, n_needle_bytes) != 0) {
+					position++;
+					n_haystack_bytes--;
+					haystack_str++;
+				}	
+				if (n_needle_bytes > n_haystack_bytes)
+					position = 0;
 			}
-			if (n_needle_bytes > n_haystack_bytes)
-				position = 0;
 		} else {
 			/*
 			 * Code below handles not only simple
@@ -376,52 +521,74 @@ position_func(struct sql_context *context, int argc, struct Mem **argv)
 			 * needle char size.
 			 */
 			haystack_str = mem_as_ustr(haystack);
-			needle_str = mem_as_ustr(needle);
+			needle_str	 = mem_as_ustr(needle);
 
 			int n_needle_chars =
 				sql_utf8_char_count(needle_str, n_needle_bytes);
 			int n_haystack_chars =
 				sql_utf8_char_count(haystack_str,
-						    n_haystack_bytes);
+							n_haystack_bytes);
+
 
 			if (n_haystack_chars < n_needle_chars) {
 				position = 0;
 				goto finish;
 			}
-			/*
-			 * Comparison window is determined by
-			 * beg_offset and end_offset. beg_offset
-			 * is offset in bytes from haystack
-			 * beginning to window beginning.
-			 * end_offset is offset in bytes from
-			 * haystack beginning to window end.
-			 */
-			int end_offset = 0;
-			for (int c = 0; c < n_needle_chars; c++) {
-				SQL_UTF8_FWD_1(haystack_str, end_offset,
-					       n_haystack_bytes);
-			}
-			int beg_offset = 0;
-			struct coll *coll = sqlGetFuncCollSeq(context);
-			int c;
-			for (c = 0; c + n_needle_chars <= n_haystack_chars; c++) {
-				if (coll->cmp((const char *) haystack_str + beg_offset,
-					      end_offset - beg_offset,
-					      (const char *) needle_str,
-					      n_needle_bytes, coll) == 0)
-					goto finish;
-				position++;
-				/* Update offsets. */
-				SQL_UTF8_FWD_1(haystack_str, beg_offset,
-					       n_haystack_bytes);
-				SQL_UTF8_FWD_1(haystack_str, end_offset,
-					       n_haystack_bytes);
+
+			prefix_table = calloc(n_needle_chars, sizeof(size_t));
+			
+			if (prefix_table != NULL) {
+				/*
+				 * KMP algorithm with unicode support
+				 * searching: time O(n + m), mem O(m)
+				 */
+				UCharIterator needle_iter, haystack_iter;
+
+				uiter_setUTF8(	&needle_iter,	needle_str,   n_needle_bytes);
+				uiter_setUTF8(&haystack_iter, haystack_str, n_haystack_bytes);
+				
+				prefix_table_gen_ustr(&needle_iter, needle_str, n_needle_bytes, n_needle_chars, prefix_table);
+				position = kmp_ustr(context, &haystack_iter, &needle_iter, n_haystack_chars, prefix_table);
+			} else {
+				/*
+				 * Naive searching substring algorithm 
+				 * searching: time O(m * n), mem O(1)
+				 *
+				 * Comparison window is determined by
+				 * beg_offset and end_offset. beg_offset
+				 * is offset in bytes from haystack
+				 * beginning to window beginning.
+				 * end_offset is offset in bytes from
+				 * haystack beginning to window end.
+				 */
+				int end_offset = 0;
+				for (int c = 0; c < n_needle_chars; c++) {
+					SQL_UTF8_FWD_1(haystack_str, end_offset,
+							   n_haystack_bytes);
+				}
+				int beg_offset = 0;
+				struct coll *coll = sqlGetFuncCollSeq(context);
+				int c;
+				for (c = 0; c + n_needle_chars <= n_haystack_chars; c++) {
+					if (coll->cmp((const char *) haystack_str + beg_offset,
+							  end_offset - beg_offset,
+							  (const char *) needle_str,
+							  n_needle_bytes, coll) == 0)
+						goto finish;
+					position++;
+					/* Update offsets. */
+					SQL_UTF8_FWD_1(haystack_str, beg_offset,
+							   n_haystack_bytes);
+					SQL_UTF8_FWD_1(haystack_str, end_offset,
+							   n_haystack_bytes);
+				}
+				/* Needle was not found in the haystack. */
+				position = 0;
 			}
-			/* Needle was not found in the haystack. */
-			position = 0;
 		}
 	}
 finish:
+	free(prefix_table);
 	assert(position >= 0);
 	sql_result_uint(context, position);
 }
@@ -1280,24 +1447,31 @@ zeroblobFunc(sql_context * context, int argc, sql_value ** argv)
 }
 
 /*
- * The replace() function.  Three arguments are all strings: call
+ * The replace() function.	Three arguments are all strings: call
  * them A, B, and C. The result is also a string which is derived
  * from A by replacing every occurrence of B with C.  The match
  * must be exact.  Collating sequences are not used.
  */
 static void
-replaceFunc(sql_context * context, int argc, sql_value ** argv)
+replaceFunc(sql_context *context, int argc, sql_value **argv)
 {
-	const unsigned char *zStr;	/* The input string A */
-	const unsigned char *zPattern;	/* The pattern string B */
-	const unsigned char *zRep;	/* The replacement string C */
-	unsigned char *zOut;	/* The output */
-	int nStr;		/* Size of zStr */
-	int nPattern;		/* Size of zPattern */
-	int nRep;		/* Size of zRep */
-	i64 nOut;		/* Maximum size of zOut */
-	int loopLimit;		/* Last zStr[] that might match zPattern[] */
-	int i, j;		/* Loop counters */
+	const unsigned char *zStr;        /* The input string A                          */
+	const unsigned char *zPattern;    /* The pattern string B                        */
+	const unsigned char *zRep;        /* The replacement string C                    */
+	unsigned char *zOut;              /* The output                                  */
+	int nStr;                         /* Size of zStr                                */
+	int nPattern;                     /* Size of zPattern                            */
+	int nRep;                         /* Size of zRep                                */
+	i64 nOut;                         /* Maximum size of zOut                        */
+	int loopLimit;                    /* Last zStr[] that might match zPattern[]     */
+	int i, j;                         /* Loop counters                               */
+	int nCnt = 0;                     /* counter for number of Pattern in Str        */
+	int iStr = 0;                     /* zStr iterator for finding Patter with KMP   */
+	int nCopiedOut = 0;               /* zOut iterator for already copied mem        */
+	int nCopiedStr = 0;               /* zStr iterator for already copied mem	     */
+	int nPatternPos = 0;              /* zStr iterator for found Pattern position    */
+	size_t *prefix_table = NULL;	  /* prefix table for kmp alg                    */
+
 
 	assert(argc == 3);
 	UNUSED_PARAMETER(argc);
@@ -1309,7 +1483,7 @@ replaceFunc(sql_context * context, int argc, sql_value ** argv)
 	zPattern = mem_as_ustr(argv[1]);
 	if (zPattern == 0) {
 		assert(mem_is_null(argv[1])
-		       || sql_context_db_handle(context)->mallocFailed);
+			   || sql_context_db_handle(context)->mallocFailed);
 		return;
 	}
 	nPattern = mem_len_unsafe(argv[1]);
@@ -1331,40 +1505,89 @@ replaceFunc(sql_context * context, int argc, sql_value ** argv)
 		return;
 	}
 	loopLimit = nStr - nPattern;
-	for (i = j = 0; i <= loopLimit; i++) {
-		if (zStr[i] != zPattern[0]
-		    || memcmp(&zStr[i], zPattern, nPattern)) {
-			zOut[j++] = zStr[i];
-		} else {
-			u8 *zOld;
-			sql *db = sql_context_db_handle(context);
-			nOut += nRep - nPattern;
-			testcase(nOut - 1 == db->aLimit[SQL_LIMIT_LENGTH]);
-			testcase(nOut - 2 == db->aLimit[SQL_LIMIT_LENGTH]);
-			if (nOut - 1 > db->aLimit[SQL_LIMIT_LENGTH]) {
-				diag_set(ClientError, ER_SQL_EXECUTE, "string "\
-					 "or binary string is too big");
-				context->is_aborted = true;
-				sql_free(zOut);
-				return;
-			}
-			zOld = zOut;
-			zOut = sql_realloc64(zOut, (int)nOut);
-			if (zOut == 0) {
-				context->is_aborted = true;
-				sql_free(zOld);
-				return;
+
+	if (prefix_table != NULL) {
+		/* 
+		 * optimized replacement logic using the KMP algorithm
+		 * requires additional O(nPattern) memory for prefix table
+		 * replacing: time O(n + m)
+		 */
+
+		/* precalculating prefix table */
+		prefix_table = calloc(nPattern, sizeof(size_t));
+		prefix_table_gen_bin(zPattern, nPattern, prefix_table);
+		/* counting the number of Pattern occurrences */
+		for (nCnt = iStr = nPatternPos = 0;;) {
+			nPatternPos = kmp_bin(zStr, zPattern, nStr, nPattern, iStr, prefix_table);
+			iStr = nPatternPos + nPattern;
+			if (nPatternPos == 0)
+				break;
+			++nCnt;
+		}
+		/* reallocationg mem for zOut */
+		nOut += (nRep - nPattern) * nCnt;
+		assert(nOut < SQL_MAX_LENGTH);
+		zOut = contextMalloc(context, (i64) nOut);
+		if (zOut == 0) {
+			return;
+		}
+		/* coping and replacing in-place */
+		for (iStr = nCopiedStr = nCopiedOut = nPatternPos = 0;;) {
+			nPatternPos = kmp_bin(zStr, zPattern, nStr, nPattern, iStr, prefix_table);
+			iStr = nPatternPos + nPattern;
+			if (nPatternPos == 0)
+				break;
+			assert(nPatternPos >= nCopiedStr);
+			memcpy(zOut + nCopiedOut, zStr + nCopiedStr, nPatternPos - nCopiedStr);
+			nCopiedOut += nPatternPos - nCopiedStr;
+			memcpy(zOut + nCopiedOut, zRep, nRep);
+			nCopiedOut += nRep;
+			nCopiedStr = nPatternPos + nPattern;
+		}
+		memcpy(zOut + nCopiedOut, zStr + nCopiedStr, nStr - nCopiedStr);
+		zOut[nOut - 1] = 0;
+		free(prefix_table);
+	} else {
+		/*
+		 * Naive replacing substring algorithm 
+		 * replacing: time O(n * m), mem O(1)
+		 * used if cannot allocate mem for prefix table
+		 */
+		for (i = j = 0; i <= loopLimit; i++) {
+			if (zStr[i] != zPattern[0]
+				|| memcmp(&zStr[i], zPattern, nPattern)) {
+				zOut[j++] = zStr[i];
+			} else {
+				u8 *zOld;
+				sql *db = sql_context_db_handle(context);
+				nOut += nRep - nPattern;
+				testcase(nOut - 1 == db->aLimit[SQL_LIMIT_LENGTH]);
+				testcase(nOut - 2 == db->aLimit[SQL_LIMIT_LENGTH]);
+				if (nOut - 1 > db->aLimit[SQL_LIMIT_LENGTH]) {
+					diag_set(ClientError, ER_SQL_EXECUTE, "string "\
+						 "or binary string is too big");
+					context->is_aborted = true;
+					sql_free(zOut);
+					return;
+				}
+				zOld = zOut;
+				zOut = sql_realloc64(zOut, (int)nOut);
+				if (zOut == 0) {
+					context->is_aborted = true;
+					sql_free(zOld);
+					return;
+				}
+				memcpy(&zOut[j], zRep, nRep);
+				j += nRep;
+				i += nPattern - 1;
 			}
-			memcpy(&zOut[j], zRep, nRep);
-			j += nRep;
-			i += nPattern - 1;
 		}
+		assert(j + nStr - i + 1 == nOut);
+		memcpy(&zOut[j], &zStr[i], nStr - i);
+		j += nStr - i;
+		assert(j <= nOut);
+		zOut[j] = 0;						  
 	}
-	assert(j + nStr - i + 1 == nOut);
-	memcpy(&zOut[j], &zStr[i], nStr - i);
-	j += nStr - i;
-	assert(j <= nOut);
-	zOut[j] = 0;
 	if (context->func->def->returns == FIELD_TYPE_STRING)
 		mem_set_str_dynamic(context->pOut, (char *)zOut, j);
 	else

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2021-11-14  9:13 [Tarantool-patches] [PATCH] sql: Replace naive position alg with KMP Lord via Tarantool-patches

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