package persist

import (
	"fmt"
	"strconv"
	"strings"

	"github.com/drs/gre-panel/internal/link"
)

// networkd file suffixes.
const (
	NetdevSuffix  = ".netdev"
	NetworkSuffix = ".network"
)

// NetdevName is the .netdev file name for a tunnel.
func NetdevName(interfaceName string) string { return interfaceName + NetdevSuffix }

// NetworkName is the .network file name for a tunnel.
func NetworkName(interfaceName string) string { return interfaceName + NetworkSuffix }

// networkdKind maps a tunnel kind onto the NetDev Kind= value. systemd-networkd
// spells the IPv6 variants the same way iproute2 does.
func networkdKind(kind string) string { return kind }

// Netdev renders the <name>.netdev file, which declares the tunnel device
// itself (§9.4).
func (r *Renderer) Netdev(spec link.TunnelSpec) string {
	var b strings.Builder

	b.WriteString(OwnershipMarker + " interface=" + spec.Name + "\n")
	b.WriteString("# This file is generated by gre-panel. Edit the tunnel in the panel; changes made\n")
	b.WriteString("# here are overwritten the next time it is applied.\n")

	b.WriteString("\n[NetDev]\n")
	fmt.Fprintf(&b, "Name=%s\n", spec.Name)
	fmt.Fprintf(&b, "Kind=%s\n", networkdKind(spec.Kind))
	fmt.Fprintf(&b, "Description=GRE Tunnel %s (managed by gre-panel)\n", spec.Name)
	if spec.Mtu > 0 {
		fmt.Fprintf(&b, "MTUBytes=%d\n", spec.Mtu)
	}

	b.WriteString("\n[Tunnel]\n")
	if spec.Local != "" {
		fmt.Fprintf(&b, "Local=%s\n", spec.Local)
	}
	if spec.Remote != "" {
		fmt.Fprintf(&b, "Remote=%s\n", spec.Remote)
	}
	// Without a bind device the tunnel does not hang off any particular link, and
	// networkd needs to be told so explicitly or it never creates the device.
	if spec.BindDevice == "" {
		b.WriteString("Independent=yes\n")
	}
	if link.IsIPv6Kind(spec.Kind) {
		hop := spec.Ttl
		if spec.HopLimit != nil {
			hop = *spec.HopLimit
		}
		if hop > 0 {
			fmt.Fprintf(&b, "TTL=%d\n", hop)
		}
		if spec.EncapLimit != nil {
			fmt.Fprintf(&b, "EncapsulationLimit=%d\n", *spec.EncapLimit)
		}
		if spec.FlowLabel != "" && spec.FlowLabel != "inherit" {
			fmt.Fprintf(&b, "IPv6FlowLabel=%s\n", spec.FlowLabel)
		}
	} else {
		if spec.Ttl > 0 {
			fmt.Fprintf(&b, "TTL=%d\n", spec.Ttl)
		} else {
			// networkd spells a TTL of zero "inherit", matching the kernel.
			b.WriteString("TTL=inherit\n")
		}
	}
	// Stated explicitly for the same reason the ip command states it: "inherit"
	// is a value, not an absence, and leaving it out would mean a fixed TOS of
	// zero on a tunnel configured to copy the inner one.
	if spec.Tos != "" && spec.Tos != "inherit" {
		fmt.Fprintf(&b, "TOS=%s\n", spec.Tos)
	} else {
		b.WriteString("TOS=inherit\n")
	}

	// The keys are written as separate input and output values whenever they
	// differ, because they are independent attributes and collapsing them would
	// quietly change an asymmetric tunnel into a symmetric one.
	switch {
	case spec.IKey != nil && spec.OKey != nil && *spec.IKey == *spec.OKey:
		fmt.Fprintf(&b, "Key=%s\n", strconv.FormatUint(uint64(*spec.IKey), 10))
	default:
		if spec.IKey != nil {
			fmt.Fprintf(&b, "InputKey=%s\n", strconv.FormatUint(uint64(*spec.IKey), 10))
		}
		if spec.OKey != nil {
			fmt.Fprintf(&b, "OutputKey=%s\n", strconv.FormatUint(uint64(*spec.OKey), 10))
		}
	}

	fmt.Fprintf(&b, "DiscoverPathMTU=%s\n", yesNo(spec.IsPathMtuDiscovery))
	if spec.HasInputSequence || spec.HasOutputSequence {
		b.WriteString("SerializeTunneledPackets=yes\n")
	}
	if spec.IsIgnoreDf {
		b.WriteString("IgnoreDontFragment=yes\n")
	}
	return b.String()
}

// Network renders the <name>.network file, which configures the addresses on
// the tunnel device (§9.4).
func (r *Renderer) Network(spec link.TunnelSpec, addresses []link.Address) string {
	var b strings.Builder

	b.WriteString(OwnershipMarker + " interface=" + spec.Name + "\n")
	b.WriteString("# This file is generated by gre-panel. Edit the tunnel in the panel; changes made\n")
	b.WriteString("# here are overwritten the next time it is applied.\n")

	b.WriteString("\n[Match]\n")
	fmt.Fprintf(&b, "Name=%s\n", spec.Name)

	b.WriteString("\n[Network]\n")
	if len(addresses) == 0 {
		b.WriteString("# This tunnel carries no addresses.\n")
	}
	for _, addr := range addresses {
		fmt.Fprintf(&b, "Address=%s\n", addr.String())
	}
	// The panel manages the layer-3 link only. Routing, NAT and firewalling are
	// deliberately out of scope, so nothing else is configured here.
	b.WriteString("ConfigureWithoutCarrier=yes\n")

	for _, addr := range addresses {
		if !addr.NeedsExplicitPeer() {
			continue
		}
		b.WriteString("\n[Address]\n")
		fmt.Fprintf(&b, "Address=%s\n", addr.String())
		fmt.Fprintf(&b, "Peer=%s\n", addr.Peer)
	}
	return b.String()
}

// UnderlayNetworkNote explains why a tunnel with a bind device needs a line on
// that device's own .network file, which the panel does not write because
// touching the underlay's configuration is out of scope.
func UnderlayNetworkNote(interfaceName, bindDevice string) string {
	return fmt.Sprintf("This tunnel is bound to %s. For systemd-networkd to create it, the .network "+
		"file for %s needs Tunnel=%s in its [Network] section. The panel does not write that file, "+
		"because changing the configuration of a physical interface is outside what it manages.",
		bindDevice, bindDevice, interfaceName)
}

func yesNo(v bool) string {
	if v {
		return "yes"
	}
	return "no"
}