int g_size = 10;
color background_color = color (80, 80, 220);
color runner = color (255, 50, 50);
color visited_color = color(220, 240, 240);
color done_color = color (100, 160, 250);
int c_size;

Cell[][] cell;
ArrayList<Cell> done = new ArrayList<Cell>();
ArrayList<Cell> visit = new ArrayList<Cell>();
Cell run_cell;

void setup() {
  size(600, 600);
  frameRate(20);
  smooth(4);
  strokeCap(ROUND);
  c_size = max(width/g_size, height/g_size);
  cell = new Cell[g_size][g_size];
  for (int i = 0; i < g_size; i++) {
    for (int j = 0; j < g_size; j++) {
      cell[i][j] = new Cell(i, j);
    }
  }
  for (int i = 0; i < g_size; i++) {
    for (int j = 0; j < g_size; j++) {
      cell[i][j].add_neighbor();
    }
  }
  run_cell = cell[0][0];
  visit.add(run_cell);
}

void draw() {
  background(background_color);
  for (int i = 0; i < g_size; i++) {
    for (int j = 0; j < g_size; j++) {
      cell[i][j].draw_cell();
      cell[i][j].draw_wall();
    }
  }
  if (visit.size() < g_size*g_size) {
    if (run_cell.check_sides()) {
      Cell chosen = run_cell.pick_neighbor();
      done.add(run_cell);
      run_cell.stacked = true;
      if (chosen.i - run_cell.i == 1) {
        run_cell.wall[1] = false;
        chosen.wall[3] = false;
      } else if (chosen.i - run_cell.i == -1) {
        run_cell.wall[3] = false;
        chosen.wall[1] = false;
      } else if (chosen.j - run_cell.j == 1) {
        run_cell.wall[2] = false;
        chosen.wall[0] = false;
      } else {
        run_cell.wall[0] = false;
        chosen.wall[2] = false;
      }
      run_cell.current = false;
      run_cell = chosen;
      run_cell.current = true;
      run_cell.visited = true;
    } else if (done.size()>0) {
      run_cell.current = false;
      run_cell = done.remove(done.size()-1);
      run_cell.stacked = false;
      run_cell.current = true;
    }
  }
}

class Cell {
  ArrayList<Cell> neighbor;
  boolean visited, stacked, current;
  boolean[] wall;
  int i, j;

  Cell(int _i, int _j) {
    i = _i;
    j = _j;
    wall = new boolean[]{true,true,true,true};
  }

  Cell pick_neighbor() {
    ArrayList<Cell> unvisited = new ArrayList<Cell>();
     for(int i = 0;  i < neighbor.size(); i++){
      Cell nb = neighbor.get(i);
     if(nb.visited == false) unvisited.add(nb);
    }
    return unvisited.get(floor(random(unvisited.size())));
  }

  void add_neighbor() {
    neighbor = new ArrayList<Cell>();
    if(i>0){neighbor.add(cell[i-1][j]);}
    if(i<g_size-1){neighbor.add(cell[i+1][j]);}
    if(j>0){neighbor.add(cell[i][j-1]);}
    if(j<g_size-1){neighbor.add(cell[i][j+1]);}
  }

    boolean check_sides() {
      for(int i = 0;  i < neighbor.size(); i++){
      Cell nb = neighbor.get(i);
     if(!nb.visited) return true;
    }
    return false;
  }

  void draw_cell() {
    noStroke();
    noFill();
    if(current) fill(runner);
    else if(stacked) fill(done_color);
    else if(visited) fill(visited_color);
    rect(j*c_size,i*c_size,c_size,c_size);
  }

  void draw_wall() {
    stroke(0);
    strokeWeight(5);
    if(wall[0]) line(j*c_size, i*c_size, j*c_size, (i+1)* c_size);
    if(wall[1]) line(j*c_size, (i+1)*c_size, (j+1)*c_size, (i+1)*c_size);
    if(wall[2]) line((j+1)*c_size, (i+1)*c_size, (j+1)*c_size, i*c_size);
    if(wall[3]) line((j+1)*c_size, i*c_size, j*c_size, i*c_size);
  }
}