Simple MIDI file reader

use strict;

# read the output from midifile, sort it into "note column" groups

my $timebase = 120;

my $beatsperbar = 3;

sub fmtm { # function to convert time to bar number

    int($_[0] / ($timebase * $beatsperbar)) + 1;

}

sub fmt { # function to convert time to beat number

    my $t = $_[0] / $timebase;

    while ($t >= $beatsperbar) { $t -= $beatsperbar; }

    $t + 1;

}

sub fmtd { # function to convert duration to beat count

    $_[0] / $timebase;

}

my %noteons;

my %noteoffs;

my %channels;

my %grouptimes;

while (<>) {

    my ($t,$c,$d,$p);

    # split events into time, channel, duration, pitch

    if (/^(\d+): Note: channel (\d+) duration (\d+) pitch (\d+)/) {

	$t = $1;

	$c = $2;

	$d = $3;

	$p = $4;

    } else {

	next;

    }

    # record note-on times, plus all active channels

    $noteons{$t}{$c}{$d}{$p}++;

    $channels{$c}++;

    # start times of note groups: whenever a note starts or ends

    $grouptimes{$t}++;

    $grouptimes{$t + $d}++;

}

my $prevt;

my %groupdurs;

# work out the duration of each note-group

for my $t (sort { $a <=> $b } keys %grouptimes) { # sort group starts numerically

    if (defined $prevt) {

	# duration of the previous note-group is that from its start

	# time to that of the current one

	$groupdurs{$prevt} = $t - $prevt;

    }

    $prevt = $t;

}

my $n = 0;

for my $t (sort { $a <=> $b } keys %groupdurs) {

    printf "NOTEGROUP:%d  Bar:%d  Beat:%.1f  Dur:%.1f",

	    $n, fmtm($t), fmt($t), fmtd($groupdurs{$t});

    # within this group we want to output a pitch number for every

    # "known" channel, i.e. everything in keys %channels.  these

    # pitches may be based on notes starting at this time (via noteons

    # hash) or notes that have started previously but not yet finished

    # (via noteoffs hash)

    print "  Midi: [";

    my @pitches; # array of all pitches (ordered by channel) in this group

    for my $ot (keys %noteoffs) {

	# prune notes that have ended already or are ending now

	if ($ot le $t) {

	    delete $noteoffs{$ot};

	}

    }

    for my $c (sort { $a <=> $b } keys %channels) {

	my @here; # array of all pitches on this channel

	for my $d (keys %{$noteons{$t}{$c}}) {

	    for my $p (keys %{$noteons{$t}{$c}{$d}}) {

		# add notes that begin at this time, indexed by end time

		$noteoffs{$t + $d}{$c}{$p}++;

	    }

	}

	for my $ot (keys %noteoffs) {

	    # note(s) still sounding, add to group if in channel

	    for my $p (keys %{$noteoffs{$ot}{$c}}) {

		push @here, $p;

	    }

	}

	if (@here > 0) {

	    # if we have any notes on this channel in this group,

	    # prepare to print them separated by spaces

	    push @pitches, (join " ", @here);

	} else {

	    # otherwise prepare to print 'rest'

	    push @pitches, "'rest'";

	}

    }

    # now print the channel note lists separated by comma

    print join ", ", @pitches;

    print "]\n";

    ++$n;

}