Add peak/RMS detection switch; sweep docs to match code

- Compressor: switchable peak / RMS detection (EnumParam<DetectionMode> in lib.rs
  -> use_rms bool in CompressorSettings; DSP stays framework-agnostic). RMS is a
  one-pole running mean of the linked squared level with a hardcoded 5 ms window,
  updated whenever active so peak<->RMS switching is seamless. New unit test
  (RMS compresses a sine less than peak); 6 tests total.
- README: reconciled Implementation Order checklists with actual progress
  (Stages 1-2 done; look-ahead/latency + basic UI pulled forward), annotated the
  project structure (implemented vs planned), and corrected the Latency and
  Denormal-flushing notes to match the code (set_latency_samples once / constant
  latency; in-code denormal flush). Overview and goals left unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/dsp/compressor.rs

@@ -26,6 +26,10 @@ const MAX_CHANNELS: usize = 2;
 /// ~ -240 dBFS; keeps `log10` away from zero without affecting audible levels.
 const LEVEL_EPS: f32 = 1e-12;
 
+/// Hardcoded RMS averaging window (one-pole time constant). Deliberately small; can be
+/// promoted to a parameter later.
+const RMS_WINDOW_MS: f32 = 5.0;
+
 /// Flush a decaying envelope value to zero once it is far below audibility, so the
 /// exponential tail can't drift into denormal range (which causes CPU spikes).
 #[inline]
@@ -51,6 +55,8 @@ pub struct CompressorSettings {
     /// How far (in samples) the detector reads *ahead* of the output, 0..=`fixed_delay`.
     /// This does NOT change the reported latency — the audio delay is always `fixed_delay`.
     pub lookahead_samples: usize,
+    /// `true` = RMS detection (running power average), `false` = naive sample peak.
+    pub use_rms: bool,
     pub bypass: bool,
 }
 
@@ -64,6 +70,10 @@ pub struct Compressor {
     /// Constant audio delay applied to every sample == the reported plugin latency.
     fixed_delay: usize,
 
+    /// RMS detector state: running mean of the squared (linked) level, plus its coefficient.
+    mean_sq: f32,
+    rms_coef: f32,
+
     /// Smooth decoupled peak-detector state, expressed as dB of **attenuation** (>= 0).
     y1: f32, // release branch (peak-with-decay)
     yl: f32, // attack-smoothed output
@@ -77,6 +87,8 @@ impl Default for Compressor {
             capacity: 0,
             write_pos: 0,
             fixed_delay: 0,
+            mean_sq: 0.0,
+            rms_coef: 0.0,
             y1: 0.0,
             yl: 0.0,
         }
@@ -95,6 +107,7 @@ impl Compressor {
         self.fixed_delay = (max_lookahead_ms * 0.001 * sample_rate).ceil() as usize;
         // +1 so the oldest (output) sample and the newest (write) sample never alias.
         self.capacity = self.fixed_delay + 1;
+        self.rms_coef = Self::time_to_coef(RMS_WINDOW_MS, sample_rate);
         let channels = num_channels.clamp(1, MAX_CHANNELS);
         self.delay = vec![vec![0.0; self.capacity]; channels];
         self.reset();
@@ -106,6 +119,7 @@ impl Compressor {
             ch.iter_mut().for_each(|s| *s = 0.0);
         }
         self.write_pos = 0;
+        self.mean_sq = 0.0;
         self.y1 = 0.0;
         self.yl = 0.0;
     }
@@ -179,7 +193,13 @@ impl Compressor {
         let gain_lin = if set.bypass {
             1.0
         } else {
-            let level_db = 20.0 * (peak + LEVEL_EPS).log10();
+            // RMS = running mean of the linked squared level over a fixed window. Updated
+            // whenever active (regardless of mode) so switching peak<->RMS is seamless.
+            self.mean_sq = flush_denormal(
+                self.rms_coef * self.mean_sq + (1.0 - self.rms_coef) * peak * peak,
+            );
+            let detector = if set.use_rms { self.mean_sq.sqrt() } else { peak };
+            let level_db = 20.0 * (detector + LEVEL_EPS).log10();
             // Desired attenuation in dB, as a positive quantity.
             let target = -Self::gain_computer(level_db, set.threshold_db, set.ratio, set.knee_db);
 
@@ -224,6 +244,7 @@ mod tests {
             release_coef: Compressor::time_to_coef(1.0, SR),
             makeup_db: 0.0,
             lookahead_samples: 0,
+            use_rms: false,
             bypass: false,
         }
     }
@@ -270,6 +291,42 @@ mod tests {
         assert_close(out[0], expected, 1e-3);
     }
 
+    #[test]
+    fn rms_compresses_a_sine_less_than_peak() {
+        // For a sine, RMS level is ~3 dB below the peak (A/√2), so RMS detection sees a lower
+        // level and applies less gain reduction -> louder output than peak detection.
+        use std::f32::consts::PI;
+
+        fn output_rms(use_rms: bool) -> f32 {
+            let mut comp = Compressor::new();
+            comp.prepare(SR, 1, MAX_LOOKAHEAD_MS);
+            let mut set = settings(-30.0, 4.0, 0.0);
+            set.use_rms = use_rms;
+            set.attack_coef = Compressor::time_to_coef(1.0, SR);
+            set.release_coef = Compressor::time_to_coef(50.0, SR);
+
+            let (amp, freq) = (0.5f32, 2000.0f32);
+            let total = SR as usize;
+            let mut out = [0.0f32];
+            let (mut acc, mut cnt) = (0.0f64, 0u32);
+            for i in 0..total {
+                let x = amp * (2.0 * PI * freq * i as f32 / SR).sin();
+                comp.process(&[x], &mut out, &set);
+                if i >= total - 4800 {
+                    // measure RMS over the last 0.1 s, after settling
+                    acc += (out[0] * out[0]) as f64;
+                    cnt += 1;
+                }
+            }
+            (acc / cnt as f64).sqrt() as f32
+        }
+
+        assert!(
+            output_rms(true) > output_rms(false),
+            "RMS detection should compress a sine less than peak"
+        );
+    }
+
     #[test]
     fn latency_is_constant_regardless_of_lookahead() {
         // Audio is always delayed by `fixed_delay` (== reported latency); the look-ahead

src/lib.rs

@@ -5,6 +5,17 @@ use std::sync::Arc;
 mod dsp;
 use dsp::compressor::{Compressor, CompressorSettings, MAX_LOOKAHEAD_MS};
 
+/// Level-detection mode for the compressor's detector.
+#[derive(Enum, PartialEq, Clone, Copy)]
+enum DetectionMode {
+    #[id = "peak"]
+    #[name = "Peak"]
+    Peak,
+    #[id = "rms"]
+    #[name = "RMS"]
+    Rms,
+}
+
 /// Codename 206 — Stage 2: a single full-band compressor with look-ahead.
 ///
 /// The `CompressorParams` struct is `#[nested]` so the exact same controls + DSP can be
@@ -33,6 +44,8 @@ struct Codename206Params {
 
 #[derive(Params)]
 struct CompressorParams {
+    #[id = "detect"]
+    pub detection: EnumParam<DetectionMode>,
     #[id = "thresh"]
     pub threshold_db: FloatParam,
     #[id = "ratio"]
@@ -80,6 +93,8 @@ impl Default for Codename206Params {
 impl Default for CompressorParams {
     fn default() -> Self {
         Self {
+            detection: EnumParam::new("Detection", DetectionMode::Peak),
+
             threshold_db: FloatParam::new(
                 "Threshold",
                 -18.0,
@@ -188,6 +203,9 @@ impl Plugin for Codename206 {
                     ui.heading(Self::NAME);
                     ui.separator();
                     egui::Grid::new("params").num_columns(2).show(ui, |ui| {
+                        ui.label("Detection");
+                        ui.add(widgets::ParamSlider::for_param(&params.comp.detection, setter));
+                        ui.end_row();
                         ui.label("Threshold");
                         ui.add(widgets::ParamSlider::for_param(&params.comp.threshold_db, setter));
                         ui.end_row();
@@ -260,6 +278,7 @@ impl Plugin for Codename206 {
             release_coef: Compressor::time_to_coef(c.release_ms.value(), self.sample_rate),
             makeup_db: 0.0,
             lookahead_samples: lookahead,
+            use_rms: c.detection.value() == DetectionMode::Rms,
             bypass: c.bypass.value(),
         };