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int total = 100;
int range = 10;
int player = 50;
void setup() {
  size(640, 480);
  frameRate(5);
}

void draw() {
  background(200);

  fill(0);
  for (int i=0; i<total; i++)
    ellipse(width/(float)total*i, height/2, 5, 5);

  ellipse(width/(float)total*player, height/2, 10, 10);
  int challenger = getChallenger(player, total, range);
  fill(255);
  ellipse(width/(float)total*challenger, height/2, 10, 10);
}

void mousePressed() {
  player = (int)(mouseX/(width/(float)total));
}

int getChallenger(int player, int total, int range) {
  int left = min(player, range);
  int right = min(total-(player+1), range);
  int offset = (int)random(left+right)-left;
  offset += offset<0?0:1;
  return player+offset;
}