fs.go

// Copyright 2009 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.

// HTTP file system request handler

package fasthttpfs

import (
	"errors"
	"fmt"
	"io"
	"io/fs"
	"mime"
	"mime/multipart"
	"net/http"
	"net/textproto"
	"net/url"
	"path"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"time"
	"unsafe"

	"github.com/valyala/fasthttp"
)

type anyDirs interface {
	len() int
	name(i int) string
	isDir(i int) bool
}

type fileInfoDirs []fs.FileInfo

func (d fileInfoDirs) len() int          { return len(d) }
func (d fileInfoDirs) isDir(i int) bool  { return d[i].IsDir() }
func (d fileInfoDirs) name(i int) string { return d[i].Name() }

type dirEntryDirs []fs.DirEntry

func (d dirEntryDirs) len() int          { return len(d) }
func (d dirEntryDirs) isDir(i int) bool  { return d[i].IsDir() }
func (d dirEntryDirs) name(i int) string { return d[i].Name() }

func dirList(ctx *fasthttp.RequestCtx, f http.File) {
	// Prefer to use ReadDir instead of Readdir,
	// because the former doesn't require calling
	// Stat on every entry of a directory on Unix.
	var dirs anyDirs
	var err error
	if d, ok := f.(fs.ReadDirFile); ok {
		var list dirEntryDirs
		list, err = d.ReadDir(-1)
		dirs = list
	} else {
		var list fileInfoDirs
		list, err = f.Readdir(-1)
		dirs = list
	}

	if err != nil {
		ctx.Logger().Printf("http: error reading directory: %v", err)
		ctx.Error("Error reading directory", fasthttp.StatusInternalServerError)
		return
	}
	sort.Slice(dirs, func(i, j int) bool { return dirs.name(i) < dirs.name(j) })

	ctx.SetContentType("text/html; charset=utf-8")
	fmt.Fprintf(ctx, "<pre>\n")
	for i, n := 0, dirs.len(); i < n; i++ {
		name := dirs.name(i)
		if dirs.isDir(i) {
			name += "/"
		}
		// name may contain '?' or '#', which must be escaped to remain
		// part of the URL path, and not indicate the start of a query
		// string or fragment.
		url := url.URL{Path: name}
		fmt.Fprintf(ctx, "<a href=\"%s\">%s</a>\n", url.String(), htmlReplacer.Replace(name))
	}
	fmt.Fprintf(ctx, "</pre>\n")
}

// ServeContent replies to the request using the content in the
// provided ReadSeeker. The main benefit of ServeContent over io.Copy
// is that it handles Range requests properly, sets the MIME type, and
// handles If-Match, If-Unmodified-Since, If-None-Match, If-Modified-Since,
// and If-Range requests.
//
// If the response's Content-Type header is not set, ServeContent
// first tries to deduce the type from name's file extension and,
// if that fails, falls back to reading the first block of the content
// and passing it to DetectContentType.
// The name is otherwise unused; in particular it can be empty and is
// never sent in the response.
//
// If modtime is not the zero time or Unix epoch, ServeContent
// includes it in a Last-Modified header in the response. If the
// request includes an If-Modified-Since header, ServeContent uses
// modtime to decide whether the content needs to be sent at all.
//
// The content's Seek method must work: ServeContent uses
// a seek to the end of the content to determine its size.
//
// If the caller has set w's ETag header formatted per RFC 7232, section 2.3,
// ServeContent uses it to handle requests using If-Match, If-None-Match, or If-Range.
//
// Note that *os.File implements the io.ReadSeeker interface.
func ServeContent(ctx *fasthttp.RequestCtx, name string, modtime time.Time, content io.ReadSeeker) {
	sizeFunc := func() (int64, error) {
		size, err := content.Seek(0, io.SeekEnd)
		if err != nil {
			return 0, errSeeker
		}
		_, err = content.Seek(0, io.SeekStart)
		if err != nil {
			return 0, errSeeker
		}
		return size, nil
	}
	serveContent(ctx, name, modtime, sizeFunc, content)
}

// errSeeker is returned by ServeContent's sizeFunc when the content
// doesn't seek properly. The underlying Seeker's error text isn't
// included in the sizeFunc reply so it's not sent over HTTP to end
// users.
var errSeeker = errors.New("seeker can't seek")

// errNoOverlap is returned by serveContent's parseRange if first-byte-pos of
// all of the byte-range-spec values is greater than the content size.
var errNoOverlap = errors.New("invalid range: failed to overlap")

// if name is empty, filename is unknown. (used for mime type, before sniffing)
// if modtime.IsZero(), modtime is unknown.
// content must be seeked to the beginning of the file.
// The sizeFunc is called at most once. Its error, if any, is sent in the HTTP response.
func serveContent(ctx *fasthttp.RequestCtx, name string, modtime time.Time, sizeFunc func() (int64, error), content io.ReadSeeker) {
	setLastModified(ctx, modtime)
	done, rangeReq := checkPreconditions(ctx, modtime)
	if done {
		return
	}

	code := fasthttp.StatusOK

	// If Content-Type isn't set, use the file's extension to find it, but
	// if the Content-Type is unset explicitly, do not sniff the type.
	ctype := b2s(ctx.Response.Header.ContentType())
	if ctype == "" || ctype == "text/plain; charset=utf-8" {
		ctype = mime.TypeByExtension(filepath.Ext(name))
		if ctype == "" {
			// read a chunk to decide between utf-8 text and binary
			var buf [sniffLen]byte
			n, _ := io.ReadFull(content, buf[:])
			ctype = http.DetectContentType(buf[:n])
			_, err := content.Seek(0, io.SeekStart) // rewind to output whole file
			if err != nil {
				ctx.Error("seeker can't seek", fasthttp.StatusInternalServerError)
				return
			}
		}
		ctx.SetContentType(ctype)
	}

	size, err := sizeFunc()
	if err != nil {
		ctx.Error(err.Error(), fasthttp.StatusInternalServerError)
		return
	}

	// handle Content-Range header.
	sendSize := size
	var sendContent io.Reader = content
	if size >= 0 {
		ranges, err := parseRange(rangeReq, size)
		if err != nil {
			if err == errNoOverlap {
				ctx.Response.Header.Set("Content-Range", fmt.Sprintf("bytes */%d", size))
			}
			ctx.Error(err.Error(), fasthttp.StatusRequestedRangeNotSatisfiable)
			return
		}
		if sumRangesSize(ranges) > size {
			// The total number of bytes in all the ranges
			// is larger than the size of the file by
			// itself, so this is probably an attack, or a
			// dumb client. Ignore the range request.
			ranges = nil
		}
		switch {
		case len(ranges) == 1:
			// RFC 7233, Section 4.1:
			// "If a single part is being transferred, the server
			// generating the 206 response MUST generate a
			// Content-Range header field, describing what range
			// of the selected representation is enclosed, and a
			// payload consisting of the range.
			// ...
			// A server MUST NOT generate a multipart response to
			// a request for a single range, since a client that
			// does not request multiple parts might not support
			// multipart responses."
			ra := ranges[0]
			if _, err := content.Seek(ra.start, io.SeekStart); err != nil {
				ctx.Error(err.Error(), fasthttp.StatusRequestedRangeNotSatisfiable)
				return
			}
			sendSize = ra.length
			code = fasthttp.StatusPartialContent
			ctx.Response.Header.Set("Content-Range", ra.contentRange(size))
		case len(ranges) > 1:
			sendSize = rangesMIMESize(ranges, ctype, size)
			code = fasthttp.StatusPartialContent

			pr, pw := io.Pipe()
			mw := multipart.NewWriter(pw)
			ctx.SetContentType("multipart/byteranges; boundary=" + mw.Boundary())
			sendContent = pr
			defer pr.Close() // cause writing goroutine to fail and exit if CopyN doesn't finish.
			go func() {
				for _, ra := range ranges {
					part, err := mw.CreatePart(ra.mimeHeader(ctype, size))
					if err != nil {
						pw.CloseWithError(err)
						return
					}
					if _, err := content.Seek(ra.start, io.SeekStart); err != nil {
						pw.CloseWithError(err)
						return
					}
					if _, err := io.CopyN(part, content, ra.length); err != nil {
						pw.CloseWithError(err)
						return
					}
				}
				mw.Close()
				pw.Close()
			}()
		}

		ctx.Response.Header.Set("Accept-Ranges", "bytes")
		if len(ctx.Response.Header.Peek("Content-Encoding")) == 0 {
			ctx.Response.Header.SetContentLength(int(sendSize))
		}
	}

	ctx.SetStatusCode(code)

	if !ctx.IsHead() {
		io.CopyN(ctx, sendContent, sendSize)
	}
}

// scanETag determines if a syntactically valid ETag is present at s. If so,
// the ETag and remaining text after consuming ETag is returned. Otherwise,
// it returns "", "".
func scanETag(s string) (etag string, remain string) {
	s = textproto.TrimString(s)
	start := 0
	if strings.HasPrefix(s, "W/") {
		start = 2
	}
	if len(s[start:]) < 2 || s[start] != '"' {
		return "", ""
	}
	// ETag is either W/"text" or "text".
	// See RFC 7232 2.3.
	for i := start + 1; i < len(s); i++ {
		c := s[i]
		switch {
		// Character values allowed in ETags.
		case c == 0x21 || c >= 0x23 && c <= 0x7E || c >= 0x80:
		case c == '"':
			return s[:i+1], s[i+1:]
		default:
			return "", ""
		}
	}
	return "", ""
}

// etagStrongMatch reports whether a and b match using strong ETag comparison.
// Assumes a and b are valid ETags.
func etagStrongMatch(a, b string) bool {
	return a == b && a != "" && a[0] == '"'
}

// etagWeakMatch reports whether a and b match using weak ETag comparison.
// Assumes a and b are valid ETags.
func etagWeakMatch(a, b string) bool {
	return strings.TrimPrefix(a, "W/") == strings.TrimPrefix(b, "W/")
}

// condResult is the result of an HTTP request precondition check.
// See https://tools.ietf.org/html/rfc7232 section 3.
type condResult int

const (
	condNone condResult = iota
	condTrue
	condFalse
)

func checkIfMatch(ctx *fasthttp.RequestCtx) condResult {
	im := b2s(ctx.Request.Header.Peek("If-Match"))
	if im == "" {
		return condNone
	}
	for {
		im = textproto.TrimString(im)
		if len(im) == 0 {
			break
		}
		if im[0] == ',' {
			im = im[1:]
			continue
		}
		if im[0] == '*' {
			return condTrue
		}
		etag, remain := scanETag(im)
		if etag == "" {
			break
		}
		if etagStrongMatch(etag, b2s(ctx.Request.Header.Peek("Etag"))) {
			return condTrue
		}
		im = remain
	}

	return condFalse
}

func checkIfUnmodifiedSince(ctx *fasthttp.RequestCtx, modtime time.Time) condResult {
	ius := b2s(ctx.Request.Header.Peek("If-Unmodified-Since"))
	if ius == "" || isZeroTime(modtime) {
		return condNone
	}
	t, err := http.ParseTime(ius)
	if err != nil {
		return condNone
	}

	// The Last-Modified header truncates sub-second precision so
	// the modtime needs to be truncated too.
	modtime = modtime.Truncate(time.Second)
	if modtime.Before(t) || modtime.Equal(t) {
		return condTrue
	}
	return condFalse
}

func checkIfNoneMatch(ctx *fasthttp.RequestCtx) condResult {
	inm := b2s(ctx.Request.Header.Peek("If-None-Match"))
	if inm == "" {
		return condNone
	}
	buf := inm
	for {
		buf = textproto.TrimString(buf)
		if len(buf) == 0 {
			break
		}
		if buf[0] == ',' {
			buf = buf[1:]
			continue
		}
		if buf[0] == '*' {
			return condFalse
		}
		etag, remain := scanETag(buf)
		if etag == "" {
			break
		}
		if etagWeakMatch(etag, b2s(ctx.Response.Header.Peek("Etag"))) {
			return condFalse
		}
		buf = remain
	}
	return condTrue
}

func checkIfModifiedSince(ctx *fasthttp.RequestCtx, modtime time.Time) condResult {
	if !ctx.IsGet() && !ctx.IsHead() {
		return condNone
	}
	ims := b2s(ctx.Request.Header.Peek("If-Modified-Since"))
	if ims == "" || isZeroTime(modtime) {
		return condNone
	}
	t, err := http.ParseTime(ims)
	if err != nil {
		return condNone
	}
	// The Last-Modified header truncates sub-second precision so
	// the modtime needs to be truncated too.
	modtime = modtime.Truncate(time.Second)
	if modtime.Before(t) || modtime.Equal(t) {
		return condFalse
	}
	return condTrue
}

func checkIfRange(ctx *fasthttp.RequestCtx, modtime time.Time) condResult {
	if !ctx.IsGet() && !ctx.IsHead() {
		return condNone
	}
	ir := b2s(ctx.Request.Header.Peek("If-Range"))
	if ir == "" {
		return condNone
	}
	etag, _ := scanETag(ir)
	if etag != "" {
		if etagStrongMatch(etag, b2s(ctx.Response.Header.Peek("Etag"))) {
			return condTrue
		} else {
			return condFalse
		}
	}
	// The If-Range value is typically the ETag value, but it may also be
	// the modtime date. See golang.org/issue/8367.
	if modtime.IsZero() {
		return condFalse
	}
	t, err := http.ParseTime(ir)
	if err != nil {
		return condFalse
	}
	if t.Unix() == modtime.Unix() {
		return condTrue
	}
	return condFalse
}

var unixEpochTime = time.Unix(0, 0)

// isZeroTime reports whether t is obviously unspecified (either zero or Unix()=0).
func isZeroTime(t time.Time) bool {
	return t.IsZero() || t.Equal(unixEpochTime)
}

func setLastModified(ctx *fasthttp.RequestCtx, modtime time.Time) {
	if !isZeroTime(modtime) {
		ctx.Response.Header.Set("Last-Modified", modtime.UTC().Format(http.TimeFormat))
	}
}

func writeNotModified(ctx *fasthttp.RequestCtx) {
	// RFC 7232 section 4.1:
	// a sender SHOULD NOT generate representation metadata other than the
	// above listed fields unless said metadata exists for the purpose of
	// guiding cache updates (e.g., Last-Modified might be useful if the
	// response does not have an ETag field).
	ctx.Response.Header.Del("Content-Type")
	ctx.Response.Header.Del("Content-Length")
	if len(ctx.Response.Header.Peek("Etag")) == 0 {
		ctx.Response.Header.Del("Last-Modified")
	}
	ctx.SetStatusCode(fasthttp.StatusNotModified)
}

// checkPreconditions evaluates request preconditions and reports whether a precondition
// resulted in sending StatusNotModified or StatusPreconditionFailed.
func checkPreconditions(ctx *fasthttp.RequestCtx, modtime time.Time) (done bool, rangeHeader string) {
	// This function carefully follows RFC 7232 section 6.
	ch := checkIfMatch(ctx)
	if ch == condNone {
		ch = checkIfUnmodifiedSince(ctx, modtime)
	}
	if ch == condFalse {
		ctx.SetStatusCode(fasthttp.StatusPreconditionFailed)
		return true, ""
	}
	switch checkIfNoneMatch(ctx) {
	case condFalse:
		if ctx.IsGet() || ctx.IsHead() {
			writeNotModified(ctx)
			return true, ""
		} else {
			ctx.SetStatusCode(fasthttp.StatusPreconditionFailed)
			return true, ""
		}
	case condNone:
		if checkIfModifiedSince(ctx, modtime) == condFalse {
			writeNotModified(ctx)
			return true, ""
		}
	}

	rangeHeader = b2s(ctx.Request.Header.Peek("Range"))
	if rangeHeader != "" && checkIfRange(ctx, modtime) == condFalse {
		rangeHeader = ""
	}
	return false, rangeHeader
}

// name is '/'-separated, not filepath.Separator.
func serveFile(ctx *fasthttp.RequestCtx, fs http.FileSystem, name string, redirect bool) {
	const indexPage = "/index.html"

	// redirect .../index.html to .../
	// can't use Redirect() because that would make the path absolute,
	// which would be a problem running under StripPrefix
	if strings.HasSuffix(b2s(ctx.Path()), indexPage) {
		localRedirect(ctx, "./")
		return
	}

	f, err := fs.Open(name)
	if err != nil {
		msg, code := toHTTPError(err)
		ctx.Error(msg, code)
		return
	}
	defer f.Close()

	d, err := f.Stat()
	if err != nil {
		msg, code := toHTTPError(err)
		ctx.Error(msg, code)
		return
	}

	if redirect {
		// redirect to canonical path: / at end of directory url
		// r.URL.Path always begins with /
		url := b2s(ctx.Path())
		if d.IsDir() {
			if url[len(url)-1] != '/' {
				localRedirect(ctx, path.Base(url)+"/")
				return
			}
		} else {
			if url[len(url)-1] == '/' {
				localRedirect(ctx, "../"+path.Base(url))
				return
			}
		}
	}

	if d.IsDir() {
		url := b2s(ctx.Path())
		// redirect if the directory name doesn't end in a slash
		if url == "" || url[len(url)-1] != '/' {
			localRedirect(ctx, path.Base(url)+"/")
			return
		}

		// use contents of index.html for directory, if present
		index := strings.TrimSuffix(name, "/") + indexPage
		ff, err := fs.Open(index)
		if err == nil {
			defer ff.Close()
			dd, err := ff.Stat()
			if err == nil {
				name = index
				d = dd
				f = ff
			}
		}
	}

	// Still a directory? (we didn't find an index.html file)
	if d.IsDir() {
		if checkIfModifiedSince(ctx, d.ModTime()) == condFalse {
			writeNotModified(ctx)
			return
		}
		setLastModified(ctx, d.ModTime())
		dirList(ctx, f)
		return
	}

	// serveContent will check modification time
	sizeFunc := func() (int64, error) { return d.Size(), nil }
	serveContent(ctx, d.Name(), d.ModTime(), sizeFunc, f)
}

// toHTTPError returns a non-specific HTTP error message and status code
// for a given non-nil error value. It's important that toHTTPError does not
// actually return err.Error(), since msg and httpStatus are returned to users,
// and historically Go's ServeContent always returned just "404 Not Found" for
// all errors. We don't want to start leaking information in error messages.
func toHTTPError(err error) (msg string, httpStatus int) {
	if errors.Is(err, fs.ErrNotExist) {
		return "404 page not found", fasthttp.StatusNotFound
	}
	if errors.Is(err, fs.ErrPermission) {
		return "403 Forbidden", fasthttp.StatusForbidden
	}
	// Default:
	return "500 Internal Server Error", fasthttp.StatusInternalServerError
}

// localRedirect gives a Moved Permanently response.
// It does not convert relative paths to absolute paths like Redirect does.
func localRedirect(ctx *fasthttp.RequestCtx, newPath string) {
	if q := b2s(ctx.URI().QueryString()); q != "" {
		newPath += "?" + q
	}
	ctx.Response.Header.Set("Location", newPath)
	ctx.Response.SetStatusCode(fasthttp.StatusMovedPermanently)
}

// ServeFile replies to the request with the contents of the named
// file or directory.
//
// If the provided file or directory name is a relative path, it is
// interpreted relative to the current directory and may ascend to
// parent directories. If the provided name is constructed from user
// input, it should be sanitized before calling ServeFile.
//
// As a precaution, ServeFile will reject requests where r.URL.Path
// contains a ".." path element; this protects against callers who
// might unsafely use filepath.Join on r.URL.Path without sanitizing
// it and then use that filepath.Join result as the name argument.
//
// As another special case, ServeFile redirects any request where r.URL.Path
// ends in "/index.html" to the same path, without the final
// "index.html". To avoid such redirects either modify the path or
// use ServeContent.
//
// Outside of those two special cases, ServeFile does not use
// r.URL.Path for selecting the file or directory to serve; only the
// file or directory provided in the name argument is used.
func ServeFile(ctx *fasthttp.RequestCtx, name string) {
	if containsDotDot(b2s(ctx.Path())) {
		// Too many programs use r.URL.Path to construct the argument to
		// serveFile. Reject the request under the assumption that happened
		// here and ".." may not be wanted.
		// Note that name might not contain "..", for example if code (still
		// incorrectly) used filepath.Join(myDir, r.URL.Path).
		ctx.Error("invalid URL path", fasthttp.StatusBadRequest)
		return
	}
	dir, file := filepath.Split(name)
	serveFile(ctx, http.Dir(dir), file, false)
}

func containsDotDot(v string) bool {
	if !strings.Contains(v, "..") {
		return false
	}
	for _, ent := range strings.FieldsFunc(v, isSlashRune) {
		if ent == ".." {
			return true
		}
	}
	return false
}

func isSlashRune(r rune) bool { return r == '/' || r == '\\' }

// FileServer returns a handler that serves HTTP requests
// with the contents of the file system rooted at root.
//
// As a special case, the returned file server redirects any request
// ending in "/index.html" to the same path, without the final
// "index.html".
//
// To use the operating system's file system implementation,
// use http.Dir:
//
//     http.Handle("/", http.FileServer(http.Dir("/tmp")))
//
// To use an fs.FS implementation, use http.FS to convert it:
//
//	http.Handle("/", http.FileServer(http.FS(fsys)))
//
func FileServer(root http.FileSystem) fasthttp.RequestHandler {
	return func(ctx *fasthttp.RequestCtx) {
		upath := b2s(ctx.Path())
		if !strings.HasPrefix(upath, "/") {
			upath = "/" + upath
			ctx.Request.URI().SetPath(upath)
		}
		serveFile(ctx, root, path.Clean(upath), true)
	}
}

// httpRange specifies the byte range to be sent to the client.
type httpRange struct {
	start, length int64
}

func (r httpRange) contentRange(size int64) string {
	return fmt.Sprintf("bytes %d-%d/%d", r.start, r.start+r.length-1, size)
}

func (r httpRange) mimeHeader(contentType string, size int64) textproto.MIMEHeader {
	return textproto.MIMEHeader{
		"Content-Range": {r.contentRange(size)},
		"Content-Type":  {contentType},
	}
}

// parseRange parses a Range header string as per RFC 7233.
// errNoOverlap is returned if none of the ranges overlap.
func parseRange(s string, size int64) ([]httpRange, error) {
	if s == "" {
		return nil, nil // header not present
	}
	const b = "bytes="
	if !strings.HasPrefix(s, b) {
		return nil, errors.New("invalid range")
	}
	var ranges []httpRange
	noOverlap := false
	for _, ra := range strings.Split(s[len(b):], ",") {
		ra = textproto.TrimString(ra)
		if ra == "" {
			continue
		}
		start, end, ok := strings.Cut(ra, "-")
		if !ok {
			return nil, errors.New("invalid range")
		}
		start, end = textproto.TrimString(start), textproto.TrimString(end)
		var r httpRange
		if start == "" {
			// If no start is specified, end specifies the
			// range start relative to the end of the file,
			// and we are dealing with <suffix-length>
			// which has to be a non-negative integer as per
			// RFC 7233 Section 2.1 "Byte-Ranges".
			if end == "" || end[0] == '-' {
				return nil, errors.New("invalid range")
			}
			i, err := strconv.ParseInt(end, 10, 64)
			if i < 0 || err != nil {
				return nil, errors.New("invalid range")
			}
			if i > size {
				i = size
			}
			r.start = size - i
			r.length = size - r.start
		} else {
			i, err := strconv.ParseInt(start, 10, 64)
			if err != nil || i < 0 {
				return nil, errors.New("invalid range")
			}
			if i >= size {
				// If the range begins after the size of the content,
				// then it does not overlap.
				noOverlap = true
				continue
			}
			r.start = i
			if end == "" {
				// If no end is specified, range extends to end of the file.
				r.length = size - r.start
			} else {
				i, err := strconv.ParseInt(end, 10, 64)
				if err != nil || r.start > i {
					return nil, errors.New("invalid range")
				}
				if i >= size {
					i = size - 1
				}
				r.length = i - r.start + 1
			}
		}
		ranges = append(ranges, r)
	}
	if noOverlap && len(ranges) == 0 {
		// The specified ranges did not overlap with the content.
		return nil, errNoOverlap
	}
	return ranges, nil
}

// countingWriter counts how many bytes have been written to it.
type countingWriter int64

func (w *countingWriter) Write(p []byte) (n int, err error) {
	*w += countingWriter(len(p))
	return len(p), nil
}

// rangesMIMESize returns the number of bytes it takes to encode the
// provided ranges as a multipart response.
func rangesMIMESize(ranges []httpRange, contentType string, contentSize int64) (encSize int64) {
	var w countingWriter
	mw := multipart.NewWriter(&w)
	for _, ra := range ranges {
		mw.CreatePart(ra.mimeHeader(contentType, contentSize))
		encSize += ra.length
	}
	mw.Close()
	encSize += int64(w)
	return
}

func sumRangesSize(ranges []httpRange) (size int64) {
	for _, ra := range ranges {
		size += ra.length
	}
	return
}

var htmlReplacer = strings.NewReplacer(
	"&", "&amp;",
	"<", "&lt;",
	">", "&gt;",
	// "&#34;" is shorter than "&quot;".
	`"`, "&#34;",
	// "&#39;" is shorter than "&apos;" and apos was not in HTML until HTML5.
	"'", "&#39;",
)

// The algorithm uses at most sniffLen bytes to make its decision.
const sniffLen = 512

func b2s(b []byte) string {
	return *(*string)(unsafe.Pointer(&b))
}