feat(weld): add sidechain test build

Weld will phase-align a bass layer to another layer it hears through the
sidechain. This first build only checks that FL Studio delivers the
sidechain to a CLAP aux port: it passes the main layer through unchanged,
meters the sidechain and offers Layer/Sum/Sum dry listening. No latency.

plugins/weld/core/src/lib.rs

@@ -0,0 +1,150 @@
+//! Weld engine: aligns the phase of the main layer to a sidechain layer so the two sum
+//! instead of cancel. Allocation-free after `new`; zero latency.
+mod settings;
+pub use settings::{Listen, Settings};
+
+/// Levels for the editor, published a few times per second.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct AnalyzerFrame {
+    /// RMS of the main input in dBFS since the last frame.
+    pub main_db: f32,
+    /// RMS of the sidechain input in dBFS since the last frame.
+    pub side_db: f32,
+    /// Peak of the sidechain input in dBFS since the last frame.
+    pub side_peak_db: f32,
+    /// True once any sidechain sample above −90 dBFS arrived since activation.
+    pub side_seen: bool,
+    pub sample_rate: f32,
+}
+impl Default for AnalyzerFrame {
+    fn default() -> Self {
+        Self {
+            main_db: -120.,
+            side_db: -120.,
+            side_peak_db: -120.,
+            side_seen: false,
+            sample_rate: 48000.,
+        }
+    }
+}
+
+fn db(power_or_amp: f32, power: bool) -> f32 {
+    let v = power_or_amp.max(1e-12);
+    if power {
+        10. * v.log10()
+    } else {
+        20. * v.log10()
+    }
+    .max(-120.)
+}
+
+#[derive(Default)]
+struct Levels {
+    main: f64,
+    side: f64,
+    peak: f32,
+    count: usize,
+}
+
+pub struct Engine {
+    rate: f32,
+    channels: usize,
+    settings: Settings,
+    gain: f32,
+    target_gain: f32,
+    gain_coef: f32,
+    fresh: bool,
+    levels: Levels,
+    side_seen: bool,
+}
+impl Engine {
+    pub fn new(rate: f32, channels: usize) -> Result<Self, &'static str> {
+        if !(rate.is_finite() && (8000.0..=768000.).contains(&rate)) {
+            return Err("unsupported sample rate");
+        }
+        if !(1..=2).contains(&channels) {
+            return Err("unsupported channel count");
+        }
+        Ok(Self {
+            rate,
+            channels,
+            settings: Settings::default(),
+            gain: 1.,
+            target_gain: 1.,
+            gain_coef: 1. - (-1. / (0.02 * rate)).exp(),
+            fresh: true,
+            levels: Levels::default(),
+            side_seen: false,
+        })
+    }
+    pub const fn latency_samples(&self) -> u32 {
+        0
+    }
+    pub fn channels(&self) -> usize {
+        self.channels
+    }
+    pub fn settings(&self) -> &Settings {
+        &self.settings
+    }
+    pub fn set_settings(&mut self, settings: Settings) -> Result<(), &'static str> {
+        settings.validate()?;
+        self.settings = settings;
+        self.target_gain = 10f32.powf(settings.output / 20.);
+        if self.fresh {
+            self.gain = self.target_gain;
+            self.fresh = false;
+        }
+        Ok(())
+    }
+    pub fn reset(&mut self) {
+        self.gain = self.target_gain;
+        self.levels = Levels::default();
+    }
+    /// One frame of main input `x` and sidechain input `side`. Mono uses index 0 only.
+    #[inline]
+    pub fn process_sample(&mut self, x: [f32; 2], side: [f32; 2]) -> [f32; 2] {
+        let n = self.channels;
+        let (mut m, mut s) = (0f32, 0f32);
+        for c in 0..n {
+            m += x[c];
+            s += side[c];
+            self.levels.peak = self.levels.peak.max(side[c].abs());
+        }
+        m /= n as f32;
+        s /= n as f32;
+        self.levels.main += (m * m) as f64;
+        self.levels.side += (s * s) as f64;
+        self.levels.count += 1;
+        if s.abs() > 3e-5 {
+            self.side_seen = true;
+        }
+        self.gain += (self.target_gain - self.gain) * self.gain_coef;
+        if self.settings.bypass {
+            return x;
+        }
+        let mut out = [0.; 2];
+        for c in 0..n {
+            let layer = x[c];
+            out[c] = self.gain
+                * match self.settings.listen {
+                    Listen::Layer => layer,
+                    Listen::Sum => layer + side[c],
+                    Listen::SumDry => x[c] + side[c],
+                };
+        }
+        out
+    }
+    /// Levels since the previous call.
+    pub fn take_analyzer(&mut self) -> AnalyzerFrame {
+        let n = self.levels.count.max(1) as f64;
+        let frame = AnalyzerFrame {
+            main_db: db((self.levels.main / n) as f32, true),
+            side_db: db((self.levels.side / n) as f32, true),
+            side_peak_db: db(self.levels.peak, false),
+            side_seen: self.side_seen,
+            sample_rate: self.rate,
+        };
+        self.levels = Levels::default();
+        frame
+    }
+}

plugins/weld/core/src/settings.rs

@@ -0,0 +1,43 @@
+//! Host-independent controls. Gains in dB.
+use serde::{Deserialize, Serialize};
+
+/// What the plugin outputs.
+#[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq, Eq, Default)]
+pub enum Listen {
+    /// The aligned main layer only. The sidechain layer reaches the mix on its own track.
+    #[default]
+    Layer,
+    /// Aligned main layer plus the sidechain layer, to judge the sum in one place.
+    Sum,
+    /// Unprocessed main layer plus the sidechain layer, to compare with `Sum`.
+    SumDry,
+}
+
+#[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq)]
+#[serde(deny_unknown_fields)]
+pub struct Settings {
+    pub listen: Listen,
+    pub output: f32,
+    pub bypass: bool,
+}
+impl Default for Settings {
+    fn default() -> Self {
+        Self {
+            listen: Listen::Layer,
+            output: 0.,
+            bypass: false,
+        }
+    }
+}
+impl Settings {
+    pub fn validate(&self) -> Result<(), &'static str> {
+        if !self.output.is_finite() {
+            return Err("non-finite setting");
+        }
+        if (-12.0..=12.).contains(&self.output) {
+            Ok(())
+        } else {
+            Err("setting out of range")
+        }
+    }
+}