package resolver

import (
	"bufio"
	"fmt"
	"os"
	"strings"
)

type NS struct {
	Name string
	Addr string
}

func LoadHints(path string) ([]NS, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, fmt.Errorf("hints file %s not found — download from https://www.internic.net/domain/named.root: %w", path, err)
	}
	defer f.Close()

	var nsNames []string
	glue := make(map[string]string)
	sc := bufio.NewScanner(f)
	for sc.Scan() {
		fields := strings.Fields(sc.Text())
		if len(fields) < 4 || fields[0] == ";" {
			continue
		}
		if fields[0] == "." && fields[2] == "NS" {
			nsNames = append(nsNames, fields[3])
		}
		if fields[2] == "A" {
			glue[fields[0]] = fields[3]
		}
	}
	if err := sc.Err(); err != nil {
		return nil, err
	}

	var hints []NS
	for _, name := range nsNames {
		hints = append(hints, NS{Name: name, Addr: glue[name]})
	}
	if len(hints) == 0 {
		return nil, fmt.Errorf("no root hints parsed from %s", path)
	}
	return hints, nil
}