package firecracker

import (
	"context"
	"encoding/binary"
	"fmt"
	"net/netip"
	"os/exec"
	"strings"
)

const (
	defaultNetworkCIDR       = "172.16.0.0/16"
	defaultNetworkPrefixBits = 30
	defaultInterfaceID       = "net0"
	defaultTapPrefix         = "fctap"
)

// NetworkAllocation describes the concrete host-local network values assigned to a machine
type NetworkAllocation struct {
	InterfaceID string
	TapName     string
	HostCIDR    netip.Prefix
	GuestCIDR   netip.Prefix
	GatewayIP   netip.Addr
	GuestMAC    string
}

// GuestIP returns the guest IP address.
func (n NetworkAllocation) GuestIP() netip.Addr {
	return n.GuestCIDR.Addr()
}

// NetworkAllocator allocates /30 tap networks to machines.
type NetworkAllocator struct {
	basePrefix netip.Prefix
}

// NewNetworkAllocator returns a new /30 allocator rooted at the provided IPv4
// prefix.
func NewNetworkAllocator(cidr string) (*NetworkAllocator, error) {
	cidr = strings.TrimSpace(cidr)
	if cidr == "" {
		cidr = defaultNetworkCIDR
	}

	prefix, err := netip.ParsePrefix(cidr)
	if err != nil {
		return nil, fmt.Errorf("parse network cidr %q: %w", cidr, err)
	}
	prefix = prefix.Masked()
	if !prefix.Addr().Is4() {
		return nil, fmt.Errorf("network cidr %q must be IPv4", cidr)
	}
	if prefix.Bits() > defaultNetworkPrefixBits {
		return nil, fmt.Errorf("network cidr %q must be no more specific than /%d", cidr, defaultNetworkPrefixBits)
	}

	return &NetworkAllocator{basePrefix: prefix}, nil
}

// Allocate chooses the first free /30 network not present in used.
func (a *NetworkAllocator) Allocate(used []NetworkAllocation) (NetworkAllocation, error) {
	if a == nil {
		return NetworkAllocation{}, fmt.Errorf("network allocator is required")
	}

	allocated := make(map[netip.Addr]struct{}, len(used))
	for _, network := range used {
		if network.GuestIP().IsValid() {
			allocated[network.GuestIP()] = struct{}{}
		}
	}

	totalSubnets := 1 << uint(defaultNetworkPrefixBits-a.basePrefix.Bits())
	for i := range totalSubnets {
		network, err := a.networkForIndex(i)
		if err != nil {
			return NetworkAllocation{}, err
		}
		if _, exists := allocated[network.GuestIP()]; exists {
			continue
		}
		return network, nil
	}

	return NetworkAllocation{}, fmt.Errorf("network cidr %q is exhausted", a.basePrefix)
}

func (a *NetworkAllocator) networkForIndex(index int) (NetworkAllocation, error) {
	if index < 0 {
		return NetworkAllocation{}, fmt.Errorf("network index must be non-negative")
	}

	base := ipv4ToUint32(a.basePrefix.Addr())
	subnetBase := base + uint32(index*4)
	hostIP := uint32ToIPv4(subnetBase + 1)
	guestIP := uint32ToIPv4(subnetBase + 2)

	return NetworkAllocation{
		InterfaceID: defaultInterfaceID,
		TapName:     fmt.Sprintf("%s%d", defaultTapPrefix, index),
		HostCIDR:    netip.PrefixFrom(hostIP, defaultNetworkPrefixBits),
		GuestCIDR:   netip.PrefixFrom(guestIP, defaultNetworkPrefixBits),
		GatewayIP:   hostIP,
		GuestMAC:    macForIPv4(guestIP),
	}, nil
}

// NetworkProvisioner prepares the host-side tap device for a machine.
type NetworkProvisioner interface {
	Ensure(context.Context, NetworkAllocation) error
	Remove(context.Context, NetworkAllocation) error
}

// IPTapProvisioner provisions tap devices through the `ip` CLI.
type IPTapProvisioner struct {
	runCommand func(context.Context, string, ...string) error
}

// NewIPTapProvisioner returns a provisioner backed by `ip`.
func NewIPTapProvisioner() *IPTapProvisioner {
	return &IPTapProvisioner{
		runCommand: func(ctx context.Context, name string, args ...string) error {
			cmd := exec.CommandContext(ctx, name, args...)
			output, err := cmd.CombinedOutput()
			if err != nil {
				return fmt.Errorf("%s %s: %w: %s", name, strings.Join(args, " "), err, strings.TrimSpace(string(output)))
			}
			return nil
		},
	}
}

// Ensure creates and brings up the tap device with the host-side address.
func (p *IPTapProvisioner) Ensure(ctx context.Context, network NetworkAllocation) error {
	if p == nil || p.runCommand == nil {
		return fmt.Errorf("network provisioner is required")
	}
	if strings.TrimSpace(network.TapName) == "" {
		return fmt.Errorf("tap name is required")
	}

	err := p.runCommand(ctx, "ip", "tuntap", "add", "dev", network.TapName, "mode", "tap")
	if err != nil {
		lower := strings.ToLower(err.Error())
		if !strings.Contains(lower, "file exists") && !strings.Contains(lower, "device or resource busy") {
			return fmt.Errorf("create tap device %q: %w", network.TapName, err)
		}
		if removeErr := p.Remove(ctx, network); removeErr != nil {
			return fmt.Errorf("remove stale tap device %q: %w", network.TapName, removeErr)
		}
		if err := p.runCommand(ctx, "ip", "tuntap", "add", "dev", network.TapName, "mode", "tap"); err != nil {
			return fmt.Errorf("create tap device %q after cleanup: %w", network.TapName, err)
		}
	}

	if err := p.runCommand(ctx, "ip", "addr", "replace", network.HostCIDR.String(), "dev", network.TapName); err != nil {
		return fmt.Errorf("assign host address to %q: %w", network.TapName, err)
	}
	if err := p.runCommand(ctx, "ip", "link", "set", "dev", network.TapName, "up"); err != nil {
		return fmt.Errorf("bring up tap device %q: %w", network.TapName, err)
	}
	return nil
}

// Remove deletes the tap device if it exists.
func (p *IPTapProvisioner) Remove(ctx context.Context, network NetworkAllocation) error {
	if p == nil || p.runCommand == nil {
		return fmt.Errorf("network provisioner is required")
	}
	if strings.TrimSpace(network.TapName) == "" {
		return nil
	}
	if err := p.runCommand(ctx, "ip", "link", "del", "dev", network.TapName); err != nil {
		lower := strings.ToLower(err.Error())
		if strings.Contains(lower, "cannot find device") || strings.Contains(lower, "does not exist") {
			return nil
		}
		return fmt.Errorf("remove tap device %q: %w", network.TapName, err)
	}
	return nil
}

func ipv4ToUint32(ip netip.Addr) uint32 {
	bytes := ip.As4()
	return binary.BigEndian.Uint32(bytes[:])
}

func uint32ToIPv4(value uint32) netip.Addr {
	var bytes [4]byte
	binary.BigEndian.PutUint32(bytes[:], value)
	return netip.AddrFrom4(bytes)
}

func macForIPv4(ip netip.Addr) string {
	bytes := ip.As4()
	return fmt.Sprintf("06:00:%02x:%02x:%02x:%02x", bytes[0], bytes[1], bytes[2], bytes[3])
}