Length of a cubic bezier curve

cubic-bezier-length.ts

function pointOnBezierCurve(
  start: number[],
  cp1: number[],
  cp2: number[],
  end: number[],
  t: number
) {
  let [ax, ay] = start
  let [bx, by] = cp1
  let [cx, cy] = cp2
  let [dx, dy] = end

  ax += (bx - ax) * t
  bx += (cx - bx) * t
  cx += (dx - cx) * t
  ax += (bx - ax) * t
  bx += (cx - bx) * t
  ay += (by - ay) * t
  by += (cy - by) * t
  cy += (dy - cy) * t
  ay += (by - ay) * t
  by += (cy - by) * t

  return vec.add([ax, ay], vec.mul(vec.sub([bx, by], [ax, ay]), t))
}

function getBezierCurveLength(
  start: number[],
  cp1: number[],
  cp2: number[],
  end: number[],
  ptCount = 40
) {
  let totDist = 0
  let prev = start

  for (var i = 1; i < ptCount; i++) {
    var pt = pointOnBezierCurve(start, cp1, cp2, end, i / ptCount)
    totDist += vec.dist(pt, prev)
    prev = pt
  }

  totDist += vec.dist(prev, end)
  return totDist
}