//go:build linux

package agent

import (
	"context"
	"errors"
	"fmt"
	"log/slog"
	"net"
	"sync"
	"syscall"
	"time"

	flockv1alpha1 "code.fritzlab.net/fritzlab/flock/pkg/api/v1alpha1"
	"github.com/vishvananda/netlink"
)

// AnycastReconciler keeps the kernel's anycast host routes and BIRD's
// advertised set in sync with (committed allocations × pod Ready).
//
// Lifecycle (per design doc):
//   - CNI ADD assigns anycast IPs to pod lo (already done in netns_linux.go).
//   - Pod transitions to Ready=True → install host /128 (or /32) route at
//     `dev flock<8hex>` and add the IP to BIRD's export filter.
//   - Pod transitions to Ready=False or DELETE → remove kernel route, remove
//     from BIRD export.
//
// Reconcile is idempotent. Triggers: AfterCommit hook, Pod informer
// UpdateFunc on Ready transitions, periodic 2s tick.
type AnycastReconciler struct {
	Node       string
	Store      *Store
	Pods       *PodCache
	NodeConfig *NodeConfigCache
	Bird       *BirdManager
	RouterID   string
	Logger     *slog.Logger

	mu         sync.Mutex
	advertised map[string]anycastTarget // canonical IP → install info
	trigger    chan struct{}
}

// anycastTarget describes the kernel route shape for one advertised
// anycast IP: which veth, and which pod eth0 IP to use as next-hop.
type anycastTarget struct {
	hostIface string
	via       net.IP
}

// NewAnycastReconciler returns a Reconciler ready to Run.
func NewAnycastReconciler(node string, store *Store, pods *PodCache, nc *NodeConfigCache, bird *BirdManager, routerID string, logger *slog.Logger) *AnycastReconciler {
	return &AnycastReconciler{
		Node:       node,
		Store:      store,
		Pods:       pods,
		NodeConfig: nc,
		Bird:       bird,
		RouterID:   routerID,
		Logger:     logger,
		advertised: map[string]anycastTarget{},
		trigger:    make(chan struct{}, 1),
	}
}

// Trigger requests one reconcile pass. Coalesces — if a pass is already
// pending, the call is a no-op.
func (r *AnycastReconciler) Trigger() {
	select {
	case r.trigger <- struct{}{}:
	default:
	}
}

// Run blocks until ctx is cancelled. Reconciles on Trigger or every 2s.
func (r *AnycastReconciler) Run(ctx context.Context) {
	t := time.NewTicker(2 * time.Second)
	defer t.Stop()
	r.reconcile() // initial pass
	for {
		select {
		case <-ctx.Done():
			return
		case <-t.C:
			r.reconcile()
		case <-r.trigger:
			r.reconcile()
		}
	}
}

func (r *AnycastReconciler) reconcile() {
	r.mu.Lock()
	defer r.mu.Unlock()

	desired := r.computeDesired()

	// Install routes that should exist but don't (or whose target changed).
	for ip, t := range desired {
		if cur, ok := r.advertised[ip]; ok && cur.hostIface == t.hostIface && cur.via.Equal(t.via) {
			continue
		}
		if err := installAnycastRoute(ip, t); err != nil {
			r.Logger.Warn("anycast install", "ip", ip, "host", t.hostIface, "via", t.via, "err", err)
			continue
		}
		r.Logger.Info("anycast advertise", "ip", ip, "host", t.hostIface, "via", t.via)
		r.advertised[ip] = t
	}
	// Remove routes that exist but shouldn't.
	for ip, t := range r.advertised {
		if _, want := desired[ip]; !want {
			if err := removeAnycastRoute(ip, t); err != nil {
				r.Logger.Warn("anycast remove", "ip", ip, "host", t.hostIface, "err", err)
			} else {
				r.Logger.Info("anycast withdraw", "ip", ip, "host", t.hostIface)
			}
			delete(r.advertised, ip)
		}
	}

	// Re-render BIRD with the active set.
	r.renderBird(desired)
}

// computeDesired walks the Store and returns the per-ip anycastTarget for
// every anycast advertisement that should be active right now. Each target
// uses the pod's own eth0 IP (same family) as the route's `via` next-hop —
// that way kernel NDP/ARP resolves the eth0 address, which IS configured
// on the pod's eth0, so the pod responds normally without proxy_ndp.
func (r *AnycastReconciler) computeDesired() map[string]anycastTarget {
	out := map[string]anycastTarget{}
	for _, a := range r.Store.Snapshot() {
		if a.State != StateCommitted || len(a.Anycast) == 0 {
			continue
		}
		pod, ok := r.Pods.Get(a.Namespace, a.PodName)
		if !ok || !podReady(pod) {
			continue
		}
		host := HostIfaceName(a.ContainerID)
		via6 := net.ParseIP(a.IP6)
		via4 := net.ParseIP(a.IP4)
		for _, ipStr := range a.Anycast {
			ip := net.ParseIP(ipStr)
			if ip == nil {
				continue
			}
			var via net.IP
			if ip.To4() != nil {
				via = via4
			} else {
				via = via6
			}
			if via == nil {
				r.Logger.Warn("anycast skipped: pod has no unicast IP of same family",
					"pod", a.Namespace+"/"+a.PodName, "anycast", ipStr)
				continue
			}
			out[canonical(ip)] = anycastTarget{hostIface: host, via: via}
		}
	}
	return out
}

func (r *AnycastReconciler) renderBird(desired map[string]anycastTarget) {
	nc := r.NodeConfig.Load()
	if nc == nil || r.Bird == nil {
		return
	}
	var v6, v4 []string
	for ipStr := range desired {
		ip := net.ParseIP(ipStr)
		if ip == nil {
			continue
		}
		if ip.To4() != nil {
			v4 = append(v4, ip.To4().String())
		} else {
			v6 = append(v6, ip.To16().String())
		}
	}
	if err := r.Bird.Render(nc, v6, v4, r.RouterID); err != nil {
		r.Logger.Warn("anycast bird render", "err", err)
	}
}

// installAnycastRoute installs `<ipStr>/<128|32> via t.via dev t.hostIface`.
// Idempotent — RouteReplace overwrites a stale entry.
func installAnycastRoute(ipStr string, t anycastTarget) error {
	ip := net.ParseIP(ipStr)
	if ip == nil {
		return fmt.Errorf("bad ip %q", ipStr)
	}
	link, err := netlink.LinkByName(t.hostIface)
	if err != nil {
		return fmt.Errorf("lookup %s: %w", t.hostIface, err)
	}
	prefix := 128
	if ip.To4() != nil {
		prefix = 32
		ip = ip.To4()
	}
	r := &netlink.Route{
		LinkIndex: link.Attrs().Index,
		Dst:       cidrFor(ip, prefix),
		Gw:        t.via,
		// SCOPE_UNIVERSE — the gateway is on a different "logical" subnet
		// than the local /128 route, but reachable on this veth. Linux is
		// happy as long as the veth has IPv6 forwarding on (it does — set
		// in configureHostSide) and the pod's eth0 has the via address
		// (also true — that's the pod's IP6/IP4 we allocated).
	}
	return netlink.RouteReplace(r)
}

// removeAnycastRoute deletes the host route. Missing routes / interfaces
// are treated as success — DEL paths can race with veth teardown.
func removeAnycastRoute(ipStr string, t anycastTarget) error {
	ip := net.ParseIP(ipStr)
	if ip == nil {
		return nil
	}
	link, err := netlink.LinkByName(t.hostIface)
	if err != nil {
		return nil
	}
	prefix := 128
	if ip.To4() != nil {
		prefix = 32
		ip = ip.To4()
	}
	r := &netlink.Route{
		LinkIndex: link.Attrs().Index,
		Dst:       cidrFor(ip, prefix),
		Gw:        t.via,
	}
	if err := netlink.RouteDel(r); err != nil {
		// ESRCH ("no such process") is netlink-speak for "no such route";
		// treat as success.
		if errors.Is(err, syscall.ESRCH) || linkNotFound(err) {
			return nil
		}
		return err
	}
	return nil
}

// _ = flockv1alpha1 to silence unused import warnings on minimal builds.
var _ = flockv1alpha1.GroupName