Re: Change default serialize precision from 100 to 17

+1 On Mon, Feb 7, 2011 at 8:26 PM, Gustavo Lopes <glopes@nebm.ist.utl.pt> wrote: > The default serialize precision is currently [1] set at 100. A little code > inspection shows precision, in this case, takes the usual meaning of number > of significant digits. > > Given that the implicit precision of a (normal) IEEE 754 double precision > number is slightly less than 16 digits [2], this is a serious overkill. Put > another way, while the mantissa is composed of 52 bits plus 1 implicit bit, > 100 decimal digits can carry up to 100*log2(10) =~ 332 bits of information, > around 6 times more. > > Given this, I propose changing the default precision to 17 (while the > precision is slightly less than 16, a 17th digit is necessary because the > first decimal digit carries little information when it is low). > > From my tests, this makes serialization and unserialization of doubles > around 3 times faster (counting the function calls to serialize/unserialize, > plus a loop variable increment and stop condition check). It also makes the > serialization data. > > Crucially, from my tests, the condition that the variable stays the same > before and after serialization+unserialization still holds. The test > include, for little endian machines, verifies this. > > If no one objects, I'll change the default precision to 17. > > //run with php -d serialize_precision=17 > $numbers = array( >        "0000000000000000", //0 >        "2d431cebe2362a3f", //.0002 >        "2e431cebe2362a3f", //.0002 + 10^-Accuracy[.0002]*1.01 >        "0000000000001000", //2^-1022. (minimum normal double) >        "0100000000001000", //2^-1022. + 10^-Accuracy[2^-1022.]*1.01 >        "ffffffffffffef7f", //2^1024. (maximum normal double) >        "feffffffffffef7f", //2^1024. - 10^-Accuracy[2^1024.] >        "0100000000000000", //minumum subnormal double >        "0200000000000000", //2nd minumum subnormal double >        "fffffffffffff000", //maximum subnormal double >        "fefffffffffff000", //2nd maximum subnormal double >        "0000000000000f7f", //+inf >        "0000000000000fff", //-inf > ); > > foreach ($numbers as $ns) { >        $num = unpack("d", pack("H*", $ns)); $num = reset($num); >        echo "number: ", sprintf("%.17e", $num), "... "; >        $num2 = unserialize(serialize($num)); >        $repr = unpack("H*", pack("d", $num2)); $repr = reset($repr); >        if ($repr == $ns) >                echo "OK\n"; >        else >                echo "mismatch\n\twas:    $ns\n\tbecame: $repr\n"; > } > > >    [1]: > http://lxr.php.net/opengrok/xref/PHP_TRUNK/main/main.c#451 >    [2]: > http://en.wikipedia.org/wiki/Double_precision_floating-point_format > -- > Gustavo Lopes > > -- > PHP Internals - PHP Runtime Development Mailing List > To unsubscribe, visit: http://www.php.net/unsub.php > >