素数ランダムウォーク

mySketch
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function setup() {
  width = 2000
  height = 2000
  N = 15000
  l = 1.5
  createCanvas(width, height);
  background(230)
  fill(200, 0, 0)
  circle(width / 2, 3 * height / 4, 5)
  
  
  
  //
  //　素数列の準備
  //
  num = []
  prime = []
  for (var i = 2; i < N; i++) {
    num.push(i)
  }
  n = 2
  while (n < Math.sqrt(N)) {
    prime.push(n)
    num = num.filter(function(value) {
      return !(value % n === 0)
    })
    n = num[0]
  }
  prime = prime.concat(num)
  
  
  //
  //　素数ランダムウォーク
  //
  direction = ["N", "E", "S", "W"]
  pre_direction = "N"
  x = width / 2
  y = 3 * height / 4
  
  for (var i = 0; i < N; i++) {
    next_direction = direction.filter(function(value) {
      return !(value === pre_direction)
    })
    if (prime.indexOf(i) > 0) {
      rand = Math.floor(Math.random() * 3)
      switch (next_direction[rand]) {
          //
          //　もと来た道を帰れないので、N-S, E-Wで逆の方向に向かう
          //
        case "N":
          line(x, y, x, y+l)
          y = y+l
          pre_direction = "S"
        break;
        
        case "E":
          line(x, y, x-l, y)
          x = x-l
          pre_direction = "W"
        break;
        
        case "S":
          line(x, y, x, y-l)
          y = y-l
          pre_direction = "N"
        break;
        
        case "W":
          line(x, y, x+l, y)
          x = x+l
          pre_direction = "E"
      }
  
    } else {
      switch (pre_direction) {
        case "N":
          line(x, y, x, y-l)
          y = y-l
        break;
        
        case "E":
          line(x, y, x+l, y)
          x = x+l
        break;
        
        case "S":
          line(x, y, x, y+l)
          y = y+l
        break;
        
        case "W":
          line(x, y, x-l, y)
          x = x-l
      }
    }
  }
}