Re: Understanding Unpack on Decimal Value

First, check pack:

> V An unsigned long (32-bit) in "VAX" (little-endian) order.

Have you noticed the pattern?

         bytes              chars
36353935 ----> 36 35 39 35  ---->  6 5 9 5
                                   | | | |
       first 4 digits      reverse | | | |
595661 -------------> 5956 ------> 6 5 9 5
[download]

It works for any input greater or equal to 1000:

#! /usr/bin/perl
use warnings;
use strict;

use Test::More;

sub what {
    my ($input) = @_;
    my $unpacked = unpack "V", $input;
    my $result = join "", map chr hex, sprintf("%x\n", $unpacked) =~ /
+(..)/g;
    is $result, reverse substr $input, 0, 4;
}

what($_) for 1000 .. 2000,
             2e4, 3e5, 4e6, 5e7, 6e8, 7e9, 8e10, 9e11, 2 ** 64 - 3;

done_testing();
[download]

Update: For numbers fitting into integer, you can also use

reverse($input) % 10_000
[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

Comment on Re: Understanding Unpack on Decimal Value Select or Download Code

Replies are listed 'Best First'.
Re^2: Understanding Unpack on Decimal Value by arblargan (Acolyte) on Nov 01, 2018 at 21:27 UTC
ASCII!! That didn't even occur to me as this portion of the function is entirely numeric. I suspect that the way this was implemented in the original script was incorrectly understood. Thank you for the clarification. One additional question, if I could... Since it returns only a portion of the result, I attempted to modify the snippet a little bit, but it still produced the same result. See below. `my $input = 0x916CD; #595661 my @unpacked = unpack "VV", $input; print(join ", ", @unpacked); >>909457717 #0x36353935` [download] From your reply, I was expecting to have the following. `Dec: 825_636_405, 14_645 Hex: 0x31363635, 0x3935` [download] Could you help me understand why this doesn't unpack into the array like I thought it would? Thanks for the help!	[reply] [d/l] [select]
Re^3: Understanding Unpack on Decimal Value by choroba (Cardinal) on Nov 01, 2018 at 21:51 UTC
You need a number greater than 9_999_999 to get more than one element back: `for my $input (9_999_990 .. 10_000_010) { my @unpacked = unpack 'VV', $input; say scalar @unpacked, " $input"; }` [download] ($q=q:Sq=~/;[c](.)(.)/;chr(-\|\|-\|5+lengthSq)`"S\|oS2"`map{chr \|+ord }map{substrSq`S_+\|`\|}3E\|-\|`7**2-3:)=~y+S\|`+$1,++print+eval$q,q,a, [download]	[reply] [d/l] [select]


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