====== Request for Comments: Add a Logical Shift Operator For Strings ======
  * Version: 1.0
  * Date: 2011-17-07
  * Author: Anthony Ferrara <ircmaxell@php.net>
  * Status: Inactive
  * First Published at: http://wiki.php.net/rfc/logicalshiftoperator


===== Introduction =====

This RFC is to discuss the addition of 2 new operators to the core, a logical left shift and a logical right shift.

==== Why do we need new operators? ====

The currently implemented left and right shift operators (''<<'' and ''>>'' respectively) perform [[http://en.wikipedia.org/wiki/Bitwise_operation#Arithmetic_shift|arithmetic shifts]].  I am proposing the introduction of a pair of [[http://en.wikipedia.org/wiki/Logical_shift|logical shift]] operators which perform their operations on arbitrary length strings.

The problem can be seen by trying to shift ''-1'' (''0xFFFFFFFF'' on 32 bit, ''0xFFFFFFFFFFFFFFFF'' on 64 bit).  If we try ''-1 %%>>%% 1'', we get back ''-1'' again.  This is due to the shift being arithmetic.  If we try ''-1 %%>>%% 2'', we will still get ''-1''.  

Additionally, the current shifts only work on strings when the string is parsable as a number.  So ''"2" %%>>%% 1'' yields ''1'' instead of the expected logical value of ''0x19''.

At present, if you want to do a logical shift on a string, you need to iterate over the string and apply the shift to each part of it (where the part is less than 4 bytes long, to prevent issues on 32 bit systems).  For example:

For example, to implement a logical left shift currently requires something like this (this example could be optimized, but the basics of it are there):

        $mask   = (0xff << (8 - $bits)) & 0xff;
        $state  = 0;
        $length = strlen($data);
        for ($i = $length - 1; $i >= 0; $i--) {
            $tmp     = ord($data[$i]);
            $result .= chr(($tmp << $bits) | $state);
            $state   = ($tmp & $mask) >> (8 - $bits);
        }
        return strrev($result);

Other bitwise operators do operate properly on strings (such as XOR - ''^'', AND - ''&'' and OR - ''|'').  So it would be complete to add support for shifting strings.

===== Proposal =====

I propose the addition of two new operators which will perform a logical shift.  The selection of the operators is a little bit more difficult as ''%%<<<%%'' is already taken by heredoc.  Perhaps something like ''b%%<<%%''...

Example:

''$foo = "a long string" {operator} 1;''

Would yield a string with the following bits:

''0xc240d8dedcce40e6e8e4d2dcce''

===== Changelog =====

 - Added note about %%<<<%% being used by heredoc already.