midi re-tempoizer

Speeds up or slows down a MIDI file by altering its set_tempo
events:

require 5;
use strict;
use warnings;
use constant DEBUG => 0;
use MIDI;

# [email protected] 2004-12-02

$ARGV[0] = $1/$2 if $ARGV[0] and $ARGV[0] =~ m[^(\d+)/(\d+)$]s;

my( $scale, $in, $out ) = @ARGV;
die "Usage:  scaletempo scalefactor infile.mid outfile.mid
   scalefactor = how much faster to make it.
       2 = twice as fast
     6/7 = slow it down to 6/7ths of its current speed
      .5 = slow it down to half of its current speed
"
  unless @ARGV == 3
  and $scale =~ m/^(?: (?: \d*\.\d+ ) | \d+ )$/sx and $scale != 0
  and length( $in ) and length( $out )
  and -e $in and -r $in
;

my $opus = MIDI::Opus->new({ 'from_file' => $in });

DEBUG and print "$in contains ", scalar($opus->tracks), " tracks.\n";

my $tweaked = 0;
foreach my $t ( $opus->tracks ) {
   DEBUG and print " Tweaking track $t\n";
   foreach my $e ( @{ $t->events_r || next } ) {
     if( $e->[0] eq 'set_tempo' ) {
       $tweaked = 1;
       DEBUG > 1 and print " event @$e\n";
       $e->[2] = int( .5 + $e->[2] / $scale );
       DEBUG > 1 and print "    => @$e\n";
     }
   }
}

unless($tweaked) {
   DEBUG and print "No track events found.  Faking it.\n";
   my $new_ev = [ 'set_tempo', 0,  int( .5 + 500_000 / $scale ) ];
   if( $opus->tracks == 1 ) {
     DEBUG and print "Prepending [@$new_ev] to the one track.\n";
     unshift @{ $opus->tracks_r->[0]->events_r
       || die "No events in track 0?!" },  $new_ev;
   } else {
     DEBUG and print "Prepending a [@$new_ev] tempo-track.\n";
     unshift @{ $opus->tracks_r },
       MIDI::Track->new( { 'events' => [$new_ev], } );
   }
}

DEBUG and print "Writing $out\n";
$opus->write_to_file( $out );
DEBUG and print "Done.\n";
exit;
__END__

--
Sean M. Burke    http://search.cpan.org/~sburke/