//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.
//
// When more than one Ready pod on this node binds the same anycast IP, the
// kernel route uses RTA_MULTIPATH so the kernel does per-flow ECMP across
// the contributing pods. This is the within-node companion to BGP-level
// ECMP across nodes.
//
// 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{}
}

// 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 nexthop set
	// changed.
	for ip, t := range desired {
		if cur, ok := r.advertised[ip]; ok && cur.equal(t) {
			continue
		}
		if err := installAnycastRoute(ip, t); err != nil {
			r.Logger.Warn("anycast install", "ip", ip, "nexthops", len(t.nexthops), "err", err)
			continue
		}
		r.Logger.Info("anycast advertise", "ip", ip, "nexthops", describeNexthops(t))
		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, "err", err)
			} else {
				r.Logger.Info("anycast withdraw", "ip", ip)
			}
			delete(r.advertised, ip)
		}
	}

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

// computeDesired delegates to the pure resolveAnycastTargets and plugs in
// the live informer-based isReady callback.
func (r *AnycastReconciler) computeDesired() map[string]anycastTarget {
	return resolveAnycastTargets(
		r.Store.Snapshot(),
		func(ns, name string) bool {
			pod, ok := r.Pods.Get(ns, name)
			return ok && podAnycastEligible(pod)
		},
		func(s string) { r.Logger.Warn(s) },
	)
}

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>` pointing at the
// nexthop set in t. With one nexthop the route is a plain via-route;
// with multiple, it's a multipath route using RTA_MULTIPATH so the
// kernel hashes flows across the constituent pods.
//
// 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)
	}
	if len(t.nexthops) == 0 {
		return fmt.Errorf("anycast %s: no nexthops", ipStr)
	}
	prefix := 128
	if ip.To4() != nil {
		prefix = 32
		ip = ip.To4()
	}
	r := &netlink.Route{Dst: cidrFor(ip, prefix)}
	if len(t.nexthops) == 1 {
		// Single nexthop — keep the route shape identical to today's
		// production form. Functionally equivalent to a 1-element
		// MultiPath but `ip route show` renders nicer for operators.
		nh := t.nexthops[0]
		link, err := netlink.LinkByName(nh.hostIface)
		if err != nil {
			return fmt.Errorf("lookup %s: %w", nh.hostIface, err)
		}
		r.LinkIndex = link.Attrs().Index
		r.Gw = nh.via
	} else {
		hops := make([]*netlink.NexthopInfo, 0, len(t.nexthops))
		for _, nh := range t.nexthops {
			link, err := netlink.LinkByName(nh.hostIface)
			if err != nil {
				return fmt.Errorf("lookup %s: %w", nh.hostIface, err)
			}
			hops = append(hops, &netlink.NexthopInfo{
				LinkIndex: link.Attrs().Index,
				Gw:        nh.via,
				Hops:      0,
			})
		}
		r.MultiPath = hops
	}
	return netlink.RouteReplace(r)
}

// removeAnycastRoute deletes the host route. Missing routes / interfaces
// are treated as success — DEL paths can race with veth teardown.
//
// Kernel route deletion matches by destination prefix; we don't need to
// re-specify the nexthop set.
func removeAnycastRoute(ipStr string, _ anycastTarget) error {
	ip := net.ParseIP(ipStr)
	if ip == nil {
		return nil
	}
	prefix := 128
	if ip.To4() != nil {
		prefix = 32
		ip = ip.To4()
	}
	r := &netlink.Route{Dst: cidrFor(ip, prefix)}
	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
}

// describeNexthops returns a compact string for log messages.
func describeNexthops(t anycastTarget) string {
	var s string
	for i, nh := range t.nexthops {
		if i > 0 {
			s += ","
		}
		s += nh.hostIface + "→" + nh.via.String()
	}
	return s
}

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