package agent

import (
	"context"
	"fmt"
	"log/slog"
	"net"
	"os"
	"path/filepath"

	"k8s.io/client-go/rest"
	"k8s.io/client-go/tools/clientcmd"
)

// SocketPath is the unix socket on which flock-agent serves RPCs from the
// CNI plugin. Mirrors pkg/cni.SocketPath; kept as a separate constant so the
// agent package has no import-cycle on the CNI package.
const SocketPath = "/run/flock/flock.sock"

// Server is the agent's runtime container: state store, kubernetes informers,
// netlink, BIRD, nftables. M1.5 wires the state store, a placeholder unix
// listener, and a NodeConfig informer.
type Server struct {
	Node       string
	Store      *Store
	NodeConfig *NodeConfigCache
	Logger     *slog.Logger
	socket     string
	restCfg    *rest.Config
}

// Config configures NewServer.
type Config struct {
	Node       string
	StatePath  string // typically /var/lib/flock/allocations.json
	Socket     string // typically /run/flock/flock.sock
	Logger     *slog.Logger
	Kubeconfig string // empty => in-cluster config
}

// NewServer constructs a Server. It does NOT start any goroutines; call Run.
func NewServer(cfg Config) (*Server, error) {
	if cfg.Node == "" {
		return nil, fmt.Errorf("Node must be set")
	}
	if cfg.StatePath == "" {
		cfg.StatePath = "/var/lib/flock/allocations.json"
	}
	if cfg.Socket == "" {
		cfg.Socket = SocketPath
	}
	if cfg.Logger == nil {
		cfg.Logger = slog.Default()
	}
	if err := os.MkdirAll(filepath.Dir(cfg.StatePath), 0o750); err != nil {
		return nil, fmt.Errorf("mkdir state dir: %w", err)
	}
	store, err := NewStore(cfg.StatePath, cfg.Node)
	if err != nil {
		return nil, fmt.Errorf("open store: %w", err)
	}

	restCfg, err := loadRestConfig(cfg.Kubeconfig)
	if err != nil {
		return nil, fmt.Errorf("load kube config: %w", err)
	}

	return &Server{
		Node:       cfg.Node,
		Store:      store,
		NodeConfig: &NodeConfigCache{},
		Logger:     cfg.Logger,
		socket:     cfg.Socket,
		restCfg:    restCfg,
	}, nil
}

func loadRestConfig(kubeconfig string) (*rest.Config, error) {
	if kubeconfig != "" {
		return clientcmd.BuildConfigFromFlags("", kubeconfig)
	}
	return rest.InClusterConfig()
}

// Run starts the agent and blocks until ctx is cancelled. M1.5 opens the
// unix listener, starts the NodeConfig informer, and waits. The RPC handler
// is still a no-op until M2.
func (s *Server) Run(ctx context.Context) error {
	if err := os.MkdirAll(filepath.Dir(s.socket), 0o750); err != nil {
		return fmt.Errorf("mkdir socket dir: %w", err)
	}
	_ = os.Remove(s.socket)
	l, err := net.Listen("unix", s.socket)
	if err != nil {
		return fmt.Errorf("listen %s: %w", s.socket, err)
	}
	defer l.Close()

	s.Logger.Info("flock-agent started",
		"node", s.Node,
		"socket", s.socket,
		"allocations", len(s.Store.Snapshot()),
	)

	// Accept loop: closes every connection immediately (M2 will dispatch).
	go func() {
		for {
			conn, err := l.Accept()
			if err != nil {
				return
			}
			_ = conn.Close()
		}
	}()

	// NodeConfig informer. Any error from the informer terminates Run.
	errCh := make(chan error, 1)
	go func() {
		errCh <- StartNodeConfigInformer(ctx, s.restCfg, s.Node, s.NodeConfig, s.Logger)
	}()

	select {
	case <-ctx.Done():
		s.Logger.Info("flock-agent stopping")
		return nil
	case err := <-errCh:
		return fmt.Errorf("informer: %w", err)
	}
}