[PATCH 3/6] string: reduce strjoin runtime, drop trailing separator

[Ahmad Fatoum]

Hi,

On 27.10.22 08:56, Sascha Hauer wrote:
> On Wed, Oct 26, 2022 at 08:42:02AM +0200, Ahmad Fatoum wrote:
>> The implementation of strjoin is a bit suboptimal. The destination
>> string is traversed from the beginning due to strcat and we have a
>> left-over separator at the end, while it should only be in-between.
>>
>> Fix this.
> 
> Rather than fixing a just introduced function I would expect a patch
> introducing strjoin() and then another one converting the time command
> over to use it.

What difference does it make?

>> +void *mempcpy(void *dest, const void *src, size_t count)
>> +{
>> +	return memcpy(dest, src, count) + count;
>> +}
>> +EXPORT_SYMBOL(mempcpy);
>>  
>>  #ifndef __HAVE_ARCH_MEMMOVE
>>  /**
>> @@ -943,7 +948,7 @@ char *strjoin(const char *separator, char **arr, size_t arrlen)
>>  {
>>  	size_t separatorlen;
>>  	int len = 1; /* '\0' */
>> -	char *buf;
>> +	char *buf, *p;
>>  	int i;
>>  
>>  	separatorlen = strlen(separator);
>> @@ -951,12 +956,14 @@ char *strjoin(const char *separator, char **arr, size_t arrlen)
>>  	for (i = 0; i < arrlen; i++)
>>  		len += strlen(arr[i]) + separatorlen;
>>  
>> -	buf = xzalloc(len);
>> +	p = buf = xmalloc(len);
>>  
>>  	for (i = 0; i < arrlen; i++) {
>> -		strcat(buf, arr[i]);
>> -		strcat(buf, separator);
>> +		p = stpcpy(p, arr[i]);
>> +		p = mempcpy(p, separator, separatorlen);
>>  	}
>>  
>> +	p[-separatorlen] = '\0';
> 
> That's a bit hard to read. How about:
> 
> 	for (i = 0; i < arrlen; i++) {
> 		p = stpcpy(p, arr[i]);
> 		if (i < arrlen - 1)
> 			p = stpcpy(p, separator);
> 	}
> 
> That would also allow you to optimize the allocated buffer size above.

I like my version more. I dislike these once-only checks inside loops.

Cheers,
Ahmad

> 
> Sascha
> 


-- 
Pengutronix e.K.                           |                             |
Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |