From 366deda83c9656c96af0f1bb459517d57a1e5a1b Mon Sep 17 00:00:00 2001
From: Robbert van der Helm <mail@robbertvanderhelm.nl>
Date: Fri, 4 Feb 2022 02:37:40 +0100
Subject: [PATCH] Add MIDI support to the sine example

---
 plugins/examples/sine/src/lib.rs | 73 +++++++++++++++++++++++++++-----
 src/plugin.rs                    | 12 +++++-
 src/util.rs                      |  6 +++
 3 files changed, 80 insertions(+), 11 deletions(-)

diff --git a/plugins/examples/sine/src/lib.rs b/plugins/examples/sine/src/lib.rs
index 039084fd..6511c538 100644
--- a/plugins/examples/sine/src/lib.rs
+++ b/plugins/examples/sine/src/lib.rs
@@ -21,7 +21,7 @@ use nih_plug::{
     formatters, util, Buffer, BufferConfig, BusConfig, Plugin, ProcessContext, ProcessStatus,
     Vst3Plugin,
 };
-use nih_plug::{FloatParam, Param, Params, Range, Smoother, SmoothingStyle};
+use nih_plug::{BoolParam, FloatParam, Param, Params, Range, Smoother, SmoothingStyle};
 use std::f32::consts;
 use std::pin::Pin;
 
@@ -32,7 +32,10 @@ struct Sine {
     params: Pin<Box<SineParams>>,
     sample_rate: f32,
 
+    /// The current phase of the sine wave, always kept between in `[0, 1]`.
     phase: f32,
+    /// The active frequency, if triggered by MIDI.
+    active_note_freq: Option<f32>,
 }
 
 #[derive(Params)]
@@ -42,6 +45,9 @@ struct SineParams {
 
     #[id = "freq"]
     pub frequency: FloatParam,
+
+    #[id = "usemid"]
+    pub use_midi: BoolParam,
 }
 
 impl Default for Sine {
@@ -51,6 +57,7 @@ impl Default for Sine {
             sample_rate: 1.0,
 
             phase: 0.0,
+            active_note_freq: None,
         }
     }
 }
@@ -83,10 +90,29 @@ impl Default for SineParams {
                 value_to_string: formatters::f32_rounded(0),
                 ..Default::default()
             },
+            use_midi: BoolParam {
+                value: false,
+                name: "Use MIDI",
+                ..Default::default()
+            },
         }
     }
 }
 
+impl Sine {
+    fn calculate_sine(&mut self, frequency: f32) -> f32 {
+        let phase_delta = frequency / self.sample_rate;
+        let sine = (self.phase * consts::TAU).sin();
+
+        self.phase += phase_delta;
+        if self.phase >= 1.0 {
+            self.phase -= 1.0;
+        }
+
+        sine
+    }
+}
+
 impl Plugin for Sine {
     const NAME: &'static str = "Sine Test Tone";
     const VENDOR: &'static str = "Moist Plugins GmbH";
@@ -120,19 +146,46 @@ impl Plugin for Sine {
         true
     }
 
-    fn process(&mut self, buffer: &mut Buffer, _context: &dyn ProcessContext) -> ProcessStatus {
-        for samples in buffer.iter_mut() {
+    fn process(&mut self, buffer: &mut Buffer, context: &dyn ProcessContext) -> ProcessStatus {
+        let mut next_event = context.next_midi_event();
+        for (sample_id, samples) in buffer.iter_mut().enumerate() {
             // Smoothing is optionally built into the parameters themselves
             let gain = self.params.gain.smoothed.next();
-            let frequency = self.params.frequency.smoothed.next();
 
-            let phase_delta = frequency / self.sample_rate;
-            let sine = (self.phase * consts::TAU).sin();
+            // This plugin can be either triggered by MIDI or controleld by a parameter
+            let sine = if self.params.use_midi.value {
+                // Act on the next MIDI event
+                'midi_events: loop {
+                    match next_event {
+                        Some(event) if event.timing() == sample_id as u32 => match event {
+                            nih_plug::NoteEvent::NoteOn { note, .. } => {
+                                // Reset the phase if this is a new note
+                                if self.active_note_freq.is_none() {
+                                    self.phase = 0.0;
+                                }
 
-            self.phase += phase_delta;
-            if self.phase >= 1.0 {
-                self.phase -= 1.0;
-            }
+                                self.active_note_freq = Some(util::midi_note_to_freq(note));
+                            }
+                            nih_plug::NoteEvent::NoteOff { note, .. } => {
+                                if self.active_note_freq == Some(util::midi_note_to_freq(note)) {
+                                    self.active_note_freq = None;
+                                }
+                            }
+                        },
+                        _ => break 'midi_events,
+                    }
+
+                    next_event = context.next_midi_event();
+                }
+
+                match self.active_note_freq {
+                    Some(frequency) => self.calculate_sine(frequency),
+                    None => 0.0,
+                }
+            } else {
+                let frequency = self.params.frequency.smoothed.next();
+                self.calculate_sine(frequency)
+            };
 
             for sample in samples {
                 *sample = sine * util::db_to_gain(gain);
diff --git a/src/plugin.rs b/src/plugin.rs
index d9e0ce86..30ed1584 100644
--- a/src/plugin.rs
+++ b/src/plugin.rs
@@ -155,7 +155,7 @@ pub enum ProcessStatus {
 }
 
 /// Event for (incoming) notes. Right now this only supports a very small subset of the MIDI
-/// specification.
+/// specification. See the util module for convenient conversion functions.
 ///
 /// All of the timings are sample offsets withing the current buffer.
 ///
@@ -175,3 +175,13 @@ pub enum NoteEvent {
         velocity: u8,
     },
 }
+
+impl NoteEvent {
+    /// Return the sample within the current buffer this event belongs to.
+    pub fn timing(&self) -> u32 {
+        match &self {
+            NoteEvent::NoteOn { timing, .. } => *timing,
+            NoteEvent::NoteOff { timing, .. } => *timing,
+        }
+    }
+}
diff --git a/src/util.rs b/src/util.rs
index 3d6ae5ae..96fbed74 100644
--- a/src/util.rs
+++ b/src/util.rs
@@ -35,6 +35,12 @@ pub fn gain_to_db(gain: f32) -> f32 {
     }
 }
 
+/// Convert a MIDI note ID to a frequency at A4 = 440 Hz equal temperament and middle C = note 60 =
+/// C4.
+pub fn midi_note_to_freq(pitch: u8) -> f32 {
+    2.0f32.powf((pitch as f32 - 69.0) / 12.0) * 440.0
+}
+
 /// A version of [std::thread::Builder::spawn_unchecked] that works on the stable compiler.
 /// Implementation courtesy of Yandros on the Rust Discord.
 pub(crate) trait ThreadSpawnUnchecked {