boolean thisGen[][] = new boolean[100][100];
boolean nextGen[][] = new boolean[100][100];

void setup() {
  size(800, 800);
  strokeWeight(1);
  frameRate(100);
  for (int i = 0; i < thisGen.length; i++) {
    for (int j = 0; j < thisGen.length; j++) {
      nextGen[i][j] = false;
      int rand = int(random(5));
      if (rand == 0) {
        thisGen[i][j] = true;
      }
    }
  }
  //glider
  //thisGen[5][5] = true;
  //thisGen[6][5] = true;
  //thisGen[6][3] = true;
  //thisGen[7][4] = true;
  //thisGen[7][5] = true;
}

void draw() {
  for (int i = 0; i < thisGen.length; i++) {
    for (int j = 0; j < thisGen.length; j++) {
      if (thisGen[i][j]) {
        fill(0);
      } else fill(255);
      rect(i*8, j*8, 8, 8);
    }
  }
  for (int i = 1; i < thisGen.length - 1; i++) {
    for (int j = 1; j < thisGen.length - 1; j++) {

      int count = 0;
      if (thisGen[i-1][j-1]) count++;  // top left
      if (thisGen[i][j-1]) count++; //top middle
      if (thisGen[i+1][j-1]) count++; // top right
      if (thisGen[i+1][j]) count++; // middle right
      if (thisGen[i+1][j+1]) count++; //bottom right
      if (thisGen[i][j+1]) count++; //bottom middle
      if (thisGen[i-1][j+1]) count++; //bottom left
      if (thisGen[i-1][j]) count++; // left middle

      if (thisGen[i][j] && count < 2) nextGen[i][j] = false;
      if (thisGen[i][j] && count > 3) nextGen[i][j] = false;
      if (thisGen[i][j] && count == 2) nextGen[i][j] = true;
      if (thisGen[i][j] && count == 3) nextGen[i][j] = true;
      if (!thisGen[i][j] && count == 3) nextGen[i][j] = true;
    }
  }

  for (int i = 0; i < thisGen.length; i++) {
    for (int j = 0; j < thisGen.length; j++) {
      thisGen[i][j] = nextGen[i][j];
    }
  }
}