package resolution

import (
	"fmt"
	"net/netip"
	"slices"
	"sort"
	"strings"

	mcat "git.novox.be/novox/mesh-sdk/go/catalogue"
)

// privateRanges are the only ranges a local network may lie in (ADR 0340 §2): RFC 1918 for IPv4 and the
// unique local addresses for IPv6. A network counts only when it lies wholly inside one of them.
var privateRanges = []struct {
	prefix netip.Prefix
	name   string
}{
	{netip.MustParsePrefix("10.0.0.0/8"), "a private range (RFC 1918, 10.0.0.0/8)"},
	{netip.MustParsePrefix("172.16.0.0/12"), "a private range (RFC 1918, 172.16.0.0/12)"},
	{netip.MustParsePrefix("192.168.0.0/16"), "a private range (RFC 1918, 192.168.0.0/16)"},
	{netip.MustParsePrefix("fc00::/7"), "the IPv6 unique local range (fc00::/7)"},
}

// JudgeLocalNetworks says, for each network a machine reported, whether it counts as its local network.
//
// **A public subnet never counts** (ADR 0340 §2): a network counts only when it lies wholly in a private
// range, so `local-network` can never open a port to the internet — a machine sitting on a public /24 has
// that /24 judged and refused, never rendered. A link-local network does not count either. And only a
// link the machine reported as facing outside it counts (§3): the private network's own link and a
// container bridge are this machine's own, not its local network, whatever their addresses.
//
// Sorted by link and then network, one entry per (link, network), so what `node` shows and what the
// filter is rendered from do not depend on the order a machine happened to say them in.
func JudgeLocalNetworks(attached []mcat.AttachedNetwork, outward []string, tunnel string) []mcat.LocalNetwork {
	seen := map[[2]string]bool{}
	var out []mcat.LocalNetwork
	for _, a := range attached {
		key := [2]string{a.Link, a.Network}
		if seen[key] {
			continue
		}
		seen[key] = true
		out = append(out, judgeOne(a, outward, tunnel))
	}
	sort.Slice(out, func(i, j int) bool {
		if out[i].Link != out[j].Link {
			return out[i].Link < out[j].Link
		}
		return out[i].Network < out[j].Network
	})
	return out
}

func judgeOne(a mcat.AttachedNetwork, outward []string, tunnel string) mcat.LocalNetwork {
	j := mcat.LocalNetwork{Link: a.Link, Network: a.Network}
	prefix, err := netip.ParsePrefix(strings.TrimSpace(a.Network))
	if err != nil {
		j.Why = "not a network the mesh can read (" + err.Error() + ")"
		return j
	}
	prefix = prefix.Masked()
	j.Network = prefix.String()
	switch {
	case tunnel != "" && a.Link == tunnel:
		j.Why = "the private network's own link, not the local network"
		return j
	case !slices.Contains(outward, a.Link):
		j.Why = fmt.Sprintf("%s does not face outside this machine (a bridge or a link of its own)", a.Link)
		return j
	case prefix.Addr().IsLoopback():
		j.Why = "loopback"
		return j
	case prefix.Addr().IsLinkLocalUnicast():
		j.Why = "link-local, never counted"
		return j
	}
	for _, r := range privateRanges {
		if r.prefix.Contains(prefix.Addr()) && prefix.Bits() >= r.prefix.Bits() {
			j.Counts = true
			j.Why = "in " + r.name
			return j
		}
	}
	if prefix.Addr().IsPrivate() {
		// Overlaps a private range without lying wholly inside it (a /7 around 10.0.0.0/8, say).
		j.Why = "not wholly in a private range, so never counted"
		return j
	}
	j.Why = "public, never counted: local-network may not open a port to the internet"
	return j
}

// CountedLocalNetworks is the networks that count, as the filter renders them: sorted, without repeats.
func CountedLocalNetworks(judged []mcat.LocalNetwork) []string {
	var out []string
	for _, j := range judged {
		if j.Counts && !slices.Contains(out, j.Network) {
			out = append(out, j.Network)
		}
	}
	sort.Strings(out)
	return out
}

// LocalNetworkPorts is every rule admitted from `local-network`, with the networks it resolved to.
func LocalNetworkPorts(rules []mcat.Rule, networks []string) []mcat.LocalNetworkPort {
	var out []mcat.LocalNetworkPort
	for _, rule := range rules {
		if rule.From != mcat.FromLocalNetwork {
			continue
		}
		out = append(out, mcat.LocalNetworkPort{Port: rule.Port, Protocol: rule.Protocol,
			Because: rule.Because, Networks: networks})
	}
	return out
}