chroma.coffee

@chromaprint ?= {}

NUM_BANDS = 12

log2 = Math.log(2)

freqToOctave = (freq, base = 440 / 16.0) ->
  Math.log(freq / base) / log2

chroma = (minFreq, maxFreq, frameSize, sampleRate, interpolate) ->
  notes = []
  features = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

  freqToIndex = (frq) -> Math.round(frameSize * frq / sampleRate)
  indexToFreq = (idx) -> (idx * sampleRate) / frameSize
  indexToOctave = (i) -> freqToOctave(indexToFreq(i))

  minIndex = Math.max(1, freqToIndex(minFreq))
  maxIndex = Math.min(frameSize / 2, freqToIndex(maxFreq))

  for i in [minIndex...maxIndex]
    octave = indexToOctave(i)
    notes[i] = NUM_BANDS * (octave - Math.floor(octave))

  (frame) ->
    energy = (i) -> frame[i] ? 0 # null/false/undefined entries are 0
    for i in [minIndex...maxIndex]
      e = energy(i)
      note = notes[i]
      unit = Math.floor(note)
      fraction = note - unit
      if interpolate

        if fraction < 0.5
          unit2  = (unit + NUM_BANDS - 1) % NUM_BANDS
          a = 0.5 + fraction
        else if fraction > 0.5
          unit2 = (unit + 1) % NUM_BANDS
          a = 1.5 - fraction
        else
          unit2 = unit
          a = 1.0
        features[unit]  += e * a
        features[unit2] += e * (1 - a)
      else
        features[unit] += e
    features

@chromaprint.chroma = chroma