perf: remove per-sample waste from the compressor/limiter hot path

Found by reading the audio-thread code, not by profiling - no profiler is
set up in this project yet. All four are algorithmic or arithmetic wins,
none change the DSP result; the existing tests still pass.

- oversampler: max_true_peak recomputed its circular-history tap index
  (including a % TAPS_PER_PHASE) inside the phase x channel loop, though
  the index depends only on pos/k - 8x redundant. Hoisted to a per-sample
  precomputed index array. This runs unconditionally every sample (it
  feeds the limiter's true-peak detector, not gated by metering), so it
  was the highest-value fix.

- limiter: the sliding-window max rescanned the entire look-ahead ring
  (~73 samples at 48kHz/1.5ms) every sample - O(window) per sample.
  Replaced with an O(1)-amortized monotonic-deque sliding max, backed by
  preallocated ring arrays and RT-safe. Covered by a new test against a
  naive brute-force reference. Also precomputed the constant true-peak
  margin gain in prepare() instead of a powf every sample.

- compressor: replaced per-sample log10/powf(10, x/20) with the
  equivalent ln/exp forms, mirroring nih-plug's own gain_to_db_fast /
  db_to_gain_fast. Called 4x/sample (3 bands + 'All').

- both: replaced % capacity ring-index math (a real integer division,
  since capacity is not a power of two) with a conditional-subtract
  wrap_add helper. Left the oversampler's % TAPS_PER_PHASE alone - that
  divisor is a compile-time constant LLVM already turns into a multiply.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LU1NRL7fAaZTtFMTTPn2b1

src/dsp/compressor.rs

@@ -24,9 +24,20 @@ pub const MAX_LOOKAHEAD_MS: f32 = 5.0;
 /// Most channels we ever process in one frame (our audio layouts are mono/stereo).
 const MAX_CHANNELS: usize = 2;
 
-/// ~ -240 dBFS; keeps `log10` away from zero without affecting audible levels.
+/// ~ -240 dBFS; keeps the level conversion away from zero without affecting audible levels.
 const LEVEL_EPS: f32 = 1e-12;
 
+/// `ln(x) * LOG10_GAIN_TO_DB == log10(x) * 20`, but a bare `ln()` runs faster than `log10()` on
+/// most architectures (mirrors nih-plug's `util::gain_to_db_fast`). Used in the per-sample level
+/// detector, called 4x/sample (once per band + 'All').
+const LOG10_GAIN_TO_DB: f32 = std::f32::consts::LOG10_E * 20.0;
+
+/// `exp(x * DB_TO_GAIN_EXP) == 10.0f32.powf(x / 20.0)`, but `exp()` runs faster than a general
+/// `powf()` on most architectures (mirrors nih-plug's `util::db_to_gain_fast`) — `powf` has to
+/// handle an arbitrary runtime base, while this bakes `ln(10)` in as a constant. Used for the
+/// per-sample wet gain, called 4x/sample.
+const DB_TO_GAIN_EXP: f32 = std::f32::consts::LN_10 / 20.0;
+
 /// Silence-floor anchor for the below-threshold shaping: at/below this level the gain change is 0
 /// (silence stays silence), and the low region fans up/down from here toward the threshold. Matches
 /// the editor gain-curve's display floor.
@@ -230,8 +241,12 @@ impl Compressor {
 
         // 2) Tap positions (circular). Output is always `fixed_delay` old; the detector reads
         //    `l` samples newer than the output, i.e. `l` samples into the output's future.
-        let out_pos = (self.write_pos + self.capacity - self.fixed_delay) % self.capacity;
-        let det_pos = (self.write_pos + self.capacity - (self.fixed_delay - l)) % self.capacity;
+        // `wrap_add` instead of `%`: capacity is rarely a power of two, so a plain modulo forces
+        // an integer division here; a conditional subtract is cheaper and behaves identically
+        // given the add is always <= capacity (true for every offset computed in this function).
+        let out_pos = wrap_add(self.write_pos, self.capacity - self.fixed_delay, self.capacity);
+        let det_pos =
+            wrap_add(self.write_pos, self.capacity - (self.fixed_delay - l), self.capacity);
 
         // 3) Linked peak detector at the look-ahead tap.
         let mut peak = 0.0f32;
@@ -245,7 +260,7 @@ impl Compressor {
         //    active (regardless of mode) so switching peak<->RMS is seamless.
         self.mean_sq = 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();
+        let level_db = (detector + LEVEL_EPS).ln() * LOG10_GAIN_TO_DB;
         // Desired attenuation in dB, as a positive quantity.
         let target = -Self::gain_computer(
             level_db,
@@ -262,7 +277,7 @@ impl Compressor {
         self.y1 = target.max(set.release_coef * self.y1 + (1.0 - set.release_coef) * target);
         self.yl = set.attack_coef * self.yl + (1.0 - set.attack_coef) * self.y1;
 
-        let wet_gain = 10.0f32.powf((set.makeup_db - self.yl) / 20.0);
+        let wet_gain = ((set.makeup_db - self.yl) * DB_TO_GAIN_EXP).exp();
 
         // 5) Dry/wet mix (parallel compression). Both paths use the same delayed input, so the
         //    blend is phase-aligned. mix = 0 -> dry passthrough (clean bypass), mix = 1 -> wet.
@@ -273,7 +288,19 @@ impl Compressor {
         }
 
         // 6) Advance the write head.
-        self.write_pos = (self.write_pos + 1) % self.capacity;
+        self.write_pos = wrap_add(self.write_pos, 1, self.capacity);
+    }
+}
+
+/// Wrap `pos + add` into `[0, capacity)`, assuming `add <= capacity` — a conditional subtract in
+/// place of `%`, since `capacity` is rarely a power of two (so `%` costs a real integer division).
+#[inline]
+fn wrap_add(pos: usize, add: usize, capacity: usize) -> usize {
+    let sum = pos + add;
+    if sum >= capacity {
+        sum - capacity
+    } else {
+        sum
     }
 }
 

src/dsp/limiter.rs

@@ -28,17 +28,97 @@ fn time_to_coef(time_ms: f32, sample_rate: f32) -> f32 {
     }
 }
 
+/// Wrap `pos + add` into `[0, capacity)`, assuming `add <= capacity` — a conditional subtract in
+/// place of `%`, since `capacity` is rarely a power of two (so `%` costs a real integer division).
+#[inline]
+fn wrap_add(pos: usize, add: usize, capacity: usize) -> usize {
+    let sum = pos + add;
+    if sum >= capacity {
+        sum - capacity
+    } else {
+        sum
+    }
+}
+
+/// O(1)-amortized fixed-window sliding maximum (the classic monotonic-deque algorithm), in place
+/// of rescanning the whole window every sample. Values are pushed one at a time; each push returns
+/// the max over the last `window` pushes (inclusive of the one just pushed).
+///
+/// Backed by two preallocated ring buffers sized to `window` (never grows past that: entries are
+/// evicted once their age reaches `window`, and a strictly-decreasing run of un-evicted entries
+/// still ages out within `window` pushes) — so `push` never allocates, keeping it RT-safe.
+struct SlidingMax {
+    val: Vec<f32>,
+    seq: Vec<u64>,
+    cap: usize,
+    head: usize,
+    len: usize,
+    next_seq: u64,
+}
+
+impl SlidingMax {
+    fn new() -> Self {
+        Self { val: Vec::new(), seq: Vec::new(), cap: 0, head: 0, len: 0, next_seq: 0 }
+    }
+
+    fn prepare(&mut self, window: usize) {
+        self.cap = window.max(1);
+        self.val = vec![0.0; self.cap];
+        self.seq = vec![0; self.cap];
+        self.reset();
+    }
+
+    fn reset(&mut self) {
+        self.head = 0;
+        self.len = 0;
+        self.next_seq = 0;
+    }
+
+    /// Push a new value; return the maximum over the last `cap` pushes (including this one).
+    fn push(&mut self, value: f32) -> f32 {
+        let t = self.next_seq;
+
+        // Values can never be the max while the new one is still in the window, so drop them.
+        while self.len > 0 {
+            let back = (self.head + self.len - 1) % self.cap;
+            if self.val[back] <= value {
+                self.len -= 1;
+            } else {
+                break;
+            }
+        }
+
+        let back = (self.head + self.len) % self.cap;
+        self.val[back] = value;
+        self.seq[back] = t;
+        self.len += 1;
+
+        // Evict entries that have aged out of the window.
+        while self.seq[self.head] + self.cap as u64 <= t {
+            self.head = (self.head + 1) % self.cap;
+            self.len -= 1;
+        }
+
+        self.next_seq = t + 1;
+        self.val[self.head]
+    }
+}
+
 pub struct Limiter {
     /// Per-channel audio delay ring.
     delay: Vec<Vec<f32>>,
-    /// Linked `|x|` history, same length as the delay ring (for the sliding maximum).
-    peaks: Vec<f32>,
+    /// O(1)-amortized sliding-window maximum over the linked `|x|` history (replaces an O(window)
+    /// full-rescan every sample).
+    sliding_max: SlidingMax,
     capacity: usize,
     write_pos: usize,
     fixed_delay: usize,
     /// Current smoothed gain (<= 1).
     gain: f32,
     attack_coef: f32,
+    /// `10^(-TRUE_PEAK_MARGIN_DB/20)`, precomputed once (the margin is a fixed constant, so
+    /// there's no reason to recompute this `powf` on every sample).
+    margin_gain: f32,
     /// 4× interpolator for true-peak (inter-sample) detection.
     oversampler: Oversampler,
 }
@@ -47,12 +127,13 @@ impl Default for Limiter {
     fn default() -> Self {
         Self {
             delay: Vec::new(),
-            peaks: Vec::new(),
+            sliding_max: SlidingMax::new(),
             capacity: 0,
             write_pos: 0,
             fixed_delay: 0,
             gain: 1.0,
             attack_coef: 0.0,
+            margin_gain: 1.0,
             oversampler: Oversampler::new(),
         }
     }
@@ -68,9 +149,10 @@ impl Limiter {
         self.fixed_delay = (LOOKAHEAD_MS * 0.001 * sample_rate).ceil() as usize;
         self.capacity = self.fixed_delay + 1;
         self.attack_coef = time_to_coef(ATTACK_MS, sample_rate);
+        self.margin_gain = 10.0f32.powf(-TRUE_PEAK_MARGIN_DB / 20.0);
         let channels = num_channels.clamp(1, MAX_CHANNELS);
         self.delay = vec![vec![0.0; self.capacity]; channels];
-        self.peaks = vec![0.0; self.capacity];
+        self.sliding_max.prepare(self.capacity);
         self.oversampler.prepare(channels);
         self.reset();
     }
@@ -79,7 +161,7 @@ impl Limiter {
         for ch in &mut self.delay {
             ch.iter_mut().for_each(|s| *s = 0.0);
         }
-        self.peaks.iter_mut().for_each(|p| *p = 0.0);
+        self.sliding_max.reset();
         self.oversampler.reset();
         self.write_pos = 0;
         self.gain = 1.0;
@@ -111,21 +193,17 @@ impl Limiter {
         let peak = sample_peak.max(self.oversampler.max_true_peak(&input[..n]));
 
         // Target a hair below the ceiling so the (slightly under-read) true peak stays under it.
-        let target_ceiling = ceiling * 10.0f32.powf(-TRUE_PEAK_MARGIN_DB / 20.0);
+        let target_ceiling = ceiling * self.margin_gain;
 
         // Write into the ring.
         for ch in 0..n {
             self.delay[ch][self.write_pos] = input[ch];
         }
-        self.peaks[self.write_pos] = peak;
 
         // Sliding maximum over the look-ahead window (= the whole ring). Because the oldest sample
         // (the one we output now) is in this window, `ceiling / window_max` applied to it can never
         // exceed the ceiling, and the gain has pre-dropped for any louder sample still to come.
-        let mut window_max = 0.0f32;
-        for &p in &self.peaks {
-            window_max = window_max.max(p);
-        }
+        let window_max = self.sliding_max.push(peak);
         let target = if window_max > target_ceiling {
             target_ceiling / window_max
         } else {
@@ -140,12 +218,14 @@ impl Limiter {
         };
 
         // Output the delayed sample, clamped to the ceiling as the hard guarantee.
-        let out_pos = (self.write_pos + 1) % self.capacity; // oldest sample = fixed_delay ago
+        // Conditional subtract instead of `%`: capacity is rarely a power of two, and the add
+        // here (1) is always <= capacity, so a plain compare-and-subtract is enough.
+        let out_pos = wrap_add(self.write_pos, 1, self.capacity); // oldest sample = fixed_delay ago
         for ch in 0..n {
             output[ch] = (self.delay[ch][out_pos] * self.gain).clamp(-ceiling, ceiling);
         }
 
-        self.write_pos = (self.write_pos + 1) % self.capacity;
+        self.write_pos = wrap_add(self.write_pos, 1, self.capacity);
     }
 }
 
@@ -160,6 +240,31 @@ mod tests {
         time_to_coef(50.0, SR)
     }
 
+    #[test]
+    fn sliding_max_matches_naive_full_rescan() {
+        // A small xorshift-ish PRNG (no extra deps) exercising both rising and falling runs.
+        let mut state = 0x1234_5678u32;
+        let mut next = || {
+            state ^= state << 13;
+            state ^= state >> 17;
+            state ^= state << 5;
+            (state % 1000) as f32 / 1000.0
+        };
+
+        let window = 7usize;
+        let mut sm = SlidingMax::new();
+        sm.prepare(window);
+        let mut history: Vec<f32> = Vec::new();
+
+        for _ in 0..2_000 {
+            let v = next();
+            history.push(v);
+            let start = history.len().saturating_sub(window);
+            let expected = history[start..].iter().cloned().fold(0.0f32, f32::max);
+            assert_eq!(sm.push(v), expected);
+        }
+    }
+
     #[test]
     fn never_exceeds_ceiling_on_spikes() {
         // Mostly silence with occasional large spikes — output must never exceed the ceiling.

src/dsp/oversampler.rs

@@ -97,15 +97,21 @@ impl Oversampler {
             self.history[ch][slot] = input[ch];
         }
 
+        // The tap index only depends on `pos` and `k`, not on `phase` or `ch` — hoist it out of
+        // those loops instead of recomputing (and re-dividing) it PHASES*MAX_CHANNELS times over.
+        let mut idx = [0usize; TAPS_PER_PHASE];
+        for (k, slot) in idx.iter_mut().enumerate() {
+            // k = 0 is the newest sample, increasing k goes back in time.
+            *slot = (self.pos + TAPS_PER_PHASE - k) % TAPS_PER_PHASE;
+        }
+
         let mut peak = 0.0f32;
         for ch in 0..n {
             let hist = &self.history[ch];
             for phase in &self.coeffs {
                 let mut acc = 0.0f32;
-                for (k, &c) in phase.iter().enumerate() {
-                    // k = 0 is the newest sample, increasing k goes back in time.
-                    let idx = (self.pos + TAPS_PER_PHASE - k) % TAPS_PER_PHASE;
-                    acc += c * hist[idx];
+                for (&c, &i) in phase.iter().zip(idx.iter()) {
+                    acc += c * hist[i];
                 }
                 peak = peak.max(acc.abs());
             }

src/lib.rs

@@ -241,7 +241,9 @@ impl Plugin for Codename206 {
             // Drive + compress each band (per-sample smoothed pre-gain & makeup), then sum.
             summed[..n].fill(0.0);
             for b in 0..3 {
-                let pre = util::db_to_gain(band_params[b].pre_gain_db.smoothed.next());
+                // `_fast` skips the below-(-100dB) branch, which pre_gain_db's -24..+36 range
+                // never reaches, in exchange for a cheaper exp() instead of a general powf().
+                let pre = util::db_to_gain_fast(band_params[b].pre_gain_db.smoothed.next());
                 for ch in 0..n {
                     band_in[b][ch] *= pre;
                 }
@@ -272,7 +274,7 @@ impl Plugin for Codename206 {
             }
 
             // 'All' aggregate channel over the summed bands (driven before its compressor).
-            let all_pre = util::db_to_gain(self.params.all.pre_gain_db.smoothed.next());
+            let all_pre = util::db_to_gain_fast(self.params.all.pre_gain_db.smoothed.next());
             for ch in 0..n {
                 summed[ch] *= all_pre;
             }