--- /dev/null
+// Copyright 2016 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 bidirule implements the Bidi Rule defined by RFC 5893.
+//
+// This package is under development. The API may change without notice and
+// without preserving backward compatibility.
+package bidirule
+
+import (
+ "errors"
+ "unicode/utf8"
+
+ "golang.org/x/text/transform"
+ "golang.org/x/text/unicode/bidi"
+)
+
+// This file contains an implementation of RFC 5893: Right-to-Left Scripts for
+// Internationalized Domain Names for Applications (IDNA)
+//
+// A label is an individual component of a domain name. Labels are usually
+// shown separated by dots; for example, the domain name "www.example.com" is
+// composed of three labels: "www", "example", and "com".
+//
+// An RTL label is a label that contains at least one character of class R, AL,
+// or AN. An LTR label is any label that is not an RTL label.
+//
+// A "Bidi domain name" is a domain name that contains at least one RTL label.
+//
+// The following guarantees can be made based on the above:
+//
+// o In a domain name consisting of only labels that satisfy the rule,
+// the requirements of Section 3 are satisfied. Note that even LTR
+// labels and pure ASCII labels have to be tested.
+//
+// o In a domain name consisting of only LDH labels (as defined in the
+// Definitions document [RFC5890]) and labels that satisfy the rule,
+// the requirements of Section 3 are satisfied as long as a label
+// that starts with an ASCII digit does not come after a
+// right-to-left label.
+//
+// No guarantee is given for other combinations.
+
+// ErrInvalid indicates a label is invalid according to the Bidi Rule.
+var ErrInvalid = errors.New("bidirule: failed Bidi Rule")
+
+type ruleState uint8
+
+const (
+ ruleInitial ruleState = iota
+ ruleLTR
+ ruleLTRFinal
+ ruleRTL
+ ruleRTLFinal
+ ruleInvalid
+)
+
+type ruleTransition struct {
+ next ruleState
+ mask uint16
+}
+
+var transitions = [...][2]ruleTransition{
+ // [2.1] The first character must be a character with Bidi property L, R, or
+ // AL. If it has the R or AL property, it is an RTL label; if it has the L
+ // property, it is an LTR label.
+ ruleInitial: {
+ {ruleLTRFinal, 1 << bidi.L},
+ {ruleRTLFinal, 1<<bidi.R | 1<<bidi.AL},
+ },
+ ruleRTL: {
+ // [2.3] In an RTL label, the end of the label must be a character with
+ // Bidi property R, AL, EN, or AN, followed by zero or more characters
+ // with Bidi property NSM.
+ {ruleRTLFinal, 1<<bidi.R | 1<<bidi.AL | 1<<bidi.EN | 1<<bidi.AN},
+
+ // [2.2] In an RTL label, only characters with the Bidi properties R,
+ // AL, AN, EN, ES, CS, ET, ON, BN, or NSM are allowed.
+ // We exclude the entries from [2.3]
+ {ruleRTL, 1<<bidi.ES | 1<<bidi.CS | 1<<bidi.ET | 1<<bidi.ON | 1<<bidi.BN | 1<<bidi.NSM},
+ },
+ ruleRTLFinal: {
+ // [2.3] In an RTL label, the end of the label must be a character with
+ // Bidi property R, AL, EN, or AN, followed by zero or more characters
+ // with Bidi property NSM.
+ {ruleRTLFinal, 1<<bidi.R | 1<<bidi.AL | 1<<bidi.EN | 1<<bidi.AN | 1<<bidi.NSM},
+
+ // [2.2] In an RTL label, only characters with the Bidi properties R,
+ // AL, AN, EN, ES, CS, ET, ON, BN, or NSM are allowed.
+ // We exclude the entries from [2.3] and NSM.
+ {ruleRTL, 1<<bidi.ES | 1<<bidi.CS | 1<<bidi.ET | 1<<bidi.ON | 1<<bidi.BN},
+ },
+ ruleLTR: {
+ // [2.6] In an LTR label, the end of the label must be a character with
+ // Bidi property L or EN, followed by zero or more characters with Bidi
+ // property NSM.
+ {ruleLTRFinal, 1<<bidi.L | 1<<bidi.EN},
+
+ // [2.5] In an LTR label, only characters with the Bidi properties L,
+ // EN, ES, CS, ET, ON, BN, or NSM are allowed.
+ // We exclude the entries from [2.6].
+ {ruleLTR, 1<<bidi.ES | 1<<bidi.CS | 1<<bidi.ET | 1<<bidi.ON | 1<<bidi.BN | 1<<bidi.NSM},
+ },
+ ruleLTRFinal: {
+ // [2.6] In an LTR label, the end of the label must be a character with
+ // Bidi property L or EN, followed by zero or more characters with Bidi
+ // property NSM.
+ {ruleLTRFinal, 1<<bidi.L | 1<<bidi.EN | 1<<bidi.NSM},
+
+ // [2.5] In an LTR label, only characters with the Bidi properties L,
+ // EN, ES, CS, ET, ON, BN, or NSM are allowed.
+ // We exclude the entries from [2.6].
+ {ruleLTR, 1<<bidi.ES | 1<<bidi.CS | 1<<bidi.ET | 1<<bidi.ON | 1<<bidi.BN},
+ },
+ ruleInvalid: {
+ {ruleInvalid, 0},
+ {ruleInvalid, 0},
+ },
+}
+
+// [2.4] In an RTL label, if an EN is present, no AN may be present, and
+// vice versa.
+const exclusiveRTL = uint16(1<<bidi.EN | 1<<bidi.AN)
+
+// From RFC 5893
+// An RTL label is a label that contains at least one character of type
+// R, AL, or AN.
+//
+// An LTR label is any label that is not an RTL label.
+
+// Direction reports the direction of the given label as defined by RFC 5893.
+// The Bidi Rule does not have to be applied to labels of the category
+// LeftToRight.
+func Direction(b []byte) bidi.Direction {
+ for i := 0; i < len(b); {
+ e, sz := bidi.Lookup(b[i:])
+ if sz == 0 {
+ i++
+ }
+ c := e.Class()
+ if c == bidi.R || c == bidi.AL || c == bidi.AN {
+ return bidi.RightToLeft
+ }
+ i += sz
+ }
+ return bidi.LeftToRight
+}
+
+// DirectionString reports the direction of the given label as defined by RFC
+// 5893. The Bidi Rule does not have to be applied to labels of the category
+// LeftToRight.
+func DirectionString(s string) bidi.Direction {
+ for i := 0; i < len(s); {
+ e, sz := bidi.LookupString(s[i:])
+ if sz == 0 {
+ i++
+ }
+ c := e.Class()
+ if c == bidi.R || c == bidi.AL || c == bidi.AN {
+ return bidi.RightToLeft
+ }
+ i += sz
+ }
+ return bidi.LeftToRight
+}
+
+// Valid reports whether b conforms to the BiDi rule.
+func Valid(b []byte) bool {
+ var t Transformer
+ if n, ok := t.advance(b); !ok || n < len(b) {
+ return false
+ }
+ return t.isFinal()
+}
+
+// ValidString reports whether s conforms to the BiDi rule.
+func ValidString(s string) bool {
+ var t Transformer
+ if n, ok := t.advanceString(s); !ok || n < len(s) {
+ return false
+ }
+ return t.isFinal()
+}
+
+// New returns a Transformer that verifies that input adheres to the Bidi Rule.
+func New() *Transformer {
+ return &Transformer{}
+}
+
+// Transformer implements transform.Transform.
+type Transformer struct {
+ state ruleState
+ hasRTL bool
+ seen uint16
+}
+
+// A rule can only be violated for "Bidi Domain names", meaning if one of the
+// following categories has been observed.
+func (t *Transformer) isRTL() bool {
+ const isRTL = 1<<bidi.R | 1<<bidi.AL | 1<<bidi.AN
+ return t.seen&isRTL != 0
+}
+
+func (t *Transformer) isFinal() bool {
+ if !t.isRTL() {
+ return true
+ }
+ return t.state == ruleLTRFinal || t.state == ruleRTLFinal || t.state == ruleInitial
+}
+
+// Reset implements transform.Transformer.
+func (t *Transformer) Reset() { *t = Transformer{} }
+
+// Transform implements transform.Transformer. This Transformer has state and
+// needs to be reset between uses.
+func (t *Transformer) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
+ if len(dst) < len(src) {
+ src = src[:len(dst)]
+ atEOF = false
+ err = transform.ErrShortDst
+ }
+ n, err1 := t.Span(src, atEOF)
+ copy(dst, src[:n])
+ if err == nil || err1 != nil && err1 != transform.ErrShortSrc {
+ err = err1
+ }
+ return n, n, err
+}
+
+// Span returns the first n bytes of src that conform to the Bidi rule.
+func (t *Transformer) Span(src []byte, atEOF bool) (n int, err error) {
+ if t.state == ruleInvalid && t.isRTL() {
+ return 0, ErrInvalid
+ }
+ n, ok := t.advance(src)
+ switch {
+ case !ok:
+ err = ErrInvalid
+ case n < len(src):
+ if !atEOF {
+ err = transform.ErrShortSrc
+ break
+ }
+ err = ErrInvalid
+ case !t.isFinal():
+ err = ErrInvalid
+ }
+ return n, err
+}
+
+// Precomputing the ASCII values decreases running time for the ASCII fast path
+// by about 30%.
+var asciiTable [128]bidi.Properties
+
+func init() {
+ for i := range asciiTable {
+ p, _ := bidi.LookupRune(rune(i))
+ asciiTable[i] = p
+ }
+}
+
+func (t *Transformer) advance(s []byte) (n int, ok bool) {
+ var e bidi.Properties
+ var sz int
+ for n < len(s) {
+ if s[n] < utf8.RuneSelf {
+ e, sz = asciiTable[s[n]], 1
+ } else {
+ e, sz = bidi.Lookup(s[n:])
+ if sz <= 1 {
+ if sz == 1 {
+ // We always consider invalid UTF-8 to be invalid, even if
+ // the string has not yet been determined to be RTL.
+ // TODO: is this correct?
+ return n, false
+ }
+ return n, true // incomplete UTF-8 encoding
+ }
+ }
+ // TODO: using CompactClass would result in noticeable speedup.
+ // See unicode/bidi/prop.go:Properties.CompactClass.
+ c := uint16(1 << e.Class())
+ t.seen |= c
+ if t.seen&exclusiveRTL == exclusiveRTL {
+ t.state = ruleInvalid
+ return n, false
+ }
+ switch tr := transitions[t.state]; {
+ case tr[0].mask&c != 0:
+ t.state = tr[0].next
+ case tr[1].mask&c != 0:
+ t.state = tr[1].next
+ default:
+ t.state = ruleInvalid
+ if t.isRTL() {
+ return n, false
+ }
+ }
+ n += sz
+ }
+ return n, true
+}
+
+func (t *Transformer) advanceString(s string) (n int, ok bool) {
+ var e bidi.Properties
+ var sz int
+ for n < len(s) {
+ if s[n] < utf8.RuneSelf {
+ e, sz = asciiTable[s[n]], 1
+ } else {
+ e, sz = bidi.LookupString(s[n:])
+ if sz <= 1 {
+ if sz == 1 {
+ return n, false // invalid UTF-8
+ }
+ return n, true // incomplete UTF-8 encoding
+ }
+ }
+ // TODO: using CompactClass results in noticeable speedup.
+ // See unicode/bidi/prop.go:Properties.CompactClass.
+ c := uint16(1 << e.Class())
+ t.seen |= c
+ if t.seen&exclusiveRTL == exclusiveRTL {
+ t.state = ruleInvalid
+ return n, false
+ }
+ switch tr := transitions[t.state]; {
+ case tr[0].mask&c != 0:
+ t.state = tr[0].next
+ case tr[1].mask&c != 0:
+ t.state = tr[1].next
+ default:
+ t.state = ruleInvalid
+ if t.isRTL() {
+ return n, false
+ }
+ }
+ n += sz
+ }
+ return n, true
+}
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