frame-hop/hop.js

Frame slicing for windowed signal processing

"use strict"

// Slices a stream of frames into a stream of overlapping windows
// The size of each frame is the same as the size o
function createHopStream(frame_size, hop_size, onFrame, max_data_size) {
  if(hop_size > frame_size) {
    throw new Error("Hop size must be smaller than frame size")
  }
  max_data_size     = max_data_size || frame_size
  var buffer        = new Float32Array(2*frame_size + max_data_size)
  var ptr           = 0
  var frame_slices  = []
  for(var j=0; j+frame_size<=buffer.length; j+=hop_size) {
    frame_slices.push(buffer.subarray(j, j+frame_size))
  }
  return function processHopData(data) {
    var i, j, k
    buffer.set(data, ptr)
    ptr += data.length
    for(i=0, j=0; j+frame_size<=ptr; ++i, j+=hop_size) {
      onFrame(frame_slices[i])
    }
    for(k=0; j<ptr; ) {
      buffer.set(frame_slices[i], k)
      var nhops = Math.ceil((k+frame_size) / hop_size)|0
      var nptr  = nhops * hop_size
      if(nptr !== k+frame_size) {
        nhops -= 1
        nptr -= hop_size
      }
      i += nhops
      j += (nptr - k)
      k  = nptr
    }
    ptr += k - j
  }
}

module.exports = createHopStream