@sebastbake/music-tempo/src/FFT.ts

Finding out tempo of the music

/**
 * Get Hamming window
 * @param {Number} bufferSize - windows size
 * @return {Array} wnd - Hamming window
 */
export function getHammingWindow(bufferSize: number) {
  const a = 25 / 46;
  const b = 21 / 46;
  const scale = 1 / bufferSize / 0.54;
  const sqrtBufferSize = Math.sqrt(bufferSize);
  const factor = (Math.PI * 2) / bufferSize;
  let wnd: number[] = [];
  for (let i = 0; i < bufferSize; i++) {
    wnd[i] = sqrtBufferSize * (scale * (a - b * Math.cos(factor * i)));
  }
  return wnd;
}
/**
 * Computes FFT and converts the results to magnitude representation
 * @param {Array} re - the real part of the input data and the magnitude of the output data
 * @param {Array} im - the imaginary part of the input data
 */
export function getSpectrum(re: number[], im: number[]) {
  const direction = -1;
  const n = re.length;
  const bits = Math.round(Math.log(n) / Math.log(2));
  const twoPI = Math.PI * 2;
  if (n != 1 << bits) throw new Error("FFT data must be power of 2");
  let localN;
  let j = 0;
  for (let i = 0; i < n - 1; i++) {
    if (i < j) {
      let temp = re[j];
      re[j] = re[i];
      re[i] = temp;
      temp = im[j];
      im[j] = im[i];
      im[i] = temp;
    }
    let k = n / 2;
    while (k >= 1 && k - 1 < j) {
      j = j - k;
      k = k / 2;
    }
    j = j + k;
  }
  for (let m = 1; m <= bits; m++) {
    localN = 1 << m;
    let Wjk_r = 1;
    let Wjk_i = 0;
    let theta = twoPI / localN;
    let Wj_r = Math.cos(theta);
    let Wj_i = direction * Math.sin(theta);
    let nby2 = localN / 2;
    for (j = 0; j < nby2; j++) {
      for (let k = j; k < n; k += localN) {
        let id = k + nby2;
        let tempr = Wjk_r * re[id] - Wjk_i * im[id];
        let tempi = Wjk_r * im[id] + Wjk_i * re[id];
        re[id] = re[k] - tempr;
        im[id] = im[k] - tempi;
        re[k] += tempr;
        im[k] += tempi;
      }
      let wtemp = Wjk_r;
      Wjk_r = Wj_r * Wjk_r - Wj_i * Wjk_i;
      Wjk_i = Wj_r * Wjk_i + Wj_i * wtemp;
    }
  }

  for (let i = 0; i < re.length; i++) {
    let pow = re[i] * re[i] + im[i] * im[i];
    //im[i] = Math.atan2(im[i], re[i]);
    re[i] = pow;
  }

  for (let i = 0; i < re.length; i++) re[i] = Math.sqrt(re[i]);
}