// Copyright 2023 The Gitea Authors. All rights reserved.
// SPDX-License-Identifier: MIT

package httplib

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

	"code.gitea.io/gitea/modules/setting"
	"code.gitea.io/gitea/modules/util"
)

type RequestContextKeyStruct struct{}

var RequestContextKey = RequestContextKeyStruct{}

func urlIsRelative(s string, u *url.URL) bool {
	// Unfortunately, browsers consider a redirect Location with preceding "//", "\\", "/\" and "\/" as meaning redirect to "http(s)://REST_OF_PATH"
	// Therefore we should ignore these redirect locations to prevent open redirects
	if len(s) > 1 && (s[0] == '/' || s[0] == '\\') && (s[1] == '/' || s[1] == '\\') {
		return false
	}
	if u == nil {
		return false // invalid URL
	}
	if u.Scheme != "" || u.Host != "" {
		return false // absolute URL with scheme or host
	}
	// Now, the URL is likely a relative URL
	// HINT: GOLANG-HTTP-REDIRECT-BUG: Golang security vulnerability: "http.Redirect" calls "path.Clean" and changes the meaning of a path
	// For example, `/a/../\b` will be changed to `/\b`, then it hits the first checked pattern and becomes an open redirect to "{current-scheme}://b"
	// For a valid relative URL, its "path" shouldn't contain `\` because such char must be escaped.
	// So if the "path" contains `\`, it is not a valid relative URL, then we can prevent open redirect.
	return !strings.Contains(u.Path, "\\")
}

// IsRelativeURL detects if a URL is relative (no scheme or host)
func IsRelativeURL(s string) bool {
	u, err := url.Parse(s)
	return err == nil && urlIsRelative(s, u)
}

func getRequestScheme(req *http.Request) string {
	// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-Proto
	if proto, ok := parseForwardedProtoValue(req.Header.Get("X-Forwarded-Proto")); ok {
		return proto
	}
	if proto, ok := parseForwardedProtoValue(req.Header.Get("X-Forwarded-Protocol")); ok {
		return proto
	}
	if proto, ok := parseForwardedProtoValue(req.Header.Get("X-Url-Scheme")); ok {
		return proto
	}
	if s := req.Header.Get("Front-End-Https"); s != "" {
		return util.Iif(s == "on", "https", "http")
	}
	if s := req.Header.Get("X-Forwarded-Ssl"); s != "" {
		return util.Iif(s == "on", "https", "http")
	}
	return ""
}

func parseForwardedProtoValue(val string) (string, bool) {
	if val == "http" || val == "https" {
		return val, true
	}
	return "", false
}

// GuessCurrentAppURL tries to guess the current full public URL (with sub-path) by http headers. It always has a '/' suffix, exactly the same as setting.AppURL
// TODO: should rename it to GuessCurrentPublicURL in the future
func GuessCurrentAppURL(ctx context.Context) string {
	return GuessCurrentHostURL(ctx) + setting.AppSubURL + "/"
}

// GuessCurrentHostURL tries to guess the current full host URL (no sub-path) by http headers, there is no trailing slash.
func GuessCurrentHostURL(ctx context.Context) string {
	// Try the best guess to get the current host URL (will be used for public URL) by http headers.
	// At the moment, if site admin doesn't configure the proxy headers correctly, then Gitea would guess wrong.
	// There are some cases:
	// 1. The reverse proxy is configured correctly, it passes "X-Forwarded-Proto/Host" headers. Perfect, Gitea can handle it correctly.
	// 2. The reverse proxy is not configured correctly, doesn't pass "X-Forwarded-Proto/Host" headers, eg: only one "proxy_pass http://gitea:3000" in Nginx.
	// 3. There is no reverse proxy.
	// Without more information, Gitea is impossible to distinguish between case 2 and case 3, then case 2 would result in
	// wrong guess like guessed public URL becomes "http://gitea:3000/" behind a "https" reverse proxy, which is not accessible by end users.
	// So we introduced "PUBLIC_URL_DETECTION" option, to control the guessing behavior to satisfy different use cases.
	req, ok := ctx.Value(RequestContextKey).(*http.Request)
	if !ok {
		return strings.TrimSuffix(setting.AppURL, setting.AppSubURL+"/")
	}
	reqScheme := getRequestScheme(req)
	if reqScheme == "" {
		// if no reverse proxy header, try to use "Host" header for absolute URL
		if setting.PublicURLDetection == setting.PublicURLAuto && req.Host != "" {
			return util.Iif(req.TLS == nil, "http://", "https://") + req.Host
		}
		// fall back to default AppURL
		return strings.TrimSuffix(setting.AppURL, setting.AppSubURL+"/")
	}
	// X-Forwarded-Host has many problems: non-standard, not well-defined (X-Forwarded-Port or not), conflicts with Host header.
	// So do not use X-Forwarded-Host, just use Host header directly.
	return reqScheme + "://" + req.Host
}

func GuessCurrentHostDomain(ctx context.Context) string {
	_, host, _ := strings.Cut(GuessCurrentHostURL(ctx), "://")
	domain, _, _ := net.SplitHostPort(host)
	return util.IfZero(domain, host)
}

// MakeAbsoluteURL tries to make a link to an absolute public URL:
// * If link is empty, it returns the current public URL.
// * If link is absolute, it returns the link.
// * Otherwise, it returns the current host URL + link, the link itself should have correct sub-path (AppSubURL) if needed.
func MakeAbsoluteURL(ctx context.Context, link string) string {
	if link == "" {
		return GuessCurrentAppURL(ctx)
	}
	if !IsRelativeURL(link) {
		return link
	}
	return GuessCurrentHostURL(ctx) + "/" + strings.TrimPrefix(link, "/")
}

type urlType int

const (
	urlTypeGiteaAbsolute     urlType = iota + 1 // "http://gitea/subpath"
	urlTypeGiteaPageRelative                    // "/subpath"
	urlTypeGiteaSiteRelative                    // "?key=val"
	urlTypeUnknown                              // "http://other"
)

func detectURLRoutePath(ctx context.Context, s string) (routePath string, ut urlType) {
	u, err := url.Parse(s)
	if err != nil {
		return "", urlTypeUnknown
	}
	cleanedPath := ""
	if u.Path != "" {
		cleanedPath = util.PathJoinRelX(u.Path)
		cleanedPath = util.Iif(cleanedPath == ".", "", "/"+cleanedPath)
	}
	if urlIsRelative(s, u) {
		if u.Path == "" {
			return "", urlTypeGiteaPageRelative
		}
		if strings.HasPrefix(strings.ToLower(cleanedPath+"/"), strings.ToLower(setting.AppSubURL+"/")) {
			return cleanedPath[len(setting.AppSubURL):], urlTypeGiteaSiteRelative
		}
		return "", urlTypeUnknown
	}
	u.Path = cleanedPath + "/"
	urlLower := strings.ToLower(u.String())
	if strings.HasPrefix(urlLower, strings.ToLower(setting.AppURL)) {
		return cleanedPath[len(setting.AppSubURL):], urlTypeGiteaAbsolute
	}
	guessedCurURL := GuessCurrentAppURL(ctx)
	if strings.HasPrefix(urlLower, strings.ToLower(guessedCurURL)) {
		return cleanedPath[len(setting.AppSubURL):], urlTypeGiteaAbsolute
	}
	return "", urlTypeUnknown
}

func IsCurrentGiteaSiteURL(ctx context.Context, s string) bool {
	_, ut := detectURLRoutePath(ctx, s)
	return ut != urlTypeUnknown
}

type GiteaSiteURL struct {
	RoutePath   string
	OwnerName   string
	RepoName    string
	RepoSubPath string
}

func ParseGiteaSiteURL(ctx context.Context, s string) *GiteaSiteURL {
	routePath, ut := detectURLRoutePath(ctx, s)
	if ut == urlTypeUnknown || ut == urlTypeGiteaPageRelative {
		return nil
	}
	ret := &GiteaSiteURL{RoutePath: routePath}
	fields := strings.SplitN(strings.TrimPrefix(ret.RoutePath, "/"), "/", 3)

	// TODO: now it only does a quick check for some known reserved paths, should do more strict checks in the future
	if fields[0] == "attachments" {
		return ret
	}
	if len(fields) < 2 {
		return ret
	}
	ret.OwnerName = fields[0]
	ret.RepoName = fields[1]
	if len(fields) == 3 {
		ret.RepoSubPath = "/" + fields[2]
	}
	return ret
}
