package corehttp

import (
	"context"
	"fmt"
	"net"
	"net/http"
	"net/url"
	"regexp"
	"strings"

	isd "github.com/jbenet/go-is-domain"
	mbase "github.com/multiformats/go-multibase"
	cid "gitlab.dms3.io/dms3/go-cid"
	core "gitlab.dms3.io/dms3/go-dms3/core"
	coreapi "gitlab.dms3.io/dms3/go-dms3/core/coreapi"
	namesys "gitlab.dms3.io/dms3/go-dms3/namesys"
	"gitlab.dms3.io/p2p/go-p2p-core/peer"

	config "gitlab.dms3.io/dms3/go-dms3-config"
	iface "gitlab.dms3.io/dms3/interface-go-dms3-core"
	options "gitlab.dms3.io/dms3/interface-go-dms3-core/options"
	nsopts "gitlab.dms3.io/dms3/interface-go-dms3-core/options/namesys"
)

var defaultPaths = []string{"/dms3/", "/dms3ns/", "/api/", "/p2p/", "/version"}

var pathGatewaySpec = &config.GatewaySpec{
	Paths:         defaultPaths,
	UseSubdomains: false,
}

var subdomainGatewaySpec = &config.GatewaySpec{
	Paths:         defaultPaths,
	UseSubdomains: true,
}

var defaultKnownGateways = map[string]*config.GatewaySpec{
	"localhost":       subdomainGatewaySpec,
	"dms3.io":         pathGatewaySpec,
	"gateway.dms3.io": pathGatewaySpec,
	"dweb.link":       subdomainGatewaySpec,
}

// Label's max length in DNS (https://tools.ietf.org/html/rfc1034#page-7)
const dnsLabelMaxLength int = 63

// HostnameOption rewrites an incoming request based on the Host header.
func HostnameOption() ServeOption {
	return func(n *core.Dms3Node, _ net.Listener, mux *http.ServeMux) (*http.ServeMux, error) {
		childMux := http.NewServeMux()

		coreAPI, err := coreapi.NewCoreAPI(n)
		if err != nil {
			return nil, err
		}

		cfg, err := n.Repo.Config()
		if err != nil {
			return nil, err
		}

		knownGateways := prepareKnownGateways(cfg.Gateway.PublicGateways)

		mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
			// Unfortunately, many (well, dms3.io) gateways use
			// DNSLink so if we blindly rewrite with DNSLink, we'll
			// break /dms3 links.
			//
			// We fix this by maintaining a list of known gateways
			// and the paths that they serve "gateway" content on.
			// That way, we can use DNSLink for everything else.

			// Support X-Forwarded-Host if added by a reverse proxy
			// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-Host
			host := r.Host
			if xHost := r.Header.Get("X-Forwarded-Host"); xHost != "" {
				host = xHost
			}

			// HTTP Host & Path check: is this one of our  "known gateways"?
			if gw, ok := isKnownHostname(host, knownGateways); ok {
				// This is a known gateway but request is not using
				// the subdomain feature.

				// Does this gateway _handle_ this path?
				if hasPrefix(r.URL.Path, gw.Paths...) {
					// It does.

					// Should this gateway use subdomains instead of paths?
					if gw.UseSubdomains {
						// Yes, redirect if applicable
						// Example: dweb.link/dms3/{cid} → {cid}.dms3.dweb.link
						newURL, err := toSubdomainURL(host, r.URL.Path, r, coreAPI)
						if err != nil {
							http.Error(w, err.Error(), http.StatusBadRequest)
							return
						}
						if newURL != "" {
							// Just to be sure single Origin can't be abused in
							// web browsers that ignored the redirect for some
							// reason, Clear-Site-Data header clears browsing
							// data (cookies, storage etc) associated with
							// hostname's root Origin
							// Note: we can't use "*" due to bug in Chromium:
							// https://bugs.chromium.org/p/chromium/issues/detail?id=898503
							w.Header().Set("Clear-Site-Data", "\"cookies\", \"storage\"")

							// Set "Location" header with redirect destination.
							// It is ignored by curl in default mode, but will
							// be respected by user agents that follow
							// redirects by default, namely web browsers
							w.Header().Set("Location", newURL)

							// Note: we continue regular gateway processing:
							// HTTP Status Code http.StatusMovedPermanently
							// will be set later, in statusResponseWriter
						}
					}

					// Not a subdomain resource, continue with path processing
					// Example: 127.0.0.1:8080/dms3/{CID}, dms3.io/dms3/{CID} etc
					childMux.ServeHTTP(w, r)
					return
				}
				// Not a whitelisted path

				// Try DNSLink, if it was not explicitly disabled for the hostname
				if !gw.NoDNSLink && isDNSLinkName(r.Context(), coreAPI, host) {
					// rewrite path and handle as DNSLink
					r.URL.Path = "/dms3ns/" + stripPort(host) + r.URL.Path
					childMux.ServeHTTP(w, withHostnameContext(r, host))
					return
				}

				// If not, resource does not exist on the hostname, return 404
				http.NotFound(w, r)
				return
			}

			// HTTP Host check: is this one of our subdomain-based "known gateways"?
			// DMS3 details extracted from the host: {rootID}.{ns}.{gwHostname}
			// /dms3/ example: {cid}.dms3.localhost:8080, {cid}.dms3.dweb.link
			// /dms3ns/ example: {p2p-key}.dms3ns.localhost:8080, {inlined-dnslink-fqdn}.dms3ns.dweb.link
			if gw, gwHostname, ns, rootID, ok := knownSubdomainDetails(host, knownGateways); ok {
				// Looks like we're using a known gateway in subdomain mode.

				// Assemble original path prefix.
				pathPrefix := "/" + ns + "/" + rootID

				// Does this gateway _handle_ subdomains AND this path?
				if !(gw.UseSubdomains && hasPrefix(pathPrefix, gw.Paths...)) {
					// If not, resource does not exist, return 404
					http.NotFound(w, r)
					return
				}

				// Check if rootID is a valid CID
				if rootCID, err := cid.Decode(rootID); err == nil {
					// Do we need to redirect root CID to a canonical DNS representation?
					dnsCID, err := toDNSLabel(rootID, rootCID)
					if err != nil {
						http.Error(w, err.Error(), http.StatusBadRequest)
						return
					}
					if !strings.HasPrefix(r.Host, dnsCID) {
						dnsPrefix := "/" + ns + "/" + dnsCID
						newURL, err := toSubdomainURL(gwHostname, dnsPrefix+r.URL.Path, r, coreAPI)
						if err != nil {
							http.Error(w, err.Error(), http.StatusBadRequest)
							return
						}
						if newURL != "" {
							// Redirect to deterministic CID to ensure CID
							// always gets the same Origin on the web
							http.Redirect(w, r, newURL, http.StatusMovedPermanently)
							return
						}
					}

					// Do we need to fix multicodec in PeerID represented as CIDv1?
					if isPeerIDNamespace(ns) {
						if rootCID.Type() != cid.P2pKey {
							newURL, err := toSubdomainURL(gwHostname, pathPrefix+r.URL.Path, r, coreAPI)
							if err != nil {
								http.Error(w, err.Error(), http.StatusBadRequest)
								return
							}
							if newURL != "" {
								// Redirect to CID fixed inside of toSubdomainURL()
								http.Redirect(w, r, newURL, http.StatusMovedPermanently)
								return
							}
						}
					}
				} else { // rootID is not a CID..

					// Check if rootID is a single DNS label with an inlined
					// DNSLink FQDN a single DNS label. We support this so
					// loading DNSLink names over TLS "just works" on public
					// HTTP gateways.
					//
					// Rationale for doing this can be found under "Option C"
					// at: https://gitlab.dms3.io/dms3/in-web-browsers/issues/169
					//
					// TLDR is:
					// https://dweb.link/dms3ns/my.v-long.example.com
					// can be loaded from a subdomain gateway with a wildcard
					// TLS cert if represented as a single DNS label:
					// https://my-v--long-example-com.dms3ns.dweb.link
					if ns == "dms3ns" && !strings.Contains(rootID, ".") {
						// if there is no TXT recordfor rootID
						if !isDNSLinkName(r.Context(), coreAPI, rootID) {
							// my-v--long-example-com → my.v-long.example.com
							dnslinkFQDN := toDNSLinkFQDN(rootID)
							if isDNSLinkName(r.Context(), coreAPI, dnslinkFQDN) {
								// update path prefix to use real FQDN with DNSLink
								pathPrefix = "/dms3ns/" + dnslinkFQDN
							}
						}
					}
				}

				// Rewrite the path to not use subdomains
				r.URL.Path = pathPrefix + r.URL.Path

				// Serve path request
				childMux.ServeHTTP(w, withHostnameContext(r, gwHostname))
				return
			}
			// We don't have a known gateway. Fallback on DNSLink lookup

			// Wildcard HTTP Host check:
			// 1. is wildcard DNSLink enabled (Gateway.NoDNSLink=false)?
			// 2. does Host header include a fully qualified domain name (FQDN)?
			// 3. does DNSLink record exist in DNS?
			if !cfg.Gateway.NoDNSLink && isDNSLinkName(r.Context(), coreAPI, host) {
				// rewrite path and handle as DNSLink
				r.URL.Path = "/dms3ns/" + stripPort(host) + r.URL.Path
				childMux.ServeHTTP(w, withHostnameContext(r, host))
				return
			}

			// else, treat it as an old school gateway, I guess.
			childMux.ServeHTTP(w, r)
		})
		return childMux, nil
	}
}

type gatewayHosts struct {
	exact    map[string]*config.GatewaySpec
	wildcard []wildcardHost
}

type wildcardHost struct {
	re   *regexp.Regexp
	spec *config.GatewaySpec
}

// Extends request context to include hostname of a canonical gateway root
// (subdomain root or dnslink fqdn)
func withHostnameContext(r *http.Request, hostname string) *http.Request {
	// This is required for links on directory listing pages to work correctly
	// on subdomain and dnslink gateways. While DNSlink could read value from
	// Host header, subdomain gateways have more comples rules (knownSubdomainDetails)
	// More: https://gitlab.dms3.io/dms3/dir-index-html/issues/42
	ctx := context.WithValue(r.Context(), "gw-hostname", hostname)
	return r.WithContext(ctx)
}

func prepareKnownGateways(publicGateways map[string]*config.GatewaySpec) gatewayHosts {
	var hosts gatewayHosts

	hosts.exact = make(map[string]*config.GatewaySpec, len(publicGateways)+len(defaultKnownGateways))

	// First, implicit defaults such as subdomain gateway on localhost
	for hostname, gw := range defaultKnownGateways {
		hosts.exact[hostname] = gw
	}

	// Then apply values from Gateway.PublicGateways, if present in the config
	for hostname, gw := range publicGateways {
		if gw == nil {
			// Remove any implicit defaults, if present. This is useful when one
			// wants to disable subdomain gateway on localhost etc.
			delete(hosts.exact, hostname)
			continue
		}
		if strings.Contains(hostname, "*") {
			// from *.domain.tld, construct a regexp that match any direct subdomain
			// of .domain.tld.
			//
			// Regexp will be in the form of ^[^.]+\.domain.tld(?::\d+)?$

			escaped := strings.ReplaceAll(hostname, ".", `\.`)
			regexed := strings.ReplaceAll(escaped, "*", "[^.]+")

			re, err := regexp.Compile(fmt.Sprintf(`^%s(?::\d+)?$`, regexed))
			if err != nil {
				log.Warn("invalid wildcard gateway hostname \"%s\"", hostname)
			}

			hosts.wildcard = append(hosts.wildcard, wildcardHost{re: re, spec: gw})
		} else {
			hosts.exact[hostname] = gw
		}
	}

	return hosts
}

// isKnownHostname checks Gateway.PublicGateways and returns matching
// GatewaySpec with gracefull fallback to version without port
func isKnownHostname(hostname string, knownGateways gatewayHosts) (gw *config.GatewaySpec, ok bool) {
	// Try hostname (host+optional port - value from Host header as-is)
	if gw, ok := knownGateways.exact[hostname]; ok {
		return gw, ok
	}
	// Also test without port
	if gw, ok = knownGateways.exact[stripPort(hostname)]; ok {
		return gw, ok
	}

	// Wildcard support. Test both with and without port.
	for _, host := range knownGateways.wildcard {
		if host.re.MatchString(hostname) {
			return host.spec, true
		}
	}

	return nil, false
}

// Parses Host header and looks for a known gateway matching subdomain host.
// If found, returns GatewaySpec and subdomain components extracted from Host
// header: {rootID}.{ns}.{gwHostname}
// Note: hostname is host + optional port
func knownSubdomainDetails(hostname string, knownGateways gatewayHosts) (gw *config.GatewaySpec, gwHostname, ns, rootID string, ok bool) {
	labels := strings.Split(hostname, ".")
	// Look for FQDN of a known gateway hostname.
	// Example: given "dist.dms3.io.dms3ns.dweb.link":
	// 1. Lookup "link" TLD in knownGateways: negative
	// 2. Lookup "dweb.link" in knownGateways: positive
	//
	// Stops when we have 2 or fewer labels left as we need at least a
	// rootId and a namespace.
	for i := len(labels) - 1; i >= 2; i-- {
		fqdn := strings.Join(labels[i:], ".")
		gw, ok := isKnownHostname(fqdn, knownGateways)
		if !ok {
			continue
		}

		ns := labels[i-1]
		if !isSubdomainNamespace(ns) {
			break
		}

		// Merge remaining labels (could be a FQDN with DNSLink)
		rootID := strings.Join(labels[:i-1], ".")
		return gw, fqdn, ns, rootID, true
	}
	// no match
	return nil, "", "", "", false
}

// isDNSLinkName returns bool if a valid DNS TXT record exist for provided host
func isDNSLinkName(ctx context.Context, dms3 iface.CoreAPI, host string) bool {
	fqdn := stripPort(host)
	if len(fqdn) == 0 && !isd.IsDomain(fqdn) {
		return false
	}
	name := "/dms3ns/" + fqdn
	// check if DNSLink exists
	depth := options.Name.ResolveOption(nsopts.Depth(1))
	_, err := dms3.Name().Resolve(ctx, name, depth)
	return err == nil || err == namesys.ErrResolveRecursion
}

func isSubdomainNamespace(ns string) bool {
	switch ns {
	case "dms3", "dms3ns", "p2p", "ld":
		return true
	default:
		return false
	}
}

func isPeerIDNamespace(ns string) bool {
	switch ns {
	case "dms3ns", "p2p":
		return true
	default:
		return false
	}
}

// Converts a CID to DNS-safe representation that fits in 63 characters
func toDNSLabel(rootID string, rootCID cid.Cid) (dnsCID string, err error) {
	// Return as-is if things fit
	if len(rootID) <= dnsLabelMaxLength {
		return rootID, nil
	}

	// Convert to Base36 and see if that helped
	rootID, err = cid.NewCidV1(rootCID.Type(), rootCID.Hash()).StringOfBase(mbase.Base36)
	if err != nil {
		return "", err
	}
	if len(rootID) <= dnsLabelMaxLength {
		return rootID, nil
	}

	// Can't win with DNS at this point, return error
	return "", fmt.Errorf("CID incompatible with DNS label length limit of 63: %s", rootID)
}

// Returns true if HTTP request involves TLS certificate.
// See https://gitlab.dms3.io/dms3/in-web-browsers/issues/169 to understand how it
// impacts DNSLink websites on public gateways.
func isHTTPSRequest(r *http.Request) bool {
	// X-Forwarded-Proto if added by a reverse proxy
	// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-Proto
	xproto := r.Header.Get("X-Forwarded-Proto")
	// Is request a native TLS (not used atm, but future-proofing)
	// or a proxied HTTPS (eg. go-dms3 behind nginx at a public gw)?
	return r.URL.Scheme == "https" || xproto == "https"
}

// Converts a FQDN to DNS-safe representation that fits in 63 characters:
// my.v-long.example.com → my-v--long-example-com
func toDNSLinkDNSLabel(fqdn string) (dnsLabel string, err error) {
	dnsLabel = strings.ReplaceAll(fqdn, "-", "--")
	dnsLabel = strings.ReplaceAll(dnsLabel, ".", "-")
	if len(dnsLabel) > dnsLabelMaxLength {
		return "", fmt.Errorf("DNSLink representation incompatible with DNS label length limit of 63: %s", dnsLabel)
	}
	return dnsLabel, nil
}

// Converts a DNS-safe representation of DNSLink FQDN to real FQDN:
// my-v--long-example-com → my.v-long.example.com
func toDNSLinkFQDN(dnsLabel string) (fqdn string) {
	fqdn = strings.ReplaceAll(dnsLabel, "--", "@") // @ placeholder is unused in DNS labels
	fqdn = strings.ReplaceAll(fqdn, "-", ".")
	fqdn = strings.ReplaceAll(fqdn, "@", "-")
	return fqdn
}

// Converts a hostname/path to a subdomain-based URL, if applicable.
func toSubdomainURL(hostname, path string, r *http.Request, dms3 iface.CoreAPI) (redirURL string, err error) {
	var scheme, ns, rootID, rest string

	query := r.URL.RawQuery
	parts := strings.SplitN(path, "/", 4)
	isHTTPS := isHTTPSRequest(r)
	safeRedirectURL := func(in string) (out string, err error) {
		safeURI, err := url.ParseRequestURI(in)
		if err != nil {
			return "", err
		}
		return safeURI.String(), nil
	}

	if isHTTPS {
		scheme = "https:"
	} else {
		scheme = "http:"
	}

	switch len(parts) {
	case 4:
		rest = parts[3]
		fallthrough
	case 3:
		ns = parts[1]
		rootID = parts[2]
	default:
		return "", nil
	}

	if !isSubdomainNamespace(ns) {
		return "", nil
	}

	// add prefix if query is present
	if query != "" {
		query = "?" + query
	}

	// Normalize problematic PeerIDs (eg. ed25519+identity) to CID representation
	if isPeerIDNamespace(ns) && !isd.IsDomain(rootID) {
		peerID, err := peer.Decode(rootID)
		// Note: PeerID CIDv1 with protobuf multicodec will fail, but we fix it
		// in the next block
		if err == nil {
			rootID = peer.ToCid(peerID).String()
		}
	}

	// If rootID is a CID, ensure it uses DNS-friendly text representation
	if rootCID, err := cid.Decode(rootID); err == nil {
		multicodec := rootCID.Type()
		var base mbase.Encoding = mbase.Base32

		// Normalizations specific to /dms3ns/{p2p-key}
		if isPeerIDNamespace(ns) {
			// Using Base36 for /dms3ns/ for consistency
			// Context: https://gitlab.dms3.io/dms3/go-dms3/pull/7441#discussion_r452372828
			base = mbase.Base36

			// PeerIDs represented as CIDv1 are expected to have p2p-key
			// multicodec (https://gitlab.dms3.io/p2p/specs/pull/209).
			// We ease the transition by fixing multicodec on the fly:
			// https://gitlab.dms3.io/dms3/go-dms3/issues/5287#issuecomment-492163929
			if multicodec != cid.P2pKey {
				multicodec = cid.P2pKey
			}
		}

		// Ensure CID text representation used in subdomain is compatible
		// with the way DNS and URIs are implemented in user agents.
		//
		// 1. Switch to CIDv1 and enable case-insensitive Base encoding
		//    to avoid issues when user agent force-lowercases the hostname
		//    before making the request
		//    (https://gitlab.dms3.io/dms3/in-web-browsers/issues/89)
		rootCID = cid.NewCidV1(multicodec, rootCID.Hash())
		rootID, err = rootCID.StringOfBase(base)
		if err != nil {
			return "", err
		}
		// 2. Make sure CID fits in a DNS label, adjust encoding if needed
		//    (https://gitlab.dms3.io/dms3/go-dms3/issues/7318)
		rootID, err = toDNSLabel(rootID, rootCID)
		if err != nil {
			return "", err
		}
	} else { // rootID is not a CID

		// Check if rootID is a FQDN with DNSLink and convert it to TLS-safe
		// representation that fits in a single DNS label.  We support this so
		// loading DNSLink names over TLS "just works" on public HTTP gateways
		// that pass 'https' in X-Forwarded-Proto to go-dms3.
		//
		// Rationale can be found under "Option C"
		// at: https://gitlab.dms3.io/dms3/in-web-browsers/issues/169
		//
		// TLDR is:
		// /dms3ns/my.v-long.example.com
		// can be loaded from a subdomain gateway with a wildcard TLS cert if
		// represented as a single DNS label:
		// https://my-v--long-example-com.dms3ns.dweb.link
		if isHTTPS && ns == "dms3ns" && strings.Contains(rootID, ".") {
			if isDNSLinkName(r.Context(), dms3, rootID) {
				// my.v-long.example.com → my-v--long-example-com
				dnsLabel, err := toDNSLinkDNSLabel(rootID)
				if err != nil {
					return "", err
				}
				// update path prefix to use real FQDN with DNSLink
				rootID = dnsLabel
			}
		}
	}

	return safeRedirectURL(fmt.Sprintf(
		"%s//%s.%s.%s/%s%s",
		scheme,
		rootID,
		ns,
		hostname,
		rest,
		query,
	))
}

func hasPrefix(path string, prefixes ...string) bool {
	for _, prefix := range prefixes {
		// Assume people are creative with trailing slashes in Gateway config
		p := strings.TrimSuffix(prefix, "/")
		// Support for both /version and /dms3/$cid
		if p == path || strings.HasPrefix(path, p+"/") {
			return true
		}
	}
	return false
}

func stripPort(hostname string) string {
	host, _, err := net.SplitHostPort(hostname)
	if err == nil {
		return host
	}
	return hostname
}