envelope.js

import { timeCoef } from './util.js'

// Branching one-pole envelope follower. Returns a stateful per-sample detector.
// Separate attack and release time constants; the classic feed-forward compressor
// topology. Peak or RMS detection selectable.
export function envelope(opts = {}) {
  let sr = opts.sampleRate || 44100
  let aa = timeCoef(opts.attack ?? 5, sr)
  let ar = timeCoef(opts.release ?? 50, sr)
  let detector = opts.detector || 'peak'
  let rmsWin = Math.max(1, opts.rmsWindow || 256)

  let rmsBuf = detector === 'rms' ? new Float32Array(rmsWin) : null
  let rmsSum = 0
  let rmsIdx = 0
  let env = 0

  return function detect(x) {
    let mag
    if (detector === 'rms') {
      let sq = x * x
      rmsSum += sq - rmsBuf[rmsIdx]
      rmsBuf[rmsIdx] = sq
      rmsIdx = (rmsIdx + 1) % rmsWin
      mag = Math.sqrt(Math.max(0, rmsSum / rmsWin))
    } else {
      mag = x < 0 ? -x : x
    }
    let c = mag > env ? aa : ar
    env = c * env + (1 - c) * mag
    return env
  }
}