package iplist

import (
	"bufio"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net"
	"net/netip"
	"slices"
	"strings"

	"go4.org/netipx"
)

const (
	// commentPrefix starts a comment line in a list file.
	commentPrefix = "#"

	// networkSeparator separates the address and the mask length of a
	// network, as in 192.0.2.0/24.
	networkSeparator = "/"

	// dshieldFieldSeparator separates the fields of a DShield line.
	dshieldFieldSeparator = "\t"

	// iCloudPrivateRelayFieldSeparator separates the fields of a line of
	// Apple's iCloud Private Relay file.
	iCloudPrivateRelayFieldSeparator = ","

	// spamhausMetadataType is the type of the line that ends a Spamhaus
	// file. That line holds the file's metadata and no network.
	spamhausMetadataType = "metadata"
)

var (
	errUnknownFormat = errors.New("unknown list format")
	errNoNetwork     = errors.New("no network on the line")
	errBadRange      = errors.New("first and last address make no range")
	errNoAddresses   = errors.New("no addresses in the document")
)

// parse reads a list file written in format and returns the addresses it
// holds. Anything it cannot read fails the whole file, so a file that is not
// the list, such as an error page, never replaces a good copy.
func parse(r io.Reader, format Format) (*netipx.IPSet, error) {
	switch format {
	case FormatLines:
		return parseLines(r, addAddressOrNetwork)
	case FormatSpamhausJSON:
		return parseLines(r, addSpamhausNetwork)
	case FormatDShield:
		return parseLines(r, addDShieldNetwork)
	case FormatOnionooJSON:
		return parseOnionoo(r)
	case FormatMullvadJSON:
		return parseMullvad(r)
	case FormatNordVPNJSON:
		return parseNordVPN(r)
	case FormatPIAJSON:
		return parsePIA(r)
	case FormatIVPNJSON:
		return parseIVPN(r)
	case FormatProtonVPN:
		return parseLinesNotEmpty(r, addAddressOrNetwork)
	case FormatICloudPrivateRelayCSV:
		return parseLinesNotEmpty(r, addICloudPrivateRelayNetwork)
	default:
		return nil, errUnknownFormat
	}
}

// parseLines reads a list file line by line, skipping blank lines and
// comments, and calls addLine with each other line.
func parseLines(
	r io.Reader, addLine func(*netipx.IPSetBuilder, string) error,
) (*netipx.IPSet, error) {
	var builder netipx.IPSetBuilder

	scanner := bufio.NewScanner(r)

	for lineNumber := 1; scanner.Scan(); lineNumber++ {
		line := strings.TrimSpace(scanner.Text())
		if line == "" || strings.HasPrefix(line, commentPrefix) {
			continue
		}

		err := addLine(&builder, line)
		if err != nil {
			return nil, fmt.Errorf("line %d: %w", lineNumber, err)
		}
	}

	err := scanner.Err()
	if err != nil {
		return nil, err
	}

	return builder.IPSet()
}

// addAddressOrNetwork adds an address, such as 192.0.2.1, or a network, such
// as 192.0.2.0/24, to builder.
func addAddressOrNetwork(builder *netipx.IPSetBuilder, text string) error {
	if strings.Contains(text, networkSeparator) {
		network, err := netip.ParsePrefix(text)
		if err != nil {
			return err
		}

		builder.AddPrefix(network)

		return nil
	}

	addr, err := netip.ParseAddr(text)
	if err != nil {
		return err
	}

	builder.Add(addr)

	return nil
}

// addSpamhausNetwork adds the network on a line of a Spamhaus file to
// builder. The metadata line adds nothing.
func addSpamhausNetwork(builder *netipx.IPSetBuilder, line string) error {
	var entry struct {
		CIDR string `json:"cidr"`
		Type string `json:"type"`
	}

	err := json.Unmarshal([]byte(line), &entry)
	if err != nil {
		return err
	}

	if entry.Type == spamhausMetadataType {
		return nil
	}

	network, err := netip.ParsePrefix(entry.CIDR)
	if err != nil {
		return err
	}

	builder.AddPrefix(network)

	return nil
}

// addDShieldNetwork adds the network on a line of a DShield file to builder.
// The line's first two fields are the network's first and last address.
func addDShieldNetwork(builder *netipx.IPSetBuilder, line string) error {
	firstField, rest, found := strings.Cut(line, dshieldFieldSeparator)
	if !found {
		return errNoNetwork
	}

	lastField, _, _ := strings.Cut(rest, dshieldFieldSeparator)

	first, err := netip.ParseAddr(firstField)
	if err != nil {
		return err
	}

	last, err := netip.ParseAddr(lastField)
	if err != nil {
		return err
	}

	network := netipx.IPRangeFrom(first, last)
	if !network.IsValid() {
		return errBadRange
	}

	builder.AddRange(network)

	return nil
}

// parseOnionoo returns every address in the exit_addresses and or_addresses
// fields of the relays in an Onionoo details document. A document with no
// addresses fails, because the Tor network always has relays.
func parseOnionoo(r io.Reader) (*netipx.IPSet, error) {
	//nolint:tagliatelle // Onionoo's own field names
	var details struct {
		Relays []struct {
			ExitAddresses []string `json:"exit_addresses"`
			ORAddresses   []string `json:"or_addresses"`
		} `json:"relays"`
	}

	err := readJSON(r, &details)
	if err != nil {
		return nil, err
	}

	var builder netipx.IPSetBuilder

	for _, relay := range details.Relays {
		for _, text := range relay.ExitAddresses {
			err = addAddress(&builder, text)
			if err != nil {
				return nil, err
			}
		}

		for _, text := range relay.ORAddresses {
			err = addAddressWithPort(&builder, text)
			if err != nil {
				return nil, err
			}
		}
	}

	addresses, err := builder.IPSet()
	if err != nil {
		return nil, err
	}

	if len(addresses.Ranges()) == 0 {
		return nil, errNoAddresses
	}

	return addresses, nil
}

// parseMullvad returns the addresses of the relays in a Mullvad document. A
// relay with no IPv6 address has null for it, and only bridges have
// ipv4_v2ray.
func parseMullvad(r io.Reader) (*netipx.IPSet, error) {
	//nolint:tagliatelle // Mullvad's own field names
	var relays []struct {
		IPv4      string `json:"ipv4_addr_in"`
		IPv6      string `json:"ipv6_addr_in"`
		IPv4V2Ray string `json:"ipv4_v2ray"`
	}

	err := readJSON(r, &relays)
	if err != nil {
		return nil, err
	}

	var texts []string

	for _, relay := range relays {
		texts = append(texts, relay.IPv4, relay.IPv6, relay.IPv4V2Ray)
	}

	return addressSet(texts)
}

// parseNordVPN returns the addresses of the servers in a NordVPN document. A
// server with no IPv6 address has an empty ipv6_station.
func parseNordVPN(r io.Reader) (*netipx.IPSet, error) {
	//nolint:tagliatelle // NordVPN's own field names
	var servers []struct {
		Station     string `json:"station"`
		IPv6Station string `json:"ipv6_station"`
	}

	err := readJSON(r, &servers)
	if err != nil {
		return nil, err
	}

	var texts []string

	for _, server := range servers {
		texts = append(texts, server.Station, server.IPv6Station)
	}

	return addressSet(texts)
}

// parsePIA returns the addresses of the servers in a Private Internet Access
// document. json.Decoder stops at the end of the JSON, so the signature after
// it is not read.
func parsePIA(r io.Reader) (*netipx.IPSet, error) {
	var list struct {
		Regions []struct {
			Servers map[string][]struct {
				IP string `json:"ip"`
			} `json:"servers"`
		} `json:"regions"`
	}

	err := json.NewDecoder(r).Decode(&list)
	if err != nil {
		return nil, err
	}

	var texts []string

	for _, region := range list.Regions {
		for _, servers := range region.Servers {
			for _, server := range servers {
				texts = append(texts, server.IP)
			}
		}
	}

	return addressSet(texts)
}

// parseIVPN returns the addresses of the hosts in an IVPN document. A host's
// ipv6 object holds only an address inside the VPN, so it is not read.
func parseIVPN(r io.Reader) (*netipx.IPSet, error) {
	type gateway struct {
		Hosts []struct {
			Host  string `json:"host"`
			V2Ray string `json:"v2ray"`
		} `json:"hosts"`
	}

	var list struct {
		WireGuard []gateway `json:"wireguard"`
		OpenVPN   []gateway `json:"openvpn"`
	}

	err := readJSON(r, &list)
	if err != nil {
		return nil, err
	}

	var texts []string

	for _, gw := range slices.Concat(list.WireGuard, list.OpenVPN) {
		for _, host := range gw.Hosts {
			texts = append(texts, host.Host, host.V2Ray)
		}
	}

	return addressSet(texts)
}

// parseLinesNotEmpty is parseLines for a list that always holds addresses, so
// a file with none fails.
func parseLinesNotEmpty(
	r io.Reader, addLine func(*netipx.IPSetBuilder, string) error,
) (*netipx.IPSet, error) {
	addresses, err := parseLines(r, addLine)
	if err != nil {
		return nil, err
	}

	if len(addresses.Ranges()) == 0 {
		return nil, errNoAddresses
	}

	return addresses, nil
}

// addICloudPrivateRelayNetwork adds the network at the start of a line of
// Apple's iCloud Private Relay file to builder.
func addICloudPrivateRelayNetwork(builder *netipx.IPSetBuilder, line string) error {
	field, _, found := strings.Cut(line, iCloudPrivateRelayFieldSeparator)
	if !found {
		return errNoNetwork
	}

	network, err := netip.ParsePrefix(field)
	if err != nil {
		return err
	}

	builder.AddPrefix(network)

	return nil
}

// readJSON reads r to its end and decodes it, as one JSON document, into v.
func readJSON(r io.Reader, v any) error {
	data, err := io.ReadAll(r)
	if err != nil {
		return err
	}

	return json.Unmarshal(data, v)
}

// addressSet returns the set of the addresses in texts, skipping empty ones.
// A list with no addresses fails, because every VPN service has servers.
func addressSet(texts []string) (*netipx.IPSet, error) {
	var builder netipx.IPSetBuilder

	for _, text := range texts {
		if text == "" {
			continue
		}

		err := addAddress(&builder, text)
		if err != nil {
			return nil, err
		}
	}

	addresses, err := builder.IPSet()
	if err != nil {
		return nil, err
	}

	if len(addresses.Ranges()) == 0 {
		return nil, errNoAddresses
	}

	return addresses, nil
}

// addAddress adds an address, such as 192.0.2.1, to builder.
func addAddress(builder *netipx.IPSetBuilder, text string) error {
	addr, err := netip.ParseAddr(text)
	if err != nil {
		return err
	}

	builder.Add(addr)

	return nil
}

// addAddressWithPort adds the address of text, such as 192.0.2.1:443 or
// [2001:db8::1]:443, to builder. Onionoo allows a list of ports after the
// address, so the part after the address is not read.
func addAddressWithPort(builder *netipx.IPSetBuilder, text string) error {
	host, _, err := net.SplitHostPort(text)
	if err != nil {
		return err
	}

	return addAddress(builder, host)
}