src/pkg/url/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.

 // Package URL parses URLs and implements query escaping.
 // See RFC 3986.
 package url

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

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

 func (e *Error) 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 EscapeError string

 func (e EscapeError) 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
 }

 // QueryUnescape does the inverse transformation of QueryEscape, converting
 // %AB into the byte 0xAB and '+' into ' ' (space). It returns an error if
 // any % is not followed by two hexadecimal digits.
 func QueryUnescape(s string) (string, os.Error) {
 	return unescape(s, encodeQueryComponent)
 }

 // unescape unescapes a string; the mode specifies
 // which section of the URL string is being unescaped.
 func unescape(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 "", EscapeError(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
 }

 // QueryEscape escapes the string so it can be safely placed
 // inside a URL query.
 func QueryEscape(s string) string {
 	return escape(s, encodeQueryComponent)
 }

 func escape(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 = unescape(u, encodeUserPassword); err != nil {
 		return "", "", err
 	}
 	if password, err = unescape(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 := escape(user, encodeUserPassword)
 	if password != "" {
 		raw += ":" + escape(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.NewError("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, ""
 }

 // Parse 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.)
 // The rawurl may be relative or absolute.
 func Parse(rawurl string) (url *URL, err os.Error) {
 	return parse(rawurl, false)
 }

 // ParseRequest parses rawurl into a URL structure.  It assumes that
 // rawurl was received from an HTTP request, so the rawurl is interpreted
 // only as an absolute URI or an absolute path.
 // The string rawurl is assumed not to have a #fragment suffix.
 // (Web browsers strip #fragment before sending the URL to a web server.)
 func ParseRequest(rawurl string) (url *URL, err os.Error) {
 	return parse(rawurl, true)
 }

 // parse parses a URL from a string in one of two contexts.  If
 // viaRequest is true, the URL is assumed to have arrived via an HTTP request,
 // in which case only absolute URLs or path-absolute relative URLs are allowed.
 // If viaRequest is false, all forms of relative URLs are allowed.
 func parse(rawurl string, viaRequest bool) (url *URL, err os.Error) {
 	var (
 		leadingSlash bool
 		path         string
 	)

 	if rawurl == "" {
 		err = os.NewError("empty url")
 		goto Error
 	}
 	url = new(URL)
 	url.Raw = rawurl

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

 	if url.Scheme != "" && !leadingSlash {
 		// 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 = unescape(path, encodeOpaque); err != nil {
 			goto Error
 		}
 		url.OpaquePath = true
 	} else {
 		if viaRequest && !leadingSlash {
 			err = os.NewError("invalid URI for request")
 			goto Error
 		}

 		// 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 != "" || !viaRequest) &&
 			strings.HasPrefix(path, "//") && !strings.HasPrefix(path, "///") {
 			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.NewError("hexadecimal escape in host")
 			goto Error
 		}
 		url.Host = rawHost

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

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

 }

 // ParseWithReference is like Parse but allows a trailing #fragment.
 func ParseWithReference(rawurlref string) (url *URL, err os.Error) {
 	// Cut off #frag.
 	rawurl, frag := split(rawurlref, '#', false)
 	if url, err = Parse(rawurl); err != nil {
 		return nil, err
 	}
 	url.Raw += frag
 	url.RawPath += frag
 	if len(frag) > 1 {
 		frag = frag[1:]
 		if url.Fragment, err = unescape(frag, encodeFragment); err != nil {
 			return nil, &Error{"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 RawAuthority.
 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 += escape(path, encodeOpaque)
 	} else {
 		result += escape(url.Path, encodePath)
 	}
 	if url.RawQuery != "" {
 		result += "?" + url.RawQuery
 	}
 	if url.Fragment != "" {
 		result += "#" + escape(url.Fragment, encodeFragment)
 	}
 	return result
 }

 // Values maps a string key to a list of values.
 // It is typically used for query parameters and form values.
 // Unlike in the http.Header map, the keys in a Values map
 // are case-sensitive.
 type Values map[string][]string

 // Get gets the first value associated with the given key.
 // If there are no values associated with the key, Get returns
 // the empty string. To access multiple values, use the map
 // directly.
 func (v Values) Get(key string) string {
 	if v == nil {
 		return ""
 	}
 	vs, ok := v[key]
 	if !ok || len(vs) == 0 {
 		return ""
 	}
 	return vs[0]
 }

 // Set sets the key to value. It replaces any existing
 // values.
 func (v Values) Set(key, value string) {
 	v[key] = []string{value}
 }

 // Add adds the key to value. It appends to any existing
 // values associated with key.
 func (v Values) Add(key, value string) {
 	v[key] = append(v[key], value)
 }

 // Del deletes the values associated with key.
 func (v Values) Del(key string) {
 	v[key] = nil, false
 }

 // ParseQuery parses the URL-encoded query string and returns
 // a map listing the values specified for each key.
 // ParseQuery always returns a non-nil map containing all the
 // valid query parameters found; err describes the first decoding error
 // encountered, if any.
 func ParseQuery(query string) (m Values, err os.Error) {
 	m = make(Values)
 	err = parseQuery(m, query)
 	return
 }

 func parseQuery(m Values, query string) (err os.Error) {
 	for _, kv := range strings.Split(query, "&") {
 		if len(kv) == 0 {
 			continue
 		}
 		kvPair := strings.SplitN(kv, "=", 2)

 		var key, value string
 		var e os.Error
 		key, e = QueryUnescape(kvPair[0])
 		if e == nil && len(kvPair) > 1 {
 			value, e = QueryUnescape(kvPair[1])
 		}
 		if e != nil {
 			err = e
 			continue
 		}
 		m[key] = append(m[key], value)
 	}
 	return err
 }

 // Encode encodes the values into ``URL encoded'' form.
 // e.g. "foo=bar&bar=baz"
 func (v Values) Encode() string {
 	if v == nil {
 		return ""
 	}
 	parts := make([]string, 0, len(v)) // will be large enough for most uses
 	for k, vs := range v {
 		prefix := QueryEscape(k) + "="
 		for _, v := range vs {
 			parts = append(parts, prefix+QueryEscape(v))
 		}
 	}
 	return strings.Join(parts, "&")
 }

 // resolvePath applies special path segments from refs and applies
 // them to base, per RFC 2396.
 func resolvePath(basepath string, refpath string) string {
 	base := strings.Split(basepath, "/")
 	refs := strings.Split(refpath, "/")
 	if len(base) == 0 {
 		base = []string{""}
 	}
 	for idx, ref := range refs {
 		switch {
 		case ref == ".":
 			base[len(base)-1] = ""
 		case ref == "..":
 			newLen := len(base) - 1
 			if newLen < 1 {
 				newLen = 1
 			}
 			base = base[0:newLen]
 			base[len(base)-1] = ""
 		default:
 			if idx == 0 || base[len(base)-1] == "" {
 				base[len(base)-1] = ref
 			} else {
 				base = append(base, ref)
 			}
 		}
 	}
 	return strings.Join(base, "/")
 }

 // IsAbs returns true if the URL is absolute.
 func (url *URL) IsAbs() bool {
 	return url.Scheme != ""
 }

 // Parse parses a URL in the context of a base URL.  The URL in ref
 // may be relative or absolute.  Parse returns nil, err on parse
 // failure, otherwise its return value is the same as ResolveReference.
 func (base *URL) Parse(ref string) (*URL, os.Error) {
 	refurl, err := Parse(ref)
 	if err != nil {
 		return nil, err
 	}
 	return base.ResolveReference(refurl), nil
 }

 // ResolveReference resolves a URI reference to an absolute URI from
 // an absolute base URI, per RFC 2396 Section 5.2.  The URI reference
 // may be relative or absolute.  ResolveReference always returns a new
 // URL instance, even if the returned URL is identical to either the
 // base or reference. If ref is an absolute URL, then ResolveReference
 // ignores base and returns a copy of ref.
 func (base *URL) ResolveReference(ref *URL) *URL {
 	url := new(URL)
 	switch {
 	case ref.IsAbs():
 		*url = *ref
 	default:
 		// relativeURI   = ( net_path | abs_path | rel_path ) [ "?" query ]
 		*url = *base
 		if ref.RawAuthority != "" {
 			// The "net_path" case.
 			url.RawAuthority = ref.RawAuthority
 			url.Host = ref.Host
 			url.RawUserinfo = ref.RawUserinfo
 		}
 		switch {
 		case url.OpaquePath:
 			url.Path = ref.Path
 			url.RawPath = ref.RawPath
 			url.RawQuery = ref.RawQuery
 		case strings.HasPrefix(ref.Path, "/"):
 			// The "abs_path" case.
 			url.Path = ref.Path
 			url.RawPath = ref.RawPath
 			url.RawQuery = ref.RawQuery
 		default:
 			// The "rel_path" case.
 			path := resolvePath(base.Path, ref.Path)
 			if !strings.HasPrefix(path, "/") {
 				path = "/" + path
 			}
 			url.Path = path
 			url.RawPath = url.Path
 			url.RawQuery = ref.RawQuery
 			if ref.RawQuery != "" {
 				url.RawPath += "?" + url.RawQuery
 			}
 		}

 		url.Fragment = ref.Fragment
 	}
 	url.Raw = url.String()
 	return url
 }

 // Query parses RawQuery and returns the corresponding values.
 func (u *URL) Query() Values {
 	v, _ := ParseQuery(u.RawQuery)
 	return v
 }

 // EncodedPath returns the URL's path in "URL path encoded" form.
 func (u *URL) EncodedPath() string {
 	return escape(u.Path, encodePath)
 }
	// 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.

	// Package URL parses URLs and implements query escaping.
	// See RFC 3986.
	package url

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

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

	func (e *Error) 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 EscapeError string

	func (e EscapeError) 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
	}

	// QueryUnescape does the inverse transformation of QueryEscape, converting
	// %AB into the byte 0xAB and '+' into ' ' (space). It returns an error if
	// any % is not followed by two hexadecimal digits.
	func QueryUnescape(s string) (string, os.Error) {
	return unescape(s, encodeQueryComponent)
	}

	// unescape unescapes a string; the mode specifies
	// which section of the URL string is being unescaped.
	func unescape(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 "", EscapeError(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
	}

	// QueryEscape escapes the string so it can be safely placed
	// inside a URL query.
	func QueryEscape(s string) string {
	return escape(s, encodeQueryComponent)
	}

	func escape(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 = unescape(u, encodeUserPassword); err != nil {
	return "", "", err
	}
	if password, err = unescape(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 := escape(user, encodeUserPassword)
	if password != "" {
	raw += ":" + escape(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.NewError("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, ""
	}

	// Parse 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.)
	// The rawurl may be relative or absolute.
	func Parse(rawurl string) (url *URL, err os.Error) {
	return parse(rawurl, false)
	}

	// ParseRequest parses rawurl into a URL structure. It assumes that
	// rawurl was received from an HTTP request, so the rawurl is interpreted
	// only as an absolute URI or an absolute path.
	// The string rawurl is assumed not to have a #fragment suffix.
	// (Web browsers strip #fragment before sending the URL to a web server.)
	func ParseRequest(rawurl string) (url *URL, err os.Error) {
	return parse(rawurl, true)
	}

	// parse parses a URL from a string in one of two contexts. If
	// viaRequest is true, the URL is assumed to have arrived via an HTTP request,
	// in which case only absolute URLs or path-absolute relative URLs are allowed.
	// If viaRequest is false, all forms of relative URLs are allowed.
	func parse(rawurl string, viaRequest bool) (url *URL, err os.Error) {
	var (
	leadingSlash bool
	path string
	)

	if rawurl == "" {
	err = os.NewError("empty url")
	goto Error
	}
	url = new(URL)
	url.Raw = rawurl

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

	if url.Scheme != "" && !leadingSlash {
	// 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 = unescape(path, encodeOpaque); err != nil {
	goto Error
	}
	url.OpaquePath = true
	} else {
	if viaRequest && !leadingSlash {
	err = os.NewError("invalid URI for request")
	goto Error
	}

	// 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 != "" \|\| !viaRequest) &&
	strings.HasPrefix(path, "//") && !strings.HasPrefix(path, "///") {
	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.NewError("hexadecimal escape in host")
	goto Error
	}
	url.Host = rawHost

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

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

	}

	// ParseWithReference is like Parse but allows a trailing #fragment.
	func ParseWithReference(rawurlref string) (url *URL, err os.Error) {
	// Cut off #frag.
	rawurl, frag := split(rawurlref, '#', false)
	if url, err = Parse(rawurl); err != nil {
	return nil, err
	}
	url.Raw += frag
	url.RawPath += frag
	if len(frag) > 1 {
	frag = frag[1:]
	if url.Fragment, err = unescape(frag, encodeFragment); err != nil {
	return nil, &Error{"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 RawAuthority.
	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 += escape(path, encodeOpaque)
	} else {
	result += escape(url.Path, encodePath)
	}
	if url.RawQuery != "" {
	result += "?" + url.RawQuery
	}
	if url.Fragment != "" {
	result += "#" + escape(url.Fragment, encodeFragment)
	}
	return result
	}

	// Values maps a string key to a list of values.
	// It is typically used for query parameters and form values.
	// Unlike in the http.Header map, the keys in a Values map
	// are case-sensitive.
	type Values map[string][]string

	// Get gets the first value associated with the given key.
	// If there are no values associated with the key, Get returns
	// the empty string. To access multiple values, use the map
	// directly.
	func (v Values) Get(key string) string {
	if v == nil {
	return ""
	}
	vs, ok := v[key]
	if !ok \|\| len(vs) == 0 {
	return ""
	}
	return vs[0]
	}

	// Set sets the key to value. It replaces any existing
	// values.
	func (v Values) Set(key, value string) {
	v[key] = []string{value}
	}

	// Add adds the key to value. It appends to any existing
	// values associated with key.
	func (v Values) Add(key, value string) {
	v[key] = append(v[key], value)
	}

	// Del deletes the values associated with key.
	func (v Values) Del(key string) {
	v[key] = nil, false
	}

	// ParseQuery parses the URL-encoded query string and returns
	// a map listing the values specified for each key.
	// ParseQuery always returns a non-nil map containing all the
	// valid query parameters found; err describes the first decoding error
	// encountered, if any.
	func ParseQuery(query string) (m Values, err os.Error) {
	m = make(Values)
	err = parseQuery(m, query)
	return
	}

	func parseQuery(m Values, query string) (err os.Error) {
	for _, kv := range strings.Split(query, "&") {
	if len(kv) == 0 {
	continue
	}
	kvPair := strings.SplitN(kv, "=", 2)

	var key, value string
	var e os.Error
	key, e = QueryUnescape(kvPair[0])
	if e == nil && len(kvPair) > 1 {
	value, e = QueryUnescape(kvPair[1])
	}
	if e != nil {
	err = e
	continue
	}
	m[key] = append(m[key], value)
	}
	return err
	}

	// Encode encodes the values into ``URL encoded'' form.
	// e.g. "foo=bar&bar=baz"
	func (v Values) Encode() string {
	if v == nil {
	return ""
	}
	parts := make([]string, 0, len(v)) // will be large enough for most uses
	for k, vs := range v {
	prefix := QueryEscape(k) + "="
	for _, v := range vs {
	parts = append(parts, prefix+QueryEscape(v))
	}
	}
	return strings.Join(parts, "&")
	}

	// resolvePath applies special path segments from refs and applies
	// them to base, per RFC 2396.
	func resolvePath(basepath string, refpath string) string {
	base := strings.Split(basepath, "/")
	refs := strings.Split(refpath, "/")
	if len(base) == 0 {
	base = []string{""}
	}
	for idx, ref := range refs {
	switch {
	case ref == ".":
	base[len(base)-1] = ""
	case ref == "..":
	newLen := len(base) - 1
	if newLen < 1 {
	newLen = 1
	}
	base = base[0:newLen]
	base[len(base)-1] = ""
	default:
	if idx == 0 \|\| base[len(base)-1] == "" {
	base[len(base)-1] = ref
	} else {
	base = append(base, ref)
	}
	}
	}
	return strings.Join(base, "/")
	}

	// IsAbs returns true if the URL is absolute.
	func (url *URL) IsAbs() bool {
	return url.Scheme != ""
	}

	// Parse parses a URL in the context of a base URL. The URL in ref
	// may be relative or absolute. Parse returns nil, err on parse
	// failure, otherwise its return value is the same as ResolveReference.
	func (base URL) Parse(ref string) (URL, os.Error) {
	refurl, err := Parse(ref)
	if err != nil {
	return nil, err
	}
	return base.ResolveReference(refurl), nil
	}

	// ResolveReference resolves a URI reference to an absolute URI from
	// an absolute base URI, per RFC 2396 Section 5.2. The URI reference
	// may be relative or absolute. ResolveReference always returns a new
	// URL instance, even if the returned URL is identical to either the
	// base or reference. If ref is an absolute URL, then ResolveReference
	// ignores base and returns a copy of ref.
	func (base URL) ResolveReference(ref URL) *URL {
	url := new(URL)
	switch {
	case ref.IsAbs():
	url = ref
	default:
	// relativeURI = ( net_path \| abs_path \| rel_path ) [ "?" query ]
	url = base
	if ref.RawAuthority != "" {
	// The "net_path" case.
	url.RawAuthority = ref.RawAuthority
	url.Host = ref.Host
	url.RawUserinfo = ref.RawUserinfo
	}
	switch {
	case url.OpaquePath:
	url.Path = ref.Path
	url.RawPath = ref.RawPath
	url.RawQuery = ref.RawQuery
	case strings.HasPrefix(ref.Path, "/"):
	// The "abs_path" case.
	url.Path = ref.Path
	url.RawPath = ref.RawPath
	url.RawQuery = ref.RawQuery
	default:
	// The "rel_path" case.
	path := resolvePath(base.Path, ref.Path)
	if !strings.HasPrefix(path, "/") {
	path = "/" + path
	}
	url.Path = path
	url.RawPath = url.Path
	url.RawQuery = ref.RawQuery
	if ref.RawQuery != "" {
	url.RawPath += "?" + url.RawQuery
	}
	}

	url.Fragment = ref.Fragment
	}
	url.Raw = url.String()
	return url
	}

	// Query parses RawQuery and returns the corresponding values.
	func (u *URL) Query() Values {
	v, _ := ParseQuery(u.RawQuery)
	return v
	}

	// EncodedPath returns the URL's path in "URL path encoded" form.
	func (u *URL) EncodedPath() string {
	return escape(u.Path, encodePath)
	}