Optimize Strings_startWith()

Use strncmp() combined with a strlen() will give better performance
than a strstr in worst case. Especially when the match prefix is a
constant and not a variable.

While we are at it, replace the match() function in linux/Battery.c,
which uses a naive algorithm, with a macro that does better job by
utilizing Strings_startWith().

    $ gcc --version | head -n 1
    gcc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4
    $ uname -m
    x86_64
    $ size htop.old htop.new
       text   data    bss    dec    hex filename
     137929  15112   3776 156817  26491 htop.old
     137784  15104   3776 156664  263f8 htop.new

Signed-off-by: Kang-Che Sung <explorer09 @ gmail.com>
This commit is contained in:
Explorer09 2016-08-11 10:49:35 +08:00
parent b269eb24b0
commit bf35921abb
4 changed files with 15 additions and 28 deletions

View File

@ -17,10 +17,15 @@ in the source distribution for its full text.
/*{ /*{
#include <stdio.h> #include <stdio.h>
#define String_startsWith(s, match) (strstr((s), (match)) == (s)) #define String_startsWith(s, match) (strncmp((s),(match),strlen(match)) == 0)
#define String_contains_i(s1, s2) (strcasestr(s1, s2) != NULL) #define String_contains_i(s1, s2) (strcasestr(s1, s2) != NULL)
}*/ }*/
/*
* String_startsWith gives better performance if strlen(match) can be computed
* at compile time (e.g. when they are immutable string literals). :)
*/
char* String_cat(const char* s1, const char* s2) { char* String_cat(const char* s1, const char* s2) {
int l1 = strlen(s1); int l1 = strlen(s1);
int l2 = strlen(s2); int l2 = strlen(s2);

View File

@ -11,9 +11,14 @@ in the source distribution for its full text.
#include <stdio.h> #include <stdio.h>
#define String_startsWith(s, match) (strstr((s), (match)) == (s)) #define String_startsWith(s, match) (strncmp((s),(match),strlen(match)) == 0)
#define String_contains_i(s1, s2) (strcasestr(s1, s2) != NULL) #define String_contains_i(s1, s2) (strcasestr(s1, s2) != NULL)
/*
* String_startsWith gives better performance if strlen(match) can be computed
* at compile time (e.g. when they are immutable string literals). :)
*/
char* String_cat(const char* s1, const char* s2); char* String_cat(const char* s1, const char* s2);
char* String_trim(const char* in); char* String_trim(const char* in);

View File

@ -179,25 +179,6 @@ static inline ssize_t xread(int fd, void *buf, size_t count) {
} }
} }
/**
* Returns a pointer to the suffix of `str` if its beginning matches `prefix`.
* Returns NULL if the prefix does not match.
* Examples:
* match("hello world", "hello "); -> "world"
* match("hello world", "goodbye "); -> NULL
*/
static inline const char* match(const char* str, const char* prefix) {
for (;;) {
if (*prefix == '\0') {
return str;
}
if (*prefix != *str) {
return NULL;
}
prefix++; str++;
}
}
static void Battery_getSysData(double* level, ACPresence* isOnAC) { static void Battery_getSysData(double* level, ACPresence* isOnAC) {
*level = 0; *level = 0;
@ -240,6 +221,8 @@ static void Battery_getSysData(double* level, ACPresence* isOnAC) {
bool full = false; bool full = false;
bool now = false; bool now = false;
while ((line = strsep(&buf, "\n")) != NULL) { while ((line = strsep(&buf, "\n")) != NULL) {
#define match(str,prefix) \
(String_startsWith(str,prefix) ? (str) + strlen(prefix) : NULL)
const char* ps = match(line, "POWER_SUPPLY_"); const char* ps = match(line, "POWER_SUPPLY_");
if (!ps) { if (!ps) {
continue; continue;
@ -266,6 +249,7 @@ static void Battery_getSysData(double* level, ACPresence* isOnAC) {
continue; continue;
} }
} }
#undef match
} else if (entryName[0] == 'A') { } else if (entryName[0] == 'A') {
if (*isOnAC != AC_ERROR) { if (*isOnAC != AC_ERROR) {
continue; continue;

View File

@ -29,13 +29,6 @@ Linux battery readings written by Ian P. Hands (iphands@gmail.com, ihands@redhat
// READ FROM /sys // READ FROM /sys
// ---------------------------------------- // ----------------------------------------
/**
* Returns a pointer to the suffix of `str` if its beginning matches `prefix`.
* Returns NULL if the prefix does not match.
* Examples:
* match("hello world", "hello "); -> "world"
* match("hello world", "goodbye "); -> NULL
*/
void Battery_getData(double* level, ACPresence* isOnAC); void Battery_getData(double* level, ACPresence* isOnAC);
#endif #endif