how to convert array of bytes to int64 in perl

Question

I have an specific problem to solve in Perl 5 (using minimal external dependencies, if possible compatible with version 5.12), related to decode an array of bytes like this:

my @data = (0x00, 0x00, 0x00, 0x03, 0x84, 0x14, 0x40, 0x32);

and I want to extract a number like this: 15100821554

I try several variations of pack / unpack without success like

say(unpack("Q", pack("C*", @data)));

print 3620916657624449024

The equivalent code in go is something like this

data := []byte{0x00, 0x00, 0x00, 0x03, 0x84, 0x14, 0x40, 0x32}
deciseconds := int64(binary.BigEndian.Uint64(data))

can be executed here: https://play.golang.org/p/I2J45H-sn-H

any ideas?

choroba · Accepted Answer · 2020-04-14 08:41:47Z

5

You were almost there, the following works:

say unpack 'Q', pack 'C*', reverse @data;
#                          ~~~~~~~

It means you need the opposite endianness, which translates to pack in the following way:

unpack 'Q>', pack 'C*', @data;
#        ^

edited Apr 14, 2020 at 8:41

answered Apr 13, 2020 at 18:05

choroba

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1 Comment

ikegami Over a year ago

The Q> solution is more straightforward, more extensible (e.g. you could use it as part of a larger pattern such as Q>*), and it works on more machines (the reverse solution, ironically, doesn't work on big-endian machines).

Polar Bear · Accepted Answer · 2020-04-14 02:21:03Z

-1

Desired result can be achieved with following code

use strict;
use warnings;
use feature 'say';

my $num = 0;
my @data = (0x00, 0x00, 0x00, 0x03, 0x84, 0x14, 0x40, 0x32);

$num = ($num<<8) + $_ for @data;

say $num;

Output

15100821554

NOTE: unpack is a correct way to make conversion

answered Apr 14, 2020 at 2:21

Polar Bear

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