Potential issue in casting numbers

[akotlar]

I've noticed some odd behavior, which may simply be the way Perl works, but which could be useful for msgpack-perl to take into account.

Also, once again I would ask that we have some handling for lower-precision floats. It doesn't matter that Perl doesn't separate a double from a float, what matters is how much space the msgpack representation takes. Spending 9 bytes for 1.0 is a waste.

use Data::MessagePack;
use Test::More;
use Scalar::Util qw/looks_like_number/;

sub toNumber {
  if (!looks_like_number($_[0])) {
    return $_[0];
  }

  return $_[0] + 0;
}

my $mp = Data::MessagePack->new();
$mp->prefer_integer();

my $numThatShouldBeInt = "-1.000000";

$converted = toNumber($numThatShouldBeInt);

$packed = $mp->pack($converted);
ok(length($packed) == 1, "Perl does something bizarre when returning numbers from subs");

# Something is very weird
$packed = $mp->pack($numThatShouldBeInt+ 0);
ok(length($packed) == 9, "The string $numThatShouldBeInt takes 9 bytes in msgpack when cast to number, as expected");

# Note that this doesn't occur with a positive number
$packed = $mp->pack("1.000000" + 0);
ok(length($packed) == 9, "The string 1.000000 takes 9 bytes in msgpack when cast to number, as expected");


$packed = $mp->pack(CORE::int($numThatShouldBeInt));
ok(length($packed) == 1, "The string $numThatShouldBeInt takes 1 bytes when cast as int, as expected");

[syohex]

Perl treats -1.0 as double(NV means double as below), so it is packaged as float64 whose packed size is 9 byte.

% perl -MDevel::Peek -e 'my $i = "-1.000000" + 0; Dump $i'  
SV = NV(0x7fb52a02d718) at 0x7fb52a02d730
  REFCNT = 1
  FLAGS = (NOK,pNOK)
  NV = -1

[akotlar]

What is interesting is that this does not seem to be the case if you pass "-1.000" + 0 to pack() (length($packed) == 1). Returning "-1.000" + 0 from a sub, and passing that to pack() indeed results in 9 bytes.

It could be useful for prefer_integer() to also check whether a double can be stored as an int, possibly with an optional flag.