offseted marker on cubic bezier curve example

<!DOCTYPE html>
<html>
<head>
<title>Cubic bezier curve length</title>
<style>
#figure1 {
  border: 1px solid black;
}

.draggable {
  cursor: move;
}

.control-point {
  opacity: 0.5;
  stroke: none;
  fill: red;
}

.split-point {
  opacity: 0.5;
  stroke: none;
  fill: purple;
}

text {
  font-size: 12px;
}
</style>
</head>
<body>
<svg id="figure1" width="400" height="400">
  <defs>
    <marker id="arrowhead" viewBox="0 0 10 10" refX="40" refY="5"
        markerWidth="10" markerHeight="10" orient="auto">
      <path d="M10 5 0 10 0 8.7 6.8 5.5 0 5.5 0 4.5 6.8 4.5 0 1.3 0 0Z"/>
    </marker>
  </defs>
  <path d="M120 160 35 200 220 260 220 40" stroke="black" fill="none" id="controlPath"/>
  <path d="M120 160C35 200 220 260 220 40" stroke="green" fill="none" id="curve"
      marker-end="url(#arrowhead)"/>
  <circle cx="120" cy="160" r="8" id="p1" class="control-point draggable"/>
  <circle cx="35" cy="200" r="8" id="p2" class="control-point draggable"/>
  <circle cx="220" cy="260" r="8" id="p3" class="control-point draggable"/>
  <circle cx="220" cy="40" r="30" id="p4" class="control-point draggable"/>
  <circle cx="-10" cy="-10" r="8" id="splitPoint" class="split-point"/>

<text x="20" y="20" id="curveLength">curve length: 0</text>
  <text x="120" y="160" dx="10" id="t1">p1: 120/160</text>
  <text x="35" y="200" dx="10" id="t2">p2: 35/200</text>
  <text x="220" y="260" dx="10" id="t3">p3: 220/260</text>
  <text x="220" y="40" dx="10" id="t4">p4: 220/40</text>
</svg>
<script src="bezier-curve.js"></script>
<script>
// This is a port to JavaScript from [Arc length | A Primer on Bézier Curves]( https://pomax.github.io/bezierinfo/#arclength )
// MIT license
var controlPath = document.getElementById('controlPath');
var curve = document.getElementById('curve');
var p1 = document.getElementById('p1');
var p2 = document.getElementById('p2');
var p3 = document.getElementById('p3');
var p4 = document.getElementById('p4');
var dragElem = null;

var svg = document.getElementById('figure1');
svg.addEventListener('mousemove', onMouseMove);
svg.addEventListener('mouseup', onMouseUp);

var elements = [p1, p2, p3, p4];
for (i = 0; i < elements.length; i++) {
  elements[i].addEventListener('mousedown', onMouseDown);
}

var mouseOffsetX, mouseOffsetY;

function getClientPointInSVG(ev) {
  var p, m;

p = svg.createSVGPoint();
  p.x = ev.clientX;
  p.y = ev.clientY;

m = dragElem.getScreenCTM();
  return p.matrixTransform(m.inverse());
}

function onMouseDown(ev) {
  var p;

dragElem = ev.target;
  p = getClientPointInSVG(ev);
  mouseOffsetX = p.x - dragElem.getAttribute('cx');
  mouseOffsetY = p.y - dragElem.getAttribute('cy');
}

function onMouseMove(ev) {
  var p, x, y, circleID, circleTextID, circleTextElem;

if (!dragElem) {
    return;
  }

p = getClientPointInSVG(ev);
  x = p.x - mouseOffsetX;
  y = p.y - mouseOffsetY;
  dragElem.setAttribute('cx', x);
  dragElem.setAttribute('cy', y);

controlPath.setAttribute('d',
    'M' + p1.getAttribute('cx') + ' ' + p1.getAttribute('cy') +
    ' ' + p2.getAttribute('cx') + ' ' + p2.getAttribute('cy') +
    ' ' + p3.getAttribute('cx') + ' ' + p3.getAttribute('cy') +
    ' ' + p4.getAttribute('cx') + ' ' + p4.getAttribute('cy'));
  curve.setAttribute('d',
    'M' + p1.getAttribute('cx') + ' ' + p1.getAttribute('cy') +
    'C' + p2.getAttribute('cx') + ' ' + p2.getAttribute('cy') +
    ' ' + p3.getAttribute('cx') + ' ' + p3.getAttribute('cy') +
    ' ' + p4.getAttribute('cx') + ' ' + p4.getAttribute('cy'));

circleID = dragElem.getAttribute('id');
  circleTextID = circleID.replace(/^p/, 't');
  circleTextElem = document.getElementById(circleTextID);
  circleTextElem.setAttribute('x', x);
  circleTextElem.setAttribute('y', y);
  circleTextElem.firstChild.data = circleID + ': ' + x + '/' + y;
  updateCurveLengthText();
  getParameterForTrim();
}

function onMouseUp(ev) {
  dragElem = null;
}

updateCurveLengthText();

function getParameterForTrim() {
  var curve = new BezierCurve(elements.map(function(elem) {
    return {x: +elem.getAttribute('cx'), y: +elem.getAttribute('cy')};
  }));
  var trimLen = 30;
  var pathTotalLength = curve.getArcLength();
  var s = pathTotalLength - trimLen;
  var t = curve.getParameterAtArcLength(s);
  var pathLen = curve.getArcLength(t);
  var p = curve.getPointAtParameter(t);
  console.log('t', t, 'pathLen', pathLen, 'diff', (pathTotalLength - pathLen));

var splitPointCircleElem = document.getElementById('splitPoint');
  splitPointCircleElem.setAttribute('cx', p.x);
  splitPointCircleElem.setAttribute('cy', p.y);
}

getParameterForTrim();
</script>
</body>
</html>