compressor.js

import { envelope } from './envelope.js'
import { writer, concat, db2lin, lin2db } from './util.js'

// Feed-forward soft-knee compressor (Giannoulis-Massberg topology).
// Envelope → log domain → gain curve → apply to input.
export default function compressor(data, opts) {
  if (!(data instanceof Float32Array)) return writer(compressorStream(data))
  let s = compressorStream(opts)
  return concat(s.write(data), s.flush())
}

export function compressorStream(opts = {}) {
  let threshold = opts.threshold ?? -20
  let ratio = opts.ratio ?? 4
  let knee = opts.knee ?? 6
  let makeupDb = opts.makeup ?? 0
  let env = envelope(opts)

  return {
    write(chunk) {
      let out = new Float32Array(chunk.length)
      for (let i = 0; i < chunk.length; i++) {
        let x = chunk[i]
        let rDb = compressorGain(lin2db(env(x)), threshold, ratio, knee)
        out[i] = x * db2lin(rDb + makeupDb)
      }
      return out
    },
    flush() { return new Float32Array(0) }
  }
}

// Soft-knee downward compression curve. Returns gain reduction in dB (≤ 0).
// Below (T - W/2): no compression. Above (T + W/2): full ratio. In knee: quadratic.
export function compressorGain(levelDb, threshold, ratio, kneeDb) {
  let d = levelDb - threshold
  if (d < -kneeDb / 2) return 0
  if (d > kneeDb / 2) return -(d * (1 - 1 / ratio))
  let x = d + kneeDb / 2
  return -(1 - 1 / ratio) * x * x / (2 * kneeDb)
}