// Package dnsname contains helpers for handling DNS names the way Jarl stores
// them: lowercase, fully qualified, with a trailing dot ("www.example.com.").
package dnsname

import (
	"errors"
	"fmt"
	"net/netip"
	"strings"

	"github.com/miekg/dns"
	"golang.org/x/net/idna"
)

// Root is the root domain name.
const Root = "."

// Fqdn lowercases name and makes sure it ends with a dot. An empty name
// becomes the root name.
func Fqdn(name string) string {
	name = strings.ToLower(strings.TrimSpace(name))
	if name == "" || name == "." {
		return Root
	}
	if !strings.HasSuffix(name, ".") || strings.HasSuffix(name, `\.`) && !strings.HasSuffix(name, `\\.`) {
		name += "."
	}
	return name
}

// Display returns name without its trailing dot, which is how names are
// shown to people. The root name is shown as ".".
func Display(name string) string {
	if name == Root || name == "" {
		return Root
	}
	return strings.TrimSuffix(name, ".")
}

// IsSubdomain reports whether child is equal to, or below, parent. Both names
// must be canonical (see Fqdn).
func IsSubdomain(child, parent string) bool {
	if parent == Root {
		return true
	}
	return child == parent || strings.HasSuffix(child, "."+parent)
}

// IsStrictSubdomain reports whether child is below parent (and not equal).
func IsStrictSubdomain(child, parent string) bool {
	return child != parent && IsSubdomain(child, parent)
}

// Parent returns the name with its leftmost label removed. The parent of a
// top-level name is the root, and the root has no parent ("").
func Parent(name string) string {
	if name == Root {
		return ""
	}
	i := strings.IndexByte(name, '.')
	if i < 0 || i == len(name)-1 {
		return Root
	}
	return name[i+1:]
}

// FirstLabel returns the leftmost label of name.
func FirstLabel(name string) string {
	if name == Root {
		return ""
	}
	if i := strings.IndexByte(name, '.'); i >= 0 {
		return name[:i]
	}
	return name
}

// CountLabels returns the number of labels in name (the root has 0).
func CountLabels(name string) int {
	if name == Root {
		return 0
	}
	return strings.Count(strings.TrimSuffix(name, "."), ".") + 1
}

// ToASCII converts an internationalized name to its ASCII (punycode) form.
// Names that are already ASCII are returned lowercased and unchanged.
func ToASCII(name string) (string, error) {
	for i := 0; i < len(name); i++ {
		if name[i] >= 0x80 {
			trailing := strings.HasSuffix(name, ".")
			a, err := idna.Lookup.ToASCII(strings.TrimSuffix(name, "."))
			if err != nil {
				return "", fmt.Errorf("invalid internationalized name %q: %v", name, err)
			}
			if trailing {
				a += "."
			}
			return strings.ToLower(a), nil
		}
	}
	return strings.ToLower(name), nil
}

// Owner resolves a user supplied owner name against zone (canonical FQDN)
// and returns the canonical FQDN.
//
//   - "" and "@" mean the zone apex.
//   - A name ending in "." is absolute.
//   - A name without a trailing dot that equals the zone name, or ends with
//     ".<zone>", is treated as absolute. Nobody wants
//     "www.example.com.example.com.", and this is how most DNS APIs behave.
//   - Anything else is relative to the zone.
func Owner(input, zone string) (string, error) {
	in := strings.TrimSpace(input)
	if in == "" || in == "@" {
		return zone, nil
	}
	// In reverse zones an address stands for its PTR name.
	if IsReverse(zone) {
		if addr, err := netip.ParseAddr(strings.TrimSuffix(in, ".")); err == nil {
			return ReverseName(addr), nil
		}
	}
	a, err := ToASCII(in)
	if err != nil {
		return "", err
	}
	in = a
	if strings.HasSuffix(in, ".") {
		return Fqdn(in), nil
	}
	zoneNoDot := strings.TrimSuffix(zone, ".")
	if zone != Root && (in == zoneNoDot || strings.HasSuffix(in, "."+zoneNoDot)) {
		return Fqdn(in), nil
	}
	if zone == Root {
		return Fqdn(in), nil
	}
	return Fqdn(in + "." + zone), nil
}

// Relative returns name relative to zone: "@" for the apex, the remaining
// labels for names inside the zone, and the FQDN for names outside it.
func Relative(name, zone string) string {
	if name == zone {
		return "@"
	}
	if zone == Root {
		return name
	}
	if strings.HasSuffix(name, "."+zone) {
		return strings.TrimSuffix(name, "."+zone)
	}
	return name
}

// Syntax errors returned by CheckOwner and CheckZoneName.
var (
	ErrEmptyLabel = errors.New("empty label (two dots in a row?)")
	ErrTooLong    = errors.New("name is longer than 255 octets")
)

// CheckOwner validates the syntax of a canonical owner name. Labels may
// contain letters, digits, hyphens, underscores and slashes (RFC 2317 style
// reverse names); a "*" label is allowed only as the leftmost label.
func CheckOwner(name string) error {
	return checkName(name, true)
}

// CheckZoneName validates the syntax of a canonical zone name. It is the same
// as CheckOwner except that wildcards are not allowed.
func CheckZoneName(name string) error {
	return checkName(name, false)
}

func checkName(name string, wildcardOK bool) error {
	if name == Root {
		return nil
	}
	if !strings.HasSuffix(name, ".") {
		return fmt.Errorf("name %q is not fully qualified", name)
	}
	// Wire length: one length octet per label plus the label bytes, plus
	// the terminating zero octet. For a dotted name that's len(name)+1.
	if len(name)+1 > 255 {
		return ErrTooLong
	}
	labels := strings.Split(strings.TrimSuffix(name, "."), ".")
	for i, l := range labels {
		if l == "" {
			return ErrEmptyLabel
		}
		if len(l) > 63 {
			return fmt.Errorf("label %q is longer than 63 octets", l)
		}
		if l == "*" {
			if !wildcardOK {
				return errors.New("wildcard labels are not allowed here")
			}
			if i != 0 {
				return errors.New("a wildcard (*) must be the leftmost label")
			}
			continue
		}
		for j := 0; j < len(l); j++ {
			c := l[j]
			switch {
			case c >= 'a' && c <= 'z', c >= '0' && c <= '9', c == '-', c == '_', c == '/':
			case c >= 'A' && c <= 'Z':
				return fmt.Errorf("label %q must be lowercase", l)
			case c == '*':
				return fmt.Errorf("label %q: '*' is only allowed as a whole leftmost label", l)
			default:
				return fmt.Errorf("label %q contains an invalid character %q", l, c)
			}
		}
	}
	return nil
}

// IsHostname reports whether name follows the RFC 952/1123 host name rules
// that BIND's check-names option enforces: every label starts and ends with
// a letter or digit and contains only letters, digits and hyphens. If
// wildcardOK is set, a leftmost "*" label is accepted. The root name is not
// a host name, except "." used as a "no service" target where callers allow it.
func IsHostname(name string, wildcardOK bool) bool {
	if name == Root || name == "" {
		return false
	}
	labels := dns.SplitDomainName(name)
	for i, l := range labels {
		if i == 0 && wildcardOK && l == "*" {
			continue
		}
		if !isHostLabel(l) {
			return false
		}
	}
	return true
}

func isHostLabel(l string) bool {
	if l == "" || len(l) > 63 {
		return false
	}
	for j := 0; j < len(l); j++ {
		c := l[j]
		alnum := c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z' || c >= '0' && c <= '9'
		if j == 0 || j == len(l)-1 {
			if !alnum {
				return false
			}
		} else if !alnum && c != '-' {
			return false
		}
	}
	return true
}

// IsMailbox reports whether name is a valid mailbox domain name, as BIND's
// check-names applies to the SOA RNAME: the first label may contain any
// printable ASCII character (it is the local part), the rest must be a host
// name.
func IsMailbox(name string) bool {
	labels := dns.SplitDomainName(name)
	if len(labels) < 2 {
		return false
	}
	for _, c := range []byte(labels[0]) {
		if c <= 0x20 || c >= 0x7f {
			return false
		}
	}
	return IsHostname(dns.Fqdn(strings.Join(labels[1:], ".")), false)
}

// MailboxFromEmail converts an email address ("first.last@example.com") to
// the domain-name form used in the SOA RNAME field
// ("first\.last.example.com."). Strings that are already in domain-name form
// (no "@") are returned as an FQDN.
func MailboxFromEmail(s string) (string, error) {
	s = strings.TrimSpace(s)
	at := strings.LastIndexByte(s, '@')
	if at < 0 {
		return Fqdn(s), nil
	}
	local, domain := s[:at], s[at+1:]
	if local == "" || domain == "" {
		return "", fmt.Errorf("invalid email address %q", s)
	}
	local = strings.ReplaceAll(local, `\`, `\\`)
	local = strings.ReplaceAll(local, ".", `\.`)
	return Fqdn(local + "." + domain), nil
}

// EmailFromMailbox converts an SOA RNAME back into email address form.
func EmailFromMailbox(rname string) string {
	labels := dns.SplitDomainName(rname)
	if len(labels) < 2 {
		return Display(rname)
	}
	local := strings.ReplaceAll(labels[0], `\.`, ".")
	return local + "@" + strings.Join(labels[1:], ".")
}

// IsReverse reports whether name is in in-addr.arpa or ip6.arpa.
func IsReverse(name string) bool {
	return IsSubdomain(name, "in-addr.arpa.") || IsSubdomain(name, "ip6.arpa.") || IsSubdomain(name, "ip6.int.")
}

// ReverseName returns the PTR owner name for addr, for example
// "10.2.0.192.in-addr.arpa.".
func ReverseName(addr netip.Addr) string {
	addr = addr.Unmap()
	if addr.Is4() {
		b := addr.As4()
		return fmt.Sprintf("%d.%d.%d.%d.in-addr.arpa.", b[3], b[2], b[1], b[0])
	}
	b := addr.As16()
	var sb strings.Builder
	const hex = "0123456789abcdef"
	for i := 15; i >= 0; i-- {
		sb.WriteByte(hex[b[i]&0xf])
		sb.WriteByte('.')
		sb.WriteByte(hex[b[i]>>4])
		sb.WriteByte('.')
	}
	sb.WriteString("ip6.arpa.")
	return sb.String()
}

// ReverseZone returns the name of the reverse zone covering prefix. IPv4
// prefixes must be /8, /16 or /24 and IPv6 prefixes a multiple of /4; other
// lengths need RFC 2317 style delegation, which must be set up by hand.
func ReverseZone(p netip.Prefix) (string, error) {
	p = p.Masked()
	addr := p.Addr()
	bits := p.Bits()
	if addr.Is4() {
		if bits != 8 && bits != 16 && bits != 24 {
			return "", fmt.Errorf("IPv4 reverse zones must be /8, /16 or /24 (got /%d); use the enclosing /24 zone instead", bits)
		}
		b := addr.As4()
		parts := []string{}
		for i := bits/8 - 1; i >= 0; i-- {
			parts = append(parts, fmt.Sprint(b[i]))
		}
		return strings.Join(parts, ".") + ".in-addr.arpa.", nil
	}
	if bits%4 != 0 || bits == 0 {
		return "", fmt.Errorf("IPv6 reverse zones must be a multiple of /4 (got /%d)", bits)
	}
	full := ReverseName(addr) // 32 nibbles
	nibbles := strings.Split(strings.TrimSuffix(full, ".ip6.arpa."), ".")
	keep := nibbles[len(nibbles)-bits/4:]
	return strings.Join(keep, ".") + ".ip6.arpa.", nil
}

// AddrFromReverse parses a full reverse-lookup owner name back into an
// address. It returns false for names that don't name a single address.
func AddrFromReverse(name string) (netip.Addr, bool) {
	switch {
	case strings.HasSuffix(name, ".in-addr.arpa."):
		parts := strings.Split(strings.TrimSuffix(name, ".in-addr.arpa."), ".")
		if len(parts) != 4 {
			return netip.Addr{}, false
		}
		s := parts[3] + "." + parts[2] + "." + parts[1] + "." + parts[0]
		a, err := netip.ParseAddr(s)
		return a, err == nil
	case strings.HasSuffix(name, ".ip6.arpa."):
		parts := strings.Split(strings.TrimSuffix(name, ".ip6.arpa."), ".")
		if len(parts) != 32 {
			return netip.Addr{}, false
		}
		var b [16]byte
		for i, n := range parts {
			if len(n) != 1 {
				return netip.Addr{}, false
			}
			v := strings.IndexByte("0123456789abcdef", n[0])
			if v < 0 {
				return netip.Addr{}, false
			}
			pos := 31 - i
			if pos%2 == 0 {
				b[pos/2] |= byte(v) << 4
			} else {
				b[pos/2] |= byte(v)
			}
		}
		return netip.AddrFrom16(b), true
	}
	return netip.Addr{}, false
}

// FileSafe turns a zone name into something safe to use as a file name.
func FileSafe(name string) string {
	if name == Root {
		return "root"
	}
	s := strings.TrimSuffix(name, ".")
	var sb strings.Builder
	for _, c := range []byte(s) {
		switch {
		case c >= 'a' && c <= 'z', c >= '0' && c <= '9', c == '-', c == '.', c == '_':
			sb.WriteByte(c)
		default:
			fmt.Fprintf(&sb, "%%%02x", c)
		}
	}
	return sb.String()
}

// BestZone returns the longest name in zones that name falls within, or ""
// if none.
func BestZone(name string, zones []string) string {
	best := ""
	for _, z := range zones {
		if IsSubdomain(name, z) && (best == "" || len(z) > len(best)) {
			best = z
		}
	}
	return best
}