//go:build linux

package resolve

import (
	"bufio"
	"io"
	"os"
	"strings"

	"git.ktf.ninja/tabledevil/gists/projects/go-tools/go/goipgrep/internal/normalize"
)

type NeighborTable struct {
	ByMAC map[string][]string
}

func LoadNeighborTable() (*NeighborTable, error) {
	f, err := os.Open("/proc/net/arp")
	if err != nil {
		return nil, err
	}
	defer f.Close()
	return parseProcNetARP(f), nil
}

func ResolveMACs(macs []string) ([]MACResolution, error) {
	tab, err := LoadNeighborTable()
	if err != nil {
		return nil, err
	}
	var res []MACResolution
	for _, mac := range macs {
		res = append(res, MACResolution{MAC: mac, IPs: append([]string(nil), tab.ByMAC[mac]...)})
	}
	return res, nil
}

type MACResolution struct {
	MAC string   `json:"mac"`
	IPs []string `json:"ips,omitempty"`
}

func parseProcNetARP(r io.Reader) *NeighborTable {
	nt := &NeighborTable{ByMAC: make(map[string][]string)}
	sc := bufio.NewScanner(r)
	first := true
	for sc.Scan() {
		line := strings.TrimSpace(sc.Text())
		if line == "" {
			continue
		}
		if first {
			// header
			first = false
			continue
		}
		fields := strings.Fields(line)
		if len(fields) < 4 {
			continue
		}
		ip := fields[0]
		macRaw := fields[3]
		mac, ok := normalize.NormalizeMAC(macRaw)
		if !ok {
			continue
		}
		// Skip incomplete/placeholder entries.
		if mac == "00:00:00:00:00:00" {
			continue
		}
		nt.ByMAC[mac] = append(nt.ByMAC[mac], ip)
	}
	return nt
}