const dist_between = 50;

const n = 5;

​

let start, end;

​

let drag = null;

​

function setup() {

  createCanvas(400, 400);

  setupPoints();

}

​

function draw() {

  background(220);

  drawPoints();

}

​

function mousePressed() {

  let m = createVector(mouseX, mouseY);

  if(end.dist(m) < 6) {

    drag = end;

  }

  if(start.dist(m) < 6) {

    drag = start;

  }

}

​

function mouseDragged() {

  if(drag) {

    drag.x = mouseX;

    drag.y = mouseY;

    doIK();

  }

}

​

function mouseReleased() {

  drag = null;

}

​

function setupPoints() {

  points = [];

  for(let i = 0; i < n; i ++) {

    points.push(createVector(width/2 - ((n - 1) * dist_between)/2 + i * dist_between, height/2));

  }

  start = createVector(points[0].x, points[0].y);

  end = createVector(points[n-1].x, points[n-1].y);

}

​

function drawPoints() {

  fill(0);

  stroke(0);

  for(let i = 0; i < n; i ++) {

    if(i > 0) {

      line(points[i].x, points[i].y, points[i-1].x, points[i-1].y);

    }

    ellipse(points[i].x, points[i].y, 5, 5);

  }

  fill(255, 0, 0);

  ellipse(end.x, end.y, 6, 6);

  fill(0, 255, 0);

  ellipse(start.x, start.y, 6, 6);

}

​

function doIK() {

  const maxIt = 6;

  let it = 0;

  while(it < maxIt) {

    let s = points[0].equals(start);

    let e = points[n - 1].equals(end);

    if(s && e) {

      break;

    } else if(s) {

      moveTowardsEnd();

    } else if(e) {

      moveTowardsStart();

    } else {

      moveTowardsEnd();

    }

    it ++;

  }

}

​

function moveTowardsStart() {

  points[0].x = start.x;

  points[0].y = start.y;

  for(let i = 1; i < n; i ++) {

    let p = points[i];

    let pre = points[i-1];

    let d = p.dist(pre)

    if(d > dist_between) {

      // move towards pre

      points[i] = moveTowards(p, pre, d - dist_between);

    }

  }

}

​

​

​

function moveTowardsEnd() {

  points[n-1].x = end.x;

  points[n-1].y = end.y;

  for(let i = n-2; i >= 0; i --) {

    let p = points[i];

    let pre = points[i+1];

    let d = p.dist(pre)

    if(d > dist_between) {

      // move towards pre

      points[i] = moveTowards(p, pre, d - dist_between);

    }

  }

}

​

function moveTowards(p, target, d) {

  let b = p5.Vector.sub(target, p);

  b.setMag(d);

  b.add(p);

  return b;

}

​

​

​

​