kladskull / stringbuilder

String Builder Class for PHP

StringBuilder.php

StringBuilders, not happening in PHP

This class and accompanying files attempts to demonstrate that a StringBuilder in PHP doesn't make sense at all, and in fact would have a detrimental effect on the performance of a PHP program. The reason being is that variables in PHP are modifiable (mutable) meaning we can modify the string where it sits in memory. I used as many speed tricks as I could to squeeze as much out of it as possible - if you can see something I missed that would improve the speed, please, let me know.

What is a String Builder?

When concatenating a string in a language such as C/C++ or Java, a new memory area has to be found and allocated to fit the original string - plus the string being added. In C, this is done by the programmer using malloc/remalloc. In languages like Java or C++ (using std::string) this is done automatically. You could guess, that concatenating a string in a loop could take a lot longer than it should with all of the reallocation and coping of memory.

A String Builder is quite simple, it preallocates a large block of memory so that when more data is added, it doesn't have to reallocate memory and copy strings around. For explanation purposes, lets say a string builder preallocates 20 bytes of empty space: (periods are used for string length visibility)

$x = '....................'; 
$xlen = 0; // the real length of our string - it starts at 0 of course

If we call the String Builders append method with the text 'x1234' it will 'insert' (not 'copy and append') the string value at position '0', add the length of the string to a variable so we know how large our string is:

$x = 'x1234...............';
$xlen = $xlen + strlen('x1234'); // the current length of our string is now 5

If we called the appender again with '5678' it would insert the value at position $xlen (which is 5).

$x = 'x12345678...........';
$xlen = $xlen + strlen('5678'); // the current length of our string is now 9

When we want the value of the string, a method like .toString() or .str() can be called that would basically return the substring of the string buffer. In our example, it would do something like this:

$x = 'x12345678...........'; //
return substr(buffer, 0, $xlen); // which of course returns 'x12345678'. 

As you can see, the original string is much larger than 9, but we only return what we inserted by specifying the string, start position (0) and the virtual length of the appended string.

Conclusion

Although a string builder can be optimal, it can also be a total bust. There are likely more reasons to just use straight concatenation than a String Builder. When a string is returned using .toString() or .str() it actually creates a copy of the string before returning it to the caller. String Builders also use a lot more memory than immutable string manipulation would, as the clear source of the speed comes from preallocating a block of memory rather than allocating exactly what it needs, and no more. The preallocation saves time by not having to call malloc/realloc every append.

Discovery!

Was this all a waste of time? I don't think so, while trying to squeeze every bit of juice out of the StringBuilder I came across something that I'll be write about next, which will out-perform PHP's internal string concatenation!

Want a Hint? StringStream.php