thanos1983:

Questions about manipulating binary numbers like that are often best solved by looking them over in binary. Using a little bit of perl:

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

my @memory = (0, 0, 5, 59);

print "Original values:\n";
printf "% 3u : %08b\n", $_, $_ for @memory;

$memory[1] = $memory[1] << 7;
$memory[2] = $memory[2] << 14;
$memory[3] = $memory[3] << 21;
print "\nAfter shifting the values\n";
printf "% 10u : %032b\n", $_, $_ for @memory;

my $t = 0;
$t += $_ for @memory;
print "\nThe sum:\n";
printf "% 10u : %032b\n", $t, $t;

[download]

Running it gives a hopefully familiar number: ;^D

Original values:
  0 : 00000000
  0 : 00000000
  5 : 00000101
 59 : 00111011

After shifting the values
         0 : 00000000000000000000000000000000
         0 : 00000000000000000000000000000000
     81920 : 00000000000000010100000000000000
 123731968 : 00000111011000000000000000000000

The sum:
 123813888 : 00000111011000010100000000000000

[download]

Update: I just read the thread, and noticed that in your code, you use the same construct several times. Additionally, you mention that you only want seven bits. So I'd suggest writing a subroutine that masks off the high bit of your bytes and packs the number for you. Something like the following, but with a better name:

sub plonk {
    my ($b0, $b1, $b2, $b3) = @_;
    return     $b0 & 0x7f
            + ($b1 & 0x7f) << 7
            + ($b2 & 0x7f) << 14
            + ($b3 & 0x7f) << 21;
}

[download]

...roboticus

When your only tool is a hammer, all problems look like your thumb.

In reply to Re^5: ID3v2 TAG Footer Reading goes wrong by roboticus
in thread ID3v2 TAG Footer Reading goes wrong by thanos1983

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.