use strict;
use warnings;
use MIDI;
use XML::Simple;
use Data::Dumper;

my $xml    = XMLin(\*DATA);
my $song   = $xml->{SONG};
my @note   = @{ $song->{NOTE} };
my $title  = $song->{title} || 'unknown';
my $author = $song->{author} || 'unknown';

my (@event, $pitch, $duration, $velocity, $instrument);

for my $note (@note) {

   # check for patch changes
   if (exists $note->{instrument}) {
      $instrument = $MIDI::patch2number{$note->{instrument}} || 1;
      push @event,[patch_change => 0, 1, $instrument];
   }

   $pitch = scrub_pitch($note->{pitch},$pitch);
   $duration = ($note->{duration} || 1) * 96;
   $velocity = ($pitch eq 'rest') ? 0 : 96;      # hacked silence

   # add note_on/note_off pair
   push @event,[note_on  => 0,1,$MIDI::note2number{$pitch},$velocity];
   push @event,[note_off => $duration,1,$MIDI::note2number{$pitch},0];
}

my $opus = MIDI::Opus->new({
   format => 0,
   ticks  => $song->{bpm},
   tracks => [ MIDI::Track->new({
      events => [
         [text_event => 0, "Title: $title"],
         [text_event => 0, "Auhtor: $author"],
         @event,
      ]
   })],
});

$opus->write_to_file($song->{name}.'.mid');

# this transforms C into C4 and C:5 into C5 (etc.)
sub scrub_pitch {
   my ($next,$prev) = @_;
   return $prev unless $next;
   my ($note,$octave) = split(':',$next,2);
   return $note if lc($note) eq 'rest';
   $octave ||= 4;
   return "$note$octave";
}

__DATA__
<?xml version="1.0" standalone="no"?>
<!DOCTYPE OPUS SYSTEM "http://4ml.org/4ml.dtd">
<OPUS>
  <SONG name="mary" title="Mary Had a Little Lamb" bpm="160">
   <!-- Define the notes here -->
   <NOTE pitch="E:5" />
   <NOTE pitch="D:5" />
   <NOTE pitch="C:5" />
   <NOTE pitch="D:5" />
   <NOTE pitch="E:5" />
   <NOTE />
   <NOTE />

   <NOTE pitch="rest" />

   <NOTE pitch="D:3" instrument="Harpsichord" />
   <NOTE />
   <NOTE />

   <NOTE pitch="rest" />

   <NOTE pitch="E:3" />
   <NOTE pitch="G:3" />
   <NOTE />

   <NOTE pitch="rest" />

   <NOTE pitch="E" instrument="Sitar" />
   <NOTE pitch="D" />
   <NOTE pitch="C" />
   <NOTE pitch="D" />
   <NOTE pitch="E" />
   <NOTE />
   <NOTE />
   <NOTE />
   <NOTE pitch="D" instrument="Bagpipe" />
   <NOTE />
   <NOTE pitch="E" />
   <NOTE pitch="D" />
   <NOTE pitch="C:3" duration="4" instrument="Church Organ" />
  </SONG>
</OPUS>