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              var agents = [];

function make_agent() {
  var a = {
    pos: new vec2(random(), random()),
    vel: vec2.random(0.001),
    size: 1/30,
    max_force: 1/400,
    max_speed: 1/100,
    view_range: 0.3,
for (var i=0; i<100; i++) make_agent();

var target = {
  pos: new vec2(random(), random()),
  pos_prev: new vec2(),
  vel: new vec2(),
  size: 0.1,
function mouse(e, p) {
  if (e == "down") {
    var pos_prev = target.pos.clone();
    target.vel = target.pos.clone().sub(pos_prev);

function update() {
  var targetspd = target.vel.len();
  for (var a of agents) {
    // given the position of the light (target.pos)
    // how do I compute the required steering?
    // target from the agent's point of view
    var rel = target.pos.clone().sub(a.pos);
    // limit this to our top speed
    var desired_velocity = rel;
    // run away?
    if (targetspd > 0.01) desired_velocity.negate();
    for (var n of agents) {
      // don't check self:
      if (a != n) {
        var rel = n.pos.clone().sub(a.pos);
        // where is the target in the agent's field of view?
        var heading = a.vel.angle(); // our direction in radians
        var relview = rel.clone().rotate(-heading);
        // will we collide?
        var in_front = relview[0] > 0;
        var in_range = relview[0] < a.view_range;
        var in_path = Math.abs(relview[1]) < (a.size/2 + n.size/2);
        if (in_front && in_range && in_path) {
          // take evasive action!
          var avoid = new vec2(0, -relview[1]);
    // locomotion:
    // subtract our current velocity
    var steering = desired_velocity.sub(a.vel);
    // forward Euler integration
    //var steering = vec2.random(0.001);
    var acceleration = steering.limit(a.max_force);

function draw() {
  draw2D.circle(target.pos, target.size);
  for (var a of agents) {
      draw2D.circle([0.25, 0.25], 0.2);
      draw2D.circle([0.25,-0.25], 0.2);
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