// Package iprange handles address ranges such as 10.0.10.101-10.0.10.199
// or 2001:db8::100-2001:db8::1ff. BIND matches only addresses and
// prefixes, so Jarl keeps a range as it was written and turns it into the
// exact set of prefixes that covers it when it writes named.conf.
package iprange

import (
	"fmt"
	"math/big"
	"net/netip"
	"strings"
)

// Range is an inclusive range of addresses of one family.
type Range struct {
	Start, End netip.Addr
}

// String returns the range as Jarl writes it: START-END.
func (r Range) String() string { return r.Start.String() + "-" + r.End.String() }

// Parse parses START-END, both full addresses of the same family. ok is
// false when s isn't meant as a range (no "-", or text before it that
// isn't an address, such as the ACL name "branch-office"); err is set when
// it is meant as one but is wrong.
func Parse(s string) (r Range, ok bool, err error) {
	left, right, found := strings.Cut(s, "-")
	if !found {
		return Range{}, false, nil
	}
	left, right = strings.TrimSpace(left), strings.TrimSpace(right)
	if !looksLikeAddr(left) {
		return Range{}, false, nil
	}
	start, err := netip.ParseAddr(left)
	if err != nil {
		return Range{}, true, fmt.Errorf("range %q: %q is not a full IP address", s, left)
	}
	end, err := netip.ParseAddr(right)
	if err != nil {
		if right == "" {
			return Range{}, true, fmt.Errorf("range %q has no end address", s)
		}
		return Range{}, true, fmt.Errorf("range %q: %q is not a full IP address (write both ends in full, e.g. 10.0.10.101-10.0.10.199)", s, right)
	}
	if start.Zone() != "" || end.Zone() != "" {
		return Range{}, true, fmt.Errorf("range %q: addresses with a zone (%%interface) can't be matched", s)
	}
	if start.Is4() != end.Is4() {
		return Range{}, true, fmt.Errorf("range %q mixes IPv4 and IPv6", s)
	}
	if end.Less(start) {
		return Range{}, true, fmt.Errorf("range %q: the start is after the end", s)
	}
	return Range{Start: start, End: end}, true, nil
}

// looksLikeAddr reports whether s could only be meant as an address: it
// has a colon, or it is digits and dots with at least one dot.
func looksLikeAddr(s string) bool {
	if strings.Contains(s, ":") {
		return true
	}
	if !strings.Contains(s, ".") {
		return false
	}
	for i := 0; i < len(s); i++ {
		if c := s[i]; !(c >= '0' && c <= '9' || c == '.') {
			return false
		}
	}
	return true
}

// Prefixes returns the fewest prefixes that together cover exactly the
// range, in address order: 10.0.10.101-10.0.10.199 is 10.0.10.101/32,
// 10.0.10.102/31, 10.0.10.104/29, 10.0.10.112/28, 10.0.10.128/26 and
// 10.0.10.192/29. That is at most 62 prefixes for IPv4 and 254 for IPv6.
func (r Range) Prefixes() []netip.Prefix {
	var out []netip.Prefix
	a := r.Start
	for {
		// The largest prefix that starts at a and ends at or before End.
		p := netip.PrefixFrom(a, a.BitLen())
		for bits := a.BitLen() - 1; bits >= 0; bits-- {
			q := netip.PrefixFrom(a, bits)
			if q.Masked().Addr() != a || r.End.Less(last(q)) {
				break
			}
			p = q
		}
		out = append(out, p)
		end := last(p)
		if end == r.End {
			return out
		}
		a = end.Next()
	}
}

// last returns the last address of a prefix.
func last(p netip.Prefix) netip.Addr {
	b := p.Addr().AsSlice()
	for i := p.Bits(); i < len(b)*8; i++ {
		b[i/8] |= 0x80 >> (i % 8)
	}
	a, _ := netip.AddrFromSlice(b)
	return a
}

// Size returns the number of addresses in the range.
func (r Range) Size() *big.Int {
	s := new(big.Int).SetBytes(r.Start.AsSlice())
	e := new(big.Int).SetBytes(r.End.AsSlice())
	return e.Sub(e, s).Add(e, big.NewInt(1))
}

// SizeText describes the size for people: "99 addresses", or a power of
// two for very large IPv6 ranges ("about 2^80 addresses").
func (r Range) SizeText() string {
	n := r.Size()
	if n.BitLen() <= 40 {
		if n.Int64() == 1 {
			return "1 address"
		}
		return n.String() + " addresses"
	}
	return fmt.Sprintf("about 2^%d addresses", n.BitLen()-1)
}

// Mapped4 is the IPv4-mapped IPv6 space; BIND can see IPv4 clients as
// addresses in it.
var Mapped4 = netip.MustParsePrefix("::ffff:0:0/96")

// TouchesMapped4 reports whether an IPv6 range overlaps ::ffff:0:0/96.
func (r Range) TouchesMapped4() bool {
	if r.Start.Is4() {
		return false
	}
	lo, hi := Mapped4.Addr(), last(Mapped4)
	return !(r.End.Less(lo) || hi.Less(r.Start))
}

// FromPrefix returns the addresses of a prefix as a range.
func FromPrefix(p netip.Prefix) Range {
	p = p.Masked()
	return Range{Start: p.Addr(), End: last(p)}
}