func (s *Service) Register(args *RegisterArgs, reply *RegisterReply) error {
 s.mu.Lock()
 defer s.mu.Unlock()
 
 server := args.ServerInfo
 for _, server := range s.Servers {
  if server.IPAddress == args.ServerInfo.IPAddress && server.Port == args.ServerInfo.Port {
   reply.Succeed = false
   return nil
  }
 }
 reply.ServerID = len(s.Servers)
 reply.Succeed = true
 s.Servers = append(s.Servers, server)
 
 fmt.Printf("Current registerd servers:\n%v\n", s.Servers)
 
 return nil
}

func (s *Service) GetServers(args *GetServersArgs, reply *GetServersReply) error {
 // return all servers
 reply.ServerInfos = s.Servers
 
 return nil
}

func (s *Server) CallPrepare(allServers []ServerInfo, proposal Proposal) PrepareReply {
 returnedReplies := make([]PrepareReply, 0)
 for _, otherS := range allServers {
  // use a go routine to call every server
  go func(otherS ServerInfo) {
   delay := rand.Intn(10)
   time.Sleep(time.Second * time.Duration(delay))
   args := PrepareArgs{s.Info, proposal.ID}
   reply := PrepareReply{}
   fmt.Printf("【Prepare】Call Prepare on %v:%v with proposal id %v\n", otherS.IPAddress, otherS.Port, args.ProposalID)
   if Call(otherS, "Server.Prepare", &args, &reply) {
    if reply.HasAcceptedProposal {
     fmt.Printf("【Prepare】%v:%v returns accepted proposal: %v\n", otherS.IPAddress, otherS.Port, reply.AcceptedProposal)
    } else {
     fmt.Printf("【Prepare】%v:%v returns empty proposal\n", otherS.IPAddress, otherS.Port)
    }
    s.mu.Lock()
    returnedReplies = append(returnedReplies, reply)
    s.mu.Unlock()
   }
  }(otherS)
 }
 for {
  // wait for responses from majority
  if len(returnedReplies) > (len(allServers))/2.0 {
   checkReplies := returnedReplies
   // three possible response
   // 1. deny the prepare, and return an empty/accepted proposal
   // as the proposal id is not higher than minProposalID on server (proposal id <= server.minProposalID)
   // 2. accept the prepare, and return an empty proposal as the server has not accept any proposal yet
   // 3. accept the prepare, and return an accepted proposal
   // check responses from majority
   // find the response with max proposal id
   acceptedProposal := NewProposal()
   for _, r := range checkReplies {
    // if any response refused the prepare, this server should resend prepare
    if !r.PrepareAccepted {
     return r
    }
    if r.HasAcceptedProposal && r.AcceptedProposal.ID > acceptedProposal.ID {
     acceptedProposal = r.AcceptedProposal
    }
   }
   // if some other server has accepted proposal, return that proposal with max proposal id
   // if no other server has accepted proposal, return an empty proposal
   return PrepareReply{HasAcceptedProposal: !acceptedProposal.IsEmpty(), AcceptedProposal: acceptedProposal, PrepareAccepted: true}
  }
  //fmt.Printf("Waiting for response from majority...\n")
  time.Sleep(time.Second * 1)
 }
}

func (s *Server) Prepare(args *PrepareArgs, reply *PrepareReply) error {
 s.mu.Lock()
 defer s.mu.Unlock()
 // 2 promises and 1 response
 // Promise 1
 // do not accept prepare request which ProposalID <= minProposalID
 // Promise 2
 // do not accept propose request which ProposalID < minProposalID
 // Response 1
 // respond with accepted proposal if any
 if reply.PrepareAccepted = args.ProposalID > s.minProposalID; reply.PrepareAccepted {
  // ready to accept the proposal with Id s.minProposalID
  s.minProposalID = args.ProposalID
 }
 reply.HasAcceptedProposal = s.readAcceptedProposal()
 reply.AcceptedProposal = s.Proposal
 return nil
}

func (s *Server) CallPropose(allServers []ServerInfo, proposal Proposal) ProposeReply {
 returnedReplies := make([]ProposeReply, 0)
 for _, otherS := range allServers {
  go func(otherS ServerInfo) {
   delay := rand.Intn(5000)
   time.Sleep(time.Millisecond * time.Duration(delay))
   args := ProposeArgs{otherS, proposal}
   reply := ProposeReply{}
   fmt.Printf("【Propose】Call Propose on %v:%v with proposal: %v\n", otherS.IPAddress, otherS.Port, args.Proposal)
   if Call(otherS, "Server.Propose", &args, &reply) {
    fmt.Printf("【Propose】%v:%v returns: %v\n", otherS.IPAddress, otherS.Port, reply)
    s.mu.Lock()
    returnedReplies = append(returnedReplies, reply)
    s.mu.Unlock()
   }
  }(otherS)
 }
 for {
  // wait for responses from majority
  if len(returnedReplies) > (len(allServers))/2.0 {
   checkReplies := returnedReplies
   for _, r := range checkReplies {
    if !r.ProposeAccepted {
     return r
    }
   }
   return checkReplies[0]
  }
  time.Sleep(time.Second * 1)
 }
}

func (s *Server) Propose(args *ProposeArgs, reply *ProposeReply) error {
 if s.minProposalID <= args.Proposal.ID {
  s.mu.Lock()
  s.minProposalID = args.Proposal.ID
  s.Proposal = args.Proposal
  s.SaveAcceptedProposal()
  s.mu.Unlock()
 
  reply.ProposeAccepted = true
 }
 
 reply.ProposalID = s.minProposalID
 
 return nil
}