package rings

import "net/netip"

func PrefixToRange(subnet netip.Prefix) (from, to netip.Addr) {
	if u, ok := addrToU32(subnet.Addr()); ok {
		shift := 32 - subnet.Bits()

		m1 := uint32((1 << shift) - 1)
		m0 := uint32(0xffffffff) & ^m1

		u0 := u & m0
		u1 := u0 + m1

		from = u32ToAddr(u0)
		to = u32ToAddr(u1)
	}

	return from, to
}

func u32ToAddr(v uint32) netip.Addr {
	return addrFrom4(
		uint(v>>24)&0xff,
		uint(v>>16)&0xff,
		uint(v>>8)&0xff,
		uint(v)&0xff,
	)
}

func addrToU32(addr netip.Addr) (uint32, bool) {
	switch {
	case !addr.IsValid():
		return 0, false
	case addr.Is4(), addr.Is4In6():
		a4 := addr.As4()
		v := uint32(a4[0])<<24 +
			uint32(a4[1])<<16 +
			uint32(a4[2])<<8 +
			uint32(a4[3])
		return v, true
	default:
		return 0, false
	}
}

func addrFrom4(a, b, c, d uint) netip.Addr {
	return netip.AddrFrom4([4]byte{
		byte(a),
		byte(b),
		byte(c),
		byte(d),
	})
}