Hi,
while playing with LongLong piddles representing timestamps I have run into
the following troubles with precision loss when passing 64bit signed int
values between perl and PDL (both directions).
Let's have 64bit number 253402300799999999 (BTW it is a number of
microseconds since UNIX epoch beginning corresponding to
9999-12-31T23:59:59.999999Z) - it occupies "only" 58bits, so we are not
even approaching 63bit edge which might become tricky when dealing with
64bit signed integers.
Demonstration code:
use PDL;
use Config;
my $perl_var = 253402300799999999;
my $pdl_var = longlong(253402300799999999);
print "PDL::VERSION: ", $PDL::VERSION, "\n";
print "ivsize : ", $Config{ivsize}, "\n";
print "use64bitint : ", $Config{use64bitint}, "\n";
print "archname : ", $Config{archname}, "\n";
print "perl 1: ", $perl_var, "\n";
print "perl 2: ", ($perl_var % 1000), "\n";
print "pdl : ", $pdl_var->info, "\n";
print "pdl 1: ", $pdl_var, "\n";
print "pdl 2: ", ($pdl_var % 1000), "\n";
print "pdl 3: ", $pdl_var->unpdl->[0], "\n";
Output:
PDL::VERSION: 2.011
ivsize : 8
use64bitint : define
archname : MSWin32-x86-multi-thread-64int
perl 1: 253402300799999999
perl 2: 999
pdl : PDL: LongLong D []
pdl 1: 253402300800000000
pdl 2: 0
pdl 3: 2.534023008e+017
From my point of view there are 2 issues:
1/ A completely legal / not even extremely large / 64bit integer value is
OK on perl side, but when turned into a piddle the value changes
(253402300799999999 > 253402300800000000). This may be a bug.
2/ When exporting a LongLong piddle via unpdl the value (64bit signed int)
is converted into double (perl's NV) even when the perl is compiled with
64bit integers. Maybe unpdl() is not the best way for extracting piddle
value into a perl array (but I really like this function :).
Thanks in advance for any hints.
--
kmx
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