Change default serialize precision from 100 to 17

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