----- Original Message -----
From: "David Tulloh" <david@xxxxxxxxxxxx>
To: <zorglub_olsen@xxxxxxxxxxx>
Cc: <php-general@xxxxxxxxxxxxx>
Sent: Tuesday, June 13, 2006 12:18 PM
Subject: Re: Help with some clever bit operations
The example starting values
$existing = 181; # = 10110101
$new = 92; # = 01011100
$mask = 15; # = 00001111
Get the bits that will be changed
$changing = $new & $mask; # = 12 = 00001100
Get the bits that won't be changed
$staying = $existing & ~$mask; # = 176 = 10110000
Combine them together
$result = $changing ^ $staying; # = 188 = 10111100
David
Though the result is the same, logically a bitwise OR (|) would be more
appropriate. The bitwise XOR flips bits which, since in this case one set
of them is always 0 produces the same result. The bitwise OR adds the bits.
An informal set of rules is:
&, as a filter, extracts those bits where you put ones in the mask
& sets to zero those bits where you put zero in the mask
| sets bits
^ flips the bits where you put a one.
$evenodd & 1 is true on odd numbers or otherwise tests for the rightmost
bit. Coupled with a right shift >> allows you to loop through a set of
bits.
for ($i=0;$i<32;$i++) {
if ($bitset & 1) {
echo "bit $i is set";
}
$bitset >>= 1;
}
Anyway, avoid the most significant bit. Since in PHP there are no unsigned
integers, you might get some funny result if anything makes PHP do an
automatic type conversion.
Satyam
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