1// Copyright 2011 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5package html
6
7import (
8 "bufio"
9 "errors"
10 "fmt"
11 "io"
12 "strings"
13)
14
15type writer interface {
16 io.Writer
17 io.ByteWriter
18 WriteString(string) (int, error)
19}
20
21// Render renders the parse tree n to the given writer.
22//
23// Rendering is done on a 'best effort' basis: calling Parse on the output of
24// Render will always result in something similar to the original tree, but it
25// is not necessarily an exact clone unless the original tree was 'well-formed'.
26// 'Well-formed' is not easily specified; the HTML5 specification is
27// complicated.
28//
29// Calling Parse on arbitrary input typically results in a 'well-formed' parse
30// tree. However, it is possible for Parse to yield a 'badly-formed' parse tree.
31// For example, in a 'well-formed' parse tree, no <a> element is a child of
32// another <a> element: parsing "<a><a>" results in two sibling elements.
33// Similarly, in a 'well-formed' parse tree, no <a> element is a child of a
34// <table> element: parsing "<p><table><a>" results in a <p> with two sibling
35// children; the <a> is reparented to the <table>'s parent. However, calling
36// Parse on "<a><table><a>" does not return an error, but the result has an <a>
37// element with an <a> child, and is therefore not 'well-formed'.
38//
39// Programmatically constructed trees are typically also 'well-formed', but it
40// is possible to construct a tree that looks innocuous but, when rendered and
41// re-parsed, results in a different tree. A simple example is that a solitary
42// text node would become a tree containing <html>, <head> and <body> elements.
43// Another example is that the programmatic equivalent of "a<head>b</head>c"
44// becomes "<html><head><head/><body>abc</body></html>".
45func Render(w io.Writer, n *Node) error {
46 if x, ok := w.(writer); ok {
47 return render(x, n)
48 }
49 buf := bufio.NewWriter(w)
50 if err := render(buf, n); err != nil {
51 return err
52 }
53 return buf.Flush()
54}
55
56// plaintextAbort is returned from render1 when a <plaintext> element
57// has been rendered. No more end tags should be rendered after that.
58var plaintextAbort = errors.New("html: internal error (plaintext abort)")
59
60func render(w writer, n *Node) error {
61 err := render1(w, n)
62 if err == plaintextAbort {
63 err = nil
64 }
65 return err
66}
67
68func render1(w writer, n *Node) error {
69 // Render non-element nodes; these are the easy cases.
70 switch n.Type {
71 case ErrorNode:
72 return errors.New("html: cannot render an ErrorNode node")
73 case TextNode:
74 return escape(w, n.Data)
75 case DocumentNode:
76 for c := n.FirstChild; c != nil; c = c.NextSibling {
77 if err := render1(w, c); err != nil {
78 return err
79 }
80 }
81 return nil
82 case ElementNode:
83 // No-op.
84 case CommentNode:
85 if _, err := w.WriteString("<!--"); err != nil {
86 return err
87 }
88 if _, err := w.WriteString(n.Data); err != nil {
89 return err
90 }
91 if _, err := w.WriteString("-->"); err != nil {
92 return err
93 }
94 return nil
95 case DoctypeNode:
96 if _, err := w.WriteString("<!DOCTYPE "); err != nil {
97 return err
98 }
99 if _, err := w.WriteString(n.Data); err != nil {
100 return err
101 }
102 if n.Attr != nil {
103 var p, s string
104 for _, a := range n.Attr {
105 switch a.Key {
106 case "public":
107 p = a.Val
108 case "system":
109 s = a.Val
110 }
111 }
112 if p != "" {
113 if _, err := w.WriteString(" PUBLIC "); err != nil {
114 return err
115 }
116 if err := writeQuoted(w, p); err != nil {
117 return err
118 }
119 if s != "" {
120 if err := w.WriteByte(' '); err != nil {
121 return err
122 }
123 if err := writeQuoted(w, s); err != nil {
124 return err
125 }
126 }
127 } else if s != "" {
128 if _, err := w.WriteString(" SYSTEM "); err != nil {
129 return err
130 }
131 if err := writeQuoted(w, s); err != nil {
132 return err
133 }
134 }
135 }
136 return w.WriteByte('>')
137 default:
138 return errors.New("html: unknown node type")
139 }
140
141 // Render the <xxx> opening tag.
142 if err := w.WriteByte('<'); err != nil {
143 return err
144 }
145 if _, err := w.WriteString(n.Data); err != nil {
146 return err
147 }
148 for _, a := range n.Attr {
149 if err := w.WriteByte(' '); err != nil {
150 return err
151 }
152 if a.Namespace != "" {
153 if _, err := w.WriteString(a.Namespace); err != nil {
154 return err
155 }
156 if err := w.WriteByte(':'); err != nil {
157 return err
158 }
159 }
160 if _, err := w.WriteString(a.Key); err != nil {
161 return err
162 }
163 if _, err := w.WriteString(`="`); err != nil {
164 return err
165 }
166 if err := escape(w, a.Val); err != nil {
167 return err
168 }
169 if err := w.WriteByte('"'); err != nil {
170 return err
171 }
172 }
173 if voidElements[n.Data] {
174 if n.FirstChild != nil {
175 return fmt.Errorf("html: void element <%s> has child nodes", n.Data)
176 }
177 _, err := w.WriteString("/>")
178 return err
179 }
180 if err := w.WriteByte('>'); err != nil {
181 return err
182 }
183
184 // Add initial newline where there is danger of a newline beging ignored.
185 if c := n.FirstChild; c != nil && c.Type == TextNode && strings.HasPrefix(c.Data, "\n") {
186 switch n.Data {
187 case "pre", "listing", "textarea":
188 if err := w.WriteByte('\n'); err != nil {
189 return err
190 }
191 }
192 }
193
194 // Render any child nodes.
195 switch n.Data {
196 case "iframe", "noembed", "noframes", "noscript", "plaintext", "script", "style", "xmp":
197 for c := n.FirstChild; c != nil; c = c.NextSibling {
198 if c.Type == TextNode {
199 if _, err := w.WriteString(c.Data); err != nil {
200 return err
201 }
202 } else {
203 if err := render1(w, c); err != nil {
204 return err
205 }
206 }
207 }
208 if n.Data == "plaintext" {
209 // Don't render anything else. <plaintext> must be the
210 // last element in the file, with no closing tag.
211 return plaintextAbort
212 }
213 default:
214 for c := n.FirstChild; c != nil; c = c.NextSibling {
215 if err := render1(w, c); err != nil {
216 return err
217 }
218 }
219 }
220
221 // Render the </xxx> closing tag.
222 if _, err := w.WriteString("</"); err != nil {
223 return err
224 }
225 if _, err := w.WriteString(n.Data); err != nil {
226 return err
227 }
228 return w.WriteByte('>')
229}
230
231// writeQuoted writes s to w surrounded by quotes. Normally it will use double
232// quotes, but if s contains a double quote, it will use single quotes.
233// It is used for writing the identifiers in a doctype declaration.
234// In valid HTML, they can't contain both types of quotes.
235func writeQuoted(w writer, s string) error {
236 var q byte = '"'
237 if strings.Contains(s, `"`) {
238 q = '\''
239 }
240 if err := w.WriteByte(q); err != nil {
241 return err
242 }
243 if _, err := w.WriteString(s); err != nil {
244 return err
245 }
246 if err := w.WriteByte(q); err != nil {
247 return err
248 }
249 return nil
250}
251
252// Section 12.1.2, "Elements", gives this list of void elements. Void elements
253// are those that can't have any contents.
254var voidElements = map[string]bool{
255 "area": true,
256 "base": true,
257 "br": true,
258 "col": true,
259 "command": true,
260 "embed": true,
261 "hr": true,
262 "img": true,
263 "input": true,
264 "keygen": true,
265 "link": true,
266 "meta": true,
267 "param": true,
268 "source": true,
269 "track": true,
270 "wbr": true,
271}