package model

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

// LinkLocal6 is IPv6's link-local prefix.
var LinkLocal6 = netip.MustParsePrefix("fe80::/10")

// Address is a parsed address entry: one address, a prefix or a range.
type Address struct {
	Prefix     netip.Prefix // a single address is a /32 or /128
	First, End netip.Addr   // a range (First valid only for ranges)
}

// IsV6 reports whether the entry is IPv6.
func (a Address) IsV6() bool {
	if a.First.IsValid() {
		return a.First.Is6()
	}
	return a.Prefix.Addr().Is6()
}

// String writes the entry as nftables and Housecarl write it:
// "10.0.0.5", "10.0.0.0/24", "10.0.0.1-10.0.0.9".
func (a Address) String() string {
	if a.First.IsValid() {
		return a.First.String() + "-" + a.End.String()
	}
	if a.Prefix.IsSingleIP() {
		return a.Prefix.Addr().String()
	}
	return a.Prefix.String()
}

// Contains reports whether the entry holds an address.
func (a Address) Contains(x netip.Addr) bool {
	x = x.Unmap()
	if a.First.IsValid() {
		return x.Is4() == a.First.Is4() && !x.Less(a.First) && !a.End.Less(x)
	}
	return a.Prefix.Contains(x)
}

// Overlaps reports whether the entry and a prefix share an address.
func (a Address) Overlaps(p netip.Prefix) bool {
	if !a.First.IsValid() {
		return a.Prefix.Overlaps(p)
	}
	if a.First.Is4() != p.Addr().Is4() {
		return false
	}
	// The prefix's first and last addresses against the range.
	first := p.Masked().Addr()
	last := first
	b := last.AsSlice()
	for i := p.Bits(); i < len(b)*8; i++ {
		b[i/8] |= 0x80 >> (i % 8)
	}
	last, _ = netip.AddrFromSlice(b)
	return !last.Less(a.First) && !a.End.Less(first)
}

// ParseAddress reads an address, a prefix (host bits must be zero) or a
// range, IPv4 or IPv6.
func ParseAddress(s string) (Address, error) {
	s = strings.TrimSpace(s)
	if a, b, ok := strings.Cut(s, "-"); ok {
		first, err1 := netip.ParseAddr(strings.TrimSpace(a))
		end, err2 := netip.ParseAddr(strings.TrimSpace(b))
		if err1 != nil || err2 != nil || first.Is4() != end.Is4() {
			return Address{}, fmt.Errorf("%q is not an address range (write it as 10.0.0.1-10.0.0.9)", s)
		}
		first, end = first.Unmap(), end.Unmap()
		if end.Less(first) {
			return Address{}, fmt.Errorf("the range %q ends before it starts", s)
		}
		if first == end {
			return Address{Prefix: netip.PrefixFrom(first, first.BitLen())}, nil
		}
		return Address{First: first, End: end}, nil
	}
	if strings.Contains(s, "/") {
		p, err := netip.ParsePrefix(s)
		if err != nil {
			return Address{}, fmt.Errorf("%q is not an address or prefix", s)
		}
		if p.Addr().Is4In6() {
			// Unmapping would leave an IPv4 address with IPv6 bits
			// (::ffff:0:0/96 as 0.0.0.0/96).
			return Address{}, fmt.Errorf("%q is an IPv4-mapped prefix, which can't be used: write IPv4 prefixes as IPv4 (10.0.0.0/8)", s)
		}
		if p.Masked() != p {
			return Address{}, fmt.Errorf("%q has host bits set; the prefix is %s", s, p.Masked())
		}
		return Address{Prefix: p}, nil
	}
	a, err := netip.ParseAddr(s)
	if err != nil || a.Zone() != "" {
		return Address{}, fmt.Errorf("%q is not an address, prefix, range or set name", s)
	}
	a = a.Unmap()
	return Address{Prefix: netip.PrefixFrom(a, a.BitLen())}, nil
}

// IsLiteral reports whether a source/destination reference is an
// address, prefix or range rather than a set name.
func IsLiteral(ref string) bool {
	_, err := ParseAddress(ref)
	return err == nil
}

// NormalizeAddress rewrites an address entry in its canonical form.
func NormalizeAddress(s string) (string, error) {
	a, err := ParseAddress(s)
	if err != nil {
		return "", err
	}
	return a.String(), nil
}