Re: Bitwise operators and check if an bit is NOT within theflag.

[Jochem Maas <jochem@xxxxxxxxxxxxx>]

Mathijs wrote:
> Jochem Maas wrote:
>> Mathijs wrote:


...

>>>
> 
> 
> Thank you very much.
> This seems to work :).

cool. heres's a couple of funcs that might help you to understand bitwise
operations better:

<?php

/* whether there is only 1 single bit set or not */
function single_bit_set(/*int*/ $i)
{
    // beware: if $i is zero !($i & ~get_ls1bit($i)) returns true;
    return $i ? !($i & ~get_ls1bit($i)): false;
}

/* return the value of the least significant bit */
function get_ls1bit(/*int*/ $i)
{
    for ($j = 1; $i && !($i & $j); $j <<= 1);
    return $i ? $j : 0;
}

/* return the value of the most significant bit */
function get_ms1bit(/*int*/ $i)
{
    $x = 0;
    for ($j = $i; $i && !($j == 1); $j >>= 1) { $x++; }
    return $i ? $j <<= $x: 0;
}

/* doesn't break when exponents are near the wordsize
 * of the machine as the native xor does, here is some example code to
 * illustrate:

php -r '
    // include function definition here
    $x = 3851235679;
    $y = 43814;
    echo "\nThis is the value we want";
    echo "\n3851262585";

    echo "\nThe result of a native xor operation on integer values is treated as a signed integer";
    echo "\n".($x ^ $y);

    echo "\nWe therefore perform the MSB separately";
    echo "\n".get_xor32($x, $y)."\n";
'

 */
function get_xor32(/*int*/ $a, /*int*/ $b)
{
       $a1 = $a & 0x7FFF0000;
       $a2 = $a & 0x0000FFFF;
       $a3 = $a & 0x80000000;
       $b1 = $b & 0x7FFF0000;
       $b2 = $b & 0x0000FFFF;
       $b3 = $b & 0x80000000;

       $c = ($a3 != $b3) ? 0x80000000 : 0;

       return (($a1 ^ $b1) |($a2 ^ $b2)) + $c;
}

function get_bit_str($var, $safety = 0)
{
    $rtn = '';

    $var = intval($var);
    if ($var < 0) { $var = 0 - $var; }

    while ($var != 0 /*&& $safety < 31*/) {
        $rtn .= ($var & 1);
        $var >>= 1;
        $safety++;
    }

    return $rtn;
}

> 

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