reverb.fut

def cap [n] (xs: [n]f64) =
  map (\x -> x / f64.maximum xs) xs

def comb' [n] (delay: i64) (intensity: f64) (xs: [n]f64): [n]f64 =
  loop xs = copy xs for i in delay..<n do
  let xs[i] = xs[i] + intensity * xs[i - delay]
  in xs

entry comb delay intensity = map (cap <-< comb' delay intensity)

def allpass [n] (delay: i64) (intensity: f64) (xs: [n]f64): [n]f64 =
  let combed = comb' delay intensity xs
  in loop xs = copy xs for i < n do
     let xs[i] = (-intensity) * xs[i] + (1 - intensity**2) * (if i < delay then 0 else combed[i-delay])
     in xs

def reverb' [n] (xs: [n]f64) =
  let cs1 = comb' 1931 0.7 xs
  let cs2 = comb' 2213 0.7 xs
  let cs3 = comb' 1747 0.7 xs
  let cs4 = comb' 1559 0.7 xs
  in map4 (\c1 c2 c3 c4 -> c1 + c2 + c3 + c4) cs1 cs2 cs3 cs4
     |> allpass 167 0.7
     |> allpass 191 0.7
     |> cap

entry reverb = map reverb'

-- Original audio:

-- > :audio ($loadaudio "priorities.mp3")

-- With comb filter:

-- > :audio (comb 441i64 0.8f64 ($loadaudio "priorities.mp3"))

-- Reverb:

-- > :audio (reverb ($loadaudio "priorities.mp3"))