What is the perl equivalent to Java's Double.doubleToLongBits()?

Question

I have two floating point numbers and I want to compute their difference in Units in Last Place (ULP). In Java this is:

Double.doubleToLongBits(a) - Double.doubleToLongBits(b)

What is the equivalent in Perl?

Answer 1

On a Perl with double-precision floats^[1] and unsigned integers at least 64 bits in size^[2], you can use the following:

unpack('Q', pack('d', $n))

For example,

$ perl -e'
   use Config qw( %Config );

   die("Not supported\n")
      if $Config{nvsize} != 8
      || $Config{uvsize} < 8;

   printf("%X\n", unpack("Q", pack("d", -0.1)));
'
BFB999999999999A

---

1. perl -V:nvsize gives 8
2. perl -V:uvsize gives 8 or higher