// urllib parses URLs and implements query escaping.
// See RFC 3986. This module generally follows RFC 3986, except where
// it deviates for compatibility reasons.
// Based off:   https://github.com/golang/go/blob/master/src/net/url/url.go
// Last commit: https://github.com/golang/go/commit/fe2ed5054176935d4adcf13e891715ccf2ee3cce
// 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.
module urllib

import strings

enum EncodingMode {
	encode_path
	encode_path_segment
	encode_host
	encode_zone
	encode_user_password
	encode_query_component
	encode_fragment
}

const (
	err_msg_escape = 'unescape: invalid URL escape'
	err_msg_parse  = 'parse: failed parsing url'
)

fn error_msg(message string, val string) string {
	mut msg := 'net.urllib.$message'
	if val != '' {
		msg = '$msg ($val)'
	}
	return msg
}

// Return true if the specified character should be escaped when
// appearing in a URL string, according to RFC 3986.
//
// Please be informed that for now should_escape does not check all
// reserved characters correctly. See golang.org/issue/5684.
fn should_escape(c byte, mode EncodingMode) bool {
	// §2.3 Unreserved characters (alphanum)
	if (`a` <= c && c <= `z`) || (`A` <= c && c <= `Z`) || (`0` <= c && c <= `9`) {
		return false
	}
	if mode == .encode_host || mode == .encode_zone {
		// §3.2.2 host allows
		// sub-delims = `!` / `$` / `&` / ``` / `(` / `)` / `*` / `+` / `,` / `;` / `=`
		// as part of reg-name.
		// We add : because we include :port as part of host.
		// We add [ ] because we include [ipv6]:port as part of host.
		// We add < > because they`re the only characters left that
		// we could possibly allow, and parse will reject them if we
		// escape them (because hosts can`t use %-encoding for
		// ASCII bytes).
		if c in [`!`, `$`, `&`, `\\`, `(`, `)`, `*`, `+`, `,`, `;`, `=`, `:`, `[`, `]`, `<`, `>`,
			`"`] {
			return false
		}
	}
	match c {
		`-`, `_`, `.`, `~` {
			// §2.3 Unreserved characters (mark)
			return false
		}
		`$`, `&`, `+`, `,`, `/`, `:`, `;`, `=`, `?`, `@` {
			// §2.2 Reserved characters (reserved)
			// Different sections of the URL allow a few of
			// the reserved characters to appear unescaped.
			match mode {
				.encode_path {
					// §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 three as well. That leaves only ? to escape.
					return c == `?`
				}
				.encode_path_segment {
					// §3.3
					// The RFC allows : @ & = + $ but saves / ; , for assigning
					// meaning to individual path segments.
					return c == `/` || c == `;` || c == `,` || c == `?`
				}
				.encode_user_password {
					// §3.2.1
					// The RFC allows `;`, `:`, `&`, `=`, `+`, `$`, and `,` 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 == `?` || c == `:`
				}
				.encode_query_component {
					// §3.4
					// The RFC reserves (so we must escape) everything.
					return true
				}
				.encode_fragment {
					// §4.1
					// The RFC text is silent but the grammar allows
					// everything, so escape nothing.
					return false
				}
				else {}
			}
		}
		else {}
	}
	if mode == .encode_fragment {
		// RFC 3986 §2.2 allows not escaping sub-delims. A subset of sub-delims are
		// included in reserved from RFC 2396 §2.2. The remaining sub-delims do not
		// need to be escaped. To minimize potential breakage, we apply two restrictions:
		// (1) we always escape sub-delims outside of the fragment, and (2) we always
		// escape single quote to avoid breaking callers that had previously assumed that
		// single quotes would be escaped. See issue #19917.
		match c {
			`!`, `(`, `)`, `*` { return false }
			else {}
		}
	}
	// Everything else must be escaped.
	return true
}

// query_unescape does the inverse transformation of query_escape,
// converting each 3-byte encoded substring of the form '%AB' into the
// hex-decoded byte 0xAB.
// It returns an error if any % is not followed by two hexadecimal
// digits.
pub fn query_unescape(s string) ?string {
	return unescape(s, .encode_query_component)
}

// path_unescape does the inverse transformation of path_escape,
// converting each 3-byte encoded substring of the form '%AB' into the
// hex-decoded byte 0xAB. It returns an error if any % is not followed
// by two hexadecimal digits.
//
// path_unescape is identical to query_unescape except that it does not
// unescape '+' to ' ' (space).
pub fn path_unescape(s string) ?string {
	return unescape(s, .encode_path_segment)
}

// unescape unescapes a string; the mode specifies
// which section of the URL string is being unescaped.
fn unescape(s_ string, mode EncodingMode) ?string {
	mut s := s_
	// Count %, check that they're well-formed.
	mut n := 0
	mut has_plus := false
	for i := 0; i < s.len; {
		x := s[i]
		match x {
			`%` {
				if s == '' {
					break
				}
				n++
				if i + 2 >= s.len || !ishex(s[i + 1]) || !ishex(s[i + 2]) {
					if mode == .encode_query_component && i + 1 < s.len {
						s = s[..i] + '%25' + s[(i + 1)..]
						i += 4 // skip the %25 and the next character
						continue
					}
					s = s[i..]
					if s.len > 3 {
						s = s[..3]
					}
					return error(error_msg(urllib.err_msg_escape, s))
				}
				// Per https://tools.ietf.org/html/rfc3986#page-21
				// in the host component %-encoding can only be used
				// for non-ASCII bytes.
				// But https://tools.ietf.org/html/rfc6874#section-2
				// introduces %25 being allowed to escape a percent sign
				// in IPv6 scoped-address literals. Yay.
				if i + 3 >= s.len && mode == .encode_host && unhex(s[i + 1]) < 8
					&& s[i..i + 3] != '%25' {
					return error(error_msg(urllib.err_msg_escape, s[i..i + 3]))
				}
				if mode == .encode_zone {
					// RFC 6874 says basically 'anything goes' for zone identifiers
					// and that even non-ASCII can be redundantly escaped,
					// but it seems prudent to restrict %-escaped bytes here to those
					// that are valid host name bytes in their unescaped form.
					// That is, you can use escaping in the zone identifier but not
					// to introduce bytes you couldn't just write directly.
					// But Windows puts spaces here! Yay.
					if i + 3 >= s.len {
						return error(error_msg('unescape: invalid escape sequence', ''))
					}
					v := ((unhex(s[i + 1]) << byte(4)) | unhex(s[i + 2]))
					if s[i..i + 3] != '%25' && v != ` ` && should_escape(v, .encode_host) {
						error(error_msg(urllib.err_msg_escape, s[i..i + 3]))
					}
				}
				i += 3
			}
			`+` {
				has_plus = mode == .encode_query_component
				i++
			}
			else {
				if (mode == .encode_host || mode == .encode_zone) && s[i] < 0x80
					&& should_escape(s[i], mode) {
					error(error_msg('unescape: invalid character in host name', s[i..i + 1]))
				}
				i++
			}
		}
	}
	if n == 0 && !has_plus {
		return '$s' // TODO `return s` once an autofree bug is fixed
	}
	if s.len < 2 * n {
		return error(error_msg('unescape: invalid escape sequence', ''))
	}
	mut t := strings.new_builder(s.len - 2 * n)
	for i := 0; i < s.len; i++ {
		x := s[i]
		match x {
			`%` {
				if i + 2 >= s.len {
					return error(error_msg('unescape: invalid escape sequence', ''))
				}
				t.write_string(((unhex(s[i + 1]) << byte(4)) | unhex(s[i + 2])).ascii_str())
				i += 2
			}
			`+` {
				if mode == .encode_query_component {
					t.write_string(' ')
				} else {
					t.write_string('+')
				}
			}
			else {
				t.write_string(s[i].ascii_str())
			}
		}
	}
	return t.str()
}

// query_escape escapes the string so it can be safely placed
// inside a URL query.
pub fn query_escape(s string) string {
	return escape(s, .encode_query_component)
}

// path_escape escapes the string so it can be safely placed inside a URL path segment,
// replacing special characters (including /) with %XX sequences as needed.
pub fn path_escape(s string) string {
	return escape(s, .encode_path_segment)
}

fn escape(s string, mode EncodingMode) string {
	mut space_count := 0
	mut hex_count := 0
	mut c := byte(0)
	for i in 0 .. s.len {
		c = s[i]
		if should_escape(c, mode) {
			if c == ` ` && mode == .encode_query_component {
				space_count++
			} else {
				hex_count++
			}
		}
	}
	if space_count == 0 && hex_count == 0 {
		return s
	}
	required := s.len + 2 * hex_count
	mut t := []byte{len: required}
	if hex_count == 0 {
		copy(t, s.bytes())
		for i in 0 .. s.len {
			if s[i] == ` ` {
				t[i] = `+`
			}
		}
		return t.bytestr()
	}
	upperhex := '0123456789ABCDEF'
	mut j := 0
	for i in 0 .. s.len {
		c1 := s[i]
		if c1 == ` ` && mode == .encode_query_component {
			t[j] = `+`
			j++
		} else if should_escape(c1, mode) {
			t[j] = `%`
			t[j + 1] = upperhex[c1 >> 4]
			t[j + 2] = upperhex[c1 & 15]
			j += 3
		} else {
			t[j] = s[i]
			j++
		}
	}
	return t.bytestr()
}

// A URL represents a parsed URL (technically, a URI reference).
//
// The general form represented is:
//
// [scheme:][//[userinfo@]host][/]path[?query][#fragment]
//
// URLs that do not start with a slash after the scheme are interpreted as:
//
// scheme:opaque[?query][#fragment]
//
// Note that the path field is stored in decoded form: /%47%6f%2f becomes /Go/.
// A consequence is that it is impossible to tell which slashes in the path were
// slashes in the raw URL and which were %2f. This distinction is rarely important,
// but when it is, the code should use raw_path, an optional field which only gets
// set if the default encoding is different from path.
//
// URL's String method uses the escaped_path method to obtain the path. See the
// escaped_path method for more details.
pub struct URL {
pub mut:
	scheme      string
	opaque      string    // encoded opaque data
	user        &Userinfo // username and password information
	host        string    // host or host:port
	path        string    // path (relative paths may omit leading slash)
	raw_path    string    // encoded path hint (see escaped_path method)
	force_query bool      // append a query ('?') even if raw_query is empty
	raw_query   string    // encoded query values, without '?'
	fragment    string    // fragment for references, without '#'
}

// user returns a Userinfo containing the provided username
// and no password set.
pub fn user(username string) &Userinfo {
	return &Userinfo{
		username: username
		password: ''
		password_set: false
	}
}

// user_password returns a Userinfo containing the provided username
// and password.
//
// 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.''
fn user_password(username string, password string) &Userinfo {
	return &Userinfo{username, password, true}
}

// The Userinfo type is an immutable encapsulation of username and
// password details for a URL. An existing Userinfo value is guaranteed
// to have a username set (potentially empty, as allowed by RFC 2396),
// and optionally a password.
struct Userinfo {
pub:
	username     string
	password     string
	password_set bool
}

fn (u &Userinfo) empty() bool {
	return isnil(u) || (u.username == '' && u.password == '')
}

// string returns the encoded userinfo information in the standard form
// of 'username[:password]'.
fn (u &Userinfo) str() string {
	if u.empty() {
		return ''
	}
	mut s := escape(u.username, .encode_user_password)
	if u.password_set {
		s += ':' + escape(u.password, .encode_user_password)
	}
	return s
}

// 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]
fn split_by_scheme(rawurl string) ?[]string {
	for i in 0 .. rawurl.len {
		c := rawurl[i]
		if (`a` <= c && c <= `z`) || (`A` <= c && c <= `Z`) {
			// do nothing
		} else if (`0` <= c && c <= `9`) || (c == `+` || c == `-` || c == `.`) {
			if i == 0 {
				return ['', rawurl]
			}
		} else if c == `:` {
			if i == 0 {
				return error(error_msg('split_by_scheme: missing protocol scheme', ''))
			}
			return [rawurl[..i], rawurl[i + 1..]]
		} else {
			// we have encountered an invalid character,
			// so there is no valid scheme
			return ['', rawurl]
		}
	}
	return ['', rawurl]
}

fn get_scheme(rawurl string) ?string {
	split := split_by_scheme(rawurl) or { return err.msg }
	return split[0]
}

// split slices s into two substrings separated by the first occurence of
// sep. If cutc is true then sep is included with the second substring.
// If sep does not occur in s then s and the empty string is returned.
fn split(s string, sep byte, cutc bool) (string, string) {
	i := s.index_byte(sep)
	if i < 0 {
		return s, ''
	}
	if cutc {
		return s[..i], s[i + 1..]
	}
	return s[..i], s[i..]
}

// parse parses rawurl into a URL structure.
//
// The rawurl may be relative (a path, without a host) or absolute
// (starting with a scheme). Trying to parse a hostname and path
// without a scheme is invalid but may not necessarily return an
// error, due to parsing ambiguities.
pub fn parse(rawurl string) ?URL {
	// Cut off #frag
	u, frag := split(rawurl, `#`, true)
	mut url := parse_url(u, false) or { return error(error_msg(urllib.err_msg_parse, u)) }
	if frag == '' {
		return url
	}
	f := unescape(frag, .encode_fragment) or { return error(error_msg(urllib.err_msg_parse,
		u)) }
	url.fragment = f
	return url
}

// parse_request_uri parses rawurl into a URL structure. It assumes that
// rawurl was received in 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.)
fn parse_request_uri(rawurl string) ?URL {
	return parse_url(rawurl, true)
}

// parse_url parses a URL from a string in one of two contexts. If
// via_request 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 via_request is false, all forms of relative URLs are allowed.
[manualfree]
fn parse_url(rawurl string, via_request bool) ?URL {
	if string_contains_ctl_byte(rawurl) {
		return error(error_msg('parse_url: invalid control character in URL', rawurl))
	}
	if rawurl == '' && via_request {
		return error(error_msg('parse_url: empty URL', rawurl))
	}
	mut url := URL{
		user: 0
	}
	if rawurl == '*' {
		url.path = '*'
		return url
	}
	// Split off possible leading 'http:', 'mailto:', etc.
	// Cannot contain escaped characters.
	p := split_by_scheme(rawurl) ?
	url.scheme = p[0]
	mut rest := p[1]
	url.scheme = url.scheme.to_lower()
	// if rest.ends_with('?') && strings.count(rest, '?') == 1 {
	if rest.ends_with('?') && !rest[..1].contains('?') {
		url.force_query = true
		rest = rest[..rest.len - 1]
	} else {
		r, raw_query := split(rest, `?`, true)
		rest = r
		url.raw_query = raw_query
	}
	if !rest.starts_with('/') {
		if url.scheme != '' {
			// We consider rootless paths per RFC 3986 as opaque.
			url.opaque = rest
			return url
		}
		if via_request {
			return error(error_msg('parse_url: invalid URI for request', ''))
		}
		// Avoid confusion with malformed schemes, like cache_object:foo/bar.
		// See golang.org/issue/16822.
		//
		// RFC 3986, §3.3:
		// In addition, a URI reference (Section 4.1) may be a relative-path reference,
		// in which case the first path segment cannot contain a colon (':') character.
		colon := rest.index(':') or { return error('there should be a : in the URL') }
		slash := rest.index('/') or { return error('there should be a / in the URL') }
		if colon >= 0 && (slash < 0 || colon < slash) {
			// First path segment has colon. Not allowed in relative URL.
			return error(error_msg('parse_url: first path segment in URL cannot contain colon',
				''))
		}
	}
	if ((url.scheme != '' || !via_request) && !rest.starts_with('///')) && rest.starts_with('//') {
		authority, r := split(rest[2..], `/`, false)
		rest = r
		a := parse_authority(authority) ?
		url.user = a.user
		url.host = a.host
	}
	// Set path and, optionally, raw_path.
	// raw_path is a hint of the encoding of path. We don't want to set it if
	// the default escaping of path is equivalent, to help make sure that people
	// don't rely on it in general.
	url.set_path(rest) ?
	return url
}

struct ParseAuthorityRes {
	user &Userinfo
	host string
}

fn parse_authority(authority string) ?ParseAuthorityRes {
	i := authority.last_index('@') or { -1 }
	mut host := ''
	mut zuser := user('')
	if i < 0 {
		h := parse_host(authority) ?
		host = h
	} else {
		h := parse_host(authority[i + 1..]) ?
		host = h
	}
	if i < 0 {
		return ParseAuthorityRes{
			host: host
			user: zuser
		}
	}
	mut userinfo := authority[..i]
	if !valid_userinfo(userinfo) {
		return error(error_msg('parse_authority: invalid userinfo', ''))
	}
	if !userinfo.contains(':') {
		u := unescape(userinfo, .encode_user_password) ?
		userinfo = u
		zuser = user(userinfo)
	} else {
		mut username, mut password := split(userinfo, `:`, true)
		u := unescape(username, .encode_user_password) ?
		username = u
		p := unescape(password, .encode_user_password) ?
		password = p
		zuser = user_password(username, password)
	}
	return ParseAuthorityRes{
		user: zuser
		host: host
	}
}

// parse_host parses host as an authority without user
// information. That is, as host[:port].
fn parse_host(host string) ?string {
	if host.starts_with('[') {
		// parse an IP-Literal in RFC 3986 and RFC 6874.
		// E.g., '[fe80::1]', '[fe80::1%25en0]', '[fe80::1]:80'.
		mut i := host.last_index(']') or {
			return error(error_msg("parse_host: missing ']' in host", ''))
		}
		mut colon_port := host[i + 1..]
		if !valid_optional_port(colon_port) {
			return error(error_msg('parse_host: invalid port $colon_port after host ',
				''))
		}
		// RFC 6874 defines that %25 (%-encoded percent) introduces
		// the zone identifier, and the zone identifier can use basically
		// any %-encoding it likes. That's different from the host, which
		// can only %-encode non-ASCII bytes.
		// We do impose some restrictions on the zone, to avoid stupidity
		// like newlines.
		if zone := host[..i].index('%25') {
			host1 := unescape(host[..zone], .encode_host) or { return err.msg }
			host2 := unescape(host[zone..i], .encode_zone) or { return err.msg }
			host3 := unescape(host[i..], .encode_host) or { return err.msg }
			return host1 + host2 + host3
		}
		if idx := host.last_index(':') {
			colon_port = host[idx..]
			if !valid_optional_port(colon_port) {
				return error(error_msg('parse_host: invalid port $colon_port after host ',
					''))
			}
		}
	}
	h := unescape(host, .encode_host) or { return err.msg }
	return h
	// host = h
	// return host
}

// set_path sets the path and raw_path fields of the URL based on the provided
// escaped path p. It maintains the invariant that raw_path is only specified
// when it differs from the default encoding of the path.
// For example:
// - set_path('/foo/bar')   will set path='/foo/bar' and raw_path=''
// - set_path('/foo%2fbar') will set path='/foo/bar' and raw_path='/foo%2fbar'
// set_path will return an error only if the provided path contains an invalid
// escaping.
pub fn (mut u URL) set_path(p string) ?bool {
	u.path = unescape(p, .encode_path) ?
	u.raw_path = if p == escape(u.path, .encode_path) { '' } else { p }
	return true
}

// escaped_path returns the escaped form of u.path.
// In general there are multiple possible escaped forms of any path.
// escaped_path returns u.raw_path when it is a valid escaping of u.path.
// Otherwise escaped_path ignores u.raw_path and computes an escaped
// form on its own.
// The String and request_uri methods use escaped_path to construct
// their results.
// In general, code should call escaped_path instead of
// reading u.raw_path directly.
pub fn (u &URL) escaped_path() string {
	if u.raw_path != '' && valid_encoded_path(u.raw_path) {
		unescape(u.raw_path, .encode_path) or { return '' }
		return u.raw_path
	}
	if u.path == '*' {
		return '*' // don't escape (Issue 11202)
	}
	return escape(u.path, .encode_path)
}

// valid_encoded_path reports whether s is a valid encoded path.
// It must not contain any bytes that require escaping during path encoding.
fn valid_encoded_path(s string) bool {
	for i in 0 .. s.len {
		// RFC 3986, Appendix A.
		// pchar = unreserved / pct-encoded / sub-delims / ':' / '@'.
		// should_escape is not quite compliant with the RFC,
		// so we check the sub-delims ourselves and let
		// should_escape handle the others.
		x := s[i]
		match x {
			`!`, `$`, `&`, `\\`, `(`, `)`, `*`, `+`, `,`, `;`, `=`, `:`, `@` {
				// ok
			}
			`[`, `]` {
				// ok - not specified in RFC 3986 but left alone by modern browsers
			}
			`%` {
				// ok - percent encoded, will decode
			}
			else {
				if should_escape(s[i], .encode_path) {
					return false
				}
			}
		}
	}
	return true
}

// valid_optional_port reports whether port is either an empty string
// or matches /^:\d*$/
fn valid_optional_port(port string) bool {
	if port == '' {
		return true
	}
	if port[0] != `:` {
		return false
	}
	for b in port[1..] {
		if b < `0` || b > `9` {
			return false
		}
	}
	return true
}

// str reassembles the URL into a valid URL string.
// The general form of the result is one of:
//
// scheme:opaque?query#fragment
// scheme://userinfo@host/path?query#fragment
//
// If u.opaque is non-empty, String uses the first form;
// otherwise it uses the second form.
// Any non-ASCII characters in host are escaped.
// To obtain the path, String uses u.escaped_path().
//
// In the second form, the following rules apply:
// - if u.scheme is empty, scheme: is omitted.
// - if u.user is nil, userinfo@ is omitted.
// - if u.host is empty, host/ is omitted.
// - if u.scheme and u.host are empty and u.user is nil,
// the entire scheme://userinfo@host/ is omitted.
// - if u.host is non-empty and u.path begins with a /,
// the form host/path does not add its own /.
// - if u.raw_query is empty, ?query is omitted.
// - if u.fragment is empty, #fragment is omitted.
pub fn (u URL) str() string {
	mut buf := strings.new_builder(200)
	if u.scheme != '' {
		buf.write_string(u.scheme)
		buf.write_string(':')
	}
	if u.opaque != '' {
		buf.write_string(u.opaque)
	} else {
		if u.scheme != '' || u.host != '' || !u.user.empty() {
			if u.host != '' || u.path != '' || !u.user.empty() {
				buf.write_string('//')
			}
			if !u.user.empty() {
				buf.write_string(u.user.str())
				buf.write_string('@')
			}
			if u.host != '' {
				buf.write_string(escape(u.host, .encode_host))
			}
		}
		path := u.escaped_path()
		if path != '' && path[0] != `/` && u.host != '' {
			buf.write_string('/')
		}
		if buf.len == 0 {
			// RFC 3986 §4.2
			// A path segment that contains a colon character (e.g., 'this:that')
			// cannot be used as the first segment of a relative-path reference, as
			// it would be mistaken for a scheme name. Such a segment must be
			// preceded by a dot-segment (e.g., './this:that') to make a relative-
			// path reference.
			i := path.index_byte(`:`)
			if i > -1 {
				// TODO remove this when autofree handles tmp
				// expressions like this
				if i > -1 && path[..i].index_byte(`/`) == -1 {
					buf.write_string('./')
				}
			}
		}
		buf.write_string(path)
	}
	if u.force_query || u.raw_query != '' {
		buf.write_string('?')
		buf.write_string(u.raw_query)
	}
	if u.fragment != '' {
		buf.write_string('#')
		buf.write_string(escape(u.fragment, .encode_fragment))
	}
	return buf.str()
}

// 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.
// 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.
//
// Query is expected to be a list of key=value settings separated by
// ampersands or semicolons. A setting without an equals sign is
// interpreted as a key set to an empty value.
pub fn parse_query(query string) ?Values {
	mut m := new_values()
	parse_query_values(mut m, query) ?
	return m
}

// parse_query_silent is the same as parse_query
// but any errors will be silent
fn parse_query_silent(query string) Values {
	mut m := new_values()
	parse_query_values(mut m, query) or {}
	return m
}

fn parse_query_values(mut m Values, query string) ?bool {
	mut had_error := false
	mut q := query
	for q != '' {
		mut key := q
		mut i := key.index_any('&;')
		if i >= 0 {
			q = key[i + 1..]
			key = key[..i]
		} else {
			q = ''
		}
		if key == '' {
			continue
		}
		mut value := ''
		if idx := key.index('=') {
			i = idx
			value = key[i + 1..]
			key = key[..i]
		}
		k := query_unescape(key) or {
			had_error = true
			continue
		}
		key = k
		v := query_unescape(value) or {
			had_error = true
			continue
		}
		value = v
		m.add(key, value)
	}
	if had_error {
		return error(error_msg('parse_query_values: failed parsing query string', ''))
	}
	return true
}

// encode encodes the values into ``URL encoded'' form
// ('bar=baz&foo=quux') sorted by key.
pub fn (v Values) encode() string {
	if v.len == 0 {
		return ''
	}
	mut buf := strings.new_builder(200)
	mut keys := []string{}
	for k, _ in v.data {
		keys << k
	}
	keys.sort()
	for k in keys {
		vs := v.data[k]
		key_kscaped := query_escape(k)
		for _, val in vs.data {
			if buf.len > 0 {
				buf.write_string('&')
			}
			buf.write_string(key_kscaped)
			buf.write_string('=')
			buf.write_string(query_escape(val))
		}
	}
	return buf.str()
}

// resolve_path applies special path segments from refs and applies
// them to base, per RFC 3986.
fn resolve_path(base string, ref string) string {
	mut full := ''
	if ref == '' {
		full = base
	} else if ref[0] != `/` {
		i := base.last_index('/') or { -1 }
		full = base[..i + 1] + ref
	} else {
		full = ref
	}
	if full == '' {
		return ''
	}
	mut dst := []string{}
	src := full.split('/')
	for _, elem in src {
		match elem {
			'.' {
				// drop
			}
			'..' {
				if dst.len > 0 {
					dst = dst[..dst.len - 1]
				}
			}
			else {
				dst << elem
			}
		}
	}
	last := src[src.len - 1]
	if last == '.' || last == '..' {
		// Add final slash to the joined path.
		dst << ''
	}
	return '/' + dst.join('/').trim_left('/')
}

// is_abs reports whether the URL is absolute.
// Absolute means that it has a non-empty scheme.
pub fn (u &URL) is_abs() bool {
	return u.scheme != ''
}

// parse parses a URL in the context of the receiver. The provided URL
// may be relative or absolute. parse returns nil, err on parse
// failure, otherwise its return value is the same as resolve_reference.
pub fn (u &URL) parse(ref string) ?URL {
	refurl := parse(ref) ?
	return u.resolve_reference(refurl)
}

// resolve_reference resolves a URI reference to an absolute URI from
// an absolute base URI u, per RFC 3986 Section 5.2. The URI reference
// may be relative or absolute. resolve_reference 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 resolve_reference
// ignores base and returns a copy of ref.
pub fn (u &URL) resolve_reference(ref &URL) ?URL {
	mut url := *ref
	if ref.scheme == '' {
		url.scheme = u.scheme
	}
	if ref.scheme != '' || ref.host != '' || !ref.user.empty() {
		// The 'absoluteURI' or 'net_path' cases.
		// We can ignore the error from set_path since we know we provided a
		// validly-escaped path.
		url.set_path(resolve_path(ref.escaped_path(), '')) ?
		return url
	}
	if ref.opaque != '' {
		url.user = user('')
		url.host = ''
		url.path = ''
		return url
	}
	if ref.path == '' && ref.raw_query == '' {
		url.raw_query = u.raw_query
		if ref.fragment == '' {
			url.fragment = u.fragment
		}
	}
	// The 'abs_path' or 'rel_path' cases.
	url.host = u.host
	url.user = u.user
	url.set_path(resolve_path(u.escaped_path(), ref.escaped_path())) ?
	return url
}

// query parses raw_query and returns the corresponding values.
// It silently discards malformed value pairs.
// To check errors use parseQuery.
pub fn (u &URL) query() Values {
	v := parse_query_silent(u.raw_query)
	return v
}

// request_uri returns the encoded path?query or opaque?query
// string that would be used in an HTTP request for u.
pub fn (u &URL) request_uri() string {
	mut result := u.opaque
	if result == '' {
		result = u.escaped_path()
		if result == '' {
			result = '/'
		}
	} else {
		if result.starts_with('//') {
			result = u.scheme + ':' + result
		}
	}
	if u.force_query || u.raw_query != '' {
		result += '?' + u.raw_query
	}
	return result
}

// hostname returns u.host, stripping any valid port number if present.
//
// If the result is enclosed in square brackets, as literal IPv6 addresses are,
// the square brackets are removed from the result.
pub fn (u &URL) hostname() string {
	host, _ := split_host_port(u.host)
	return host
}

// port returns the port part of u.host, without the leading colon.
// If u.host doesn't contain a port, port returns an empty string.
pub fn (u &URL) port() string {
	_, port := split_host_port(u.host)
	return port
}

// split_host_port separates host and port. If the port is not valid, it returns
// the entire input as host, and it doesn't check the validity of the host.
// Per RFC 3986, it requires ports to be numeric.
fn split_host_port(hostport string) (string, string) {
	mut host := hostport
	mut port := ''
	colon := host.last_index_byte(`:`)
	if colon != -1 {
		if valid_optional_port(host[colon..]) {
			port = host[colon + 1..]
			host = host[..colon]
		}
	}
	if host.starts_with('[') && host.ends_with(']') {
		host = host[1..host.len - 1]
	}
	return host, port
}

// valid_userinfo reports whether s is a valid userinfo string per RFC 3986
// Section 3.2.1:
// userinfo    = *( unreserved / pct-encoded / sub-delims / ':' )
// unreserved  = ALPHA / DIGIT / '-' / '.' / '_' / '~'
// sub-delims  = '!' / '$' / '&' / ''' / '(' / ')'
// / '*' / '+' / ',' / ';' / '='
//
// It doesn't validate pct-encoded. The caller does that via fn unescape.
pub fn valid_userinfo(s string) bool {
	for r in s {
		if `A` <= r && r <= `Z` {
			continue
		}
		if `a` <= r && r <= `z` {
			continue
		}
		if `0` <= r && r <= `9` {
			continue
		}
		match r {
			`-`, `.`, `_`, `:`, `~`, `!`, `$`, `&`, `\\`, `(`, `)`, `*`, `+`, `,`, `;`, `=`, `%`,
			`@` {
				continue
			}
			else {
				return false
			}
		}
	}
	return true
}

// string_contains_ctl_byte reports whether s contains any ASCII control character.
fn string_contains_ctl_byte(s string) bool {
	for i in 0 .. s.len {
		b := s[i]
		if b < ` ` || b == 0x7f {
			return true
		}
	}
	return false
}

pub fn ishex(c byte) bool {
	if `0` <= c && c <= `9` {
		return true
	} else if `a` <= c && c <= `f` {
		return true
	} else if `A` <= c && c <= `F` {
		return true
	}
	return false
}

fn unhex(c byte) byte {
	if `0` <= c && c <= `9` {
		return c - `0`
	} else if `a` <= c && c <= `f` {
		return c - `a` + 10
	} else if `A` <= c && c <= `F` {
		return c - `A` + 10
	}
	return 0
}