// Package network provides network-related utilities for LocalGo
package network

import (
	"errors"
	"fmt"
	"net"
	"strings"
)

// systemInterface is a network interface paired with its IPv4 addresses.
// It exists so interface enumeration can be performed without netlink, which
// the Android/Termux sandbox blocks (net.Interfaces() returns EPERM there).
type systemInterface struct {
	net.Interface
	addrs []*net.IPNet
}

// systemInterfaces returns all system interfaces with their IPv4 addresses.
// It uses netlink-based enumeration (net.Interfaces); on android it
// transparently falls back to ioctl-based enumeration when netlink is
// unavailable (see platformInterfaceFallback).
func systemInterfaces() ([]systemInterface, error) {
	ifs, err := net.Interfaces()
	if err != nil {
		return platformInterfaceFallback(err)
	}

	out := make([]systemInterface, 0, len(ifs))
	for _, i := range ifs {
		addrs, err := i.Addrs()
		if err != nil {
			continue
		}
		si := systemInterface{Interface: i}
		for _, a := range addrs {
			if ipn, ok := a.(*net.IPNet); ok && ipn.IP.To4() != nil {
				si.addrs = append(si.addrs, ipn)
			}
		}
		out = append(out, si)
	}
	return out, nil
}

// systemInterfaceByName returns the interface with the given name, or an error.
func systemInterfaceByName(name string) (*systemInterface, error) {
	ifs, err := systemInterfaces()
	if err != nil {
		return nil, err
	}
	for i := range ifs {
		if ifs[i].Name == name {
			return &ifs[i], nil
		}
	}
	return nil, fmt.Errorf("interface %q not found", name)
}

// EnumerateInterfaces returns all system interfaces. Unlike net.Interfaces,
// this works even where netlink is blocked (e.g. Android/Termux).
func EnumerateInterfaces() ([]net.Interface, error) {
	ifs, err := systemInterfaces()
	if err != nil {
		return nil, err
	}
	out := make([]net.Interface, 0, len(ifs))
	for _, i := range ifs {
		out = append(out, i.Interface)
	}
	return out, nil
}

// InterfaceByName returns the named system interface. Unlike net.InterfaceByName,
// this works even where netlink is blocked (e.g. Android/Termux).
func InterfaceByName(name string) (*net.Interface, error) {
	si, err := systemInterfaceByName(name)
	if err != nil {
		return nil, err
	}
	iface := si.Interface
	return &iface, nil
}

// GetLocalIP returns the primary non-loopback IP address of the machine
func GetLocalIP() (string, error) {
	ifs, err := systemInterfaces()
	if err != nil {
		return "", err
	}

	for _, i := range ifs {
		if (i.Flags&net.FlagUp) == 0 || (i.Flags&net.FlagLoopback) != 0 {
			continue
		}
		for _, ipn := range i.addrs {
			if !ipn.IP.IsLoopback() {
				return ipn.IP.String(), nil
			}
		}
	}

	return "", errors.New("no suitable local IP address found")
}

// GetLocalIPAddresses returns a list of local IP addresses for all non-loopback interfaces
func GetLocalIPAddresses() ([]net.IP, error) {
	var ips []net.IP
	ifs, err := systemInterfaces()
	if err != nil {
		return nil, err
	}

	for _, i := range ifs {
		// Skip down and loopback interfaces
		if (i.Flags&net.FlagUp) == 0 || (i.Flags&net.FlagLoopback) != 0 {
			continue
		}
		for _, ipn := range i.addrs {
			ips = append(ips, ipn.IP)
		}
	}
	return ips, nil
}

// GetInterfaceIPs returns all IP addresses for a specific network interface
func GetInterfaceIPs(interfaceName string) ([]string, error) {
	iface, err := systemInterfaceByName(interfaceName)
	if err != nil {
		return nil, err
	}

	var ips []string
	for _, ipn := range iface.addrs {
		ips = append(ips, ipn.IP.String())
	}
	return ips, nil
}

// FormatAddress formats an IP address and port into a URL
func FormatAddress(ip net.IP, port int, https bool) string {
	protocol := "http"
	if https {
		protocol = "https"
	}

	ipStr := ip.String()

	// Check if it's an IPv6 address
	if strings.Contains(ipStr, ":") {
		return fmt.Sprintf("%s://[%s]:%d", protocol, ipStr, port)
	}

	return fmt.Sprintf("%s://%s:%d", protocol, ipStr, port)
}

// GetPreferredOutboundIP gets the preferred IP address for outbound connections
func GetPreferredOutboundIP() (net.IP, error) {
	// Connect to a public IP (no actual connection is made)
	conn, err := net.Dial("udp", "8.8.8.8:80")
	if err != nil {
		return nil, fmt.Errorf("failed to determine preferred outbound IP: %w", err)
	}
	defer conn.Close()

	// Get the local address
	localAddr := conn.LocalAddr().(*net.UDPAddr)

	return localAddr.IP, nil
}

// ParseCIDRRange parses a CIDR notation (e.g. "192.168.1.0/24") and returns
// all usable host IPs in that range (network and broadcast addresses excluded).
func ParseCIDRRange(cidr string) ([]net.IP, error) {
	_, ipnet, err := net.ParseCIDR(cidr)
	if err != nil {
		return nil, fmt.Errorf("invalid CIDR %q: %w", cidr, err)
	}

	ip4 := ipnet.IP.To4()
	if ip4 == nil {
		return nil, fmt.Errorf("CIDR %q is not an IPv4 range", cidr)
	}

	ones, bits := ipnet.Mask.Size()
	hostBits := bits - ones
	if hostBits < 2 || hostBits > 30 {
		return nil, fmt.Errorf("CIDR %q prefix length must be /8–/30", cidr)
	}

	base := uint32(ip4[0])<<24 | uint32(ip4[1])<<16 | uint32(ip4[2])<<8 | uint32(ip4[3])
	totalHosts := (1 << hostBits) - 2
	var ips []net.IP
	for i := 1; i <= totalHosts; i++ {
		addr := base + uint32(i)
		ips = append(ips, net.IPv4(byte(addr>>24), byte(addr>>16), byte(addr>>8), byte(addr)))
	}
	return ips, nil
}

// GetSubnetIPs returns all IP addresses in the same /24 subnet as the given IP
func GetSubnetIPs(ip net.IP) []net.IP {
	ip4 := ip.To4()
	if ip4 == nil {
		return nil
	}

	var ips []net.IP
	// Assume /24 subnet for local discovery (most common)
	// Iterate 1-254
	for i := 1; i <= 254; i++ {
		// Create a copy of the base IP
		newIP := make(net.IP, len(ip4))
		copy(newIP, ip4)
		newIP[3] = byte(i)

		// Skip if it matches the original IP (optional, but scan logic might want to include self for testing)
		// Keeping self allows "finding" the local node which confirms scan is running.
		ips = append(ips, newIP)
	}
	return ips
}

// GetInterfaceIPNet returns the IPv4 network (IP + subnet mask) for the named interface.
// Returns nil if the interface has no IPv4 address.
func GetInterfaceIPNet(ifaceName string) (*net.IPNet, error) {
	iface, err := systemInterfaceByName(ifaceName)
	if err != nil {
		return nil, fmt.Errorf("interface %q: %w", ifaceName, err)
	}
	if len(iface.addrs) > 0 {
		return iface.addrs[0], nil
	}
	return nil, fmt.Errorf("interface %q has no IPv4 address", ifaceName)
}

// GetUsableSubnetIPsFromIP returns all usable host IPs in the subnet of the
// interface that owns the given IP, respecting its actual netmask. Falls back
// to a flat /24 scan if the interface cannot be determined.
func GetUsableSubnetIPsFromIP(ip net.IP) ([]net.IP, error) {
	ipStr := ip.String()
	ifs, err := systemInterfaces()
	if err != nil {
		return GetSubnetIPs(ip), nil
	}
	for _, i := range ifs {
		if (i.Flags&net.FlagUp) == 0 || (i.Flags&net.FlagLoopback) != 0 {
			continue
		}
		for _, ipn := range i.addrs {
			if ipn.IP.String() == ipStr {
				return GetUsableSubnetIPs(i.Name)
			}
		}
	}
	return GetSubnetIPs(ip), nil
}

// GetUsableSubnetIPs returns all usable host IPs in the subnet of the named
// interface, respecting its actual netmask. Subnets with more than 1022 hosts
// (larger than /22) are capped at /22 to keep scanning practical. Network and
// broadcast addresses are excluded.
func GetUsableSubnetIPs(ifaceName string) ([]net.IP, error) {
	ipnet, err := GetInterfaceIPNet(ifaceName)
	if err != nil {
		return nil, err
	}

	ip4 := ipnet.IP.To4()
	if ip4 == nil {
		return nil, fmt.Errorf("interface %q has no IPv4 address", ifaceName)
	}

	ones, bits := ipnet.Mask.Size()
	hostBits := bits - ones

	// Cap at /22 for practical scanning (max 1022 usable hosts)
	effectiveMask := ipnet.Mask
	if hostBits > 10 {
		effectiveMask = net.CIDRMask(22, bits)
		hostBits = bits - 22
	}

	if hostBits < 2 {
		return nil, fmt.Errorf("interface %q subnet prefix /%d is too small for scanning", ifaceName, bits-hostBits)
	}

	maskBits := []byte{effectiveMask[0], effectiveMask[1], effectiveMask[2], effectiveMask[3]}
	base := uint32(ip4[0])<<24 | uint32(ip4[1])<<16 | uint32(ip4[2])<<8 | uint32(ip4[3])
	base &= uint32(maskBits[0])<<24 | uint32(maskBits[1])<<16 | uint32(maskBits[2])<<8 | uint32(maskBits[3])

	totalHosts := (1 << hostBits) - 2
	var ips []net.IP
	for i := 1; i <= totalHosts; i++ {
		addr := base + uint32(i)
		ips = append(ips, net.IPv4(byte(addr>>24), byte(addr>>16), byte(addr>>8), byte(addr)))
	}
	return ips, nil
}