package namedconf

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

// AMKind is the kind of an address match list element.
type AMKind int

const (
	AMPrefix AMKind = iota // an address or prefix
	AMKey                  // key <name>
	AMName                 // an ACL name (or built-in ACL)
	AMNested               // { ... }
	AMOther                // anything else (e.g. geoip), passed through
)

// Builtin ACL names.
var BuiltinACLs = map[string]bool{"any": true, "none": true, "localhost": true, "localnets": true}

// AMElement is one element of an address match list.
type AMElement struct {
	Negated bool
	Kind    AMKind
	Prefix  netip.Prefix // AMPrefix
	Name    string       // AMKey, AMName
	Nested  []AMElement  // AMNested
	Raw     string       // AMOther
}

// String returns the element in canonical named.conf syntax, without the
// trailing semicolon.
func (e AMElement) String() string {
	var s string
	switch e.Kind {
	case AMPrefix:
		if e.Prefix.IsSingleIP() {
			s = e.Prefix.Addr().String()
		} else {
			s = e.Prefix.String()
		}
	case AMKey:
		s = "key " + quoteIfNeeded(e.Name)
	case AMName:
		s = quoteIfNeeded(e.Name)
	case AMNested:
		parts := make([]string, len(e.Nested))
		for i, n := range e.Nested {
			parts[i] = n.String() + ";"
		}
		s = "{ " + strings.Join(parts, " ") + " }"
	case AMOther:
		s = e.Raw
	}
	if e.Negated {
		return "!" + s
	}
	return s
}

// IsBuiltin reports whether the element is one of the built-in ACL names.
func (e AMElement) IsBuiltin() bool { return e.Kind == AMName && BuiltinACLs[e.Name] }

func quoteIfNeeded(s string) string {
	for i := 0; i < len(s); i++ {
		c := s[i]
		if !(c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z' || c >= '0' && c <= '9' || c == '-' || c == '_' || c == '.') {
			return QuoteString(s)
		}
	}
	if s == "" {
		return `""`
	}
	return s
}

// ParseAMList parses the statements of an address match list block.
func ParseAMList(b *Block) ([]AMElement, error) {
	var out []AMElement
	for _, st := range b.Stmts {
		e, err := parseAMItems(st.Items)
		if err != nil {
			return nil, &Error{Pos: st.Pos, Msg: err.Error()}
		}
		out = append(out, e)
	}
	return out, nil
}

// ParseAMElement parses a single element given as text, such as
// "!192.168.1.0/24", "key xfer" or "trusted".
func ParseAMElement(s string) (AMElement, error) {
	s = strings.TrimSpace(strings.TrimSuffix(strings.TrimSpace(s), ";"))
	if s == "" {
		return AMElement{}, errors.New("empty element")
	}
	st, err := ParseStatement(s)
	if err != nil {
		var pe *Error
		if errors.As(err, &pe) {
			return AMElement{}, errors.New(pe.Msg)
		}
		return AMElement{}, err
	}
	return parseAMItems(st.Items)
}

// ParseAMListString parses a whole list given as "{ a; b; }" or "a; b".
func ParseAMListString(s string) ([]AMElement, error) {
	s = strings.TrimSpace(s)
	s = strings.TrimSuffix(s, ";")
	s = strings.TrimSpace(s)
	if !strings.HasPrefix(s, "{") {
		s = "{ " + s + "; }"
	}
	st, err := ParseStatement("x " + s)
	if err != nil {
		return nil, err
	}
	b := st.FirstBlock()
	if b == nil || len(st.Items) != 2 {
		return nil, errors.New("malformed address match list")
	}
	return ParseAMList(b)
}

func parseAMItems(items []Item) (AMElement, error) {
	var e AMElement
	if len(items) > 0 && items[0].Bang {
		e.Negated = true
		items = items[1:]
	}
	if len(items) == 0 {
		return e, errors.New("empty address match element")
	}
	first := items[0]
	if first.Block != nil {
		if len(items) != 1 {
			return e, errors.New("unexpected text after nested address match list")
		}
		nested, err := ParseAMList(first.Block)
		if err != nil {
			return e, err
		}
		e.Kind = AMNested
		e.Nested = nested
		return e, nil
	}
	if first.Bang {
		return e, errors.New("double negation")
	}
	word := first.Value.Text
	lw := strings.ToLower(word)
	if !first.Value.Quoted && lw == "key" {
		if len(items) != 2 || items[1].Value == nil {
			return e, errors.New("'key' must be followed by a key name")
		}
		e.Kind = AMKey
		e.Name = strings.ToLower(strings.TrimSuffix(items[1].Value.Text, "."))
		return e, nil
	}
	if len(items) > 1 {
		// geoip and other multi-word elements are passed through.
		if lw == "geoip" {
			e.Kind = AMOther
			e.Raw = Compact(items)
			return e, nil
		}
		return e, fmt.Errorf("unexpected text after %q", word)
	}
	if !first.Value.Quoted || BuiltinACLs[lw] {
		if p, ok, err := parsePrefix(word); ok {
			if err != nil {
				return e, err
			}
			e.Kind = AMPrefix
			e.Prefix = p
			return e, nil
		}
	}
	if BuiltinACLs[lw] {
		e.Kind = AMName
		e.Name = lw
		return e, nil
	}
	if err := CheckACLName(word); err != nil {
		return e, err
	}
	e.Kind = AMName
	e.Name = word
	return e, nil
}

// CheckACLName validates the name of a user-defined ACL.
func CheckACLName(name string) error {
	if name == "" {
		return errors.New("ACL name is empty")
	}
	if len(name) > 64 {
		return errors.New("ACL name is longer than 64 characters")
	}
	for i := 0; i < len(name); i++ {
		c := name[i]
		if !(c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z' || c >= '0' && c <= '9' || c == '-' || c == '_' || c == '.') {
			return fmt.Errorf("ACL name %q may only contain letters, digits, '-', '_' and '.'", name)
		}
	}
	if c := name[0]; c >= '0' && c <= '9' {
		// Avoid names that look like addresses.
		if _, ok, _ := parsePrefix(name); ok {
			return fmt.Errorf("ACL name %q looks like an address", name)
		}
	}
	return nil
}

// parsePrefix parses an address or prefix as BIND does, including the IPv4
// shorthand where trailing zero octets are omitted ("10/8", "192.168/16").
// ok is false if s doesn't look like an address at all. Prefixes with host
// bits set are rejected, matching named's "address/prefix length mismatch".
func parsePrefix(s string) (netip.Prefix, bool, error) {
	addrPart, lenPart, hasLen := strings.Cut(s, "/")
	if strings.Contains(addrPart, ":") {
		a, err := netip.ParseAddr(addrPart)
		if err != nil {
			return netip.Prefix{}, true, fmt.Errorf("invalid IPv6 address %q", addrPart)
		}
		if a.Zone() != "" {
			return netip.Prefix{}, true, fmt.Errorf("zoned IPv6 address %q is not allowed", addrPart)
		}
		return finishPrefix(s, a, lenPart, hasLen, 128)
	}
	// IPv4 (possibly shorthand). Must be digits and dots only.
	if addrPart == "" {
		return netip.Prefix{}, false, nil
	}
	for i := 0; i < len(addrPart); i++ {
		if c := addrPart[i]; !(c >= '0' && c <= '9' || c == '.') {
			return netip.Prefix{}, false, nil
		}
	}
	octets := strings.Split(addrPart, ".")
	if len(octets) > 4 {
		return netip.Prefix{}, true, fmt.Errorf("invalid IPv4 address %q", addrPart)
	}
	if len(octets) < 4 && !hasLen {
		return netip.Prefix{}, true, fmt.Errorf("invalid IPv4 address %q (abbreviated addresses need a prefix length)", addrPart)
	}
	var b [4]byte
	for i, o := range octets {
		if o == "" || len(o) > 3 {
			return netip.Prefix{}, true, fmt.Errorf("invalid IPv4 address %q", addrPart)
		}
		n, err := strconv.Atoi(o)
		if err != nil || n > 255 {
			return netip.Prefix{}, true, fmt.Errorf("invalid IPv4 address %q", addrPart)
		}
		b[i] = byte(n)
	}
	return finishPrefix(s, netip.AddrFrom4(b), lenPart, hasLen, 32)
}

func finishPrefix(orig string, a netip.Addr, lenPart string, hasLen bool, max int) (netip.Prefix, bool, error) {
	bits := max
	if hasLen {
		n, err := strconv.Atoi(lenPart)
		if err != nil || n < 0 || n > max {
			return netip.Prefix{}, true, fmt.Errorf("invalid prefix length in %q", orig)
		}
		bits = n
	}
	p := netip.PrefixFrom(a, bits)
	if p.Masked() != p {
		return netip.Prefix{}, true, fmt.Errorf("%q: address/prefix length mismatch (host bits are set; did you mean %s?)", orig, p.Masked())
	}
	return p, true, nil
}

// ParseAddr parses a plain IP address (no prefix) as used by forwarders,
// primaries and similar options.
func ParseAddr(s string) (netip.Addr, error) {
	a, err := netip.ParseAddr(s)
	if err != nil {
		return netip.Addr{}, fmt.Errorf("invalid IP address %q", s)
	}
	return a, nil
}

// AMRefs returns the ACL names and key names referenced by elements,
// including nested lists. Built-in ACLs are not included.
func AMRefs(elems []AMElement) (acls, keys []string) {
	var walk func([]AMElement)
	walk = func(es []AMElement) {
		for _, e := range es {
			switch e.Kind {
			case AMName:
				if !BuiltinACLs[e.Name] {
					acls = append(acls, e.Name)
				}
			case AMKey:
				keys = append(keys, e.Name)
			case AMNested:
				walk(e.Nested)
			}
		}
	}
	walk(elems)
	return acls, keys
}

// Shadowed returns human-readable warnings for elements that can never
// match because an earlier element always matches first (BIND uses first
// match). Only address/prefix elements and "any" are considered.
func Shadowed(elems []AMElement) []string {
	var warns []string
	for i, e := range elems {
		if e.Kind != AMPrefix && e.Kind != AMName {
			continue
		}
		for j := 0; j < i; j++ {
			prev := elems[j]
			if prev.Kind == AMName && prev.Name == "any" {
				warns = append(warns, fmt.Sprintf("element %q never matches: %q before it already matches every address", e.String(), prev.String()))
				break
			}
			if prev.Kind == AMPrefix && e.Kind == AMPrefix &&
				prev.Prefix.Addr().Is4() == e.Prefix.Addr().Is4() &&
				prev.Prefix.Bits() <= e.Prefix.Bits() && prev.Prefix.Contains(e.Prefix.Addr()) {
				warns = append(warns, fmt.Sprintf("element %q never matches: %q before it already covers it (first match wins; put the more specific element first)", e.String(), prev.String()))
				break
			}
		}
	}
	return warns
}

// ContainsAny reports whether elems contain a non-negated "any" before any
// negated element would restrict it, which makes the list match everyone.
// It is used for security lints (open resolvers, open transfers).
func ContainsAny(elems []AMElement) bool {
	for _, e := range elems {
		if e.Negated {
			return false
		}
		if e.Kind == AMName && e.Name == "any" {
			return true
		}
		if e.Kind == AMPrefix && e.Prefix.Bits() == 0 {
			return true
		}
	}
	return false
}