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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 implements an HTML5-compliant tokenizer and parser.
INCOMPLETE.

Tokenization is done by creating a Tokenizer for an io.Reader r. It is the
caller's responsibility to ensure that r provides UTF-8 encoded HTML.

        z := html.NewTokenizer(r)

Given a Tokenizer z, the HTML is tokenized by repeatedly calling z.Next(),
which parses the next token and returns its type, or an error:

        for {
                tt := z.Next()
                if tt == html.ErrorToken {
                        // ...
                        return ...
                }
                // Process the current token.
        }

There are two APIs for retrieving the current token. The high-level API is to
call Token; the low-level API is to call Text or TagName / TagAttr. Both APIs
allow optionally calling Raw after Next but before Token, Text, TagName, or
TagAttr. In EBNF notation, the valid call sequence per token is:

        Next {Raw} [ Token | Text | TagName {TagAttr} ]

Token returns an independent data structure that completely describes a token.
Entities (such as "<") are unescaped, tag names and attribute keys are
lower-cased, and attributes are collected into a []Attribute. For example:

        for {
                if z.Next() == html.ErrorToken {
                        // Returning io.EOF indicates success.
                        return z.Err()
                }
                emitToken(z.Token())
        }

The low-level API performs fewer allocations and copies, but the contents of
the []byte values returned by Text, TagName and TagAttr may change on the next
call to Next. For example, to extract an HTML page's anchor text:

        depth := 0
        for {
                tt := z.Next()
                switch tt {
                case ErrorToken:
                        return z.Err()
                case TextToken:
                        if depth > 0 {
                                // emitBytes should copy the []byte it receives,
                                // if it doesn't process it immediately.
                                emitBytes(z.Text())
                        }
                case StartTagToken, EndTagToken:
                        tn, _ := z.TagName()
                        if len(tn) == 1 && tn[0] == 'a' {
                                if tt == StartTagToken {
                                        depth++
                                } else {
                                        depth--
                                }
                        }
                }
        }

Parsing is done by calling Parse with an io.Reader, which returns the root of
the parse tree (the document element) as a *Node. It is the caller's
responsibility to ensure that the Reader provides UTF-8 encoded HTML. For
example, to process each anchor node in depth-first order:

        doc, err := html.Parse(r)
        if err != nil {
                // ...
        }
        var f func(*html.Node)
        f = func(n *html.Node) {
                if n.Type == html.ElementNode && n.Data == "a" {
                        // Do something with n...
                }
                for _, c := range n.Child {
                        f(c)
                }
        }
        f(doc)

The relevant specifications include:
http://www.whatwg.org/specs/web-apps/current-work/multipage/syntax.html and
http://www.whatwg.org/specs/web-apps/current-work/multipage/tokenization.html
*/
package html

// The tokenization algorithm implemented by this package is not a line-by-line
// transliteration of the relatively verbose state-machine in the WHATWG
// specification. A more direct approach is used instead, where the program
// counter implies the state, such as whether it is tokenizing a tag or a text
// node. Specification compliance is verified by checking expected and actual
// outputs over a test suite rather than aiming for algorithmic fidelity.

// TODO(nigeltao): Does a DOM API belong in this package or a separate one?
// TODO(nigeltao): How does parsing interact with a JavaScript engine?