internal/codewalk/codewalk.go - website - Git at Google

 // Copyright 2010 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 codewalk implements support for codewalk documents.
 //
 // The /doc/codewalk/ tree is synthesized from codewalk descriptions,
 // files named _content/doc/codewalk/*.xml.
 // For an example and a description of the format, see
 // https://golang.org/doc/codewalk/codewalk.
 // That page is itself a codewalk; the source code for it is
 // _content/doc/codewalk/codewalk.xml.
 package codewalk

 import (
 	"encoding/xml"
 	"errors"
 	"fmt"
 	"io"
 	"io/fs"
 	"log"
 	"net/http"
 	"os"
 	"path"
 	"regexp"
 	"sort"
 	"strconv"
 	"strings"
 	"unicode/utf8"

 	"golang.org/x/website/internal/backport/html/template"
 	"golang.org/x/website/internal/web"
 )

 type server struct {
 	fsys fs.FS
 	site *web.Site
 }

 // NewServer returns a new server handling codewalk documents.
 func NewServer(fsys fs.FS, site *web.Site) http.Handler {
 	return &server{fsys, site}
 }

 // Handler for /doc/codewalk/ and below.
 func (s *server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	relpath := path.Clean(r.URL.Path[1:])

 	r.ParseForm()
 	if f := r.FormValue("fileprint"); f != "" {
 		s.codewalkFileprint(w, r, f)
 		return
 	}

 	// If directory exists, serve list of code walks.
 	dir, err := fs.Stat(s.fsys, relpath)
 	if err == nil && dir.IsDir() {
 		s.codewalkDir(w, r, relpath)
 		return
 	}

 	// If file exists, serve using standard file server.
 	if err == nil {
 		s.site.ServeHTTP(w, r)
 		return
 	}

 	// Otherwise append .xml and hope to find
 	// a codewalk description, but before trim
 	// the trailing /.
 	cw, err := s.loadCodewalk(relpath + ".xml")
 	if err != nil {
 		log.Print(err)
 		s.site.ServeError(w, r, err)
 		return
 	}

 	// Canonicalize the path and redirect if changed
 	if redir(w, r) {
 		return
 	}

 	s.site.ServePage(w, r, web.Page{
 		"title":    "Codewalk: " + cw.Title,
 		"tabTitle": cw.Title,
 		"layout":   "codewalk",
 		"codewalk": cw,
 	})
 }

 func redir(w http.ResponseWriter, r *http.Request) (redirected bool) {
 	canonical := path.Clean(r.URL.Path)
 	if !strings.HasSuffix(canonical, "/") {
 		canonical += "/"
 	}
 	if r.URL.Path != canonical {
 		url := *r.URL
 		url.Path = canonical
 		http.Redirect(w, r, url.String(), http.StatusMovedPermanently)
 		redirected = true
 	}
 	return
 }

 // A codewalk represents a single codewalk read from an XML file.
 type codewalk struct {
 	Title string      `xml:"title,attr"`
 	File  []string    `xml:"file"`
 	Step  []*codestep `xml:"step"`
 }

 // A codestep is a single step in a codewalk.
 type codestep struct {
 	// Filled in from XML
 	Src   string `xml:"src,attr"`
 	Title string `xml:"title,attr"`
 	XML   string `xml:",innerxml"`

 	// Derived from Src; not in XML.
 	Err    error
 	File   string
 	Lo     int
 	LoByte int
 	Hi     int
 	HiByte int
 	Data   []byte
 }

 func (c *codestep) HTML() template.HTML {
 	return template.HTML(c.XML)
 }

 // String method for printing in template.
 // Formats file address nicely.
 func (st *codestep) String() string {
 	s := st.File
 	if st.Lo != 0 || st.Hi != 0 {
 		s += fmt.Sprintf(":%d", st.Lo)
 		if st.Lo != st.Hi {
 			s += fmt.Sprintf(",%d", st.Hi)
 		}
 	}
 	return s
 }

 // loadCodewalk reads a codewalk from the named XML file.
 func (s *server) loadCodewalk(filename string) (*codewalk, error) {
 	f, err := s.fsys.Open(filename)
 	if err != nil {
 		return nil, err
 	}
 	defer f.Close()
 	cw := new(codewalk)
 	d := xml.NewDecoder(f)
 	d.Entity = xml.HTMLEntity
 	err = d.Decode(cw)
 	if err != nil {
 		return nil, &os.PathError{Op: "parsing", Path: filename, Err: err}
 	}

 	// Compute file list, evaluate line numbers for addresses.
 	m := make(map[string]bool)
 	for _, st := range cw.Step {
 		i := strings.Index(st.Src, ":")
 		if i < 0 {
 			i = len(st.Src)
 		}
 		filename := st.Src[0:i]
 		data, err := fs.ReadFile(s.fsys, filename)
 		if err != nil {
 			st.Err = err
 			continue
 		}
 		if i < len(st.Src) {
 			lo, hi, err := addrToByteRange(st.Src[i+1:], 0, data)
 			if err != nil {
 				st.Err = err
 				continue
 			}
 			// Expand match to line boundaries.
 			for lo > 0 && data[lo-1] != '\n' {
 				lo--
 			}
 			for hi < len(data) && (hi == 0 || data[hi-1] != '\n') {
 				hi++
 			}
 			st.Lo = byteToLine(data, lo)
 			st.Hi = byteToLine(data, hi-1)
 		}
 		st.Data = data
 		st.File = filename
 		m[filename] = true
 	}

 	// Make list of files
 	cw.File = make([]string, len(m))
 	i := 0
 	for f := range m {
 		cw.File[i] = f
 		i++
 	}
 	sort.Strings(cw.File)

 	return cw, nil
 }

 // codewalkDir serves the codewalk directory listing.
 // It scans the directory for subdirectories or files named *.xml
 // and prepares a table.
 func (s *server) codewalkDir(w http.ResponseWriter, r *http.Request, relpath string) {
 	type elem struct {
 		Name  string
 		Title string
 	}

 	dir, err := fs.ReadDir(s.fsys, relpath)
 	if err != nil {
 		log.Print(err)
 		s.site.ServeError(w, r, err)
 		return
 	}
 	var v []interface{}
 	for _, fi := range dir {
 		name := fi.Name()
 		if fi.IsDir() {
 			v = append(v, &elem{name + "/", ""})
 		} else if strings.HasSuffix(name, ".xml") {
 			cw, err := s.loadCodewalk(relpath + "/" + name)
 			if err != nil {
 				continue
 			}
 			v = append(v, &elem{name[0 : len(name)-len(".xml")], cw.Title})
 		}
 	}

 	s.site.ServePage(w, r, web.Page{
 		"title":  "Codewalks",
 		"layout": "codewalkdir",
 		"dirs":   v,
 	})
 }

 // codewalkFileprint serves requests with ?fileprint=f&lo=lo&hi=hi.
 // The filename f has already been retrieved and is passed as an argument.
 // Lo and hi are the numbers of the first and last line to highlight
 // in the response.  This format is used for the middle window pane
 // of the codewalk pages.  It is a separate iframe and does not get
 // the usual godoc HTML wrapper.
 func (s *server) codewalkFileprint(w http.ResponseWriter, r *http.Request, f string) {
 	relpath := strings.Trim(path.Clean(f), "/")
 	data, err := fs.ReadFile(s.fsys, relpath)
 	if err != nil {
 		log.Print(err)
 		s.site.ServeError(w, r, err)
 		return
 	}
 	lo, _ := strconv.Atoi(r.FormValue("lo"))
 	hi, _ := strconv.Atoi(r.FormValue("hi"))
 	if hi < lo {
 		hi = lo
 	}
 	lo = lineToByte(data, lo)
 	hi = lineToByte(data, hi+1)

 	// Put the mark 4 lines before lo, so that the iframe
 	// shows a few lines of context before the highlighted
 	// section.
 	n := 4
 	mark := lo
 	for ; mark > 0 && n > 0; mark-- {
 		if data[mark-1] == '\n' {
 			if n--; n == 0 {
 				break
 			}
 		}
 	}

 	io.WriteString(w, `<style type="text/css">@import "/doc/codewalk/codewalk.css";</style><pre>`)
 	template.HTMLEscape(w, data[0:mark])
 	io.WriteString(w, "<a name='mark'></a>")
 	template.HTMLEscape(w, data[mark:lo])
 	if lo < hi {
 		io.WriteString(w, "<div class='codewalkhighlight'>")
 		template.HTMLEscape(w, data[lo:hi])
 		io.WriteString(w, "</div>")
 	}
 	template.HTMLEscape(w, data[hi:])
 	io.WriteString(w, "</pre>")
 }

 // addrToByte evaluates the given address starting at offset start in data.
 // It returns the lo and hi byte offset of the matched region within data.
 // See https://9p.io/sys/doc/sam/sam.html Table II
 // for details on the syntax.
 func addrToByteRange(addr string, start int, data []byte) (lo, hi int, err error) {
 	var (
 		dir        byte
 		prevc      byte
 		charOffset bool
 	)
 	lo = start
 	hi = start
 	for addr != "" && err == nil {
 		c := addr[0]
 		switch c {
 		default:
 			err = errors.New("invalid address syntax near " + string(c))
 		case ',':
 			if len(addr) == 1 {
 				hi = len(data)
 			} else {
 				_, hi, err = addrToByteRange(addr[1:], hi, data)
 			}
 			return

 		case '+', '-':
 			if prevc == '+' || prevc == '-' {
 				lo, hi, err = addrNumber(data, lo, hi, prevc, 1, charOffset)
 			}
 			dir = c

 		case '$':
 			lo = len(data)
 			hi = len(data)
 			if len(addr) > 1 {
 				dir = '+'
 			}

 		case '#':
 			charOffset = true

 		case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
 			var i int
 			for i = 1; i < len(addr); i++ {
 				if addr[i] < '0' || addr[i] > '9' {
 					break
 				}
 			}
 			var n int
 			n, err = strconv.Atoi(addr[0:i])
 			if err != nil {
 				break
 			}
 			lo, hi, err = addrNumber(data, lo, hi, dir, n, charOffset)
 			dir = 0
 			charOffset = false
 			prevc = c
 			addr = addr[i:]
 			continue

 		case '/':
 			var i, j int
 		Regexp:
 			for i = 1; i < len(addr); i++ {
 				switch addr[i] {
 				case '\\':
 					i++
 				case '/':
 					j = i + 1
 					break Regexp
 				}
 			}
 			if j == 0 {
 				j = i
 			}
 			pattern := addr[1:i]
 			lo, hi, err = addrRegexp(data, lo, hi, dir, pattern)
 			prevc = c
 			addr = addr[j:]
 			continue
 		}
 		prevc = c
 		addr = addr[1:]
 	}

 	if err == nil && dir != 0 {
 		lo, hi, err = addrNumber(data, lo, hi, dir, 1, charOffset)
 	}
 	if err != nil {
 		return 0, 0, err
 	}
 	return lo, hi, nil
 }

 // addrNumber applies the given dir, n, and charOffset to the address lo, hi.
 // dir is '+' or '-', n is the count, and charOffset is true if the syntax
 // used was #n.  Applying +n (or +#n) means to advance n lines
 // (or characters) after hi.  Applying -n (or -#n) means to back up n lines
 // (or characters) before lo.
 // The return value is the new lo, hi.
 func addrNumber(data []byte, lo, hi int, dir byte, n int, charOffset bool) (int, int, error) {
 	switch dir {
 	case 0:
 		lo = 0
 		hi = 0
 		fallthrough

 	case '+':
 		if charOffset {
 			pos := hi
 			for ; n > 0 && pos < len(data); n-- {
 				_, size := utf8.DecodeRune(data[pos:])
 				pos += size
 			}
 			if n == 0 {
 				return pos, pos, nil
 			}
 			break
 		}
 		// find next beginning of line
 		if hi > 0 {
 			for hi < len(data) && data[hi-1] != '\n' {
 				hi++
 			}
 		}
 		lo = hi
 		if n == 0 {
 			return lo, hi, nil
 		}
 		for ; hi < len(data); hi++ {
 			if data[hi] != '\n' {
 				continue
 			}
 			switch n--; n {
 			case 1:
 				lo = hi + 1
 			case 0:
 				return lo, hi + 1, nil
 			}
 		}

 	case '-':
 		if charOffset {
 			// Scan backward for bytes that are not UTF-8 continuation bytes.
 			pos := lo
 			for ; pos > 0 && n > 0; pos-- {
 				if data[pos]&0xc0 != 0x80 {
 					n--
 				}
 			}
 			if n == 0 {
 				return pos, pos, nil
 			}
 			break
 		}
 		// find earlier beginning of line
 		for lo > 0 && data[lo-1] != '\n' {
 			lo--
 		}
 		hi = lo
 		if n == 0 {
 			return lo, hi, nil
 		}
 		for ; lo >= 0; lo-- {
 			if lo > 0 && data[lo-1] != '\n' {
 				continue
 			}
 			switch n--; n {
 			case 1:
 				hi = lo
 			case 0:
 				return lo, hi, nil
 			}
 		}
 	}

 	return 0, 0, errors.New("address out of range")
 }

 // addrRegexp searches for pattern in the given direction starting at lo, hi.
 // The direction dir is '+' (search forward from hi) or '-' (search backward from lo).
 // Backward searches are unimplemented.
 func addrRegexp(data []byte, lo, hi int, dir byte, pattern string) (int, int, error) {
 	re, err := regexp.Compile(pattern)
 	if err != nil {
 		return 0, 0, err
 	}
 	if dir == '-' {
 		// Could implement reverse search using binary search
 		// through file, but that seems like overkill.
 		return 0, 0, errors.New("reverse search not implemented")
 	}
 	m := re.FindIndex(data[hi:])
 	if len(m) > 0 {
 		m[0] += hi
 		m[1] += hi
 	} else if hi > 0 {
 		// No match.  Wrap to beginning of data.
 		m = re.FindIndex(data)
 	}
 	if len(m) == 0 {
 		return 0, 0, errors.New("no match for " + pattern)
 	}
 	return m[0], m[1], nil
 }

 // lineToByte returns the byte index of the first byte of line n.
 // Line numbers begin at 1.
 func lineToByte(data []byte, n int) int {
 	if n <= 1 {
 		return 0
 	}
 	n--
 	for i, c := range data {
 		if c == '\n' {
 			if n--; n == 0 {
 				return i + 1
 			}
 		}
 	}
 	return len(data)
 }

 // byteToLine returns the number of the line containing the byte at index i.
 func byteToLine(data []byte, i int) int {
 	l := 1
 	for j, c := range data {
 		if j == i {
 			return l
 		}
 		if c == '\n' {
 			l++
 		}
 	}
 	return l
 }
	// Copyright 2010 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 codewalk implements support for codewalk documents.
	//
	// The /doc/codewalk/ tree is synthesized from codewalk descriptions,
	// files named _content/doc/codewalk/*.xml.
	// For an example and a description of the format, see
	// https://golang.org/doc/codewalk/codewalk.
	// That page is itself a codewalk; the source code for it is
	// _content/doc/codewalk/codewalk.xml.
	package codewalk

	import (
	"encoding/xml"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"log"
	"net/http"
	"os"
	"path"
	"regexp"
	"sort"
	"strconv"
	"strings"
	"unicode/utf8"

	"golang.org/x/website/internal/backport/html/template"
	"golang.org/x/website/internal/web"
	)

	type server struct {
	fsys fs.FS
	site *web.Site
	}

	// NewServer returns a new server handling codewalk documents.
	func NewServer(fsys fs.FS, site *web.Site) http.Handler {
	return &server{fsys, site}
	}

	// Handler for /doc/codewalk/ and below.
	func (s server) ServeHTTP(w http.ResponseWriter, r http.Request) {
	relpath := path.Clean(r.URL.Path[1:])

	r.ParseForm()
	if f := r.FormValue("fileprint"); f != "" {
	s.codewalkFileprint(w, r, f)
	return
	}

	// If directory exists, serve list of code walks.
	dir, err := fs.Stat(s.fsys, relpath)
	if err == nil && dir.IsDir() {
	s.codewalkDir(w, r, relpath)
	return
	}

	// If file exists, serve using standard file server.
	if err == nil {
	s.site.ServeHTTP(w, r)
	return
	}

	// Otherwise append .xml and hope to find
	// a codewalk description, but before trim
	// the trailing /.
	cw, err := s.loadCodewalk(relpath + ".xml")
	if err != nil {
	log.Print(err)
	s.site.ServeError(w, r, err)
	return
	}

	// Canonicalize the path and redirect if changed
	if redir(w, r) {
	return
	}

	s.site.ServePage(w, r, web.Page{
	"title": "Codewalk: " + cw.Title,
	"tabTitle": cw.Title,
	"layout": "codewalk",
	"codewalk": cw,
	})
	}

	func redir(w http.ResponseWriter, r *http.Request) (redirected bool) {
	canonical := path.Clean(r.URL.Path)
	if !strings.HasSuffix(canonical, "/") {
	canonical += "/"
	}
	if r.URL.Path != canonical {
	url := *r.URL
	url.Path = canonical
	http.Redirect(w, r, url.String(), http.StatusMovedPermanently)
	redirected = true
	}
	return
	}

	// A codewalk represents a single codewalk read from an XML file.
	type codewalk struct {
	Title string `xml:"title,attr"`
	File []string `xml:"file"`
	Step []*codestep `xml:"step"`
	}

	// A codestep is a single step in a codewalk.
	type codestep struct {
	// Filled in from XML
	Src string `xml:"src,attr"`
	Title string `xml:"title,attr"`
	XML string `xml:",innerxml"`

	// Derived from Src; not in XML.
	Err error
	File string
	Lo int
	LoByte int
	Hi int
	HiByte int
	Data []byte
	}

	func (c *codestep) HTML() template.HTML {
	return template.HTML(c.XML)
	}

	// String method for printing in template.
	// Formats file address nicely.
	func (st *codestep) String() string {
	s := st.File
	if st.Lo != 0 \|\| st.Hi != 0 {
	s += fmt.Sprintf(":%d", st.Lo)
	if st.Lo != st.Hi {
	s += fmt.Sprintf(",%d", st.Hi)
	}
	}
	return s
	}

	// loadCodewalk reads a codewalk from the named XML file.
	func (s server) loadCodewalk(filename string) (codewalk, error) {
	f, err := s.fsys.Open(filename)
	if err != nil {
	return nil, err
	}
	defer f.Close()
	cw := new(codewalk)
	d := xml.NewDecoder(f)
	d.Entity = xml.HTMLEntity
	err = d.Decode(cw)
	if err != nil {
	return nil, &os.PathError{Op: "parsing", Path: filename, Err: err}
	}

	// Compute file list, evaluate line numbers for addresses.
	m := make(map[string]bool)
	for _, st := range cw.Step {
	i := strings.Index(st.Src, ":")
	if i < 0 {
	i = len(st.Src)
	}
	filename := st.Src[0:i]
	data, err := fs.ReadFile(s.fsys, filename)
	if err != nil {
	st.Err = err
	continue
	}
	if i < len(st.Src) {
	lo, hi, err := addrToByteRange(st.Src[i+1:], 0, data)
	if err != nil {
	st.Err = err
	continue
	}
	// Expand match to line boundaries.
	for lo > 0 && data[lo-1] != '\n' {
	lo--
	}
	for hi < len(data) && (hi == 0 \|\| data[hi-1] != '\n') {
	hi++
	}
	st.Lo = byteToLine(data, lo)
	st.Hi = byteToLine(data, hi-1)
	}
	st.Data = data
	st.File = filename
	m[filename] = true
	}

	// Make list of files
	cw.File = make([]string, len(m))
	i := 0
	for f := range m {
	cw.File[i] = f
	i++
	}
	sort.Strings(cw.File)

	return cw, nil
	}

	// codewalkDir serves the codewalk directory listing.
	// It scans the directory for subdirectories or files named *.xml
	// and prepares a table.
	func (s server) codewalkDir(w http.ResponseWriter, r http.Request, relpath string) {
	type elem struct {
	Name string
	Title string
	}

	dir, err := fs.ReadDir(s.fsys, relpath)
	if err != nil {
	log.Print(err)
	s.site.ServeError(w, r, err)
	return
	}
	var v []interface{}
	for _, fi := range dir {
	name := fi.Name()
	if fi.IsDir() {
	v = append(v, &elem{name + "/", ""})
	} else if strings.HasSuffix(name, ".xml") {
	cw, err := s.loadCodewalk(relpath + "/" + name)
	if err != nil {
	continue
	}
	v = append(v, &elem{name[0 : len(name)-len(".xml")], cw.Title})
	}
	}

	s.site.ServePage(w, r, web.Page{
	"title": "Codewalks",
	"layout": "codewalkdir",
	"dirs": v,
	})
	}

	// codewalkFileprint serves requests with ?fileprint=f&lo=lo&hi=hi.
	// The filename f has already been retrieved and is passed as an argument.
	// Lo and hi are the numbers of the first and last line to highlight
	// in the response. This format is used for the middle window pane
	// of the codewalk pages. It is a separate iframe and does not get
	// the usual godoc HTML wrapper.
	func (s server) codewalkFileprint(w http.ResponseWriter, r http.Request, f string) {
	relpath := strings.Trim(path.Clean(f), "/")
	data, err := fs.ReadFile(s.fsys, relpath)
	if err != nil {
	log.Print(err)
	s.site.ServeError(w, r, err)
	return
	}
	lo, _ := strconv.Atoi(r.FormValue("lo"))
	hi, _ := strconv.Atoi(r.FormValue("hi"))
	if hi < lo {
	hi = lo
	}
	lo = lineToByte(data, lo)
	hi = lineToByte(data, hi+1)

	// Put the mark 4 lines before lo, so that the iframe
	// shows a few lines of context before the highlighted
	// section.
	n := 4
	mark := lo
	for ; mark > 0 && n > 0; mark-- {
	if data[mark-1] == '\n' {
	if n--; n == 0 {
	break
	}
	}
	}

	io.WriteString(w, `<style type="text/css">@import "/doc/codewalk/codewalk.css";</style><pre>`)
	template.HTMLEscape(w, data[0:mark])
	io.WriteString(w, "<a name='mark'></a>")
	template.HTMLEscape(w, data[mark:lo])
	if lo < hi {
	io.WriteString(w, "<div class='codewalkhighlight'>")
	template.HTMLEscape(w, data[lo:hi])
	io.WriteString(w, "</div>")
	}
	template.HTMLEscape(w, data[hi:])
	io.WriteString(w, "</pre>")
	}

	// addrToByte evaluates the given address starting at offset start in data.
	// It returns the lo and hi byte offset of the matched region within data.
	// See https://9p.io/sys/doc/sam/sam.html Table II
	// for details on the syntax.
	func addrToByteRange(addr string, start int, data []byte) (lo, hi int, err error) {
	var (
	dir byte
	prevc byte
	charOffset bool
	)
	lo = start
	hi = start
	for addr != "" && err == nil {
	c := addr[0]
	switch c {
	default:
	err = errors.New("invalid address syntax near " + string(c))
	case ',':
	if len(addr) == 1 {
	hi = len(data)
	} else {
	_, hi, err = addrToByteRange(addr[1:], hi, data)
	}
	return

	case '+', '-':
	if prevc == '+' \|\| prevc == '-' {
	lo, hi, err = addrNumber(data, lo, hi, prevc, 1, charOffset)
	}
	dir = c

	case '$':
	lo = len(data)
	hi = len(data)
	if len(addr) > 1 {
	dir = '+'
	}

	case '#':
	charOffset = true

	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
	var i int
	for i = 1; i < len(addr); i++ {
	if addr[i] < '0' \|\| addr[i] > '9' {
	break
	}
	}
	var n int
	n, err = strconv.Atoi(addr[0:i])
	if err != nil {
	break
	}
	lo, hi, err = addrNumber(data, lo, hi, dir, n, charOffset)
	dir = 0
	charOffset = false
	prevc = c
	addr = addr[i:]
	continue

	case '/':
	var i, j int
	Regexp:
	for i = 1; i < len(addr); i++ {
	switch addr[i] {
	case '\\':
	i++
	case '/':
	j = i + 1
	break Regexp
	}
	}
	if j == 0 {
	j = i
	}
	pattern := addr[1:i]
	lo, hi, err = addrRegexp(data, lo, hi, dir, pattern)
	prevc = c
	addr = addr[j:]
	continue
	}
	prevc = c
	addr = addr[1:]
	}

	if err == nil && dir != 0 {
	lo, hi, err = addrNumber(data, lo, hi, dir, 1, charOffset)
	}
	if err != nil {
	return 0, 0, err
	}
	return lo, hi, nil
	}

	// addrNumber applies the given dir, n, and charOffset to the address lo, hi.
	// dir is '+' or '-', n is the count, and charOffset is true if the syntax
	// used was #n. Applying +n (or +#n) means to advance n lines
	// (or characters) after hi. Applying -n (or -#n) means to back up n lines
	// (or characters) before lo.
	// The return value is the new lo, hi.
	func addrNumber(data []byte, lo, hi int, dir byte, n int, charOffset bool) (int, int, error) {
	switch dir {
	case 0:
	lo = 0
	hi = 0
	fallthrough

	case '+':
	if charOffset {
	pos := hi
	for ; n > 0 && pos < len(data); n-- {
	_, size := utf8.DecodeRune(data[pos:])
	pos += size
	}
	if n == 0 {
	return pos, pos, nil
	}
	break
	}
	// find next beginning of line
	if hi > 0 {
	for hi < len(data) && data[hi-1] != '\n' {
	hi++
	}
	}
	lo = hi
	if n == 0 {
	return lo, hi, nil
	}
	for ; hi < len(data); hi++ {
	if data[hi] != '\n' {
	continue
	}
	switch n--; n {
	case 1:
	lo = hi + 1
	case 0:
	return lo, hi + 1, nil
	}
	}

	case '-':
	if charOffset {
	// Scan backward for bytes that are not UTF-8 continuation bytes.
	pos := lo
	for ; pos > 0 && n > 0; pos-- {
	if data[pos]&0xc0 != 0x80 {
	n--
	}
	}
	if n == 0 {
	return pos, pos, nil
	}
	break
	}
	// find earlier beginning of line
	for lo > 0 && data[lo-1] != '\n' {
	lo--
	}
	hi = lo
	if n == 0 {
	return lo, hi, nil
	}
	for ; lo >= 0; lo-- {
	if lo > 0 && data[lo-1] != '\n' {
	continue
	}
	switch n--; n {
	case 1:
	hi = lo
	case 0:
	return lo, hi, nil
	}
	}
	}

	return 0, 0, errors.New("address out of range")
	}

	// addrRegexp searches for pattern in the given direction starting at lo, hi.
	// The direction dir is '+' (search forward from hi) or '-' (search backward from lo).
	// Backward searches are unimplemented.
	func addrRegexp(data []byte, lo, hi int, dir byte, pattern string) (int, int, error) {
	re, err := regexp.Compile(pattern)
	if err != nil {
	return 0, 0, err
	}
	if dir == '-' {
	// Could implement reverse search using binary search
	// through file, but that seems like overkill.
	return 0, 0, errors.New("reverse search not implemented")
	}
	m := re.FindIndex(data[hi:])
	if len(m) > 0 {
	m[0] += hi
	m[1] += hi
	} else if hi > 0 {
	// No match. Wrap to beginning of data.
	m = re.FindIndex(data)
	}
	if len(m) == 0 {
	return 0, 0, errors.New("no match for " + pattern)
	}
	return m[0], m[1], nil
	}

	// lineToByte returns the byte index of the first byte of line n.
	// Line numbers begin at 1.
	func lineToByte(data []byte, n int) int {
	if n <= 1 {
	return 0
	}
	n--
	for i, c := range data {
	if c == '\n' {
	if n--; n == 0 {
	return i + 1
	}
	}
	}
	return len(data)
	}

	// byteToLine returns the number of the line containing the byte at index i.
	func byteToLine(data []byte, i int) int {
	l := 1
	for j, c := range data {
	if j == i {
	return l
	}
	if c == '\n' {
	l++
	}
	}
	return l
	}