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// Copyright 2010 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 html

import (
        "bytes"
        "io"
        "strconv"
        "strings"
)

// A TokenType is the type of a Token.
type TokenType int

const (
        // ErrorToken means that an error occurred during tokenization.
        ErrorToken TokenType = iota
        // TextToken means a text node.
        TextToken
        // A StartTagToken looks like <a>.
        StartTagToken
        // An EndTagToken looks like </a>.
        EndTagToken
        // A SelfClosingTagToken tag looks like <br/>.
        SelfClosingTagToken
        // A CommentToken looks like <!--x-->.
        CommentToken
        // A DoctypeToken looks like <!DOCTYPE x>
        DoctypeToken
)

// String returns a string representation of the TokenType.
func (t TokenType) String() string {
        switch t {
        case ErrorToken:
                return "Error"
        case TextToken:
                return "Text"
        case StartTagToken:
                return "StartTag"
        case EndTagToken:
                return "EndTag"
        case SelfClosingTagToken:
                return "SelfClosingTag"
        case CommentToken:
                return "Comment"
        case DoctypeToken:
                return "Doctype"
        }
        return "Invalid(" + strconv.Itoa(int(t)) + ")"
}

// An Attribute is an attribute namespace-key-value triple. Namespace is
// non-empty for foreign attributes like xlink, Key is alphabetic (and hence
// does not contain escapable characters like '&', '<' or '>'), and Val is
// unescaped (it looks like "a<b" rather than "a&lt;b").
//
// Namespace is only used by the parser, not the tokenizer.
type Attribute struct {
        Namespace, Key, Val string
}

// A Token consists of a TokenType and some Data (tag name for start and end
// tags, content for text, comments and doctypes). A tag Token may also contain
// a slice of Attributes. Data is unescaped for all Tokens (it looks like "a<b"
// rather than "a&lt;b").
type Token struct {
        Type TokenType
        Data string
        Attr []Attribute
}

// tagString returns a string representation of a tag Token's Data and Attr.
func (t Token) tagString() string {
        if len(t.Attr) == 0 {
                return t.Data
        }
        buf := bytes.NewBufferString(t.Data)
        for _, a := range t.Attr {
                buf.WriteByte(' ')
                buf.WriteString(a.Key)
                buf.WriteString(`="`)
                escape(buf, a.Val)
                buf.WriteByte('"')
        }
        return buf.String()
}

// String returns a string representation of the Token.
func (t Token) String() string {
        switch t.Type {
        case ErrorToken:
                return ""
        case TextToken:
                return EscapeString(t.Data)
        case StartTagToken:
                return "<" + t.tagString() + ">"
        case EndTagToken:
                return "</" + t.tagString() + ">"
        case SelfClosingTagToken:
                return "<" + t.tagString() + "/>"
        case CommentToken:
                return "<!--" + t.Data + "-->"
        case DoctypeToken:
                return "<!DOCTYPE " + t.Data + ">"
        }
        return "Invalid(" + strconv.Itoa(int(t.Type)) + ")"
}

// span is a range of bytes in a Tokenizer's buffer. The start is inclusive,
// the end is exclusive.
type span struct {
        start, end int
}

// A Tokenizer returns a stream of HTML Tokens.
type Tokenizer struct {
        // r is the source of the HTML text.
        r io.Reader
        // tt is the TokenType of the current token.
        tt TokenType
        // err is the first error encountered during tokenization. It is possible
        // for tt != Error && err != nil to hold: this means that Next returned a
        // valid token but the subsequent Next call will return an error token.
        // For example, if the HTML text input was just "plain", then the first
        // Next call would set z.err to io.EOF but return a TextToken, and all
        // subsequent Next calls would return an ErrorToken.
        // err is never reset. Once it becomes non-nil, it stays non-nil.
        err error
        // buf[raw.start:raw.end] holds the raw bytes of the current token.
        // buf[raw.end:] is buffered input that will yield future tokens.
        raw span
        buf []byte
        // buf[data.start:data.end] holds the raw bytes of the current token's data:
        // a text token's text, a tag token's tag name, etc.
        data span
        // pendingAttr is the attribute key and value currently being tokenized.
        // When complete, pendingAttr is pushed onto attr. nAttrReturned is
        // incremented on each call to TagAttr.
        pendingAttr   [2]span
        attr          [][2]span
        nAttrReturned int
        // rawTag is the "script" in "</script>" that closes the next token. If
        // non-empty, the subsequent call to Next will return a raw or RCDATA text
        // token: one that treats "<p>" as text instead of an element.
        // rawTag's contents are lower-cased.
        rawTag string
        // textIsRaw is whether the current text token's data is not escaped.
        textIsRaw bool
}

// Err returns the error associated with the most recent ErrorToken token.
// This is typically io.EOF, meaning the end of tokenization.
func (z *Tokenizer) Err() error {
        if z.tt != ErrorToken {
                return nil
        }
        return z.err
}

// readByte returns the next byte from the input stream, doing a buffered read
// from z.r into z.buf if necessary. z.buf[z.raw.start:z.raw.end] remains a contiguous byte
// slice that holds all the bytes read so far for the current token.
// It sets z.err if the underlying reader returns an error.
// Pre-condition: z.err == nil.
func (z *Tokenizer) readByte() byte {
        if z.raw.end >= len(z.buf) {
                // Our buffer is exhausted and we have to read from z.r.
                // We copy z.buf[z.raw.start:z.raw.end] to the beginning of z.buf. If the length
                // z.raw.end - z.raw.start is more than half the capacity of z.buf, then we
                // allocate a new buffer before the copy.
                c := cap(z.buf)
                d := z.raw.end - z.raw.start
                var buf1 []byte
                if 2*d > c {
                        buf1 = make([]byte, d, 2*c)
                } else {
                        buf1 = z.buf[:d]
                }
                copy(buf1, z.buf[z.raw.start:z.raw.end])
                if x := z.raw.start; x != 0 {
                        // Adjust the data/attr spans to refer to the same contents after the copy.
                        z.data.start -= x
                        z.data.end -= x
                        z.pendingAttr[0].start -= x
                        z.pendingAttr[0].end -= x
                        z.pendingAttr[1].start -= x
                        z.pendingAttr[1].end -= x
                        for i := range z.attr {
                                z.attr[i][0].start -= x
                                z.attr[i][0].end -= x
                                z.attr[i][1].start -= x
                                z.attr[i][1].end -= x
                        }
                }
                z.raw.start, z.raw.end, z.buf = 0, d, buf1[:d]
                // Now that we have copied the live bytes to the start of the buffer,
                // we read from z.r into the remainder.
                n, err := z.r.Read(buf1[d:cap(buf1)])
                if err != nil {
                        z.err = err
                        return 0
                }
                z.buf = buf1[:d+n]
        }
        x := z.buf[z.raw.end]
        z.raw.end++
        return x
}

// skipWhiteSpace skips past any white space.
func (z *Tokenizer) skipWhiteSpace() {
        if z.err != nil {
                return
        }
        for {
                c := z.readByte()
                if z.err != nil {
                        return
                }
                switch c {
                case ' ', '\n', '\r', '\t', '\f':
                        // No-op.
                default:
                        z.raw.end--
                        return
                }
        }
}

// readRawOrRCDATA reads until the next "</foo>", where "foo" is z.rawTag and
// is typically something like "script" or "textarea".
func (z *Tokenizer) readRawOrRCDATA() {
loop:
        for {
                c := z.readByte()
                if z.err != nil {
                        break loop
                }
                if c != '<' {
                        continue loop
                }
                c = z.readByte()
                if z.err != nil {
                        break loop
                }
                if c != '/' {
                        continue loop
                }
                for i := 0; i < len(z.rawTag); i++ {
                        c = z.readByte()
                        if z.err != nil {
                                break loop
                        }
                        if c != z.rawTag[i] && c != z.rawTag[i]-('a'-'A') {
                                continue loop
                        }
                }
                c = z.readByte()
                if z.err != nil {
                        break loop
                }
                switch c {
                case ' ', '\n', '\r', '\t', '\f', '/', '>':
                        // The 3 is 2 for the leading "</" plus 1 for the trailing character c.
                        z.raw.end -= 3 + len(z.rawTag)
                        break loop
                case '<':
                        // Step back one, to catch "</foo</foo>".
                        z.raw.end--
                }
        }
        z.data.end = z.raw.end
        // A textarea's or title's RCDATA can contain escaped entities.
        z.textIsRaw = z.rawTag != "textarea" && z.rawTag != "title"
        z.rawTag = ""
}

// readComment reads the next comment token starting with "<!--". The opening
// "<!--" has already been consumed.
func (z *Tokenizer) readComment() {
        z.data.start = z.raw.end
        defer func() {
                if z.data.end < z.data.start {
                        // It's a comment with no data, like <!-->.
                        z.data.end = z.data.start
                }
        }()
        for dashCount := 2; ; {
                c := z.readByte()
                if z.err != nil {
                        // Ignore up to two dashes at EOF.
                        if dashCount > 2 {
                                dashCount = 2
                        }
                        z.data.end = z.raw.end - dashCount
                        return
                }
                switch c {
                case '-':
                        dashCount++
                        continue
                case '>':
                        if dashCount >= 2 {
                                z.data.end = z.raw.end - len("-->")
                                return
                        }
                case '!':
                        if dashCount >= 2 {
                                c = z.readByte()
                                if z.err != nil {
                                        z.data.end = z.raw.end
                                        return
                                }
                                if c == '>' {
                                        z.data.end = z.raw.end - len("--!>")
                                        return
                                }
                        }
                }
                dashCount = 0
        }
}

// readUntilCloseAngle reads until the next ">".
func (z *Tokenizer) readUntilCloseAngle() {
        z.data.start = z.raw.end
        for {
                c := z.readByte()
                if z.err != nil {
                        z.data.end = z.raw.end
                        return
                }
                if c == '>' {
                        z.data.end = z.raw.end - len(">")
                        return
                }
        }
}

// readMarkupDeclaration reads the next token starting with "<!". It might be
// a "<!--comment-->", a "<!DOCTYPE foo>", or "<!a bogus comment". The opening
// "<!" has already been consumed.
func (z *Tokenizer) readMarkupDeclaration() TokenType {
        z.data.start = z.raw.end
        var c [2]byte
        for i := 0; i < 2; i++ {
                c[i] = z.readByte()
                if z.err != nil {
                        z.data.end = z.raw.end
                        return CommentToken
                }
        }
        if c[0] == '-' && c[1] == '-' {
                z.readComment()
                return CommentToken
        }
        z.raw.end -= 2
        const s = "DOCTYPE"
        for i := 0; i < len(s); i++ {
                c := z.readByte()
                if z.err != nil {
                        z.data.end = z.raw.end
                        return CommentToken
                }
                if c != s[i] && c != s[i]+('a'-'A') {
                        // Back up to read the fragment of "DOCTYPE" again.
                        z.raw.end = z.data.start
                        z.readUntilCloseAngle()
                        return CommentToken
                }
        }
        if z.skipWhiteSpace(); z.err != nil {
                z.data.start = z.raw.end
                z.data.end = z.raw.end
                return DoctypeToken
        }
        z.readUntilCloseAngle()
        return DoctypeToken
}

// startTagIn returns whether the start tag in z.buf[z.data.start:z.data.end]
// case-insensitively matches any element of ss.
func (z *Tokenizer) startTagIn(ss ...string) bool {
loop:
        for _, s := range ss {
                if z.data.end-z.data.start != len(s) {
                        continue loop
                }
                for i := 0; i < len(s); i++ {
                        c := z.buf[z.data.start+i]
                        if 'A' <= c && c <= 'Z' {
                                c += 'a' - 'A'
                        }
                        if c != s[i] {
                                continue loop
                        }
                }
                return true
        }
        return false
}

// readStartTag reads the next start tag token. The opening "<a" has already
// been consumed, where 'a' means anything in [A-Za-z].
func (z *Tokenizer) readStartTag() TokenType {
        z.attr = z.attr[:0]
        z.nAttrReturned = 0
        // Read the tag name and attribute key/value pairs.
        z.readTagName()
        if z.skipWhiteSpace(); z.err != nil {
                return ErrorToken
        }
        for {
                c := z.readByte()
                if z.err != nil || c == '>' {
                        break
                }
                z.raw.end--
                z.readTagAttrKey()
                z.readTagAttrVal()
                // Save pendingAttr if it has a non-empty key.
                if z.pendingAttr[0].start != z.pendingAttr[0].end {
                        z.attr = append(z.attr, z.pendingAttr)
                }
                if z.skipWhiteSpace(); z.err != nil {
                        break
                }
        }
        // Several tags flag the tokenizer's next token as raw.
        c, raw := z.buf[z.data.start], false
        if 'A' <= c && c <= 'Z' {
                c += 'a' - 'A'
        }
        switch c {
        case 'i':
                raw = z.startTagIn("iframe")
        case 'n':
                raw = z.startTagIn("noembed", "noframes", "noscript")
        case 'p':
                raw = z.startTagIn("plaintext")
        case 's':
                raw = z.startTagIn("script", "style")
        case 't':
                raw = z.startTagIn("textarea", "title")
        case 'x':
                raw = z.startTagIn("xmp")
        }
        if raw {
                z.rawTag = strings.ToLower(string(z.buf[z.data.start:z.data.end]))
        }
        // Look for a self-closing token like "<br/>".
        if z.err == nil && z.buf[z.raw.end-2] == '/' {
                return SelfClosingTagToken
        }
        return StartTagToken
}

// readEndTag reads the next end tag token. The opening "</a" has already
// been consumed, where 'a' means anything in [A-Za-z].
func (z *Tokenizer) readEndTag() {
        z.attr = z.attr[:0]
        z.nAttrReturned = 0
        z.readTagName()
        for {
                c := z.readByte()
                if z.err != nil || c == '>' {
                        return
                }
        }
}

// readTagName sets z.data to the "div" in "<div k=v>". The reader (z.raw.end)
// is positioned such that the first byte of the tag name (the "d" in "<div")
// has already been consumed.
func (z *Tokenizer) readTagName() {
        z.data.start = z.raw.end - 1
        for {
                c := z.readByte()
                if z.err != nil {
                        z.data.end = z.raw.end
                        return
                }
                switch c {
                case ' ', '\n', '\r', '\t', '\f':
                        z.data.end = z.raw.end - 1
                        return
                case '/', '>':
                        z.raw.end--
                        z.data.end = z.raw.end
                        return
                }
        }
}

// readTagAttrKey sets z.pendingAttr[0] to the "k" in "<div k=v>".
// Precondition: z.err == nil.
func (z *Tokenizer) readTagAttrKey() {
        z.pendingAttr[0].start = z.raw.end
        for {
                c := z.readByte()
                if z.err != nil {
                        z.pendingAttr[0].end = z.raw.end
                        return
                }
                switch c {
                case ' ', '\n', '\r', '\t', '\f', '/':
                        z.pendingAttr[0].end = z.raw.end - 1
                        return
                case '=', '>':
                        z.raw.end--
                        z.pendingAttr[0].end = z.raw.end
                        return
                }
        }
}

// readTagAttrVal sets z.pendingAttr[1] to the "v" in "<div k=v>".
func (z *Tokenizer) readTagAttrVal() {
        z.pendingAttr[1].start = z.raw.end
        z.pendingAttr[1].end = z.raw.end
        if z.skipWhiteSpace(); z.err != nil {
                return
        }
        c := z.readByte()
        if z.err != nil {
                return
        }
        if c != '=' {
                z.raw.end--
                return
        }
        if z.skipWhiteSpace(); z.err != nil {
                return
        }
        quote := z.readByte()
        if z.err != nil {
                return
        }
        switch quote {
        case '>':
                z.raw.end--
                return

        case '\'', '"':
                z.pendingAttr[1].start = z.raw.end
                for {
                        c := z.readByte()
                        if z.err != nil {
                                z.pendingAttr[1].end = z.raw.end
                                return
                        }
                        if c == quote {
                                z.pendingAttr[1].end = z.raw.end - 1
                                return
                        }
                }

        default:
                z.pendingAttr[1].start = z.raw.end - 1
                for {
                        c := z.readByte()
                        if z.err != nil {
                                z.pendingAttr[1].end = z.raw.end
                                return
                        }
                        switch c {
                        case ' ', '\n', '\r', '\t', '\f':
                                z.pendingAttr[1].end = z.raw.end - 1
                                return
                        case '>':
                                z.raw.end--
                                z.pendingAttr[1].end = z.raw.end
                                return
                        }
                }
        }
}

// Next scans the next token and returns its type.
func (z *Tokenizer) Next() TokenType {
        if z.err != nil {
                z.tt = ErrorToken
                return z.tt
        }
        z.raw.start = z.raw.end
        z.data.start = z.raw.end
        z.data.end = z.raw.end
        if z.rawTag != "" {
                if z.rawTag == "plaintext" {
                        // Read everything up to EOF.
                        for z.err == nil {
                                z.readByte()
                        }
                        z.textIsRaw = true
                } else {
                        z.readRawOrRCDATA()
                }
                if z.data.end > z.data.start {
                        z.tt = TextToken
                        return z.tt
                }
        }
        z.textIsRaw = false

loop:
        for {
                c := z.readByte()
                if z.err != nil {
                        break loop
                }
                if c != '<' {
                        continue loop
                }

                // Check if the '<' we have just read is part of a tag, comment
                // or doctype. If not, it's part of the accumulated text token.
                c = z.readByte()
                if z.err != nil {
                        break loop
                }
                var tokenType TokenType
                switch {
                case 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z':
                        tokenType = StartTagToken
                case c == '/':
                        tokenType = EndTagToken
                case c == '!' || c == '?':
                        // We use CommentToken to mean any of "<!--actual comments-->",
                        // "<!DOCTYPE declarations>" and "<?xml processing instructions?>".
                        tokenType = CommentToken
                default:
                        continue
                }

                // We have a non-text token, but we might have accumulated some text
                // before that. If so, we return the text first, and return the non-
                // text token on the subsequent call to Next.
                if x := z.raw.end - len("<a"); z.raw.start < x {
                        z.raw.end = x
                        z.data.end = x
                        z.tt = TextToken
                        return z.tt
                }
                switch tokenType {
                case StartTagToken:
                        z.tt = z.readStartTag()
                        return z.tt
                case EndTagToken:
                        c = z.readByte()
                        if z.err != nil {
                                break loop
                        }
                        if c == '>' {
                                // "</>" does not generate a token at all.
                                // Reset the tokenizer state and start again.
                                z.raw.start = z.raw.end
                                z.data.start = z.raw.end
                                z.data.end = z.raw.end
                                continue loop
                        }
                        if 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' {
                                z.readEndTag()
                                z.tt = EndTagToken
                                return z.tt
                        }
                        z.raw.end--
                        z.readUntilCloseAngle()
                        z.tt = CommentToken
                        return z.tt
                case CommentToken:
                        if c == '!' {
                                z.tt = z.readMarkupDeclaration()
                                return z.tt
                        }
                        z.raw.end--
                        z.readUntilCloseAngle()
                        z.tt = CommentToken
                        return z.tt
                }
        }
        if z.raw.start < z.raw.end {
                z.data.end = z.raw.end
                z.tt = TextToken
                return z.tt
        }
        z.tt = ErrorToken
        return z.tt
}

// Raw returns the unmodified text of the current token. Calling Next, Token,
// Text, TagName or TagAttr may change the contents of the returned slice.
func (z *Tokenizer) Raw() []byte {
        return z.buf[z.raw.start:z.raw.end]
}

// Text returns the unescaped text of a text, comment or doctype token. The
// contents of the returned slice may change on the next call to Next.
func (z *Tokenizer) Text() []byte {
        switch z.tt {
        case TextToken, CommentToken, DoctypeToken:
                s := z.buf[z.data.start:z.data.end]
                z.data.start = z.raw.end
                z.data.end = z.raw.end
                if !z.textIsRaw {
                        s = unescape(s)
                }
                return s
        }
        return nil
}

// TagName returns the lower-cased name of a tag token (the `img` out of
// `<IMG SRC="foo">`) and whether the tag has attributes.
// The contents of the returned slice may change on the next call to Next.
func (z *Tokenizer) TagName() (name []byte, hasAttr bool) {
        if z.data.start < z.data.end {
                switch z.tt {
                case StartTagToken, EndTagToken, SelfClosingTagToken:
                        s := z.buf[z.data.start:z.data.end]
                        z.data.start = z.raw.end
                        z.data.end = z.raw.end
                        return lower(s), z.nAttrReturned < len(z.attr)
                }
        }
        return nil, false
}

// TagAttr returns the lower-cased key and unescaped value of the next unparsed
// attribute for the current tag token and whether there are more attributes.
// The contents of the returned slices may change on the next call to Next.
func (z *Tokenizer) TagAttr() (key, val []byte, moreAttr bool) {
        if z.nAttrReturned < len(z.attr) {
                switch z.tt {
                case StartTagToken, SelfClosingTagToken:
                        x := z.attr[z.nAttrReturned]
                        z.nAttrReturned++
                        key = z.buf[x[0].start:x[0].end]
                        val = z.buf[x[1].start:x[1].end]
                        return lower(key), unescape(val), z.nAttrReturned < len(z.attr)
                }
        }
        return nil, nil, false
}

// Token returns the next Token. The result's Data and Attr values remain valid
// after subsequent Next calls.
func (z *Tokenizer) Token() Token {
        t := Token{Type: z.tt}
        switch z.tt {
        case TextToken, CommentToken, DoctypeToken:
                t.Data = string(z.Text())
        case StartTagToken, SelfClosingTagToken:
                var attr []Attribute
                name, moreAttr := z.TagName()
                for moreAttr {
                        var key, val []byte
                        key, val, moreAttr = z.TagAttr()
                        attr = append(attr, Attribute{"", string(key), string(val)})
                }
                t.Data = string(name)
                t.Attr = attr
        case EndTagToken:
                name, _ := z.TagName()
                t.Data = string(name)
        }
        return t
}

// NewTokenizer returns a new HTML Tokenizer for the given Reader.
// The input is assumed to be UTF-8 encoded.
func NewTokenizer(r io.Reader) *Tokenizer {
        return &Tokenizer{
                r:   r,
                buf: make([]byte, 0, 4096),
        }
}