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QByteArrayView: use memmem() to search for substrings

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Let's just use whatever your C library provides, on the assumption that
it is optimized. Because it does the memchr() call when the size of the
needle is 1, we also skip the 1-char findByteArray() call.

It's been available on Linux since glibc 2.0, FreeBSD since 6.0, OpenBSD
5.4, NetBSD, Apple OSes, etc. even Solaris. If your OS doesn't have it,
you should ask your vendor to add it or consider upgrading to an OS that
already has it.

The glibc implementation[1] also uses a hashing for short needles and
the Two Way string search algorithm it describes as "a bad character
shift table similar to the Boyer-Moore algorithm" for longer ones. The
FreeBSD implementation[2] (which its man page says came from MUSL) uses
1-, 2-, 3-, and 4-byte search specializations before using the Two Way
search algorithm too.

[1] https://codebrowser.dev/glibc/glibc/string/memmem.c.html
[2] https://github.com/freebsd/freebsd-src/blob/main/lib/libc/string/memmem.c

Change-Id: If05cb740b64f42eba21efffd17d101e24528f7fd
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
This commit is contained in:
Thiago Macieira 2024-05-19 16:43:24 -05:00
parent e673e5a257
commit a98715c127
3 changed files with 33 additions and 0 deletions
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20
src/corelib/configure.cmake
View File

@ -324,6 +324,22 @@ linkat(AT_FDCWD, \"foo\", AT_FDCWD, \"bar\", AT_SYMLINK_FOLLOW);
}
")
# memmem
qt_config_compile_test(memmem
LABEL "memmem()"
CODE
#define _APPLE_SAUCE 1 /* Apple doesn't require anything */
"#define _BSD_SOURCE 1 /* For FreeBSD */
#define _GNU_SOURCE 1 /* For glibc, Bionic */
#include <string.h>
int main(void)
{
const void *r = memmem(\"abc\", 3, \"bc\", 2);
(void)r;
return 0;
}")
# memrchr
qt_config_compile_test(memrchr
LABEL "memrchr()"
@ -569,6 +585,10 @@ qt_feature("std-atomic64" PUBLIC
LABEL "64 bit atomic operations"
CONDITION WrapAtomic_FOUND
)
qt_feature("memmem" PRIVATE
LABEL "C library function memmem()"
CONDITION TEST_memmem
)
qt_feature("memrchr" PRIVATE
LABEL "C library function memrchr()"
CONDITION TEST_memrchr

1
src/corelib/global/qconfig-bootstrapped.h
View File

@ -72,6 +72,7 @@
# define QT_FEATURE_linkat -1
#endif
#define QT_FEATURE_lttng -1
#define QT_FEATURE_memmem -1
#define QT_FEATURE_memrchr -1
#define QT_NO_QOBJECT
#define QT_FEATURE_process -1

12
src/corelib/text/qbytearraymatcher.cpp
View File

@ -3,6 +3,11 @@
#include "qbytearraymatcher.h"
#include <qtconfiginclude.h>
#ifndef QT_BOOTSTRAPPED
# include <private/qtcore-config_p.h>
#endif
#include <limits.h>
QT_BEGIN_NAMESPACE
@ -238,8 +243,10 @@ qsizetype QtPrivate::findByteArray(QByteArrayView haystack, qsizetype from, QByt
const auto haystack0 = haystack.data();
const auto l = haystack.size();
const auto sl = needle.size();
#if !QT_CONFIG(memmem)
if (sl == 1)
return findByteArray(haystack, from, needle.front());
#endif
if (from < 0)
from += l;
@ -250,6 +257,11 @@ qsizetype QtPrivate::findByteArray(QByteArrayView haystack, qsizetype from, QByt
if (!l)
return -1;
#if QT_CONFIG(memmem)
auto where = memmem(haystack0 + from, l - from, needle.data(), sl);
return where ? static_cast<const char *>(where) - haystack0 : -1;
#endif
/*
We use the Boyer-Moore algorithm in cases where the overhead
for the skip table should pay off, otherwise we use a simple