Thanks for all the suggestions - now posting an update for anyone who stumbles on this.

I started with something like almut's state machine but didn't like the way the structure of the file I was trying to read is then encoded into the script structure - feels like poor code/data separation.

What I've ended up with is more like BrowserUk's suggestions. The crucial point (which I didn't really realise when I originally posted) was that the file format defines how many words should be read for each "variable" in the file, although in most cases that length is actually defined by algebra involving previously-read variables. So we can do something like this:

my %v; # this hash holds all the data read

# define the names of the "variables" to read from the file
# *** in the correct order ***
my @varnames = qw/this that theother/;

# define where the length to read is not a default
my %varlengths = ('that' => 2, 'theother' = '$v{this}*2'}

foreach my $var (@varnames){

 # get the length to read
 my $readlength;
 if(! exists($varlengths{$var})
  $readlength = 1; # default
 } else {
  $readlength = eval($varlengths{$var}); # compute it
 }
 
 # read into %v
 if ($readlength == 1){
  # read a scalar
  $v{$var} = unpack TEMPL1, read( $file, $readlength );
 } else {
  # read into an anon. array
  $v{$var} = [ unpack TEMPL1, read( $file, $readlength ) ];
 }

}

[download]

In reply to Re: Reading binary files - program structure by Anonymous Monk
in thread Reading binary files - program structure by Anonymous Monk

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.