package dnsforward

import (
	"fmt"
	"net"
	"net/netip"
	"os"
	"strings"
	"time"

	"github.com/AdguardTeam/AdGuardHome/internal/aghnet"
	"github.com/AdguardTeam/dnsproxy/proxy"
	"github.com/AdguardTeam/dnsproxy/upstream"
	"github.com/AdguardTeam/golibs/errors"
	"github.com/AdguardTeam/golibs/log"
	"github.com/AdguardTeam/golibs/netutil"
	"github.com/AdguardTeam/golibs/stringutil"
	"golang.org/x/exp/maps"
	"golang.org/x/exp/slices"
)

const (
	// errNotDomainSpecific is returned when the upstream should be
	// domain-specific, but isn't.
	errNotDomainSpecific errors.Error = "not a domain-specific upstream"

	// errMissingSeparator is returned when the domain-specific part of the
	// upstream configuration line isn't closed.
	errMissingSeparator errors.Error = "missing separator"

	// errDupSeparator is returned when the domain-specific part of the upstream
	// configuration line contains more than one ending separator.
	errDupSeparator errors.Error = "duplicated separator"

	// errNoDefaultUpstreams is returned when there are no default upstreams
	// specified in the upstream configuration.
	errNoDefaultUpstreams errors.Error = "no default upstreams specified"

	// errWrongResponse is returned when the checked upstream replies in an
	// unexpected way.
	errWrongResponse errors.Error = "wrong response"
)

// loadUpstreams parses upstream DNS servers from the configured file or from
// the configuration itself.
func (s *Server) loadUpstreams() (upstreams []string, err error) {
	if s.conf.UpstreamDNSFileName == "" {
		return stringutil.FilterOut(s.conf.UpstreamDNS, IsCommentOrEmpty), nil
	}

	var data []byte
	data, err = os.ReadFile(s.conf.UpstreamDNSFileName)
	if err != nil {
		return nil, fmt.Errorf("reading upstream from file: %w", err)
	}

	upstreams = stringutil.SplitTrimmed(string(data), "\n")

	log.Debug("dnsforward: got %d upstreams in %q", len(upstreams), s.conf.UpstreamDNSFileName)

	return stringutil.FilterOut(upstreams, IsCommentOrEmpty), nil
}

// prepareUpstreamSettings sets upstream DNS server settings.
func (s *Server) prepareUpstreamSettings(boot upstream.Resolver) (err error) {
	// Load upstreams either from the file, or from the settings
	var upstreams []string
	upstreams, err = s.loadUpstreams()
	if err != nil {
		return fmt.Errorf("loading upstreams: %w", err)
	}

	s.conf.UpstreamConfig, err = s.prepareUpstreamConfig(upstreams, defaultDNS, &upstream.Options{
		Bootstrap:    boot,
		Timeout:      s.conf.UpstreamTimeout,
		HTTPVersions: UpstreamHTTPVersions(s.conf.UseHTTP3Upstreams),
		PreferIPv6:   s.conf.BootstrapPreferIPv6,
		// Use a customized set of RootCAs, because Go's default mechanism of
		// loading TLS roots does not always work properly on some routers so we're
		// loading roots manually and pass it here.
		//
		// See [aghtls.SystemRootCAs].
		//
		// TODO(a.garipov): Investigate if that's true.
		RootCAs:      s.conf.TLSv12Roots,
		CipherSuites: s.conf.TLSCiphers,
	})
	if err != nil {
		return fmt.Errorf("preparing upstream config: %w", err)
	}

	return nil
}

// prepareUpstreamConfig returns the upstream configuration based on upstreams
// and configuration of s.
func (s *Server) prepareUpstreamConfig(
	upstreams []string,
	defaultUpstreams []string,
	opts *upstream.Options,
) (uc *proxy.UpstreamConfig, err error) {
	uc, err = proxy.ParseUpstreamsConfig(upstreams, opts)
	if err != nil {
		return nil, fmt.Errorf("parsing upstream config: %w", err)
	}

	if len(uc.Upstreams) == 0 && defaultUpstreams != nil {
		log.Info("dnsforward: warning: no default upstreams specified, using %v", defaultUpstreams)
		var defaultUpstreamConfig *proxy.UpstreamConfig
		defaultUpstreamConfig, err = proxy.ParseUpstreamsConfig(defaultUpstreams, opts)
		if err != nil {
			return nil, fmt.Errorf("parsing default upstreams: %w", err)
		}

		uc.Upstreams = defaultUpstreamConfig.Upstreams
	}

	return uc, nil
}

// UpstreamHTTPVersions returns the HTTP versions for upstream configuration
// depending on configuration.
func UpstreamHTTPVersions(http3 bool) (v []upstream.HTTPVersion) {
	if !http3 {
		return upstream.DefaultHTTPVersions
	}

	return []upstream.HTTPVersion{
		upstream.HTTPVersion3,
		upstream.HTTPVersion2,
		upstream.HTTPVersion11,
	}
}

// setProxyUpstreamMode sets the upstream mode and related settings in conf
// based on provided parameters.
func setProxyUpstreamMode(
	conf *proxy.Config,
	upstreamMode UpstreamMode,
	fastestTimeout time.Duration,
) (err error) {
	switch upstreamMode {
	case UpstreamModeParallel:
		conf.UpstreamMode = proxy.UModeParallel
	case UpstreamModeFastestAddr:
		conf.UpstreamMode = proxy.UModeFastestAddr
		conf.FastestPingTimeout = fastestTimeout
	case UpstreamModeLoadBalance:
		conf.UpstreamMode = proxy.UModeLoadBalance
	default:
		return fmt.Errorf("unexpected value %q", upstreamMode)
	}

	return nil
}

// createBootstrap returns a bootstrap resolver based on the configuration of s.
// boots are the upstream resolvers that should be closed after use.  r is the
// actual bootstrap resolver, which may include the system hosts.
//
// TODO(e.burkov):  This function currently returns a resolver and a slice of
// the upstream resolvers, which are essentially the same.  boots are returned
// for being able to close them afterwards, but it introduces an implicit
// contract that r could only be used before that.  Anyway, this code should
// improve when the [proxy.UpstreamConfig] will become an [upstream.Resolver]
// and be used here.
func (s *Server) createBootstrap(
	addrs []string,
	opts *upstream.Options,
) (r upstream.Resolver, boots []*upstream.UpstreamResolver, err error) {
	if len(addrs) == 0 {
		addrs = defaultBootstrap
	}

	boots, err = aghnet.ParseBootstraps(addrs, opts)
	if err != nil {
		// Don't wrap the error, since it's informative enough as is.
		return nil, nil, err
	}

	var parallel upstream.ParallelResolver
	for _, b := range boots {
		parallel = append(parallel, upstream.NewCachingResolver(b))
	}

	if s.etcHosts != nil {
		r = upstream.ConsequentResolver{s.etcHosts, parallel}
	} else {
		r = parallel
	}

	return r, boots, nil
}

// IsCommentOrEmpty returns true if s starts with a "#" character or is empty.
// This function is useful for filtering out non-upstream lines from upstream
// configs.
func IsCommentOrEmpty(s string) (ok bool) {
	return len(s) == 0 || s[0] == '#'
}

// newUpstreamConfig validates upstreams and returns an appropriate upstream
// configuration or nil if it can't be built.
//
// TODO(e.burkov):  Perhaps proxy.ParseUpstreamsConfig should validate upstreams
// slice already so that this function may be considered useless.
func newUpstreamConfig(upstreams []string) (conf *proxy.UpstreamConfig, err error) {
	// No need to validate comments and empty lines.
	upstreams = stringutil.FilterOut(upstreams, IsCommentOrEmpty)
	if len(upstreams) == 0 {
		// Consider this case valid since it means the default server should be
		// used.
		return nil, nil
	}

	err = validateUpstreamConfig(upstreams)
	if err != nil {
		// Don't wrap the error since it's informative enough as is.
		return nil, err
	}

	conf, err = proxy.ParseUpstreamsConfig(
		upstreams,
		&upstream.Options{
			Bootstrap: net.DefaultResolver,
			Timeout:   DefaultTimeout,
		},
	)
	if err != nil {
		// Don't wrap the error since it's informative enough as is.
		return nil, err
	} else if len(conf.Upstreams) == 0 {
		return nil, errNoDefaultUpstreams
	}

	return conf, nil
}

// validateUpstreamConfig validates each upstream from the upstream
// configuration and returns an error if any upstream is invalid.
//
// TODO(e.burkov):  Merge with [upstreamConfigValidator] somehow.
func validateUpstreamConfig(conf []string) (err error) {
	for _, u := range conf {
		var ups []string
		var isSpecific bool
		ups, isSpecific, err = splitUpstreamLine(u)
		if err != nil {
			// Don't wrap the error since it's informative enough as is.
			return err
		}

		for _, addr := range ups {
			_, err = validateUpstream(addr, isSpecific)
			if err != nil {
				return fmt.Errorf("validating upstream %q: %w", addr, err)
			}
		}
	}

	return nil
}

// ValidateUpstreams validates each upstream and returns an error if any
// upstream is invalid or if there are no default upstreams specified.
//
// TODO(e.burkov):  Merge with [upstreamConfigValidator] somehow.
func ValidateUpstreams(upstreams []string) (err error) {
	_, err = newUpstreamConfig(upstreams)

	return err
}

// ValidateUpstreamsPrivate validates each upstream and returns an error if any
// upstream is invalid or if there are no default upstreams specified.  It also
// checks each domain of domain-specific upstreams for being ARPA pointing to
// a locally-served network.  privateNets must not be nil.
func ValidateUpstreamsPrivate(upstreams []string, privateNets netutil.SubnetSet) (err error) {
	conf, err := newUpstreamConfig(upstreams)
	if err != nil {
		return fmt.Errorf("creating config: %w", err)
	}

	if conf == nil {
		return nil
	}

	keys := maps.Keys(conf.DomainReservedUpstreams)
	slices.Sort(keys)

	var errs []error
	for _, domain := range keys {
		var subnet netip.Prefix
		subnet, err = extractARPASubnet(domain)
		if err != nil {
			errs = append(errs, err)

			continue
		}

		if !privateNets.Contains(subnet.Addr()) {
			errs = append(
				errs,
				fmt.Errorf("arpa domain %q should point to a locally-served network", domain),
			)
		}
	}

	return errors.Annotate(errors.Join(errs...), "checking domain-specific upstreams: %w")
}

// protocols are the supported URL schemes for upstreams.
var protocols = []string{"h3", "https", "quic", "sdns", "tcp", "tls", "udp"}

// validateUpstream returns an error if u alongside with domains is not a valid
// upstream configuration.  useDefault is true if the upstream is
// domain-specific and is configured to point at the default upstream server
// which is validated separately.  The upstream is considered domain-specific
// only if domains is at least not nil.
func validateUpstream(u string, isSpecific bool) (useDefault bool, err error) {
	// The special server address '#' means that default server must be used.
	if useDefault = u == "#" && isSpecific; useDefault {
		return useDefault, nil
	}

	// Check if the upstream has a valid protocol prefix.
	//
	// TODO(e.burkov):  Validate the domain name.
	if proto, _, ok := strings.Cut(u, "://"); ok {
		if !slices.Contains(protocols, proto) {
			return false, fmt.Errorf("bad protocol %q", proto)
		}
	} else if _, err = netip.ParseAddr(u); err == nil {
		return false, nil
	} else if _, err = netip.ParseAddrPort(u); err == nil {
		return false, nil
	}

	return false, err
}

// splitUpstreamLine returns the upstreams and the specified domains.  domains
// is nil when the upstream is not domains-specific.  Otherwise it may also be
// empty.
func splitUpstreamLine(upstreamStr string) (upstreams []string, isSpecific bool, err error) {
	if !strings.HasPrefix(upstreamStr, "[/") {
		return []string{upstreamStr}, false, nil
	}

	defer func() { err = errors.Annotate(err, "splitting upstream line %q: %w", upstreamStr) }()

	doms, ups, found := strings.Cut(upstreamStr[2:], "/]")
	if !found {
		return nil, false, errMissingSeparator
	} else if strings.Contains(ups, "/]") {
		return nil, false, errDupSeparator
	}

	for i, host := range strings.Split(doms, "/") {
		if host == "" {
			continue
		}

		err = netutil.ValidateDomainName(strings.TrimPrefix(host, "*."))
		if err != nil {
			return nil, false, fmt.Errorf("domain at index %d: %w", i, err)
		}

		isSpecific = true
	}

	return strings.Fields(ups), isSpecific, nil
}