Arduinocade

// FFT from dsp.js, see below
var FFT = function(bufferSize) {
  this.bufferSize   = bufferSize;
  this.spectrum     = new Float32Array(bufferSize/2);
  this.real         = new Float32Array(bufferSize);
  this.imag         = new Float32Array(bufferSize);
  this.reverseTable = new Uint32Array(bufferSize);
  this.sinTable     = new Float32Array(bufferSize);
  this.cosTable     = new Float32Array(bufferSize);

  var limit = 1,
      bit = bufferSize >> 1;

  while ( limit < bufferSize ) {
    for ( var i = 0; i < limit; i++ ) {
      this.reverseTable[i + limit] = this.reverseTable[i] + bit;
    }

    limit = limit << 1;
    bit = bit >> 1;
  }

  for ( var i = 0; i < bufferSize; i++ ) {
    this.sinTable[i] = Math.sin(-Math.PI/i);
    this.cosTable[i] = Math.cos(-Math.PI/i);
  }
};

FFT.prototype.forward = function(buffer) {
  var bufferSize   = this.bufferSize,
      cosTable     = this.cosTable,
      sinTable     = this.sinTable,
      reverseTable = this.reverseTable,
      real         = this.real,
      imag         = this.imag,
      spectrum     = this.spectrum;

  if ( bufferSize !== buffer.length ) {
    throw "Supplied buffer is not the same size as defined FFT. FFT Size: " + bufferSize + " Buffer Size: " + buffer.length;
  }

  for (var i = 0; i < bufferSize; i++) {
    real[i] = buffer[reverseTable[i]];
    imag[i] = 0;
  }

  var halfSize = 1,
      phaseShiftStepReal, 
      phaseShiftStepImag,
      currentPhaseShiftReal,
      currentPhaseShiftImag,
      off,
      tr,
      ti,
      tmpReal,  
      i;

  while ( halfSize < bufferSize ) {
    phaseShiftStepReal = cosTable[halfSize];
    phaseShiftStepImag = sinTable[halfSize];
    currentPhaseShiftReal = 1.0;
    currentPhaseShiftImag = 0.0;

    for ( var fftStep = 0; fftStep < halfSize; fftStep++ ) {
      i = fftStep;

      while ( i < bufferSize ) {
        off = i + halfSize;
        tr = (currentPhaseShiftReal * real[off]) - (currentPhaseShiftImag * imag[off]);
        ti = (currentPhaseShiftReal * imag[off]) + (currentPhaseShiftImag * real[off]);

        real[off] = real[i] - tr;
        imag[off] = imag[i] - ti;
        real[i] += tr;
        imag[i] += ti;

        i += halfSize << 1;
      }

      tmpReal = currentPhaseShiftReal;
      currentPhaseShiftReal = (tmpReal * phaseShiftStepReal) - (currentPhaseShiftImag * phaseShiftStepImag);
      currentPhaseShiftImag = (tmpReal * phaseShiftStepImag) + (currentPhaseShiftImag * phaseShiftStepReal);
    }

    halfSize = halfSize << 1;
  }

  i = bufferSize/2;
  while(i--)
    spectrum[i] = 2 * Math.sqrt(real[i] * real[i] + imag[i] * imag[i]) / bufferSize;
};