Arc pen

/*
{
  "id": "hreyajl6hh",
  "label": "Arc Pen",
  "icon": "ノ",
  "parameters": [
    {
      "name": "strokeColor",
      "type": "color",
      "default": "#282a2e"
    },
    {
      "name": "strokeWidth",
      "type": "float",
      "default": 2
    }
  ]
}
*/

let path, lastPos, lastTangent
let guideExtend, guideTangent, guideCircle

const HALF_PI = Math.PI / 2

function drawCircumCircle(A, B, C) {
	const a = B.getDistance(C)
	const b = C.getDistance(A)
	const c = A.getDistance(B)
	
	const a2 = a ** 2
	const b2 = b ** 2
	const c2 = c ** 2
	
	const aLen = a2 * (-a2 + b2 + c2)
  const bLen = b2 * ( a2 - b2 + c2)
  const cLen = c2 * ( a2 + b2 - c2)
  const center = A.multiply(aLen)
  	.add(B.multiply(bLen))
  	.add(C.multiply(cLen))
  	.multiply(1 / (aLen + bLen + cLen))
  
  const radius = A.getDistance(center)
  
  return new Circle(center, radius)
}


function computeMid() {
	const dir = mouse.subtract(lastPos).normalize()
	const dist = mouse.getDistance(lastPos)
	
	const phi = lastTangent.getAngleInRadians(dir) / 2
	
	const mDist = (dist / 2) / Math.cos(phi)
	const mDir = lastTangent.add(dir).normalize()
	
	return lastPos.add(mDir.multiply(mDist))
}

function move() {
	updateGuide()
}


function press() {
	if (!path) {
		lastPos = mouse
	}
}

function drag() {
	if (!path) {
		if (guideTangent) guideTangent.remove()
		
		guideTangent = Guide.addLine(lastPos, mouse, 'stroke')
		return
	}
	updateGuide()	
}


function release() {
	
	if (!path) {
		if (guideTangent) guideTangent.remove()
		
		path = new Path()
		path.strokeWidth = strokeWidth
		path.strokeColor = strokeColor
		path.moveTo(lastPos)
		
		lastTangent =
			mouse.subtract(lastPos).normalize() ||
			new Point(1, 0)
		return
	}
	
	if (guideExtend) guideExtend.remove()
	
	const mid = computeMid()
	path.arcTo(mid, mouse)
	
	lastPos = mouse
	lastTangent = path.getTangentAt(path.length)
}




function updateGuide() {
	if (!path) return
	
	const mid = computeMid()
	
	if (guideExtend) guideExtend.remove()
	guideExtend = new Arc(lastPos, mid, mouse)
	guideExtend.strokeWidth = strokeWidth
	guideExtend.strokeColor = strokeColor
	
	if (guideCircle) guideCircle.remove()
	guideCircle = drawCircumCircle(lastPos, mid, mouse)
	Guide.add(guideCircle)
}

function end() {
	if (guideExtend) guideExtend.remove()
	if (guideTangent) guideTangent.remove()
	if (guideCircle) guideCircle.remove()
	
	path = null;
}