src/pkg/http/url.go - go - Git at Google

 // 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.

 // Parse URLs (actually URIs, but that seems overly pedantic).
 // RFC 3986

 package http

 import (
 	"os"
 	"strconv"
 	"strings"
 )

 // URLError reports an error and the operation and URL that caused it.
 type URLError struct {
 	Op    string
 	URL   string
 	Error os.Error
 }

 func (e *URLError) String() string { return e.Op + " " + e.URL + ": " + e.Error.String() }

 func ishex(c byte) bool {
 	switch {
 	case '0' <= c && c <= '9':
 		return true
 	case 'a' <= c && c <= 'f':
 		return true
 	case 'A' <= c && c <= 'F':
 		return true
 	}
 	return false
 }

 func unhex(c byte) byte {
 	switch {
 	case '0' <= c && c <= '9':
 		return c - '0'
 	case 'a' <= c && c <= 'f':
 		return c - 'a' + 10
 	case 'A' <= c && c <= 'F':
 		return c - 'A' + 10
 	}
 	return 0
 }

 type encoding int

 const (
 	encodePath encoding = 1 + iota
 	encodeUserPassword
 	encodeQueryComponent
 	encodeFragment
 	encodeOpaque
 )


 type URLEscapeError string

 func (e URLEscapeError) String() string {
 	return "invalid URL escape " + strconv.Quote(string(e))
 }

 // Return true if the specified character should be escaped when
 // appearing in a URL string, according to RFC 2396.
 // When 'all' is true the full range of reserved characters are matched.
 func shouldEscape(c byte, mode encoding) bool {
 	// RFC 2396 §2.3 Unreserved characters (alphanum)
 	if 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z' || '0' <= c && c <= '9' {
 		return false
 	}
 	switch c {
 	case '-', '_', '.', '!', '~', '*', '\'', '(', ')': // §2.3 Unreserved characters (mark)
 		return false

 	case '$', '&', '+', ',', '/', ':', ';', '=', '?', '@': // §2.2 Reserved characters (reserved)
 		// Different sections of the URL allow a few of
 		// the reserved characters to appear unescaped.
 		switch mode {
 		case encodePath: // §3.3
 			// The RFC allows : @ & = + $ , but saves / ; for assigning
 			// meaning to individual path segments.  This package
 			// only manipulates the path as a whole, so we allow those
 			// last two as well.  Clients that need to distinguish between
 			// `/foo;y=z/bar` and `/foo%3by=z/bar` will have to re-decode RawPath.
 			// That leaves only ? to escape.
 			return c == '?'

 		case encodeUserPassword: // §3.2.2
 			// The RFC allows ; : & = + $ , in userinfo, so we must escape only @ and /.
 			// The parsing of userinfo treats : as special so we must escape that too.
 			return c == '@' || c == '/' || c == ':'

 		case encodeQueryComponent: // §3.4
 			// The RFC reserves (so we must escape) everything.
 			return true

 		case encodeFragment: // §4.1
 			// The RFC text is silent but the grammar allows
 			// everything, so escape nothing.
 			return false

 		case encodeOpaque: // §3 opaque_part
 			// The RFC allows opaque_part to use all characters
 			// except that the leading / must be escaped.
 			// (We implement that case in String.)
 			return false
 		}
 	}

 	// Everything else must be escaped.
 	return true
 }

 // CanonicalPath applies the algorithm specified in RFC 2396 to
 // simplify the path, removing unnecessary  . and .. elements.
 func CanonicalPath(path string) string {
 	buf := []byte(path)
 	a := buf[0:0]
 	// state helps to find /.. ^.. ^. and /. patterns.
 	// state == 1 - prev char is '/' or beginning of the string.
 	// state > 1  - prev state > 0 and prev char was '.'
 	// state == 0 - otherwise
 	state := 1
 	cnt := 0
 	for _, v := range buf {
 		switch v {
 		case '/':
 			s := state
 			state = 1
 			switch s {
 			case 2:
 				a = a[0 : len(a)-1]
 				continue
 			case 3:
 				if cnt > 0 {
 					i := len(a) - 4
 					for ; i >= 0 && a[i] != '/'; i-- {
 					}
 					a = a[0 : i+1]
 					cnt--
 					continue
 				}
 			default:
 				if len(a) > 0 {
 					cnt++
 				}
 			}
 		case '.':
 			if state > 0 {
 				state++
 			}
 		default:
 			state = 0
 		}
 		l := len(a)
 		a = a[0 : l+1]
 		a[l] = v
 	}
 	switch {
 	case state == 2:
 		a = a[0 : len(a)-1]
 	case state == 3 && cnt > 0:
 		i := len(a) - 4
 		for ; i >= 0 && a[i] != '/'; i-- {
 		}
 		a = a[0 : i+1]
 	}
 	return string(a)
 }

 // URLUnescape unescapes a string in ``URL encoded'' form,
 // converting %AB into the byte 0xAB and '+' into ' ' (space).
 // It returns an error if any % is not followed
 // by two hexadecimal digits.
 // Despite the name, this encoding applies only to individual
 // components of the query portion of the URL.
 func URLUnescape(s string) (string, os.Error) {
 	return urlUnescape(s, encodeQueryComponent)
 }

 // urlUnescape is like URLUnescape but mode specifies
 // which section of the URL is being unescaped.
 func urlUnescape(s string, mode encoding) (string, os.Error) {
 	// Count %, check that they're well-formed.
 	n := 0
 	hasPlus := false
 	for i := 0; i < len(s); {
 		switch s[i] {
 		case '%':
 			n++
 			if i+2 >= len(s) || !ishex(s[i+1]) || !ishex(s[i+2]) {
 				s = s[i:]
 				if len(s) > 3 {
 					s = s[0:3]
 				}
 				return "", URLEscapeError(s)
 			}
 			i += 3
 		case '+':
 			hasPlus = mode == encodeQueryComponent
 			i++
 		default:
 			i++
 		}
 	}

 	if n == 0 && !hasPlus {
 		return s, nil
 	}

 	t := make([]byte, len(s)-2*n)
 	j := 0
 	for i := 0; i < len(s); {
 		switch s[i] {
 		case '%':
 			t[j] = unhex(s[i+1])<<4 | unhex(s[i+2])
 			j++
 			i += 3
 		case '+':
 			if mode == encodeQueryComponent {
 				t[j] = ' '
 			} else {
 				t[j] = '+'
 			}
 			j++
 			i++
 		default:
 			t[j] = s[i]
 			j++
 			i++
 		}
 	}
 	return string(t), nil
 }

 // URLEscape converts a string into ``URL encoded'' form.
 // Despite the name, this encoding applies only to individual
 // components of the query portion of the URL.
 func URLEscape(s string) string {
 	return urlEscape(s, encodeQueryComponent)
 }

 func urlEscape(s string, mode encoding) string {
 	spaceCount, hexCount := 0, 0
 	for i := 0; i < len(s); i++ {
 		c := s[i]
 		if shouldEscape(c, mode) {
 			if c == ' ' && mode == encodeQueryComponent {
 				spaceCount++
 			} else {
 				hexCount++
 			}
 		}
 	}

 	if spaceCount == 0 && hexCount == 0 {
 		return s
 	}

 	t := make([]byte, len(s)+2*hexCount)
 	j := 0
 	for i := 0; i < len(s); i++ {
 		switch c := s[i]; {
 		case c == ' ' && mode == encodeQueryComponent:
 			t[j] = '+'
 			j++
 		case shouldEscape(c, mode):
 			t[j] = '%'
 			t[j+1] = "0123456789abcdef"[c>>4]
 			t[j+2] = "0123456789abcdef"[c&15]
 			j += 3
 		default:
 			t[j] = s[i]
 			j++
 		}
 	}
 	return string(t)
 }

 // UnescapeUserinfo parses the RawUserinfo field of a URL
 // as the form user or user:password and unescapes and returns
 // the two halves.
 //
 // This functionality should only be used with legacy web sites.
 // RFC 2396 warns that interpreting Userinfo this way
 // ``is NOT RECOMMENDED, because the passing of authentication
 // information in clear text (such as URI) has proven to be a
 // security risk in almost every case where it has been used.''
 func UnescapeUserinfo(rawUserinfo string) (user, password string, err os.Error) {
 	u, p := split(rawUserinfo, ':', true)
 	if user, err = urlUnescape(u, encodeUserPassword); err != nil {
 		return "", "", err
 	}
 	if password, err = urlUnescape(p, encodeUserPassword); err != nil {
 		return "", "", err
 	}
 	return
 }

 // EscapeUserinfo combines user and password in the form
 // user:password (or just user if password is empty) and then
 // escapes it for use as the URL.RawUserinfo field.
 //
 // This functionality should only be used with legacy web sites.
 // RFC 2396 warns that interpreting Userinfo this way
 // ``is NOT RECOMMENDED, because the passing of authentication
 // information in clear text (such as URI) has proven to be a
 // security risk in almost every case where it has been used.''
 func EscapeUserinfo(user, password string) string {
 	raw := urlEscape(user, encodeUserPassword)
 	if password != "" {
 		raw += ":" + urlEscape(password, encodeUserPassword)
 	}
 	return raw
 }

 // A URL represents a parsed URL (technically, a URI reference).
 // The general form represented is:
 //	scheme://[userinfo@]host/path[?query][#fragment]
 // The Raw, RawAuthority, RawPath, and RawQuery fields are in "wire format"
 // (special characters must be hex-escaped if not meant to have special meaning).
 // All other fields are logical values; '+' or '%' represent themselves.
 //
 // The various Raw values are supplied in wire format because
 // clients typically have to split them into pieces before further
 // decoding.
 type URL struct {
 	Raw          string // the original string
 	Scheme       string // scheme
 	RawAuthority string // [userinfo@]host
 	RawUserinfo  string // userinfo
 	Host         string // host
 	RawPath      string // /path[?query][#fragment]
 	Path         string // /path
 	OpaquePath   bool   // path is opaque (unrooted when scheme is present)
 	RawQuery     string // query
 	Fragment     string // fragment
 }

 // Maybe rawurl is of the form scheme:path.
 // (Scheme must be [a-zA-Z][a-zA-Z0-9+-.]*)
 // If so, return scheme, path; else return "", rawurl.
 func getscheme(rawurl string) (scheme, path string, err os.Error) {
 	for i := 0; i < len(rawurl); i++ {
 		c := rawurl[i]
 		switch {
 		case 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z':
 		// do nothing
 		case '0' <= c && c <= '9' || c == '+' || c == '-' || c == '.':
 			if i == 0 {
 				return "", rawurl, nil
 			}
 		case c == ':':
 			if i == 0 {
 				return "", "", os.ErrorString("missing protocol scheme")
 			}
 			return rawurl[0:i], rawurl[i+1:], nil
 		default:
 			// we have encountered an invalid character,
 			// so there is no valid scheme
 			return "", rawurl, nil
 		}
 	}
 	return "", rawurl, nil
 }

 // Maybe s is of the form t c u.
 // If so, return t, c u (or t, u if cutc == true).
 // If not, return s, "".
 func split(s string, c byte, cutc bool) (string, string) {
 	for i := 0; i < len(s); i++ {
 		if s[i] == c {
 			if cutc {
 				return s[0:i], s[i+1:]
 			}
 			return s[0:i], s[i:]
 		}
 	}
 	return s, ""
 }

 // ParseURL parses rawurl into a URL structure.
 // The string rawurl is assumed not to have a #fragment suffix.
 // (Web browsers strip #fragment before sending the URL to a web server.)
 func ParseURL(rawurl string) (url *URL, err os.Error) {
 	if rawurl == "" {
 		err = os.ErrorString("empty url")
 		goto Error
 	}
 	url = new(URL)
 	url.Raw = rawurl

 	// Split off possible leading "http:", "mailto:", etc.
 	// Cannot contain escaped characters.
 	var path string
 	if url.Scheme, path, err = getscheme(rawurl); err != nil {
 		goto Error
 	}

 	if url.Scheme != "" && (len(path) == 0 || path[0] != '/') {
 		// RFC 2396:
 		// Absolute URI (has scheme) with non-rooted path
 		// is uninterpreted.  It doesn't even have a ?query.
 		// This is the case that handles mailto:name@example.com.
 		url.RawPath = path

 		if url.Path, err = urlUnescape(path, encodeOpaque); err != nil {
 			goto Error
 		}
 		url.OpaquePath = true
 	} else {
 		// Split off query before parsing path further.
 		url.RawPath = path
 		path, query := split(path, '?', false)
 		if len(query) > 1 {
 			url.RawQuery = query[1:]
 		}

 		// Maybe path is //authority/path
 		if url.Scheme != "" && len(path) > 2 && path[0:2] == "//" {
 			url.RawAuthority, path = split(path[2:], '/', false)
 			url.RawPath = url.RawPath[2+len(url.RawAuthority):]
 		}

 		// Split authority into userinfo@host.
 		// If there's no @, split's default is wrong.  Check explicitly.
 		var rawHost string
 		if strings.Index(url.RawAuthority, "@") < 0 {
 			rawHost = url.RawAuthority
 		} else {
 			url.RawUserinfo, rawHost = split(url.RawAuthority, '@', true)
 		}

 		// We leave RawAuthority only in raw form because clients
 		// of common protocols should be using Userinfo and Host
 		// instead.  Clients that wish to use RawAuthority will have to
 		// interpret it themselves: RFC 2396 does not define the meaning.

 		if strings.Contains(rawHost, "%") {
 			// Host cannot contain escaped characters.
 			err = os.ErrorString("hexadecimal escape in host")
 			goto Error
 		}
 		url.Host = rawHost

 		if url.Path, err = urlUnescape(path, encodePath); err != nil {
 			goto Error
 		}
 	}
 	return url, nil

 Error:
 	return nil, &URLError{"parse", rawurl, err}

 }

 // ParseURLReference is like ParseURL but allows a trailing #fragment.
 func ParseURLReference(rawurlref string) (url *URL, err os.Error) {
 	// Cut off #frag.
 	rawurl, frag := split(rawurlref, '#', false)
 	if url, err = ParseURL(rawurl); err != nil {
 		return nil, err
 	}
 	url.Raw += frag
 	url.RawPath += frag
 	if len(frag) > 1 {
 		frag = frag[1:]
 		if url.Fragment, err = urlUnescape(frag, encodeFragment); err != nil {
 			return nil, &URLError{"parse", rawurl, err}
 		}
 	}
 	return url, nil
 }

 // String reassembles url into a valid URL string.
 //
 // There are redundant fields stored in the URL structure:
 // the String method consults Scheme, Path, Host, RawUserinfo,
 // RawQuery, and Fragment, but not Raw, RawPath or Authority.
 func (url *URL) String() string {
 	result := ""
 	if url.Scheme != "" {
 		result += url.Scheme + ":"
 	}
 	if url.Host != "" || url.RawUserinfo != "" {
 		result += "//"
 		if url.RawUserinfo != "" {
 			// hide the password, if any
 			info := url.RawUserinfo
 			if i := strings.Index(info, ":"); i >= 0 {
 				info = info[0:i] + ":******"
 			}
 			result += info + "@"
 		}
 		result += url.Host
 	}
 	if url.OpaquePath {
 		path := url.Path
 		if strings.HasPrefix(path, "/") {
 			result += "%2f"
 			path = path[1:]
 		}
 		result += urlEscape(path, encodeOpaque)
 	} else {
 		result += urlEscape(url.Path, encodePath)
 	}
 	if url.RawQuery != "" {
 		result += "?" + url.RawQuery
 	}
 	if url.Fragment != "" {
 		result += "#" + urlEscape(url.Fragment, encodeFragment)
 	}
 	return result
 }

 // EncodeQuery encodes the query represented as a multimap.
 func EncodeQuery(m map[string][]string) string {
 	parts := make([]string, 0, len(m)) // will be large enough for most uses
 	for k, vs := range m {
 		prefix := URLEscape(k) + "="
 		for _, v := range vs {
 			parts = append(parts, prefix+URLEscape(v))
 		}
 	}
 	return strings.Join(parts, "&")
 }
	// 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.

	// Parse URLs (actually URIs, but that seems overly pedantic).
	// RFC 3986

	package http

	import (
	"os"
	"strconv"
	"strings"
	)

	// URLError reports an error and the operation and URL that caused it.
	type URLError struct {
	Op string
	URL string
	Error os.Error
	}

	func (e *URLError) String() string { return e.Op + " " + e.URL + ": " + e.Error.String() }

	func ishex(c byte) bool {
	switch {
	case '0' <= c && c <= '9':
	return true
	case 'a' <= c && c <= 'f':
	return true
	case 'A' <= c && c <= 'F':
	return true
	}
	return false
	}

	func unhex(c byte) byte {
	switch {
	case '0' <= c && c <= '9':
	return c - '0'
	case 'a' <= c && c <= 'f':
	return c - 'a' + 10
	case 'A' <= c && c <= 'F':
	return c - 'A' + 10
	}
	return 0
	}

	type encoding int

	const (
	encodePath encoding = 1 + iota
	encodeUserPassword
	encodeQueryComponent
	encodeFragment
	encodeOpaque
	)


	type URLEscapeError string

	func (e URLEscapeError) String() string {
	return "invalid URL escape " + strconv.Quote(string(e))
	}

	// Return true if the specified character should be escaped when
	// appearing in a URL string, according to RFC 2396.
	// When 'all' is true the full range of reserved characters are matched.
	func shouldEscape(c byte, mode encoding) bool {
	// RFC 2396 §2.3 Unreserved characters (alphanum)
	if 'A' <= c && c <= 'Z' \|\| 'a' <= c && c <= 'z' \|\| '0' <= c && c <= '9' {
	return false
	}
	switch c {
	case '-', '_', '.', '!', '~', '*', '\'', '(', ')': // §2.3 Unreserved characters (mark)
	return false

	case '$', '&', '+', ',', '/', ':', ';', '=', '?', '@': // §2.2 Reserved characters (reserved)
	// Different sections of the URL allow a few of
	// the reserved characters to appear unescaped.
	switch mode {
	case encodePath: // §3.3
	// The RFC allows : @ & = + $ , but saves / ; for assigning
	// meaning to individual path segments. This package
	// only manipulates the path as a whole, so we allow those
	// last two as well. Clients that need to distinguish between
	// `/foo;y=z/bar` and `/foo%3by=z/bar` will have to re-decode RawPath.
	// That leaves only ? to escape.
	return c == '?'

	case encodeUserPassword: // §3.2.2
	// The RFC allows ; : & = + $ , in userinfo, so we must escape only @ and /.
	// The parsing of userinfo treats : as special so we must escape that too.
	return c == '@' \|\| c == '/' \|\| c == ':'

	case encodeQueryComponent: // §3.4
	// The RFC reserves (so we must escape) everything.
	return true

	case encodeFragment: // §4.1
	// The RFC text is silent but the grammar allows
	// everything, so escape nothing.
	return false

	case encodeOpaque: // §3 opaque_part
	// The RFC allows opaque_part to use all characters
	// except that the leading / must be escaped.
	// (We implement that case in String.)
	return false
	}
	}

	// Everything else must be escaped.
	return true
	}

	// CanonicalPath applies the algorithm specified in RFC 2396 to
	// simplify the path, removing unnecessary . and .. elements.
	func CanonicalPath(path string) string {
	buf := []byte(path)
	a := buf[0:0]
	// state helps to find /.. ^.. ^. and /. patterns.
	// state == 1 - prev char is '/' or beginning of the string.
	// state > 1 - prev state > 0 and prev char was '.'
	// state == 0 - otherwise
	state := 1
	cnt := 0
	for _, v := range buf {
	switch v {
	case '/':
	s := state
	state = 1
	switch s {
	case 2:
	a = a[0 : len(a)-1]
	continue
	case 3:
	if cnt > 0 {
	i := len(a) - 4
	for ; i >= 0 && a[i] != '/'; i-- {
	}
	a = a[0 : i+1]
	cnt--
	continue
	}
	default:
	if len(a) > 0 {
	cnt++
	}
	}
	case '.':
	if state > 0 {
	state++
	}
	default:
	state = 0
	}
	l := len(a)
	a = a[0 : l+1]
	a[l] = v
	}
	switch {
	case state == 2:
	a = a[0 : len(a)-1]
	case state == 3 && cnt > 0:
	i := len(a) - 4
	for ; i >= 0 && a[i] != '/'; i-- {
	}
	a = a[0 : i+1]
	}
	return string(a)
	}

	// URLUnescape unescapes a string in ``URL encoded'' form,
	// converting %AB into the byte 0xAB and '+' into ' ' (space).
	// It returns an error if any % is not followed
	// by two hexadecimal digits.
	// Despite the name, this encoding applies only to individual
	// components of the query portion of the URL.
	func URLUnescape(s string) (string, os.Error) {
	return urlUnescape(s, encodeQueryComponent)
	}

	// urlUnescape is like URLUnescape but mode specifies
	// which section of the URL is being unescaped.
	func urlUnescape(s string, mode encoding) (string, os.Error) {
	// Count %, check that they're well-formed.
	n := 0
	hasPlus := false
	for i := 0; i < len(s); {
	switch s[i] {
	case '%':
	n++
	if i+2 >= len(s) \|\| !ishex(s[i+1]) \|\| !ishex(s[i+2]) {
	s = s[i:]
	if len(s) > 3 {
	s = s[0:3]
	}
	return "", URLEscapeError(s)
	}
	i += 3
	case '+':
	hasPlus = mode == encodeQueryComponent
	i++
	default:
	i++
	}
	}

	if n == 0 && !hasPlus {
	return s, nil
	}

	t := make([]byte, len(s)-2*n)
	j := 0
	for i := 0; i < len(s); {
	switch s[i] {
	case '%':
	t[j] = unhex(s[i+1])<<4 \| unhex(s[i+2])
	j++
	i += 3
	case '+':
	if mode == encodeQueryComponent {
	t[j] = ' '
	} else {
	t[j] = '+'
	}
	j++
	i++
	default:
	t[j] = s[i]
	j++
	i++
	}
	}
	return string(t), nil
	}

	// URLEscape converts a string into ``URL encoded'' form.
	// Despite the name, this encoding applies only to individual
	// components of the query portion of the URL.
	func URLEscape(s string) string {
	return urlEscape(s, encodeQueryComponent)
	}

	func urlEscape(s string, mode encoding) string {
	spaceCount, hexCount := 0, 0
	for i := 0; i < len(s); i++ {
	c := s[i]
	if shouldEscape(c, mode) {
	if c == ' ' && mode == encodeQueryComponent {
	spaceCount++
	} else {
	hexCount++
	}
	}
	}

	if spaceCount == 0 && hexCount == 0 {
	return s
	}

	t := make([]byte, len(s)+2*hexCount)
	j := 0
	for i := 0; i < len(s); i++ {
	switch c := s[i]; {
	case c == ' ' && mode == encodeQueryComponent:
	t[j] = '+'
	j++
	case shouldEscape(c, mode):
	t[j] = '%'
	t[j+1] = "0123456789abcdef"[c>>4]
	t[j+2] = "0123456789abcdef"[c&15]
	j += 3
	default:
	t[j] = s[i]
	j++
	}
	}
	return string(t)
	}

	// UnescapeUserinfo parses the RawUserinfo field of a URL
	// as the form user or user:password and unescapes and returns
	// the two halves.
	//
	// This functionality should only be used with legacy web sites.
	// RFC 2396 warns that interpreting Userinfo this way
	// ``is NOT RECOMMENDED, because the passing of authentication
	// information in clear text (such as URI) has proven to be a
	// security risk in almost every case where it has been used.''
	func UnescapeUserinfo(rawUserinfo string) (user, password string, err os.Error) {
	u, p := split(rawUserinfo, ':', true)
	if user, err = urlUnescape(u, encodeUserPassword); err != nil {
	return "", "", err
	}
	if password, err = urlUnescape(p, encodeUserPassword); err != nil {
	return "", "", err
	}
	return
	}

	// EscapeUserinfo combines user and password in the form
	// user:password (or just user if password is empty) and then
	// escapes it for use as the URL.RawUserinfo field.
	//
	// This functionality should only be used with legacy web sites.
	// RFC 2396 warns that interpreting Userinfo this way
	// ``is NOT RECOMMENDED, because the passing of authentication
	// information in clear text (such as URI) has proven to be a
	// security risk in almost every case where it has been used.''
	func EscapeUserinfo(user, password string) string {
	raw := urlEscape(user, encodeUserPassword)
	if password != "" {
	raw += ":" + urlEscape(password, encodeUserPassword)
	}
	return raw
	}

	// A URL represents a parsed URL (technically, a URI reference).
	// The general form represented is:
	// scheme://[userinfo@]host/path[?query][#fragment]
	// The Raw, RawAuthority, RawPath, and RawQuery fields are in "wire format"
	// (special characters must be hex-escaped if not meant to have special meaning).
	// All other fields are logical values; '+' or '%' represent themselves.
	//
	// The various Raw values are supplied in wire format because
	// clients typically have to split them into pieces before further
	// decoding.
	type URL struct {
	Raw string // the original string
	Scheme string // scheme
	RawAuthority string // [userinfo@]host
	RawUserinfo string // userinfo
	Host string // host
	RawPath string // /path[?query][#fragment]
	Path string // /path
	OpaquePath bool // path is opaque (unrooted when scheme is present)
	RawQuery string // query
	Fragment string // fragment
	}

	// Maybe rawurl is of the form scheme:path.
	// (Scheme must be [a-zA-Z][a-zA-Z0-9+-.]*)
	// If so, return scheme, path; else return "", rawurl.
	func getscheme(rawurl string) (scheme, path string, err os.Error) {
	for i := 0; i < len(rawurl); i++ {
	c := rawurl[i]
	switch {
	case 'a' <= c && c <= 'z' \|\| 'A' <= c && c <= 'Z':
	// do nothing
	case '0' <= c && c <= '9' \|\| c == '+' \|\| c == '-' \|\| c == '.':
	if i == 0 {
	return "", rawurl, nil
	}
	case c == ':':
	if i == 0 {
	return "", "", os.ErrorString("missing protocol scheme")
	}
	return rawurl[0:i], rawurl[i+1:], nil
	default:
	// we have encountered an invalid character,
	// so there is no valid scheme
	return "", rawurl, nil
	}
	}
	return "", rawurl, nil
	}

	// Maybe s is of the form t c u.
	// If so, return t, c u (or t, u if cutc == true).
	// If not, return s, "".
	func split(s string, c byte, cutc bool) (string, string) {
	for i := 0; i < len(s); i++ {
	if s[i] == c {
	if cutc {
	return s[0:i], s[i+1:]
	}
	return s[0:i], s[i:]
	}
	}
	return s, ""
	}

	// ParseURL parses rawurl into a URL structure.
	// The string rawurl is assumed not to have a #fragment suffix.
	// (Web browsers strip #fragment before sending the URL to a web server.)
	func ParseURL(rawurl string) (url *URL, err os.Error) {
	if rawurl == "" {
	err = os.ErrorString("empty url")
	goto Error
	}
	url = new(URL)
	url.Raw = rawurl

	// Split off possible leading "http:", "mailto:", etc.
	// Cannot contain escaped characters.
	var path string
	if url.Scheme, path, err = getscheme(rawurl); err != nil {
	goto Error
	}

	if url.Scheme != "" && (len(path) == 0 \|\| path[0] != '/') {
	// RFC 2396:
	// Absolute URI (has scheme) with non-rooted path
	// is uninterpreted. It doesn't even have a ?query.
	// This is the case that handles mailto:name@example.com.
	url.RawPath = path

	if url.Path, err = urlUnescape(path, encodeOpaque); err != nil {
	goto Error
	}
	url.OpaquePath = true
	} else {
	// Split off query before parsing path further.
	url.RawPath = path
	path, query := split(path, '?', false)
	if len(query) > 1 {
	url.RawQuery = query[1:]
	}

	// Maybe path is //authority/path
	if url.Scheme != "" && len(path) > 2 && path[0:2] == "//" {
	url.RawAuthority, path = split(path[2:], '/', false)
	url.RawPath = url.RawPath[2+len(url.RawAuthority):]
	}

	// Split authority into userinfo@host.
	// If there's no @, split's default is wrong. Check explicitly.
	var rawHost string
	if strings.Index(url.RawAuthority, "@") < 0 {
	rawHost = url.RawAuthority
	} else {
	url.RawUserinfo, rawHost = split(url.RawAuthority, '@', true)
	}

	// We leave RawAuthority only in raw form because clients
	// of common protocols should be using Userinfo and Host
	// instead. Clients that wish to use RawAuthority will have to
	// interpret it themselves: RFC 2396 does not define the meaning.

	if strings.Contains(rawHost, "%") {
	// Host cannot contain escaped characters.
	err = os.ErrorString("hexadecimal escape in host")
	goto Error
	}
	url.Host = rawHost

	if url.Path, err = urlUnescape(path, encodePath); err != nil {
	goto Error
	}
	}
	return url, nil

	Error:
	return nil, &URLError{"parse", rawurl, err}

	}

	// ParseURLReference is like ParseURL but allows a trailing #fragment.
	func ParseURLReference(rawurlref string) (url *URL, err os.Error) {
	// Cut off #frag.
	rawurl, frag := split(rawurlref, '#', false)
	if url, err = ParseURL(rawurl); err != nil {
	return nil, err
	}
	url.Raw += frag
	url.RawPath += frag
	if len(frag) > 1 {
	frag = frag[1:]
	if url.Fragment, err = urlUnescape(frag, encodeFragment); err != nil {
	return nil, &URLError{"parse", rawurl, err}
	}
	}
	return url, nil
	}

	// String reassembles url into a valid URL string.
	//
	// There are redundant fields stored in the URL structure:
	// the String method consults Scheme, Path, Host, RawUserinfo,
	// RawQuery, and Fragment, but not Raw, RawPath or Authority.
	func (url *URL) String() string {
	result := ""
	if url.Scheme != "" {
	result += url.Scheme + ":"
	}
	if url.Host != "" \|\| url.RawUserinfo != "" {
	result += "//"
	if url.RawUserinfo != "" {
	// hide the password, if any
	info := url.RawUserinfo
	if i := strings.Index(info, ":"); i >= 0 {
	info = info[0:i] + ":******"
	}
	result += info + "@"
	}
	result += url.Host
	}
	if url.OpaquePath {
	path := url.Path
	if strings.HasPrefix(path, "/") {
	result += "%2f"
	path = path[1:]
	}
	result += urlEscape(path, encodeOpaque)
	} else {
	result += urlEscape(url.Path, encodePath)
	}
	if url.RawQuery != "" {
	result += "?" + url.RawQuery
	}
	if url.Fragment != "" {
	result += "#" + urlEscape(url.Fragment, encodeFragment)
	}
	return result
	}

	// EncodeQuery encodes the query represented as a multimap.
	func EncodeQuery(m map[string][]string) string {
	parts := make([]string, 0, len(m)) // will be large enough for most uses
	for k, vs := range m {
	prefix := URLEscape(k) + "="
	for _, v := range vs {
	parts = append(parts, prefix+URLEscape(v))
	}
	}
	return strings.Join(parts, "&")
	}