// Copyright 2014 Manu Martinez-Almeida. All rights reserved.
// Use of this source code is governed by a MIT style
// license that can be found in the LICENSE file.

package gin

import (
	"fmt"
	"html/template"
	"net"
	"net/http"
	"os"
	"path"
	"regexp"
	"strings"
	"sync"

	"github.com/gin-gonic/gin/internal/bytesconv"
	"github.com/gin-gonic/gin/render"

	"github.com/quic-go/quic-go/http3"
	"golang.org/x/net/http2"
	"golang.org/x/net/http2/h2c"
)

const defaultMultipartMemory = 32 << 20 // 32 MB
const escapedColon = "\\:"
const colon = ":"
const backslash = "\\"

var (
	default404Body = []byte("404 page not found")
	default405Body = []byte("405 method not allowed")
)

var defaultPlatform string

var defaultTrustedCIDRs = []*net.IPNet{
	{ // 0.0.0.0/0 (IPv4)
		IP:   net.IP{0x0, 0x0, 0x0, 0x0},
		Mask: net.IPMask{0x0, 0x0, 0x0, 0x0},
	},
	{ // ::/0 (IPv6)
		IP:   net.IP{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
		Mask: net.IPMask{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
	},
}

var regSafePrefix = regexp.MustCompile("[^a-zA-Z0-9/-]+")
var regRemoveRepeatedChar = regexp.MustCompile("/{2,}")

// HandlerFunc defines the handler used by gin middleware as return value.
type HandlerFunc func(*Context)

// OptionFunc defines the function to change the default configuration
type OptionFunc func(*Engine)

// HandlersChain defines a HandlerFunc slice.
type HandlersChain []HandlerFunc

// Last returns the last handler in the chain. i.e. the last handler is the main one.
func (c HandlersChain) Last() HandlerFunc {
	if length := len(c); length > 0 {
		return c[length-1]
	}
	return nil
}

// RouteInfo represents a request route's specification which contains method and path and its handler.
type RouteInfo struct {
	Method      string
	Path        string
	Handler     string
	HandlerFunc HandlerFunc
}

// RoutesInfo defines a RouteInfo slice.
type RoutesInfo []RouteInfo

// Trusted platforms
const (
	// PlatformGoogleAppEngine when running on Google App Engine. Trust X-Appengine-Remote-Addr
	// for determining the client's IP
	PlatformGoogleAppEngine = "X-Appengine-Remote-Addr"
	// PlatformCloudflare when using Cloudflare's CDN. Trust CF-Connecting-IP for determining
	// the client's IP
	PlatformCloudflare = "CF-Connecting-IP"
	// PlatformFlyIO when running on Fly.io. Trust Fly-Client-IP for determining the client's IP
	PlatformFlyIO = "Fly-Client-IP"
)

// Engine is the framework's instance, it contains the muxer, middleware and configuration settings.
// Create an instance of Engine, by using New() or Default()
type Engine struct {
	RouterGroup

	// RedirectTrailingSlash enables automatic redirection if the current route can't be matched but a
	// handler for the path with (without) the trailing slash exists.
	// For example if /foo/ is requested but a route only exists for /foo, the
	// client is redirected to /foo with http status code 301 for GET requests
	// and 307 for all other request methods.
	RedirectTrailingSlash bool

	// RedirectFixedPath if enabled, the router tries to fix the current request path, if no
	// handle is registered for it.
	// First superfluous path elements like ../ or // are removed.
	// Afterwards the router does a case-insensitive lookup of the cleaned path.
	// If a handle can be found for this route, the router makes a redirection
	// to the corrected path with status code 301 for GET requests and 307 for
	// all other request methods.
	// For example /FOO and /..//Foo could be redirected to /foo.
	// RedirectTrailingSlash is independent of this option.
	RedirectFixedPath bool

	// HandleMethodNotAllowed if enabled, the router checks if another method is allowed for the
	// current route, if the current request can not be routed.
	// If this is the case, the request is answered with 'Method Not Allowed'
	// and HTTP status code 405.
	// If no other Method is allowed, the request is delegated to the NotFound
	// handler.
	HandleMethodNotAllowed bool

	// ForwardedByClientIP if enabled, client IP will be parsed from the request's headers that
	// match those stored at `(*gin.Engine).RemoteIPHeaders`. If no IP was
	// fetched, it falls back to the IP obtained from
	// `(*gin.Context).Request.RemoteAddr`.
	ForwardedByClientIP bool

	// AppEngine was deprecated.
	// Deprecated: USE `TrustedPlatform` WITH VALUE `gin.PlatformGoogleAppEngine` INSTEAD
	// #726 #755 If enabled, it will trust some headers starting with
	// 'X-AppEngine...' for better integration with that PaaS.
	AppEngine bool

	// UseRawPath if enabled, the url.RawPath will be used to find parameters.
	UseRawPath bool

	// UnescapePathValues if true, the path value will be unescaped.
	// If UseRawPath is false (by default), the UnescapePathValues effectively is true,
	// as url.Path gonna be used, which is already unescaped.
	UnescapePathValues bool

	// RemoveExtraSlash a parameter can be parsed from the URL even with extra slashes.
	// See the PR #1817 and issue #1644
	RemoveExtraSlash bool

	// RemoteIPHeaders list of headers used to obtain the client IP when
	// `(*gin.Engine).ForwardedByClientIP` is `true` and
	// `(*gin.Context).Request.RemoteAddr` is matched by at least one of the
	// network origins of list defined by `(*gin.Engine).SetTrustedProxies()`.
	RemoteIPHeaders []string

	// TrustedPlatform if set to a constant of value gin.Platform*, trusts the headers set by
	// that platform, for example to determine the client IP
	TrustedPlatform string

	// MaxMultipartMemory value of 'maxMemory' param that is given to http.Request's ParseMultipartForm
	// method call.
	MaxMultipartMemory int64

	// UseH2C enable h2c support.
	UseH2C bool

	// ContextWithFallback enable fallback Context.Deadline(), Context.Done(), Context.Err() and Context.Value() when Context.Request.Context() is not nil.
	ContextWithFallback bool

	delims           render.Delims
	secureJSONPrefix string
	HTMLRender       render.HTMLRender
	FuncMap          template.FuncMap
	allNoRoute       HandlersChain
	allNoMethod      HandlersChain
	noRoute          HandlersChain
	noMethod         HandlersChain
	pool             sync.Pool
	trees            methodTrees
	maxParams        uint16
	maxSections      uint16
	trustedProxies   []string
	trustedCIDRs     []*net.IPNet
}

var _ IRouter = (*Engine)(nil)

// New returns a new blank Engine instance without any middleware attached.
// By default, the configuration is:
// - RedirectTrailingSlash:  true
// - RedirectFixedPath:      false
// - HandleMethodNotAllowed: false
// - ForwardedByClientIP:    true
// - UseRawPath:             false
// - UnescapePathValues:     true
func New(opts ...OptionFunc) *Engine {
	debugPrintWARNINGNew()
	engine := &Engine{
		RouterGroup: RouterGroup{
			Handlers: nil,
			basePath: "/",
			root:     true,
		},
		FuncMap:                template.FuncMap{},
		RedirectTrailingSlash:  true,
		RedirectFixedPath:      false,
		HandleMethodNotAllowed: false,
		ForwardedByClientIP:    true,
		RemoteIPHeaders:        []string{"X-Forwarded-For", "X-Real-IP"},
		TrustedPlatform:        defaultPlatform,
		UseRawPath:             false,
		RemoveExtraSlash:       false,
		UnescapePathValues:     true,
		MaxMultipartMemory:     defaultMultipartMemory,
		trees:                  make(methodTrees, 0, 9),
		delims:                 render.Delims{Left: "{{", Right: "}}"},
		secureJSONPrefix:       "while(1);",
		trustedProxies:         []string{"0.0.0.0/0", "::/0"},
		trustedCIDRs:           defaultTrustedCIDRs,
	}
	engine.RouterGroup.engine = engine
	engine.pool.New = func() any {
		return engine.allocateContext(engine.maxParams)
	}
	return engine.With(opts...)
}

// Default returns an Engine instance with the Logger and Recovery middleware already attached.
func Default(opts ...OptionFunc) *Engine {
	debugPrintWARNINGDefault()
	engine := New()
	engine.Use(Logger(), Recovery())
	return engine.With(opts...)
}

func (engine *Engine) Handler() http.Handler {
	if !engine.UseH2C {
		return engine
	}

	h2s := &http2.Server{}
	return h2c.NewHandler(engine, h2s)
}

func (engine *Engine) allocateContext(maxParams uint16) *Context {
	v := make(Params, 0, maxParams)
	skippedNodes := make([]skippedNode, 0, engine.maxSections)
	return &Context{engine: engine, params: &v, skippedNodes: &skippedNodes}
}

// Delims sets template left and right delims and returns an Engine instance.
func (engine *Engine) Delims(left, right string) *Engine {
	engine.delims = render.Delims{Left: left, Right: right}
	return engine
}

// SecureJsonPrefix sets the secureJSONPrefix used in Context.SecureJSON.
func (engine *Engine) SecureJsonPrefix(prefix string) *Engine {
	engine.secureJSONPrefix = prefix
	return engine
}

// LoadHTMLGlob loads HTML files identified by glob pattern
// and associates the result with HTML renderer.
func (engine *Engine) LoadHTMLGlob(pattern string) {
	left := engine.delims.Left
	right := engine.delims.Right
	templ := template.Must(template.New("").Delims(left, right).Funcs(engine.FuncMap).ParseGlob(pattern))

	if IsDebugging() {
		debugPrintLoadTemplate(templ)
		engine.HTMLRender = render.HTMLDebug{Glob: pattern, FuncMap: engine.FuncMap, Delims: engine.delims}
		return
	}

	engine.SetHTMLTemplate(templ)
}

// LoadHTMLFiles loads a slice of HTML files
// and associates the result with HTML renderer.
func (engine *Engine) LoadHTMLFiles(files ...string) {
	if IsDebugging() {
		engine.HTMLRender = render.HTMLDebug{Files: files, FuncMap: engine.FuncMap, Delims: engine.delims}
		return
	}

	templ := template.Must(template.New("").Delims(engine.delims.Left, engine.delims.Right).Funcs(engine.FuncMap).ParseFiles(files...))
	engine.SetHTMLTemplate(templ)
}

// SetHTMLTemplate associate a template with HTML renderer.
func (engine *Engine) SetHTMLTemplate(templ *template.Template) {
	if len(engine.trees) > 0 {
		debugPrintWARNINGSetHTMLTemplate()
	}

	engine.HTMLRender = render.HTMLProduction{Template: templ.Funcs(engine.FuncMap)}
}

// SetFuncMap sets the FuncMap used for template.FuncMap.
func (engine *Engine) SetFuncMap(funcMap template.FuncMap) {
	engine.FuncMap = funcMap
}

// NoRoute adds handlers for NoRoute. It returns a 404 code by default.
func (engine *Engine) NoRoute(handlers ...HandlerFunc) {
	engine.noRoute = handlers
	engine.rebuild404Handlers()
}

// NoMethod sets the handlers called when Engine.HandleMethodNotAllowed = true.
func (engine *Engine) NoMethod(handlers ...HandlerFunc) {
	engine.noMethod = handlers
	engine.rebuild405Handlers()
}

// Use attaches a global middleware to the router. i.e. the middleware attached through Use() will be
// included in the handlers chain for every single request. Even 404, 405, static files...
// For example, this is the right place for a logger or error management middleware.
func (engine *Engine) Use(middleware ...HandlerFunc) IRoutes {
	engine.RouterGroup.Use(middleware...)
	engine.rebuild404Handlers()
	engine.rebuild405Handlers()
	return engine
}

// With returns a Engine with the configuration set in the OptionFunc.
func (engine *Engine) With(opts ...OptionFunc) *Engine {
	for _, opt := range opts {
		opt(engine)
	}

	return engine
}

func (engine *Engine) rebuild404Handlers() {
	engine.allNoRoute = engine.combineHandlers(engine.noRoute)
}

func (engine *Engine) rebuild405Handlers() {
	engine.allNoMethod = engine.combineHandlers(engine.noMethod)
}

func (engine *Engine) addRoute(method, path string, handlers HandlersChain) {
	assert1(path[0] == '/', "path must begin with '/'")
	assert1(method != "", "HTTP method can not be empty")
	assert1(len(handlers) > 0, "there must be at least one handler")

	debugPrintRoute(method, path, handlers)

	root := engine.trees.get(method)
	if root == nil {
		root = new(node)
		root.fullPath = "/"
		engine.trees = append(engine.trees, methodTree{method: method, root: root})
	}
	root.addRoute(path, handlers)

	if paramsCount := countParams(path); paramsCount > engine.maxParams {
		engine.maxParams = paramsCount
	}

	if sectionsCount := countSections(path); sectionsCount > engine.maxSections {
		engine.maxSections = sectionsCount
	}
}

// Routes returns a slice of registered routes, including some useful information, such as:
// the http method, path and the handler name.
func (engine *Engine) Routes() (routes RoutesInfo) {
	for _, tree := range engine.trees {
		routes = iterate("", tree.method, routes, tree.root)
	}
	return routes
}

func iterate(path, method string, routes RoutesInfo, root *node) RoutesInfo {
	path += root.path
	if len(root.handlers) > 0 {
		handlerFunc := root.handlers.Last()
		routes = append(routes, RouteInfo{
			Method:      method,
			Path:        path,
			Handler:     nameOfFunction(handlerFunc),
			HandlerFunc: handlerFunc,
		})
	}
	for _, child := range root.children {
		routes = iterate(path, method, routes, child)
	}
	return routes
}

func (engine *Engine) prepareTrustedCIDRs() ([]*net.IPNet, error) {
	if engine.trustedProxies == nil {
		return nil, nil
	}

	cidr := make([]*net.IPNet, 0, len(engine.trustedProxies))
	for _, trustedProxy := range engine.trustedProxies {
		if !strings.Contains(trustedProxy, "/") {
			ip := parseIP(trustedProxy)
			if ip == nil {
				return cidr, &net.ParseError{Type: "IP address", Text: trustedProxy}
			}

			switch len(ip) {
			case net.IPv4len:
				trustedProxy += "/32"
			case net.IPv6len:
				trustedProxy += "/128"
			}
		}
		_, cidrNet, err := net.ParseCIDR(trustedProxy)
		if err != nil {
			return cidr, err
		}
		cidr = append(cidr, cidrNet)
	}
	return cidr, nil
}

// SetTrustedProxies set a list of network origins (IPv4 addresses,
// IPv4 CIDRs, IPv6 addresses or IPv6 CIDRs) from which to trust
// request's headers that contain alternative client IP when
// `(*gin.Engine).ForwardedByClientIP` is `true`. `TrustedProxies`
// feature is enabled by default, and it also trusts all proxies
// by default. If you want to disable this feature, use
// Engine.SetTrustedProxies(nil), then Context.ClientIP() will
// return the remote address directly.
func (engine *Engine) SetTrustedProxies(trustedProxies []string) error {
	engine.trustedProxies = trustedProxies
	return engine.parseTrustedProxies()
}

// isUnsafeTrustedProxies checks if Engine.trustedCIDRs contains all IPs, it's not safe if it has (returns true)
func (engine *Engine) isUnsafeTrustedProxies() bool {
	return engine.isTrustedProxy(net.ParseIP("0.0.0.0")) || engine.isTrustedProxy(net.ParseIP("::"))
}

// parseTrustedProxies parse Engine.trustedProxies to Engine.trustedCIDRs
func (engine *Engine) parseTrustedProxies() error {
	trustedCIDRs, err := engine.prepareTrustedCIDRs()
	engine.trustedCIDRs = trustedCIDRs
	return err
}

// isTrustedProxy will check whether the IP address is included in the trusted list according to Engine.trustedCIDRs
func (engine *Engine) isTrustedProxy(ip net.IP) bool {
	if engine.trustedCIDRs == nil {
		return false
	}
	for _, cidr := range engine.trustedCIDRs {
		if cidr.Contains(ip) {
			return true
		}
	}
	return false
}

// validateHeader will parse X-Forwarded-For header and return the trusted client IP address
func (engine *Engine) validateHeader(header string) (clientIP string, valid bool) {
	if header == "" {
		return "", false
	}
	items := strings.Split(header, ",")
	for i := len(items) - 1; i >= 0; i-- {
		ipStr := strings.TrimSpace(items[i])
		ip := net.ParseIP(ipStr)
		if ip == nil {
			break
		}

		// X-Forwarded-For is appended by proxy
		// Check IPs in reverse order and stop when find untrusted proxy
		if (i == 0) || (!engine.isTrustedProxy(ip)) {
			return ipStr, true
		}
	}
	return "", false
}

// updateRouteTree do update to the route tree recursively
func updateRouteTree(n *node) {
	n.path = strings.ReplaceAll(n.path, escapedColon, colon)
	n.fullPath = strings.ReplaceAll(n.fullPath, escapedColon, colon)
	n.indices = strings.ReplaceAll(n.indices, backslash, colon)
	if n.children == nil {
		return
	}
	for _, child := range n.children {
		updateRouteTree(child)
	}
}

// updateRouteTrees do update to the route trees
func (engine *Engine) updateRouteTrees() {
	for _, tree := range engine.trees {
		updateRouteTree(tree.root)
	}
}

// parseIP parse a string representation of an IP and returns a net.IP with the
// minimum byte representation or nil if input is invalid.
func parseIP(ip string) net.IP {
	parsedIP := net.ParseIP(ip)

	if ipv4 := parsedIP.To4(); ipv4 != nil {
		// return ip in a 4-byte representation
		return ipv4
	}

	// return ip in a 16-byte representation or nil
	return parsedIP
}

// Run attaches the router to a http.Server and starts listening and serving HTTP requests.
// It is a shortcut for http.ListenAndServe(addr, router)
// Note: this method will block the calling goroutine indefinitely unless an error happens.
func (engine *Engine) Run(addr ...string) (err error) {
	defer func() { debugPrintError(err) }()

	if engine.isUnsafeTrustedProxies() {
		debugPrint("[WARNING] You trusted all proxies, this is NOT safe. We recommend you to set a value.\n" +
			"Please check https://github.com/gin-gonic/gin/blob/master/docs/doc.md#dont-trust-all-proxies for details.")
	}
	engine.updateRouteTrees()
	address := resolveAddress(addr)
	debugPrint("Listening and serving HTTP on %s\n", address)
	err = http.ListenAndServe(address, engine.Handler())
	return
}

// RunTLS attaches the router to a http.Server and starts listening and serving HTTPS (secure) requests.
// It is a shortcut for http.ListenAndServeTLS(addr, certFile, keyFile, router)
// Note: this method will block the calling goroutine indefinitely unless an error happens.
func (engine *Engine) RunTLS(addr, certFile, keyFile string) (err error) {
	debugPrint("Listening and serving HTTPS on %s\n", addr)
	defer func() { debugPrintError(err) }()

	if engine.isUnsafeTrustedProxies() {
		debugPrint("[WARNING] You trusted all proxies, this is NOT safe. We recommend you to set a value.\n" +
			"Please check https://github.com/gin-gonic/gin/blob/master/docs/doc.md#dont-trust-all-proxies for details.")
	}

	err = http.ListenAndServeTLS(addr, certFile, keyFile, engine.Handler())
	return
}

// RunUnix attaches the router to a http.Server and starts listening and serving HTTP requests
// through the specified unix socket (i.e. a file).
// Note: this method will block the calling goroutine indefinitely unless an error happens.
func (engine *Engine) RunUnix(file string) (err error) {
	debugPrint("Listening and serving HTTP on unix:/%s", file)
	defer func() { debugPrintError(err) }()

	if engine.isUnsafeTrustedProxies() {
		debugPrint("[WARNING] You trusted all proxies, this is NOT safe. We recommend you to set a value.\n" +
			"Please check https://github.com/gin-gonic/gin/blob/master/docs/doc.md#dont-trust-all-proxies for details.")
	}

	listener, err := net.Listen("unix", file)
	if err != nil {
		return
	}
	defer listener.Close()
	defer os.Remove(file)

	err = http.Serve(listener, engine.Handler())
	return
}

// RunFd attaches the router to a http.Server and starts listening and serving HTTP requests
// through the specified file descriptor.
// Note: this method will block the calling goroutine indefinitely unless an error happens.
func (engine *Engine) RunFd(fd int) (err error) {
	debugPrint("Listening and serving HTTP on fd@%d", fd)
	defer func() { debugPrintError(err) }()

	if engine.isUnsafeTrustedProxies() {
		debugPrint("[WARNING] You trusted all proxies, this is NOT safe. We recommend you to set a value.\n" +
			"Please check https://github.com/gin-gonic/gin/blob/master/docs/doc.md#dont-trust-all-proxies for details.")
	}

	f := os.NewFile(uintptr(fd), fmt.Sprintf("fd@%d", fd))
	listener, err := net.FileListener(f)
	if err != nil {
		return
	}
	defer listener.Close()
	err = engine.RunListener(listener)
	return
}

// RunQUIC attaches the router to a http.Server and starts listening and serving QUIC requests.
// It is a shortcut for http3.ListenAndServeQUIC(addr, certFile, keyFile, router)
// Note: this method will block the calling goroutine indefinitely unless an error happens.
func (engine *Engine) RunQUIC(addr, certFile, keyFile string) (err error) {
	debugPrint("Listening and serving QUIC on %s\n", addr)
	defer func() { debugPrintError(err) }()

	if engine.isUnsafeTrustedProxies() {
		debugPrint("[WARNING] You trusted all proxies, this is NOT safe. We recommend you to set a value.\n" +
			"Please check https://pkg.go.dev/github.com/gin-gonic/gin#readme-don-t-trust-all-proxies for details.")
	}

	err = http3.ListenAndServeQUIC(addr, certFile, keyFile, engine.Handler())
	return
}

// RunListener attaches the router to a http.Server and starts listening and serving HTTP requests
// through the specified net.Listener
func (engine *Engine) RunListener(listener net.Listener) (err error) {
	debugPrint("Listening and serving HTTP on listener what's bind with address@%s", listener.Addr())
	defer func() { debugPrintError(err) }()

	if engine.isUnsafeTrustedProxies() {
		debugPrint("[WARNING] You trusted all proxies, this is NOT safe. We recommend you to set a value.\n" +
			"Please check https://github.com/gin-gonic/gin/blob/master/docs/doc.md#dont-trust-all-proxies for details.")
	}

	err = http.Serve(listener, engine.Handler())
	return
}

// ServeHTTP conforms to the http.Handler interface.
func (engine *Engine) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	c := engine.pool.Get().(*Context)
	c.writermem.reset(w)
	c.Request = req
	c.reset()

	engine.handleHTTPRequest(c)

	engine.pool.Put(c)
}

// HandleContext re-enters a context that has been rewritten.
// This can be done by setting c.Request.URL.Path to your new target.
// Disclaimer: You can loop yourself to deal with this, use wisely.
func (engine *Engine) HandleContext(c *Context) {
	oldIndexValue := c.index
	c.reset()
	engine.handleHTTPRequest(c)

	c.index = oldIndexValue
}

func (engine *Engine) handleHTTPRequest(c *Context) {
	httpMethod := c.Request.Method
	rPath := c.Request.URL.Path
	unescape := false
	if engine.UseRawPath && len(c.Request.URL.RawPath) > 0 {
		rPath = c.Request.URL.RawPath
		unescape = engine.UnescapePathValues
	}

	if engine.RemoveExtraSlash {
		rPath = cleanPath(rPath)
	}

	// Find root of the tree for the given HTTP method
	t := engine.trees
	for i, tl := 0, len(t); i < tl; i++ {
		if t[i].method != httpMethod {
			continue
		}
		root := t[i].root
		// Find route in tree
		value := root.getValue(rPath, c.params, c.skippedNodes, unescape)
		if value.params != nil {
			c.Params = *value.params
		}
		if value.handlers != nil {
			c.handlers = value.handlers
			c.fullPath = value.fullPath
			c.Next()
			c.writermem.WriteHeaderNow()
			return
		}
		if httpMethod != http.MethodConnect && rPath != "/" {
			if value.tsr && engine.RedirectTrailingSlash {
				redirectTrailingSlash(c)
				return
			}
			if engine.RedirectFixedPath && redirectFixedPath(c, root, engine.RedirectFixedPath) {
				return
			}
		}
		break
	}

	if engine.HandleMethodNotAllowed {
		// According to RFC 7231 section 6.5.5, MUST generate an Allow header field in response
		// containing a list of the target resource's currently supported methods.
		allowed := make([]string, 0, len(t)-1)
		for _, tree := range engine.trees {
			if tree.method == httpMethod {
				continue
			}
			if value := tree.root.getValue(rPath, nil, c.skippedNodes, unescape); value.handlers != nil {
				allowed = append(allowed, tree.method)
			}
		}
		if len(allowed) > 0 {
			c.handlers = engine.allNoMethod
			c.writermem.Header().Set("Allow", strings.Join(allowed, ", "))
			serveError(c, http.StatusMethodNotAllowed, default405Body)
			return
		}
	}

	c.handlers = engine.allNoRoute
	serveError(c, http.StatusNotFound, default404Body)
}

var mimePlain = []string{MIMEPlain}

func serveError(c *Context, code int, defaultMessage []byte) {
	c.writermem.status = code
	c.Next()
	if c.writermem.Written() {
		return
	}
	if c.writermem.Status() == code {
		c.writermem.Header()["Content-Type"] = mimePlain
		_, err := c.Writer.Write(defaultMessage)
		if err != nil {
			debugPrint("cannot write message to writer during serve error: %v", err)
		}
		return
	}
	c.writermem.WriteHeaderNow()
}

func redirectTrailingSlash(c *Context) {
	req := c.Request
	p := req.URL.Path
	if prefix := path.Clean(c.Request.Header.Get("X-Forwarded-Prefix")); prefix != "." {
		prefix = regSafePrefix.ReplaceAllString(prefix, "")
		prefix = regRemoveRepeatedChar.ReplaceAllString(prefix, "/")

		p = prefix + "/" + req.URL.Path
	}
	req.URL.Path = p + "/"
	if length := len(p); length > 1 && p[length-1] == '/' {
		req.URL.Path = p[:length-1]
	}
	redirectRequest(c)
}

func redirectFixedPath(c *Context, root *node, trailingSlash bool) bool {
	req := c.Request
	rPath := req.URL.Path

	if fixedPath, ok := root.findCaseInsensitivePath(cleanPath(rPath), trailingSlash); ok {
		req.URL.Path = bytesconv.BytesToString(fixedPath)
		redirectRequest(c)
		return true
	}
	return false
}

func redirectRequest(c *Context) {
	req := c.Request
	rPath := req.URL.Path
	rURL := req.URL.String()

	code := http.StatusMovedPermanently // Permanent redirect, request with GET method
	if req.Method != http.MethodGet {
		code = http.StatusTemporaryRedirect
	}
	debugPrint("redirecting request %d: %s --> %s", code, rPath, rURL)
	http.Redirect(c.Writer, req, rURL, code)
	c.writermem.WriteHeaderNow()
}