package resolver

import (
	"context"
	"fmt"
	"math/rand"
	"strings"
	"sync"
	"time"

	"github.com/hashicorp/consul/agent/metadata"
	"github.com/hashicorp/consul/agent/router"
	"google.golang.org/grpc/resolver"
)

var registerLock sync.Mutex

// RegisterWithGRPC registers the ServerResolverBuilder as a grpc/resolver.
// This function exists to synchronize registrations with a lock.
// grpc/resolver.Register expects all registration to happen at init and does
// not allow for concurrent registration. This function exists to support
// parallel testing.
func RegisterWithGRPC(b *ServerResolverBuilder) {
	registerLock.Lock()
	defer registerLock.Unlock()
	resolver.Register(b)
}

// Nodes provides a count of the number of nodes in the cluster. It is very
// likely implemented by serf to return the number of LAN members.
type Nodes interface {
	NumNodes() int
}

// ServerResolverBuilder tracks the current server list and keeps any
// ServerResolvers updated when changes occur.
type ServerResolverBuilder struct {
	// datacenter of the local agent.
	datacenter string
	// scheme used to query the server. Defaults to consul. Used to support
	// parallel testing because gRPC registers resolvers globally.
	scheme string
	// servers is an index of Servers by Server.ID
	servers map[string]*metadata.Server
	// resolvers is an index of connections to the serverResolver which manages
	// addresses of servers for that connection.
	resolvers map[resolver.ClientConn]*serverResolver
	// nodes provides the number of nodes in the cluster.
	nodes Nodes
	// lock for servers and resolvers.
	lock sync.RWMutex
}

var _ resolver.Builder = (*ServerResolverBuilder)(nil)

type Config struct {
	// Datacenter of the local agent.
	Datacenter string
	// Scheme used to connect to the server. Defaults to consul.
	Scheme string
}

func NewServerResolverBuilder(cfg Config, nodes Nodes) *ServerResolverBuilder {
	if cfg.Scheme == "" {
		cfg.Scheme = "consul"
	}
	return &ServerResolverBuilder{
		scheme:     cfg.Scheme,
		datacenter: cfg.Datacenter,
		nodes:      nodes,
		servers:    make(map[string]*metadata.Server),
		resolvers:  make(map[resolver.ClientConn]*serverResolver),
	}
}

// Run periodically reshuffles the order of server addresses  within the
// resolvers to ensure the load is balanced across servers.
//
// TODO: this looks very similar to agent/router.Manager.Start, which is the
// only other caller of ComputeRebalanceTimer. Are the values passed to these
// two functions different enough that we need separate goroutines to rebalance?
// or could we have a single thing handle the timers, and call both rebalance
// functions?
func (s *ServerResolverBuilder) Run(ctx context.Context) {
	// Compute the rebalance timer based on the number of local servers and nodes.
	rebalanceDuration := router.ComputeRebalanceTimer(s.serversInDC(s.datacenter), s.nodes.NumNodes())
	timer := time.NewTimer(rebalanceDuration)

	for {
		select {
		case <-timer.C:
			s.rebalanceResolvers()

			// Re-compute the wait duration.
			newTimerDuration := router.ComputeRebalanceTimer(s.serversInDC(s.datacenter), s.nodes.NumNodes())
			timer.Reset(newTimerDuration)
		case <-ctx.Done():
			timer.Stop()
			return
		}
	}
}

// rebalanceResolvers shuffles the server list for resolvers in all datacenters.
func (s *ServerResolverBuilder) rebalanceResolvers() {
	s.lock.RLock()
	defer s.lock.RUnlock()

	for _, resolver := range s.resolvers {
		// Shuffle the list of addresses using the last list given to the resolver.
		resolver.addrLock.Lock()
		addrs := resolver.addrs
		rand.Shuffle(len(addrs), func(i, j int) {
			addrs[i], addrs[j] = addrs[j], addrs[i]
		})
		// Pass the shuffled list to the resolver.
		resolver.updateAddrsLocked(addrs)
		resolver.addrLock.Unlock()
	}
}

// serversInDC returns the number of servers in the given datacenter.
func (s *ServerResolverBuilder) serversInDC(dc string) int {
	s.lock.RLock()
	defer s.lock.RUnlock()

	var serverCount int
	for _, server := range s.servers {
		if server.Datacenter == dc {
			serverCount++
		}
	}

	return serverCount
}

// ServerForAddr returns server metadata for a server with the specified address.
func (s *ServerResolverBuilder) ServerForAddr(addr string) (*metadata.Server, error) {
	s.lock.RLock()
	defer s.lock.RUnlock()

	for _, server := range s.servers {
		if server.Addr.String() == addr {
			return server, nil
		}
	}
	return nil, fmt.Errorf("failed to find Consul server for address %q", addr)
}

// Build returns a new serverResolver for the given ClientConn. The resolver
// will keep the ClientConn's state updated based on updates from Serf.
func (s *ServerResolverBuilder) Build(target resolver.Target, cc resolver.ClientConn, _ resolver.BuildOption) (resolver.Resolver, error) {
	s.lock.Lock()
	defer s.lock.Unlock()

	// If there's already a resolver for this connection, return it.
	// TODO(streaming): how would this happen since we already cache connections in ClientConnPool?
	if resolver, ok := s.resolvers[cc]; ok {
		return resolver, nil
	}

	// Make a new resolver for the dc and add it to the list of active ones.
	datacenter := strings.TrimPrefix(target.Endpoint, "server.")
	resolver := &serverResolver{
		datacenter: datacenter,
		clientConn: cc,
		close: func() {
			s.lock.Lock()
			defer s.lock.Unlock()
			delete(s.resolvers, cc)
		},
	}
	resolver.updateAddrs(s.getDCAddrs(datacenter))

	s.resolvers[cc] = resolver
	return resolver, nil
}

func (s *ServerResolverBuilder) Scheme() string { return s.scheme }

// AddServer updates the resolvers' states to include the new server's address.
func (s *ServerResolverBuilder) AddServer(server *metadata.Server) {
	s.lock.Lock()
	defer s.lock.Unlock()

	s.servers[server.ID] = server

	addrs := s.getDCAddrs(server.Datacenter)
	for _, resolver := range s.resolvers {
		if resolver.datacenter == server.Datacenter {
			resolver.updateAddrs(addrs)
		}
	}
}

// RemoveServer updates the resolvers' states with the given server removed.
func (s *ServerResolverBuilder) RemoveServer(server *metadata.Server) {
	s.lock.Lock()
	defer s.lock.Unlock()

	delete(s.servers, server.ID)

	addrs := s.getDCAddrs(server.Datacenter)
	for _, resolver := range s.resolvers {
		if resolver.datacenter == server.Datacenter {
			resolver.updateAddrs(addrs)
		}
	}
}

// getDCAddrs returns a list of the server addresses for the given datacenter.
// This method requires that lock is held for reads.
func (s *ServerResolverBuilder) getDCAddrs(dc string) []resolver.Address {
	var addrs []resolver.Address
	for _, server := range s.servers {
		if server.Datacenter != dc {
			continue
		}

		addrs = append(addrs, resolver.Address{
			Addr:       server.Addr.String(),
			Type:       resolver.Backend,
			ServerName: server.Name,
		})
	}
	return addrs
}

// serverResolver is a grpc Resolver that will keep a grpc.ClientConn up to date
// on the list of server addresses to use.
type serverResolver struct {
	// datacenter that can be reached by the clientConn. Used by ServerResolverBuilder
	// to filter resolvers for those in a specific datacenter.
	datacenter string

	// clientConn that this resolver is providing addresses for.
	clientConn resolver.ClientConn

	// close is used by ServerResolverBuilder to remove this resolver from the
	// index of resolvers. It is called by grpc when the connection is closed.
	close func()

	// addrs stores the list of addresses passed to updateAddrs, so that they
	// can be rebalanced periodically by ServerResolverBuilder.
	addrs    []resolver.Address
	addrLock sync.Mutex
}

var _ resolver.Resolver = (*serverResolver)(nil)

// updateAddrs updates this serverResolver's ClientConn to use the given set of
// addrs.
func (r *serverResolver) updateAddrs(addrs []resolver.Address) {
	r.addrLock.Lock()
	defer r.addrLock.Unlock()
	r.updateAddrsLocked(addrs)
}

// updateAddrsLocked updates this serverResolver's ClientConn to use the given
// set of addrs. addrLock must be held by caller.
func (r *serverResolver) updateAddrsLocked(addrs []resolver.Address) {
	// Only pass the first address initially, which will cause the
	// balancer to spin down the connection for its previous first address
	// if it is different. If we don't do this, it will keep using the old
	// first address as long as it is still in the list, making it impossible to
	// rebalance until that address is removed.
	var firstAddr []resolver.Address
	if len(addrs) > 0 {
		firstAddr = []resolver.Address{addrs[0]}
	}
	r.clientConn.UpdateState(resolver.State{Addresses: firstAddr})

	// Call UpdateState again with the entire list of addrs in case we need them
	// for failover.
	r.clientConn.UpdateState(resolver.State{Addresses: addrs})

	r.addrs = addrs
}

func (r *serverResolver) Close() {
	r.close()
}

// ResolveNow is not used
func (*serverResolver) ResolveNow(_ resolver.ResolveNowOption) {}