php expression question

Question

function createHash($string) {
    $check1 = $this->stringToNumber($string, 0x1505, 0x21);
    $check2 = $this->stringToNumber($string, 0, 0x1003F);

    $factor = 4;
    $halfFactor = $factor/2;

    $check1 >>= $halfFactor;
    $check1 = (($check1 >> $factor) & 0x3FFFFC0 ) | ($check1 & 0x3F);
    $check1 = (($check1 >> $factor) & 0x3FFC00 ) | ($check1 & 0x3FF);
    $check1 = (($check1 >> $factor) & 0x3C000 ) | ($check1 & 0x3FFF);  

    $calc1 = (((($check1 & 0x3C0) << $factor) | 
                ($check1 & 0x3C)) << $halfFactor ) | 
                ($check2 & 0xF0F );
    $calc2 = (((($check1 & 0xFFFFC000) << $factor) |
                ($check1 & 0x3C00)) << 0xA) | 
                ($check2 & 0xF0F0000 );

    return ($calc1 | $calc2);
}

>>= what does this expression stand for? it looks very strange to me. I couldn't find any questions on google.

deceze · Accepted Answer · 2011-06-04 04:28:11Z

2

>> means 'Right shift' and the statement you had pointed out - it means

$check1 = $check1 >>$halfFactor

edited Jun 4, 2011 at 4:28

deceze♦

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answered Jun 4, 2011 at 3:51

Legolas

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Nicklas A. · Accepted Answer · 2011-06-04 03:59:07Z

1

It's the equivalent of += for >>.

>> is bitwise shift on a binary level.
If an integer/byte has the value 0000 1000 performing >> 1 on it would make it's new value 0000 0100, it would slide the bits right inserting zeroes to the left.
>> 2 would make it 0000 0010 etc.

The effective result would be the same as dividing it by 4 as >> X == / 2^X

That code is the same as:

$check1 = $check1 >> $halfFactor

edited Jun 4, 2011 at 3:59

answered Jun 4, 2011 at 3:54

Nicklas A.

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Yuri Stuken · Accepted Answer · 2011-06-04 03:52:31Z

1

It is shift right assignment operator. See this and this.

answered Jun 4, 2011 at 3:52

Yuri Stuken

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Tadeck · Accepted Answer · 2011-06-04 03:53:02Z

0

See documentation (bitwise operators).

This code:

$check1 >>= $halfFactor;

actually means something like that: divide $check1 by 2 ^ $halfFactor times and assign result to $check1.

answered Jun 4, 2011 at 3:53

Tadeck

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