package netsplit

import (
	`math/big`
	`net`
	"net/netip"

	`github.com/projectdiscovery/mapcidr`
	"go4.org/netipx"
)

/*
	Split splits the network defined in a HostSplitter alongside its configuration and performs the subnetting.
	This strategy attempts to split the network into subnets of equal number of hosts.

	remaining may or may not be nil depending on if the number of hosts can fit cleanly within equal network sizes on boundaries.

	An ErrBadNumHosts will be returned if the number of hosts does not match the *addressable* range in a prefix.
*/
func (h *HostSplitter) Split() (nets []*netip.Prefix, remaining *netipx.IPSet, err error) {

	var tgt *big.Int
	var hosts *big.Int
	var sub netip.Prefix
	var subPtr *netip.Prefix
	var split []*net.IPNet
	var ipsb *netipx.IPSetBuilder = new(netipx.IPSetBuilder)

	if h == nil || h.NumberHosts == 0 || h.BaseSplitter == nil || h.network == nil {
		return
	}

	if split, err = mapcidr.SplitIPNetByNumber(h.network, int(h.NumberHosts)); err != nil {
		return
	}

	tgt = big.NewInt(0)
	tgt.SetUint64(uint64(h.NumberHosts))

	nets = make([]*netip.Prefix, len(split))
	for idx, n := range split {
		sub, _ = netipx.FromStdIPNet(n)
		hosts = mapcidr.CountIPsInCIDR(false, false, n)
		if hosts == nil || tgt.Cmp(hosts) != 0 {
			err = &SplitErr{
				Wrapped:            ErrBadNumHosts,
				Nets:               nets,
				Remaining:          remaining,
				LastSubnet:         &sub,
				RequestedPrefixLen: uint8(sub.Bits()),
			}
			ipsb.AddPrefix(sub)
		} else {
			subPtr = new(netip.Prefix)
			*subPtr = sub
			nets = append(nets, subPtr)
		}

		nets[idx] = new(netip.Prefix)
		*nets[idx] = sub
	}

	if remaining, err = ipsb.IPSet(); err != nil {
		return
	}

	return
}