# Some constants from the BBC executable
my $melodyoffset=0x4F00;
my $melodyrefs=0x4E5D;
my $endmelrefs=0x4E90;
my $bbcbase=0x1100;


# The melody is a set of segments define as the addresses listed at $melodyrefs. There is
# a segment before those defined starting at the initial value of $melodyoffset
# Within the segments are a set of value pairs
# The first value is the BBC pitch value (which we'll convert to a midi value (see below)
# The second is a duration
# The end of segments is defined by the pitch value being 0xFF

my $track1offset=0x4D00;
my $track1parts=0x4e92; # One more than it appears since it's pre incremented


open (INFILE,"<xor2");
binmode INFILE;
read (INFILE, $buffer, 65536);
close INFILE;

# Create a mapping between the bbc pitch values and the midi pitch values
# Effectively the pitch value on the BBC starts at 1 and increments by 4 for each semitone
# The midi ones increment by one with 23 corresponding to 1 on the bbc
# one oddity exists with the value of 0 being a semitone below 1 on the bbc

my %bbctomidi;
my $midinote=35;
for ($i=1;$i<254;$i=$i+4) {
	$bbctomidi{$i}=$midinote++;
}
$bbctomidi{0}=34;

# Setup outputbuffer with midi header recording 2 ticks per quarter note
my $outbuffer="MThd".chr(0).chr(0).chr(0).chr(6).chr(0).chr(0);
$outbuffer=$outbuffer.chr(0).chr(3); # Number of tracks
$outbuffer=$outbuffer.chr(0).chr(8); # Ticks per quarter note


# Track 1 header, we don't know the length now so set it to 0
my $trkbuffer="MTrk".chr(0).chr(0).chr(0).chr(0);

my $tracklen=0;
my $count=0;

while ($track1offset != 0x4D50) {
	my $bbcpitch=ord(substr($buffer,$track1offset-$bbcbase,1));
	my $bbcduration=3;
	#keydown, wait no ticks before doing it velocity of 0x50 (Made up value)
	$trkbuffer=$trkbuffer.chr(0);
	$trkbuffer=$trkbuffer.chr(0x90).chr($bbctomidi{$bbcpitch}).chr(0x50);
	#keyup, wait the duration before doing it
	$trkbuffer=$trkbuffer.chr($bbcduration*2).chr(0x80).chr($bbctomidi{$bbcpitch}).chr(0x50);			
	
	$tracklen=$tracklen+8;
	$track1offset=$track1offset+1;
	#print $bbcpitch." ".$bbcduration."\n";
	$count++;
	if ($count == 4 ) {
		$track1offset=0x4D00 + ord(substr($buffer,$track1parts-$bbcbase,1));
		$track1parts=$track1parts+1;
		$count=0;
	}
}

substr($trkbuffer,4,1)=chr(($tracklen>>24) & 0xFF);
substr($trkbuffer,5,1)=chr(($tracklen>>16) & 0xFF);
substr($trkbuffer,6,1)=chr(($tracklen>>8) & 0xFF);
substr($trkbuffer,7,1)=chr(($tracklen) & 0xFF);
$outbuffer=$outbuffer.$trkbuffer;


# Track 2 header, we don't know the length now so set it to 0
$trkbuffer="MTrk".chr(0).chr(0).chr(0).chr(0);

$tracklen=0;
$count=0;
$track1offset=0x4D00;  # We don't use this for the sound but we do so we know when to reset
$track1parts=0x4E92; # This is a very simple track

my $track2offset=0x4D04;

while ($track1offset != 0x4D50) {
	$bbcpitch=ord(substr($buffer,$track2offset-$bbcbase,1));
	$bbcduration=2;
	#keydown, wait no ticks before doing it velocity of 0x50 (Made up value)
	$trkbuffer=$trkbuffer.chr(0);
	$trkbuffer=$trkbuffer.chr(0x90).chr($bbctomidi{$bbcpitch}).chr(0x50);
	#keyup, wait the duration before doing it
	$trkbuffer=$trkbuffer.chr($bbcduration*2).chr(0x80).chr($bbctomidi{$bbcpitch}).chr(0x50);			
	
	$tracklen=$tracklen+8;
	$track2offset=$track2offset+1;
	#print $bbcpitch." ".$bbcduration."\n";
	$count++;
	if ($count == 6 ) {
		$track1offset=0x4D00 + ord(substr($buffer,$track1parts-$bbcbase,1));
		$track1parts=$track1parts+1;
		$count=0;
		$track2offset=0x4D04;
	}
}

substr($trkbuffer,4,1)=chr(($tracklen>>24) & 0xFF);
substr($trkbuffer,5,1)=chr(($tracklen>>16) & 0xFF);
substr($trkbuffer,6,1)=chr(($tracklen>>8) & 0xFF);
substr($trkbuffer,7,1)=chr(($tracklen) & 0xFF);
$outbuffer=$outbuffer.$trkbuffer;



# Now the melody track

$trkbuffer="MTrk".chr(0).chr(0).chr(0).chr(0);

$tracklen=0;
$count=0;

while ($melodyrefs < $endmelrefs+2 ) {
	$bbcpitch=ord(substr($buffer,$melodyoffset-$bbcbase,1));
	$bbcduration=ord(substr($buffer,($melodyoffset-$bbcbase)+1,1));
	if ($bbcpitch != 0xFF) {
		#keydown, wait no ticks before doing it velocity of 0x50 (Made up value)
		if ($count !=0) {
			$trkbuffer=$trkbuffer.chr(0);
		} 
		else
		{
			$trkbuffer=$trkbuffer.chr(24*2);  # Pause before first note on this track
		}
		$trkbuffer=$trkbuffer.chr(0x90).chr($bbctomidi{$bbcpitch}).chr(0x50);
		#keyup, wait the duration before doing it
		$trkbuffer=$trkbuffer.chr($bbcduration*2).chr(0x80).chr($bbctomidi{$bbcpitch}).chr(0x50);			
		
		$tracklen=$tracklen+8;
		$melodyoffset=$melodyoffset+2;
		$count++;
	} 
	else 
	{
		#print "Updating ref\n";
		my $lb=ord(substr($buffer,$melodyrefs-$bbcbase,1));
		my $hb=ord(substr($buffer,$melodyrefs-$bbcbase+1,1));
		$melodyoffset= ($hb<<8) + $lb;
		#print $melodyoffset."\n";
		$melodyrefs=$melodyrefs+2;
	}
}

substr($trkbuffer,4,1)=chr(($tracklen>>24) & 0xFF);
substr($trkbuffer,5,1)=chr(($tracklen>>16) & 0xFF);
substr($trkbuffer,6,1)=chr(($tracklen>>8) & 0xFF);
substr($trkbuffer,7,1)=chr(($tracklen) & 0xFF);
$outbuffer=$outbuffer.$trkbuffer;






open (OUTFILE,">xor.mid");
binmode OUTFILE;
print OUTFILE $outbuffer	;
close OUTFILE;