linux-kernel - [PATCH 1/1] lib: use sunday algorithm to do strstr() and strnstr()

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Date:   Mon, 23 Jul 2018 01:37:15 +0800
From:   Zhaoxiu Zeng <zengzhaoxiu@....com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>
Subject: [PATCH 1/1] lib: use sunday algorithm to do strstr() and strnstr()

From: Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>

The Sunday algorithm is a variation of Boyer-Moore algorithm, it is easy and fast.
For the Sunday algorithm, to see
	http://www.inf.fh-flensburg.de/lang/algorithmen/pattern/sundayen.htm

Signed-off-by: Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>
---
 lib/string.c | 92 +++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 81 insertions(+), 11 deletions(-)

diff --git a/lib/string.c b/lib/string.c
index 2c0900a5d51a..ed4ab9ee3373 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -898,19 +898,51 @@ EXPORT_SYMBOL(memscan);
  */
 char *strstr(const char *s1, const char *s2)
 {
-	size_t l1, l2;
+	size_t l2;
+	const char *pchk = s1;
 
+#ifdef __HAVE_ARCH_STRLEN
 	l2 = strlen(s2);
+#else
+	for (l2 = 0; s2[l2] != 0; l2++)
+		/* nothing */;
+#endif
 	if (!l2)
 		return (char *)s1;
-	l1 = strlen(s1);
-	while (l1 >= l2) {
-		l1--;
-		if (!memcmp(s1, s2, l2))
+
+	/*
+	 * Sunday matching
+	 */
+	while (1) {
+		size_t i;
+		char k;
+
+		/* compare */
+		for (i = 0; i < l2; i++) {
+			if (s1[i] != s2[i])
+				break;
+		}
+		if (i >= l2)
 			return (char *)s1;
-		s1++;
+
+		/* if s1 terminate early? */
+		if (pchk < s1 + i)
+			pchk = s1 + i;
+		do {
+			k = *pchk;
+			if (k == 0)
+				return NULL;
+		} while (pchk++ != s1 + l2);
+
+		/* find the k's last presence in s2 (k = s1[l2]) */
+		for (i = l2; --i != (size_t)-1; ) {
+			if (k == s2[i])
+				break;
+		}
+
+		/* shift */
+		s1 += l2 - i;
 	}
-	return NULL;
 }
 EXPORT_SYMBOL(strstr);
 #endif
@@ -925,16 +957,54 @@ EXPORT_SYMBOL(strstr);
 char *strnstr(const char *s1, const char *s2, size_t len)
 {
 	size_t l2;
+	const char *pchk = s1;
 
+#ifdef __HAVE_ARCH_STRLEN
 	l2 = strlen(s2);
+#else
+	for (l2 = 0; s2[l2] != 0; l2++)
+		/* nothing */;
+#endif
 	if (!l2)
 		return (char *)s1;
+
+	/*
+	 * Sunday matching
+	 */
 	while (len >= l2) {
-		len--;
-		if (!memcmp(s1, s2, l2))
-			return (char *)s1;
-		s1++;
+		size_t i;
+		char k;
+
+		/* compare */
+		for (i = 0; i < l2; i++) {
+			if (s1[i] != s2[i])
+				break;
+		}
+		if (i >= l2)
+			return (char *)s1;
+		if (len == l2)
+			break;
+
+		/* if s1 terminate early? */
+		if (pchk < s1 + i)
+			pchk = s1 + i;
+		do {
+			k = *pchk;
+			if (k == 0)
+				return NULL;
+		} while (pchk++ != s1 + l2);
+
+		/* find the k's last presence in s2 (k = s1[l2]) */
+		for (i = l2; --i != (size_t)-1; ) {
+			if (k == s2[i])
+				break;
+		}
+
+		/* shift */
+		s1  += l2 - i;
+		len -= l2 - i;
 	}
+
 	return NULL;
 }
 EXPORT_SYMBOL(strnstr);
-- 
2.17.1