// Package netdetect chooses which network interfaces rtnetmon should monitor.
//
// The host-specific queries (enumerating interfaces, reading the routing
// table) live behind small data types so the selection logic and the route
// parsers are pure functions that can be unit-tested on any OS with fake
// data. Only the real routing-table query is build-tagged per platform.
package netdetect

import (
	"encoding/hex"
	"errors"
	"fmt"
	"net"
	"sort"
	"strings"
)

// Selection failures. Those needing the offending interface names are
// wrapped with %w at the call site.
var (
	errUnsupportedOS = errors.New("unsupported operating system")
	errOneOfPair     = errors.New(
		"found only one of the configured interfaces; " +
			"rtnetmon needs both, or neither (default route only)")
	errNoDefaultRoute = errors.New(
		"no default route found; rtnetmon needs one internet interface")
	errManyDefaultRoutes = errors.New(
		"multiple default-route interfaces found; rtnetmon supports only one")
	errNoPhysRoute = errors.New(
		"no physical default-route interface found; " +
			"rtnetmon needs one internet interface")
	errManyPhysRoutes = errors.New(
		"multiple physical default-route interfaces found; " +
			"rtnetmon supports only one")
)

// Interface is a network interface reduced to what detection needs.
type Interface struct {
	Name string
	Up   bool
	IPv4 []string
}

// Route is one routing-table entry reduced to what detection needs. Scoped
// marks a macOS interface-scoped route (flag I), which only traffic bound to
// that interface uses.
type Route struct {
	Iface   string
	Gateway string
	Default bool
	Scoped  bool
}

// Pane names one interface to display, with its label.
type Pane struct {
	Name  string
	Label string
}

// Flags carries the user's --iface/--label choices and whether each label was
// set explicitly on the command line.
type Flags struct {
	IfaceA    string
	LabelA    string
	IfaceB    string
	LabelB    string
	LabelASet bool
	LabelBSet bool
}

// Interfaces enumerates the host's interfaces and their IPv4 addresses. This
// uses the standard library and is the same on every platform.
func Interfaces() ([]Interface, error) {
	ifs, err := net.Interfaces()
	if err != nil {
		return nil, fmt.Errorf("listing interfaces: %w", err)
	}

	out := make([]Interface, 0, len(ifs))
	for _, ifi := range ifs {
		var v4 []string

		addrs, _ := ifi.Addrs()
		for _, a := range addrs {
			if n, ok := a.(*net.IPNet); ok && n.IP.To4() != nil {
				v4 = append(v4, n.IP.String())
			}
		}

		out = append(out, Interface{
			Name: ifi.Name,
			Up:   ifi.Flags&net.FlagUp != 0,
			IPv4: v4,
		})
	}

	return out, nil
}

// Select decides which one or two interfaces to monitor for the given OS.
// See the README's supported matrix for the exact rules.
func Select(goos string, ifaces []Interface, routes []Route, f Flags) ([]Pane, error) {
	switch goos {
	case "linux":
		return selectLinux(ifaces, routes, f)
	case "darwin":
		return selectDarwin(ifaces, routes, f)
	default:
		return nil, fmt.Errorf("%w: %q", errUnsupportedOS, goos)
	}
}

// selectLinux keeps today's behavior: if both configured interfaces exist,
// monitor them as two panes; if neither exists, monitor the single
// default-route interface; anything else is an error.
func selectLinux(ifaces []Interface, routes []Route, f Flags) ([]Pane, error) {
	haveA := hasInterface(ifaces, f.IfaceA)
	haveB := hasInterface(ifaces, f.IfaceB)

	switch {
	case haveA && haveB:
		return []Pane{
			{Name: f.IfaceA, Label: f.LabelA},
			{Name: f.IfaceB, Label: f.LabelB},
		}, nil
	case !haveA && !haveB:
		name, err := onlyDefaultRoute(routes)
		if err != nil {
			return nil, err
		}

		return []Pane{{Name: name, Label: singleLabel(f)}}, nil
	default:
		return nil, fmt.Errorf("%w (%q, %q)", errOneOfPair, f.IfaceA, f.IfaceB)
	}
}

// selectDarwin monitors the physical default-route interface, plus a VPN
// tunnel as the primary pane while one is connected.
func selectDarwin(ifaces []Interface, routes []Route, f Flags) ([]Pane, error) {
	vpn := findVPN(ifaces, routes)

	phys, err := onlyPhysicalDefaultRoute(routes, vpn)
	if err != nil {
		return nil, err
	}

	if vpn == "" {
		return []Pane{{Name: phys, Label: singleLabel(f)}}, nil
	}

	return []Pane{
		{Name: vpn, Label: labelOr(f.LabelA, f.LabelASet, "VPN")},
		{Name: phys, Label: labelOr(f.LabelB, f.LabelBSet, "default route")},
	}, nil
}

// findVPN returns the name of the connected VPN tunnel, or "" if there is
// none. A tunnel is the VPN only while it carries the default route; one that
// is merely up, even with a routable address (Tailscale without an exit node,
// or a tunnel a disconnected client left behind), is not. A default route
// scoped to the tunnel does not count: only traffic bound there uses it.
func findVPN(ifaces []Interface, routes []Route) string {
	routeIfaces := map[string]bool{}

	for _, r := range routes {
		if r.Default && !r.Scoped {
			routeIfaces[r.Iface] = true
		}
	}

	sorted := append([]Interface(nil), ifaces...)
	sort.Slice(sorted, func(i, j int) bool { return sorted[i].Name < sorted[j].Name })

	for _, ifi := range sorted {
		if !isTunnel(ifi.Name) {
			continue
		}

		if routeIfaces[ifi.Name] {
			return ifi.Name
		}
	}

	return ""
}

// onlyDefaultRoute returns the single default-route interface, or an error if
// there is not exactly one.
func onlyDefaultRoute(routes []Route) (string, error) {
	names := defaultRouteIfaces(routes, "")

	switch len(names) {
	case 1:
		return names[0], nil
	case 0:
		return "", errNoDefaultRoute
	default:
		return "", fmt.Errorf("%w (%s)",
			errManyDefaultRoutes, strings.Join(names, ", "))
	}
}

// onlyPhysicalDefaultRoute returns the single non-tunnel default-route
// interface (ignoring the VPN), or an error if there is not exactly one.
func onlyPhysicalDefaultRoute(routes []Route, vpn string) (string, error) {
	names := defaultRouteIfaces(routes, vpn)

	switch len(names) {
	case 1:
		return names[0], nil
	case 0:
		return "", errNoPhysRoute
	default:
		return "", fmt.Errorf("%w (%s)",
			errManyPhysRoutes, strings.Join(names, ", "))
	}
}

// defaultRouteIfaces returns the sorted, unique interface names that carry a
// default route, excluding the named VPN interface and any other tunnel.
// Scoped routes count: while a VPN holds the default route, the physical
// interface keeps only a default route scoped to itself.
func defaultRouteIfaces(routes []Route, vpn string) []string {
	seen := map[string]bool{}

	var names []string

	for _, r := range routes {
		if !r.Default || r.Iface == vpn || isTunnel(r.Iface) || seen[r.Iface] {
			continue
		}

		seen[r.Iface] = true
		names = append(names, r.Iface)
	}

	sort.Strings(names)

	return names
}

// hasInterface reports whether an interface with the given name exists.
func hasInterface(ifaces []Interface, name string) bool {
	for _, ifi := range ifaces {
		if ifi.Name == name {
			return true
		}
	}

	return false
}

// isTunnel reports whether the interface name is a macOS userspace tunnel
// (utunN), which is what Mullvad and other WireGuard/OpenVPN clients use.
func isTunnel(name string) bool {
	return strings.HasPrefix(name, "utun")
}

// singleLabel is the label for a lone pane: the explicit --labelA if given,
// otherwise a plain description.
func singleLabel(f Flags) string {
	return labelOr(f.LabelA, f.LabelASet, "default route")
}

// labelOr returns value when it was set explicitly, otherwise fallback.
func labelOr(value string, set bool, fallback string) string {
	if set {
		return value
	}

	return fallback
}

// parseIPRoute reads `ip -4 route show default` output. Every printed line is
// a default route.
func parseIPRoute(out string) []Route {
	var routes []Route

	for _, line := range strings.Split(out, "\n") {
		fields := strings.Fields(line)
		if len(fields) == 0 || fields[0] != "default" {
			continue
		}

		r := Route{Default: true}

		for i := range len(fields) - 1 {
			switch fields[i] {
			case "dev":
				r.Iface = fields[i+1]
			case "via":
				r.Gateway = fields[i+1]
			}
		}

		if r.Iface != "" {
			routes = append(routes, r)
		}
	}

	return routes
}

// parseProcNetRoute reads /proc/net/route. A default route has destination
// 00000000; the gateway is a little-endian hex IPv4 address.
func parseProcNetRoute(out string) []Route {
	var routes []Route

	for i, line := range strings.Split(out, "\n") {
		if i == 0 { // column header
			continue
		}

		fields := strings.Fields(line)
		if len(fields) < 3 || fields[1] != "00000000" {
			continue
		}

		routes = append(routes, Route{
			Iface:   fields[0],
			Gateway: hexToIP(fields[2]),
			Default: true,
		})
	}

	return routes
}

// parseNetstat reads `netstat -rn -f inet` output (macOS); the Netif column
// (field 4) names the interface. A row whose destination is "default" is a
// default route, scoped when its flags include I. A "0/1" row and a "128.0/1"
// row on one interface together also make a default route there: VPN clients
// that leave the physical default in place send all traffic through them.
func parseNetstat(out string) []Route {
	const columns = 4 // Destination, Gateway, Flags, Netif; Expire is optional

	var routes []Route

	var lowHalf []string // interfaces with a 0/1 row

	highHalf := map[string]bool{} // interfaces with a 128.0/1 row

	for _, line := range strings.Split(out, "\n") {
		fields := strings.Fields(line)
		if len(fields) < columns {
			continue
		}

		dest, gateway, flags, iface := fields[0], fields[1], fields[2], fields[3]

		switch dest {
		case "default":
			routes = append(routes, Route{
				Iface:   iface,
				Gateway: gateway,
				Default: true,
				Scoped:  strings.Contains(flags, "I"),
			})
		case "0/1":
			lowHalf = append(lowHalf, iface)
		case "128.0/1":
			highHalf[iface] = true
		}
	}

	for _, iface := range lowHalf {
		if highHalf[iface] {
			routes = append(routes, Route{Iface: iface, Default: true})
		}
	}

	return routes
}

// hexToIP converts a little-endian hex IPv4 address (as found in
// /proc/net/route) to dotted-quad form.
func hexToIP(h string) string {
	b, err := hex.DecodeString(h)
	if err != nil || len(b) != net.IPv4len {
		return ""
	}

	// /proc/net/route stores the address little-endian.
	return net.IPv4(b[3], b[2], b[1], b[0]).String()
}