// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package redirect provides hooks to register HTTP handlers that redirect old
// godoc paths to their new equivalents and assist in accessing the issue
// tracker, wiki, code review system, etc.
package redirect // import "golang.org/x/website/internal/redirect"

import (
	"context"
	_ "embed"
	"fmt"
	"html/template"
	"net/http"
	"regexp"
	"strconv"
	"strings"
	"sync"
	"time"

	"golang.org/x/net/context/ctxhttp"
)

// Register registers HTTP handlers that redirect old godoc paths to their new
// equivalents and assist in accessing the issue tracker, wiki, code review
// system, etc.
func Register(mux *http.ServeMux) {
	handlePathRedirects(mux, pkgRedirects, "/pkg/")
	handlePathRedirects(mux, cmdRedirects, "/cmd/")
	for prefix, redirect := range prefixHelpers {
		p := "/" + prefix + "/"
		mux.Handle(p, PrefixHandler(p, redirect))
	}
	for path, redirect := range redirects {
		mux.Handle(path, Handler(redirect))
	}
	for path, redirect := range blogRedirects {
		mux.Handle("/blog"+path, Handler("/blog/"+redirect))
	}
	for _, path := range newIssueRedirects {
		mux.Handle(path, newIssueHandler(path))
	}
	// NB: /src/pkg (sans trailing slash) is the index of packages.
	mux.HandleFunc("/src/pkg/", srcPkgHandler)
	mux.HandleFunc("/cl/", clHandler)
	mux.HandleFunc("/change/", changeHandler)
	mux.HandleFunc("/design/", designHandler)
}

func handlePathRedirects(mux *http.ServeMux, redirects map[string]string, prefix string) {
	for source, target := range redirects {
		h := Handler(prefix + target + "/")
		p := prefix + source
		mux.Handle(p, h)
		mux.Handle(p+"/", h)
	}
}

// Packages that were renamed between r60 and go1.
var pkgRedirects = map[string]string{
	"asn1":              "encoding/asn1",
	"big":               "math/big",
	"cmath":             "math/cmplx",
	"csv":               "encoding/csv",
	"exec":              "os/exec",
	"exp/template/html": "html/template",
	"gob":               "encoding/gob",
	"http":              "net/http",
	"http/cgi":          "net/http/cgi",
	"http/fcgi":         "net/http/fcgi",
	"http/httptest":     "net/http/httptest",
	"http/pprof":        "net/http/pprof",
	"json":              "encoding/json",
	"mail":              "net/mail",
	"rand":              "math/rand",
	"rpc":               "net/rpc",
	"rpc/jsonrpc":       "net/rpc/jsonrpc",
	"scanner":           "text/scanner",
	"smtp":              "net/smtp",
	"tabwriter":         "text/tabwriter",
	"template":          "text/template",
	"template/parse":    "text/template/parse",
	"url":               "net/url",
	"utf16":             "unicode/utf16",
	"utf8":              "unicode/utf8",
	"xml":               "encoding/xml",
}

// Commands that were renamed between r60 and go1.
var cmdRedirects = map[string]string{
	"gofix":     "fix",
	"goinstall": "go",
	"gopack":    "pack",
	"gotest":    "go",
	"govet":     "vet",
	"goyacc":    "yacc",
}

var redirects = map[string]string{
	"/build":      "https://build.golang.org",
	"/change":     "https://go.googlesource.com/go",
	"/cl":         "https://go-review.googlesource.com",
	"/cmd/godoc/": "https://pkg.go.dev/golang.org/x/tools/cmd/godoc",
	"/issue":      "https://github.com/golang/go/issues",
	"/issues":     "https://github.com/golang/go/issues",
	"/design":     "https://go.googlesource.com/proposal/+/master/design",

	// Be nice to people who are looking in the wrong place.
	"/pkg/C/":   "/cmd/cgo/",
	"/doc/mem":  "/ref/mem",
	"/doc/spec": "/ref/spec",

	"/wiki": "https://github.com/golang/go/wiki",

	"/doc/articles/c_go_cgo.html":                    "/blog/c-go-cgo",
	"/doc/articles/concurrency_patterns.html":        "/blog/go-concurrency-patterns-timing-out-and",
	"/doc/articles/defer_panic_recover.html":         "/blog/defer-panic-and-recover",
	"/doc/articles/error_handling.html":              "/blog/error-handling-and-go",
	"/doc/articles/gobs_of_data.html":                "/blog/gobs-of-data",
	"/doc/articles/godoc_documenting_go_code.html":   "/blog/godoc-documenting-go-code",
	"/doc/articles/gos_declaration_syntax.html":      "/blog/gos-declaration-syntax",
	"/doc/articles/image_draw.html":                  "/blog/go-imagedraw-package",
	"/doc/articles/image_package.html":               "/blog/go-image-package",
	"/doc/articles/json_and_go.html":                 "/blog/json-and-go",
	"/doc/articles/json_rpc_tale_of_interfaces.html": "/blog/json-rpc-tale-of-interfaces",
	"/doc/articles/laws_of_reflection.html":          "/blog/laws-of-reflection",
	"/doc/articles/slices_usage_and_internals.html":  "/blog/go-slices-usage-and-internals",
	"/doc/go_for_cpp_programmers.html":               "/wiki/GoForCPPProgrammers",
	"/doc/go_tutorial.html":                          "/tour",
}

var newIssueRedirects = [...]string{
	"/issue/new",
	"/issue/new/",
	"/issues/new",
	"/issues/new/",
}

var prefixHelpers = map[string]string{
	"issue":  "https://github.com/golang/go/issues/",
	"issues": "https://github.com/golang/go/issues/",
	"wiki":   "https://github.com/golang/go/wiki/",
}

func Handler(target string) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		url := target
		if qs := r.URL.RawQuery; qs != "" {
			url += "?" + qs
		}
		http.Redirect(w, r, url, http.StatusMovedPermanently)
	})
}

// validPrefixID is used to validate issue and wiki path suffixes
var validPrefixID = regexp.MustCompile(`^[A-Za-z0-9\-._]*/?$`)

func PrefixHandler(prefix, baseURL string) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if p := r.URL.Path; p == prefix {
			// redirect /prefix/ to /prefix
			http.Redirect(w, r, p[:len(p)-1], http.StatusFound)
			return
		}
		id := r.URL.Path[len(prefix):]
		if !validPrefixID.MatchString(id) {
			http.Error(w, "Not found", http.StatusNotFound)
			return
		}
		target := baseURL + id
		http.Redirect(w, r, target, http.StatusFound)
	})
}

// newIssueHandler handles /issue/new and similar requests,
// redirecting to a "New Issue" UI in the main Go issue tracker.
func newIssueHandler(source string) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if r.URL.Path != source {
			http.Error(w, "Not found", http.StatusNotFound)
			return
		}
		target := "https://github.com/golang/go/issues/new"
		if qs := r.URL.RawQuery; qs == "" {
			// There are many "go.dev/issue/new" links that led to a good experience
			// of reporting an issue when there was a single issue template.
			// As of CL 366736 there are many templates, and the same URL results
			// in an empty new issue UI, which defeats having any templates.
			//
			// Handle this case specially and redirect to "/new/choose" instead,
			// at least until GitHub changes their behavior. See go.dev/issue/29839.
			target += "/choose"
		} else {
			// Query options like ?title=...&body=...&labels=... only work on /new.
			target += "?" + qs
		}
		http.Redirect(w, r, target, http.StatusFound)
	})
}

// Redirect requests from the old "/src/pkg/foo" to the new "/src/foo".
// See https://golang.org/s/go14nopkg
func srcPkgHandler(w http.ResponseWriter, r *http.Request) {
	r.URL.Path = "/src/" + r.URL.Path[len("/src/pkg/"):]
	http.Redirect(w, r, r.URL.String(), http.StatusMovedPermanently)
}

// validCLID is used to validate cl path suffixes. It supports both the
// bare ID, as well as the patchset syntax (i.e. 1234/2.)
var validCLID = regexp.MustCompile(`^[0-9]+(/[0-9]+)?/?$`)

func clHandler(w http.ResponseWriter, r *http.Request) {
	const prefix = "/cl/"
	if p := r.URL.Path; p == prefix {
		// redirect /prefix/ to /prefix
		http.Redirect(w, r, p[:len(p)-1], http.StatusFound)
		return
	}
	id := r.URL.Path[len(prefix):]

	// Some shorteners blindly rewrite go-review.googlesource.com/ to go.dev/cl/
	// but Gerrit has changed the URL schema to start with c/<repo>/+/<id>
	// instead of just <id>. So we now see URLs like go.dev/cl/c/go/+/12345.
	// Assume that the leading c/ means it is for Gerrit and blindly redirect.
	if strings.HasPrefix(id, "c/") {
		http.Redirect(w, r, "https://go-review.googlesource.com/"+id, http.StatusFound)
		return
	}

	// support /cl/152700045/, which is used in commit 0edafefc36.
	id = strings.TrimSuffix(id, "/")
	if !validCLID.MatchString(id) {
		http.Error(w, "Not found", http.StatusNotFound)
		return
	}

	// If the ID contains a slash, it is likely pointing towards a
	// specific patchset. In that case, prefix the id with 'c/',
	// which Gerrit uses to indicate a specific revision.
	if strings.Contains(id, "/") {
		id = "c/" + id
	}

	target := ""

	if n, err := strconv.Atoi(id); err == nil && isRietveldCL(n) {
		// Issue 28836: if this Rietveld CL happens to
		// also be a Gerrit CL, render a disambiguation HTML
		// page with two links instead. We need to make a
		// Gerrit API call to figure that out, but we cache
		// known Gerrit CLs so it's done at most once per CL.
		if ok, err := isGerritCL(r.Context(), n); err == nil && ok {
			w.Header().Set("Content-Type", "text/html; charset=utf-8")
			clDisambiguationHTML.Execute(w, n)
			return
		}

		target = "https://codereview.appspot.com/" + id
	} else {
		target = "https://go-review.googlesource.com/" + id
	}
	http.Redirect(w, r, target, http.StatusFound)
}

var clDisambiguationHTML = template.Must(template.New("").Parse(`<!DOCTYPE html>
<html lang="en">
	<head>
		<title>Go CL {{.}} Disambiguation</title>
		<meta name="viewport" content="width=device-width">
	</head>
	<body>
		CL number {{.}} exists in both Gerrit (the current code review system)
		and Rietveld (the previous code review system). Please make a choice:

		<ul>
			<li><a href="https://go-review.googlesource.com/{{.}}">Gerrit CL {{.}}</a></li>
			<li><a href="https://codereview.appspot.com/{{.}}">Rietveld CL {{.}}</a></li>
		</ul>
	</body>
</html>`))

// isGerritCL reports whether a Gerrit CL with the specified numeric change ID (e.g., "4247")
// is known to exist by querying the Gerrit API at https://go-review.googlesource.com.
// isGerritCL uses gerritCLCache as a cache of Gerrit CL IDs that exist.
func isGerritCL(ctx context.Context, id int) (bool, error) {
	// Check cache first.
	gerritCLCache.Lock()
	ok := gerritCLCache.exist[id]
	gerritCLCache.Unlock()
	if ok {
		return true, nil
	}

	// Query the Gerrit API Get Change endpoint, as documented at
	// https://gerrit-review.googlesource.com/Documentation/rest-api-changes.html#get-change.
	ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
	defer cancel()
	resp, err := ctxhttp.Get(ctx, nil, fmt.Sprintf("https://go-review.googlesource.com/changes/%d", id))
	if err != nil {
		return false, err
	}
	resp.Body.Close()
	switch resp.StatusCode {
	case http.StatusOK:
		// A Gerrit CL with this ID exists. Add it to cache.
		gerritCLCache.Lock()
		gerritCLCache.exist[id] = true
		gerritCLCache.Unlock()
		return true, nil
	case http.StatusNotFound:
		// A Gerrit CL with this ID doesn't exist. It may get created in the future.
		return false, nil
	default:
		return false, fmt.Errorf("unexpected status code: %v", resp.Status)
	}
}

var gerritCLCache = struct {
	sync.Mutex
	exist map[int]bool // exist is a set of Gerrit CL IDs that are known to exist.
}{exist: make(map[int]bool)}

//go:embed hg-git-mapping.bin
var hgGitMappingBin []byte

var changeMap = hashMap(hgGitMappingBin)

func changeHandler(w http.ResponseWriter, r *http.Request) {
	const prefix = "/change/"
	if p := r.URL.Path; p == prefix {
		// redirect /prefix/ to /prefix
		http.Redirect(w, r, p[:len(p)-1], http.StatusFound)
		return
	}
	hash := r.URL.Path[len(prefix):]
	target := "https://go.googlesource.com/go/+/" + hash
	if git := changeMap.Lookup(hash); git > 0 {
		target = fmt.Sprintf("https://go.googlesource.com/%v/+/%v", git.Repo(), git.Hash())
	}
	http.Redirect(w, r, target, http.StatusFound)
}

func designHandler(w http.ResponseWriter, r *http.Request) {
	const prefix = "/design/"
	if p := r.URL.Path; p == prefix {
		// redirect /prefix/ to /prefix
		http.Redirect(w, r, p[:len(p)-1], http.StatusFound)
		return
	}
	name := r.URL.Path[len(prefix):]
	target := "https://go.googlesource.com/proposal/+/master/design/" + name + ".md"
	http.Redirect(w, r, target, http.StatusFound)
}