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Date: Tue, 08 Feb 2011 01:26:09 -0000
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Subject: Change default serialize precision from 100 to 17
From: glopes@nebm.ist.utl.pt ("Gustavo Lopes")

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