Add an abstraction for the Hard Vacuum algorithm

plugins/aw_soft_vacuum/src/hard_vacuum.rs

@@ -0,0 +1,32 @@
+// Soft Vacuum: Airwindows Hard Vacuum port with oversampling
+// Copyright (C) 2023 Robbert van der Helm
+// Copyright (c) 2018 Chris Johnson
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+/// Single-channel port of the Hard Vacuum algorithm from
+/// <https://github.com/airwindows/airwindows/blob/283343b9e90c28fdb583f27e198f882f268b051b/plugins/LinuxVST/src/HardVacuum/HardVacuumProc.cpp>.
+#[derive(Debug, Default)]
+pub struct HardVacuum {
+    last_sample: f32,
+}
+
+impl HardVacuum {
+    /// Reset the processor's state. In this case this only resets the discrete derivative
+    /// calculation. Doesn't make a huge difference but it's still useful to make the effect
+    /// deterministic.
+    pub fn reset(&mut self) {
+        self.last_sample = 0.0;
+    }
+}

plugins/aw_soft_vacuum/src/lib.rs

@@ -18,8 +18,14 @@ use std::sync::Arc;
 
 use nih_plug::prelude::*;
 
+mod hard_vacuum;
+
 struct SoftVacuum {
     params: Arc<SoftVacuumParams>,
+
+    /// Stores implementations of the Hard Vacuum algorithm for each channel, since each channel
+    /// needs to maintain its own state.
+    hard_vacuum_processors: Vec<hard_vacuum::HardVacuum>,
 }
 
 #[derive(Params)]
@@ -35,6 +41,8 @@ impl Default for SoftVacuum {
     fn default() -> Self {
         Self {
             params: Arc::new(SoftVacuumParams::default()),
+
+            hard_vacuum_processors: Vec::new(),
         }
     }
 }
@@ -69,15 +77,24 @@ impl Plugin for SoftVacuum {
 
     fn initialize(
         &mut self,
-        _audio_io_layout: &AudioIOLayout,
+        audio_io_layout: &AudioIOLayout,
-        buffer_config: &BufferConfig,
+        _buffer_config: &BufferConfig,
         _context: &mut impl InitContext<Self>,
     ) -> bool {
+        let num_channels = audio_io_layout
+            .main_output_channels
+            .expect("Plugin was initialized without any outputs")
+            .get() as usize;
+        self.hard_vacuum_processors
+            .resize_with(num_channels, hard_vacuum::HardVacuum::default);
+
         true
     }
 
     fn reset(&mut self) {
-        // TODO:
+        for hard_vacuum in &mut self.hard_vacuum_processors {
+            hard_vacuum.reset();
+        }
     }
 
     fn process(