monitor dp
{
   enum {thinking, hungry, eating} state[5];
   condition self[5];

   void pickup(int i) {
      state[i] = hungry;
      test(i);
      if (state[i] != eating)
         self[i].wait();
   }

   void putdown(int i) {
      state[i] = thinking;
      test( (i+4)%5 );
      test( (i+1)%5 );
   }

   void test(int i) {
      if ((state[(i+4)%5] != eating) &&
          (state[i] == hungry) &&
          (state[(i+1)%5] != eating)) {
         state[i] = eating;
         self[i].signal();
      }
   }

   void init() {
      for (int i = 0; i < 5; i++)
         state[i] = thinking;
   }
}