web audio analyser's getFloatTimeDomainData buffer offset wrt buffers at other times and wrt buffer of 'complete file'

var soundFile = 'https://mathheadinclouds.github.io/audio/sounds/la.mp3';
var audioContext = null;
var isPlaying = false;
var sourceNode = null;
var analyser = null;
var theBuffer = null;
var reconstructedBuffer = null;
var soundRequest = null;
var loopCounter = -1;
var FFT_SIZE = 2048;
var rafID = null;
var buffers = [];
var timesSamples = [];
var timeSampleDiffs = [];
var leadingWaste = 0;

window.addEventListener('load', function() {
  soundRequest = new XMLHttpRequest();
  soundRequest.open("GET", soundFile, true);
  soundRequest.responseType = "arraybuffer";
  //soundRequest.onload = function(evt) {}
  soundRequest.send();
  var btn = document.createElement('button');
  btn.textContent = 'go';
  btn.addEventListener('click', function(evt) {
    goButtonClick(this, evt)
  });
  document.body.appendChild(btn);
});

function goButtonClick(elt, evt) {
  initAudioContext(togglePlayback);
  elt.parentElement.removeChild(elt);
}

function initAudioContext(callback) {
  audioContext = new AudioContext();
  audioContext.decodeAudioData(soundRequest.response, function(buffer) {
    theBuffer = buffer;
    callback();
  });
}

function createAnalyser() {
  analyser = audioContext.createAnalyser();
  analyser.fftSize = FFT_SIZE;
}

function startWithSourceNode() {
  sourceNode.connect(analyser);
  analyser.connect(audioContext.destination);
  sourceNode.start(0);
  isPlaying = true;
  sourceNode.addEventListener('ended', function(evt) {
    sourceNode = null;
    analyser = null;
    isPlaying = false;
    loopCounter = -1;
    window.cancelAnimationFrame(rafID);
    console.log('buffer length', theBuffer.length);
    console.log('reconstructedBuffer length', reconstructedBuffer.length);
    console.log('audio callback called counter', buffers.length);
    console.log('root mean square error', Math.sqrt(checkResult() / theBuffer.length));
    console.log('lengths of time between requestAnimationFrame callbacks, measured in audio samples:');
    console.log(timeSampleDiffs);
    console.log(
      timeSampleDiffs.filter(function(val) {
        return val === 384
      }).length,
      timeSampleDiffs.filter(function(val) {
        return val === 512
      }).length,
      timeSampleDiffs.filter(function(val) {
        return val === 640
      }).length,
      timeSampleDiffs.filter(function(val) {
        return val === 768
      }).length,
      timeSampleDiffs.filter(function(val) {
        return val === 896
      }).length,
      '*',
      timeSampleDiffs.filter(function(val) {
        return val > 896
      }).length,
      timeSampleDiffs.filter(function(val) {
        return val < 384
      }).length
    );
    console.log(
      timeSampleDiffs.filter(function(val) {
        return val === 384
      }).length +
      timeSampleDiffs.filter(function(val) {
        return val === 512
      }).length +
      timeSampleDiffs.filter(function(val) {
        return val === 640
      }).length +
      timeSampleDiffs.filter(function(val) {
        return val === 768
      }).length +
      timeSampleDiffs.filter(function(val) {
        return val === 896
      }).length
    )
  });
  myAudioCallback();
}

function togglePlayback() {
  sourceNode = audioContext.createBufferSource();
  sourceNode.buffer = theBuffer;
  createAnalyser();
  startWithSourceNode();
}

function myAudioCallback(time) {
  ++loopCounter;
  if (!buffers[loopCounter]) {
    buffers[loopCounter] = new Float32Array(FFT_SIZE);
  }
  var buf = buffers[loopCounter];
  analyser.getFloatTimeDomainData(buf);
  var now = audioContext.currentTime;
  var nowSamp = Math.round(audioContext.sampleRate * now);
  timesSamples[loopCounter] = nowSamp;
  var j, sampDiff;
  if (loopCounter === 0) {
    console.log('start sample: ', nowSamp);
    reconstructedBuffer = new Float32Array(theBuffer.length + FFT_SIZE + nowSamp);
    leadingWaste = nowSamp;
    for (j = 0; j < FFT_SIZE; j++) {
      reconstructedBuffer[nowSamp + j] = buf[j];
    }
  } else {
    sampDiff = nowSamp - timesSamples[loopCounter - 1];
    timeSampleDiffs.push(sampDiff);
    var expectedEqual = FFT_SIZE - sampDiff;
    for (j = 0; j < expectedEqual; j++) {
      if (reconstructedBuffer[nowSamp + j] !== buf[j]) {
        console.error('unexpected error', loopCounter, j);
        // debugger;
      }
    }
    for (j = expectedEqual; j < FFT_SIZE; j++) {
      reconstructedBuffer[nowSamp + j] = buf[j];
    }
    //console.log(loopCounter, nowSamp, sampDiff);
  }
  rafID = window.requestAnimationFrame(myAudioCallback);
}

function checkResult() {
  var ch0 = theBuffer.getChannelData(0);
  var ch1 = theBuffer.getChannelData(1);
  var sum = 0;
  var idxDelta = leadingWaste + FFT_SIZE;
  for (var i = 0; i < theBuffer.length; i++) {
    var samp0 = ch0[i];
    var samp1 = ch1[i];
    var samp = (samp0 + samp1) / 2;
    var check = reconstructedBuffer[i + idxDelta];
    var diff = samp - check;
    var sqDiff = diff * diff;
    sum += sqDiff;
  }
  return sum;
}