package agent

import (
	"net"
	"sort"
)

// anycastNexthop is one (host-side veth, pod-eth0-IP) pair the kernel route
// can use as a multipath nexthop.
type anycastNexthop struct {
	hostIface string
	via       net.IP
}

// anycastTarget describes the kernel route shape for one advertised anycast
// IP. When more than one Ready pod on this node binds the same anycast IP,
// every Ready pod contributes a nexthop and the kernel does per-flow ECMP
// across them.
//
// nexthops is sorted by canonical(via) for deterministic comparison and
// stable kernel-route ordering across reconcile passes — the
// AnycastReconciler skips kernel writes when the new and old targets are
// equal, which only works if the slice order is stable.
type anycastTarget struct {
	nexthops []anycastNexthop
}

// equal reports whether two targets describe the same kernel route.
// Both sides are expected to be sorted (the canonical constructor sorts).
func (t anycastTarget) equal(o anycastTarget) bool {
	if len(t.nexthops) != len(o.nexthops) {
		return false
	}
	for i := range t.nexthops {
		if t.nexthops[i].hostIface != o.nexthops[i].hostIface {
			return false
		}
		if !t.nexthops[i].via.Equal(o.nexthops[i].via) {
			return false
		}
	}
	return true
}

// resolveAnycastTargets walks the committed allocation set and returns the
// desired kernel-route shape for every anycast IP that has at least one
// Ready local pod binding it. Multiple Ready pods sharing the same anycast
// IP collapse into a single multi-nexthop target so the kernel can
// per-flow ECMP across them.
//
// Pure: no kernel calls, no informer access. Pods are surfaced via the
// isReady callback so the reconciler can plug in its informer; tests can
// pass any function that satisfies the signature.
//
// warn is invoked for human-facing skip reasons (e.g. anycast with no
// unicast of same family). nil-safe — pass nil to silently drop.
func resolveAnycastTargets(
	allocations []Allocation,
	isReady func(namespace, name string) bool,
	warn func(string),
) map[string]anycastTarget {
	if warn == nil {
		warn = func(string) {}
	}
	out := map[string]anycastTarget{}
	for _, a := range allocations {
		if a.State != StateCommitted || len(a.Anycast) == 0 {
			continue
		}
		if !isReady(a.Namespace, a.PodName) {
			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 {
				warn("anycast " + ipStr + " skipped: pod " +
					a.Namespace + "/" + a.PodName +
					" has no unicast of same family")
				continue
			}
			key := canonical(ip)
			t := out[key]
			t.nexthops = append(t.nexthops, anycastNexthop{hostIface: host, via: via})
			out[key] = t
		}
	}
	// Sort each target's nexthops for stable comparison + stable kernel
	// ordering. Sort key is canonical(via) — sufficient for stability
	// because (host, via) pairs are 1:1 (one veth per pod, one v6+v4 per
	// pod, so via uniquely identifies the nexthop).
	for k, t := range out {
		sort.Slice(t.nexthops, func(i, j int) bool {
			return canonical(t.nexthops[i].via) < canonical(t.nexthops[j].via)
		})
		out[k] = t
	}
	return out
}