erlang midi parser

-module(midi).
-export([parse/1, parse_file/1]).

-define(BEINT, big-unsigned-integer-unit).
-define(BEBIT, 1/?BEINT:1).
-define(BEINT7, 1/?BEINT:7).
-define(BEINT16, 1/?BEINT:16).
-define(BEINT32, 1/?BEINT:32).

-define(MIDI_HEADER, 16#4d546864:?BEINT32).
-define(TRACK_HEADER, 16#4d54726b:?BEINT32).

%-define(dprint(Format, Args), io:format(Format, Args)).
-define(dprint(Format, Args), ok).

parse_var_length(Data, Vl_accum) ->
case Data of
	<<1:?BEBIT, VChunk:?BEINT7, Rest/binary>>
	-> parse_var_length(Rest, VChunk bor (Vl_accum bsr 7));
	<<0:?BEBIT, VChunk:?BEINT7, Rest/binary>>
	-> {ok, VChunk bor (Vl_accum bsr 7), Rest};
	_ -> {error, badvlen}
end.

parse_meta(Data) ->
case parse_var_length(Data, 0) of
	{ok, MetaLength, MetaRest} ->
		case MetaRest of 
			<<MetaData:MetaLength/binary, Rest/binary>> ->
				?dprint("Meta length ~w~n", [MetaLength]),
				parse_event(Rest);
			_ -> {error, badmeta}
		end;
	_ -> {error, badmeta}
end.

note_name(NoteNumber) ->
	lists:nth(NoteNumber rem 12,
	["C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B"]
).

parse_note(EventType, NoteNumber, Velocity) ->
case EventType of
	_ when EventType >= 8 andalso EventType =< 10
	-> io:format("Note ~p number=~w velocity=~w~n",
		[note_name(NoteNumber), NoteNumber, Velocity]);
	_ -> ok
end.

parse_channel_event(EventType, Channel, Data) ->
case Data of
	<<Param1:1/?BEINT:8, Param2:1/?BEINT:8, Rest/binary>>
		when EventType =/= 16#C andalso EventType =/= 16#d
	-> ?dprint("Event type=~w channel=~w param1=~w param2=~w~n",
		[EventType, Channel, Param1, Param2]),
		parse_note(EventType, Param1, Param2),
		parse_event(Rest);
	<<Param1:1/?BEINT:8, Rest/binary>>
	-> ?dprint("Event type=~w channel=~w param1=~w~n", [EventType, Channel, Param1]),
		parse_event(Rest);
	_ -> {error, badevent}
end.

parse_event_data(Data) ->
case Data of
	<<16#ff:1/?BEINT:8, EventType:1/?BEINT:8, Rest/binary>> ->
		?dprint("Meta type=~w~n", [EventType]),
		parse_meta(Rest);
	<<EventType:1/?BEINT:4, Channel:1/?BEINT:4, Rest/binary>>
		when EventType =/= 15 andalso Channel =/= 15
	->
		parse_channel_event(EventType, Channel, Rest);
	_ -> {error, badevent}
end.

parse_event(RawData) ->
case parse_var_length(RawData, 0) of
	{ok, DeltaTime, Data} -> parse_event_data(Data);
	_ -> {error, badevent}
end.

parse_track(Bin) ->
case Bin of
	<<?TRACK_HEADER, ChunkSize:?BEINT32,
	  TrackData:ChunkSize/binary, Rest/binary>>
	  -> ?dprint("Track size=~w~n", [ChunkSize]),
	  parse_event(TrackData),
	  parse_track(Rest);
	_ -> {error, badtrack}
end.

frames_per_second(Fps) ->
case Fps of
	<<FrameCount:?BEINT7, TicksPerFrame:1/?BEINT:8>> ->
		?dprint("midi: FrameCount=~w, TicksPerFrame=~w~n", [FrameCount, TicksPerFrame]);
	_ -> {error, badfps}
end.

time_division(Div) ->
case Div of
	<<0:?BEBIT, TicksPerBeat:1/?BEINT:15>> ->
		?dprint("midi: TicksPerBeat=~w~n", [TicksPerBeat]);
	<<1:?BEBIT, FPSBin:15/bitstring>> ->
		frames_per_second(FPSBin);
	_ -> {error, badtimediv}
end.

parse(Mid) ->
case Mid of 
	<<?MIDI_HEADER, 6:?BEINT32, FileFormat:?BEINT16, TrackCount:?BEINT16,
	  TimeDivision:2/binary, Rest/binary>>
	->
	  	?dprint("midi: format=~w, track count=~w~n", [FileFormat, TrackCount]),
	  	time_division(TimeDivision),
		parse_track(Rest),
		ok;
	_ -> {error, badformat}
end.

parse_file(Name) ->
case file:read_file(Name) of
	{ok, Data} -> parse(Data);
	err -> err
end.