package agent

import (
	"context"
	"os/exec"
	"strings"
)

// CollectFIB returns the device's reachable prefixes from the kernel FIB — which
// includes FRR-installed routes — by shelling out to `ip route`. The control
// plane uses these to scope which routers enforce a rule. On a host without
// iproute2 it returns nil, and the control plane falls back to over-approximation.
func CollectFIB(ctx context.Context) []string {
	var prefixes []string
	for _, spec := range []struct {
		args []string
		def  string
	}{
		{[]string{"route", "show"}, "0.0.0.0/0"},
		{[]string{"-6", "route", "show"}, "::/0"},
	} {
		out, err := exec.CommandContext(ctx, "ip", spec.args...).Output()
		if err != nil {
			continue
		}
		prefixes = append(prefixes, parseRoutes(string(out), spec.def)...)
	}
	return dedup(prefixes)
}

// routeTypeKeywords are leading tokens in `ip route` output that precede the
// actual destination (e.g. "unreachable 10.0.0.0/8").
var routeTypeKeywords = map[string]bool{
	"unreachable": true, "blackhole": true, "prohibit": true, "throw": true,
	"local": true, "broadcast": true, "multicast": true, "anycast": true, "nat": true,
}

// parseRoutes extracts destination prefixes from `ip route show` output.
// defaultPrefix is substituted for a "default" route (family-specific).
func parseRoutes(output, defaultPrefix string) []string {
	var out []string
	for _, line := range strings.Split(output, "\n") {
		fields := strings.Fields(line)
		if len(fields) == 0 {
			continue
		}
		dst := fields[0]
		// ECMP routes emit continuation "nexthop ..." lines with no destination.
		if dst == "nexthop" {
			continue
		}
		if routeTypeKeywords[dst] {
			if len(fields) < 2 {
				continue
			}
			dst = fields[1]
		}
		if dst == "default" {
			out = append(out, defaultPrefix)
			continue
		}
		out = append(out, normalizePrefix(dst))
	}
	return out
}

// normalizePrefix turns a bare host address into a host prefix (/32 or /128).
func normalizePrefix(dst string) string {
	if strings.Contains(dst, "/") {
		return dst
	}
	if strings.Contains(dst, ":") {
		return dst + "/128"
	}
	return dst + "/32"
}