midi-keys/src/parser.rs

use nom::{
    bytes::complete::{tag, take, take_till},
    IResult,
};

use crate::midi::{Message, ParsedMessage, SystemCommon, SystemRealtime, VoiceCategory, VoiceMessage};

pub fn parse_message(bytes: &[u8]) -> IResult<&[u8], Message> {
    let raw = Vec::from(bytes);
    let (bytes, status_byte) = take(1usize)(bytes)?;
    let status_byte = status_byte[0];

    // TODO: implement running status; see [1].
    // [1]: http://midi.teragonaudio.com/tech/midispec/run.htm

    if status_byte < 0xF0 {
        let (bytes, vm) = parse_voice_message(status_byte, bytes)?;
        Ok((bytes, Message::new(raw, ParsedMessage::Voice(vm))))
    } else if status_byte < 0xf8 {
        let (bytes, sc) = parse_system_common(status_byte, bytes)?;
        Ok((bytes, Message::new(raw, ParsedMessage::System(sc))))
    } else {
        let sr = parse_system_realtime(status_byte);
        Ok((bytes, Message::new(raw, ParsedMessage::Realtime(sr))))
    }
}

/// The highest order bit is only ever set for status bytes, so we can test
/// for to tell whether or not we've found a status byte. This is needed for
/// finding the end of dynamic-length messages, such as SysEx, which is
/// considered aborted if another status byte comes before 0xf7.
fn is_status_byte(b: u8) -> bool {
    (b & 0x80) != 0
}

fn parse_system_common(status_byte: u8, bytes: &[u8]) -> IResult<&[u8], SystemCommon> {
    match status_byte {
        0xf0 => parse_system_exclusive(bytes),
        0xf1 => one_byte_message(bytes, |time_code| SystemCommon::MidiTimeCode { time_code }),
        0xf2 => parse_song_position_pointer(bytes),
        0xf3 => one_byte_message(bytes, |song_number| SystemCommon::SongSelect {
            song_number,
        }),
        0xf6 => Ok((bytes, SystemCommon::TuneRequest)),
        _ => Ok((bytes, SystemCommon::Unknown)),
    }
}

pub fn parse_system_realtime(status_byte: u8) -> SystemRealtime {
    match status_byte {
        0xf8 => SystemRealtime::Clock,
        0xf9 => SystemRealtime::Tick,
        0xfa => SystemRealtime::Start,
        0xfb => SystemRealtime::Continue,
        0xfc => SystemRealtime::Stop,
        0xfe => SystemRealtime::ActiveSense,
        0xff => SystemRealtime::Reset,
        _ => SystemRealtime::Unknown,
    }
}

pub fn parse_voice_message(status_byte: u8, remainder: &[u8]) -> IResult<&[u8], VoiceMessage> {
    let category_nibble = 0xf0 & status_byte;
    let channel = 0x0f & status_byte;

    let (remainder, category) = match category_nibble {
        0x80 => parse_voice_note(remainder, true)?,
        0x90 => parse_voice_note(remainder, false)?,
        0xa0 => two_byte_message(remainder, |note, pressure| VoiceCategory::AfterTouch {
            note,
            pressure,
        })?,
        0xb0 => two_byte_message(remainder, |controller, value| {
            VoiceCategory::ControlChange { controller, value }
        })?,
        0xc0 => one_byte_message(remainder, |value| VoiceCategory::ProgramChange { value })?,
        0xd0 => one_byte_message(remainder, |pressure| VoiceCategory::ChannelPressure {
            pressure,
        })?,
        0xe0 => parse_pitch_wheel(remainder)?,
        _ => (remainder, VoiceCategory::Unknown),
    };

    Ok((remainder, VoiceMessage::new(category, channel)))
}

pub fn parse_voice_note(bytes: &[u8], off: bool) -> IResult<&[u8], VoiceCategory> {
    two_byte_message(bytes, |note, velocity| {
        if velocity == 0 || off {
            VoiceCategory::NoteOff { note, velocity }
        } else {
            VoiceCategory::NoteOn { note, velocity }
        }
    })
}

pub fn parse_pitch_wheel(bytes: &[u8]) -> IResult<&[u8], VoiceCategory> {
    let (bytes, value) = take_14_bit_value(bytes)?;
    Ok((bytes, VoiceCategory::PitchWheel { value }))
}

pub fn parse_system_exclusive(bytes: &[u8]) -> IResult<&[u8], SystemCommon> {
    let (remainder, data) = take_till(is_status_byte)(bytes)?;
    let (remainder, _) = tag([0xf7])(remainder)?;

    let data: Vec<u8> = data.into();

    Ok((remainder, SystemCommon::SystemExclusive { data }))
}

pub fn parse_song_position_pointer(bytes: &[u8]) -> IResult<&[u8], SystemCommon> {
    let (remainder, value) = take_14_bit_value(bytes)?;
    Ok((remainder, SystemCommon::SongPositionPointer { value }))
}

pub fn one_byte_message<T, F>(bytes: &[u8], f: F) -> IResult<&[u8], T>
where
    F: Fn(u8) -> T,
{
    let (bytes, b) = take(1usize)(bytes)?;
    Ok((bytes, f(b[0])))
}

pub fn two_byte_message<T, F>(bytes: &[u8], f: F) -> IResult<&[u8], T>
where
    F: Fn(u8, u8) -> T,
{
    let (bytes, b) = take(2usize)(bytes)?;
    Ok((bytes, f(b[0], b[1])))
}

pub fn take_14_bit_value(bytes: &[u8]) -> IResult<&[u8], u16> {
    let (bytes, db) = take(2usize)(bytes)?;
    let value = ((db[0] as u16) << 7) | db[1] as u16;

    Ok((bytes, value))
}

#[cfg(test)]
mod tests {
    use crate::{log::load_raw_log, midi::VoiceMessage};

    use super::*;

    #[test]
    fn parses_note_on_message() {
        let msg = [0x90, 0x24, 0x51];
        let (remainder, parsed) = parse_message(&msg).unwrap();

        let note = 0x24;
        let velocity = 0x51;
        let expected = Message::Voice(VoiceMessage::new(
            VoiceCategory::NoteOn { note, velocity },
            0,
        ));
        assert_eq!(parsed, expected,);
        assert_eq!(remainder.len(), 0);
    }

    #[test]
    fn parse_log_from_windsynth() {
        // I played a few notes on my Roland AE-20 and saved it to a log file.
        // This test just reads that in and ensures that everything parses. It
        // doesn't check for *correct* parsing, but this is sufficient for much
        // of our needs.

        let filename = "assets/windsynth.log";
        let entries = load_raw_log(filename).unwrap();

        assert_eq!(1260, entries.len());
        for entry in entries {
            let parsed = parse_message(&entry.message);
            assert!(
                parsed.is_ok(),
                "failed to parse message: {:?}; {:?}",
                &entry.message,
                parsed
            );
        }
    }
}