I'm using a subroutine I found as part of a program to convert a file with thousands of IP ranges to the minimum number of CIDR blocks. I have tested it and can't find a situation where it doesn't work.

The problems is I would like to understand HOW IT WORKS. Here is the declaration, calling statement and the subroutine:

my (@chunks, @fbits, @lbits);
.....
.....
do_chunk(\@chunks, \@fbits, \@lbits);
....
....

sub do_chunk
{
my ($chunks, $fbits, $lbits) = @_;
my (@prefix, $idx1, $idx2, $size);
    
# Find common prefix.  After that, next bit is 0 for $fbits and 1 for
# $lbits is 1.  A split a this point guarantees the longest suffix.
        
$idx1 = 0;
$idx1++
while ($idx1 <= $#$fbits and $$fbits[$idx1] eq $$lbits[$idx1]);
@prefix = @$fbits[0 .. $idx1 - 1];
    
$idx2 = $#$fbits;
$idx2--
while ($idx2 >= $idx1 and $$fbits[$idx2] eq '0' and $$lbits[$idx2] eq 
+'1');
    
# Split if $fbits and $lbits disagree on the length of the chunk.
        
if ($idx2 >= $idx1)
{
$size = $#$fbits - $idx1;
do_chunk ($chunks, $fbits, [ @prefix, (split //, '0' . '1' x $size) ])
+;
do_chunk ($chunks, [ @prefix, (split //, '1' . '0' x $size) ], $lbits)
+;
}
else
{
$size = $#$fbits - $idx2;
push @$chunks, [ (bits2num( [ @prefix, (split //, '0' x $size) ])), @$
+fbits - $size ];
}
}

[download]

Now for my questions:

  1. What does the "\" do in "\@" in the calling arguments?
  2. In the subroutine I'm confused by the use of $#$, $$ and @$
  3. Last (for now) since this subroutine recursively calls itself, how will the second call shown below ever get executed?
if ($idx2 >= $idx1)
{
$size = $#$fbits - $idx1;
do_chunk ($chunks, $fbits, [ @prefix, (split //, '0' . '1' x $size) ])
+;
do_chunk ($chunks, [ @prefix, (split //, '1' . '0' x $size) ], $lbits)
+;
}

[download]

Thanks in advance

In reply to Total Syntax Confusion Plus by RobertJ

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.