Source: render.go in package golang.org/x/net/html


package html

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"strings"
)

type writer interface {
	io.Writer
	io.ByteWriter
	WriteString(string) (int, error)
}

Render renders the parse tree n to the given writer. Rendering is done on a 'best effort' basis: calling Parse on the output of Render will always result in something similar to the original tree, but it is not necessarily an exact clone unless the original tree was 'well-formed'. 'Well-formed' is not easily specified; the HTML5 specification is complicated. Calling Parse on arbitrary input typically results in a 'well-formed' parse tree. However, it is possible for Parse to yield a 'badly-formed' parse tree. For example, in a 'well-formed' parse tree, no <a> element is a child of another <a> element: parsing "<a><a>" results in two sibling elements. Similarly, in a 'well-formed' parse tree, no <a> element is a child of a <table> element: parsing "<p><table><a>" results in a <p> with two sibling children; the <a> is reparented to the <table>'s parent. However, calling Parse on "<a><table><a>" does not return an error, but the result has an <a> element with an <a> child, and is therefore not 'well-formed'. Programmatically constructed trees are typically also 'well-formed', but it is possible to construct a tree that looks innocuous but, when rendered and re-parsed, results in a different tree. A simple example is that a solitary text node would become a tree containing <html>, <head> and <body> elements. Another example is that the programmatic equivalent of "a<head>b</head>c" becomes "<html><head><head/><body>abc</body></html>".

func Render(w io.Writer, n *Node) error {
	if x, ok := w.(writer); ok {
		return render(x, n)
	}
	buf := bufio.NewWriter(w)
	if err := render(buf, n); err != nil {
		return err
	}
	return buf.Flush()
}

plaintextAbort is returned from render1 when a <plaintext> element has been rendered. No more end tags should be rendered after that.

var plaintextAbort = errors.New("html: internal error (plaintext abort)")

func render(w writer, n *Node) error {
	err := render1(w, n)
	if err == plaintextAbort {
		err = nil
	}
	return err
}

Render non-element nodes; these are the easy cases.

	switch n.Type {
	case ErrorNode:
		return errors.New("html: cannot render an ErrorNode node")
	case TextNode:
		return escape(w, n.Data)
	case DocumentNode:
		for c := n.FirstChild; c != nil; c = c.NextSibling {
			if err := render1(w, c); err != nil {
				return err
			}
		}
		return nil

No-op.

	case CommentNode:
		if _, err := w.WriteString("<!--"); err != nil {
			return err
		}
		if _, err := w.WriteString(n.Data); err != nil {
			return err
		}
		if _, err := w.WriteString("-->"); err != nil {
			return err
		}
		return nil
	case DoctypeNode:
		if _, err := w.WriteString("<!DOCTYPE "); err != nil {
			return err
		}
		if _, err := w.WriteString(n.Data); err != nil {
			return err
		}
		if n.Attr != nil {
			var p, s string
			for _, a := range n.Attr {
				switch a.Key {
				case "public":
					p = a.Val
				case "system":
					s = a.Val
				}
			}
			if p != "" {
				if _, err := w.WriteString(" PUBLIC "); err != nil {
					return err
				}
				if err := writeQuoted(w, p); err != nil {
					return err
				}
				if s != "" {
					if err := w.WriteByte(' '); err != nil {
						return err
					}
					if err := writeQuoted(w, s); err != nil {
						return err
					}
				}
			} else if s != "" {
				if _, err := w.WriteString(" SYSTEM "); err != nil {
					return err
				}
				if err := writeQuoted(w, s); err != nil {
					return err
				}
			}
		}
		return w.WriteByte('>')
	case RawNode:
		_, err := w.WriteString(n.Data)
		return err
	default:
		return errors.New("html: unknown node type")
	}

Render the <xxx> opening tag.

	if err := w.WriteByte('<'); err != nil {
		return err
	}
	if _, err := w.WriteString(n.Data); err != nil {
		return err
	}
	for _, a := range n.Attr {
		if err := w.WriteByte(' '); err != nil {
			return err
		}
		if a.Namespace != "" {
			if _, err := w.WriteString(a.Namespace); err != nil {
				return err
			}
			if err := w.WriteByte(':'); err != nil {
				return err
			}
		}
		if _, err := w.WriteString(a.Key); err != nil {
			return err
		}
		if _, err := w.WriteString(`="`); err != nil {
			return err
		}
		if err := escape(w, a.Val); err != nil {
			return err
		}
		if err := w.WriteByte('"'); err != nil {
			return err
		}
	}
	if voidElements[n.Data] {
		if n.FirstChild != nil {
			return fmt.Errorf("html: void element <%s> has child nodes", n.Data)
		}
		_, err := w.WriteString("/>")
		return err
	}
	if err := w.WriteByte('>'); err != nil {
		return err
	}

Add initial newline where there is danger of a newline beging ignored.

	if c := n.FirstChild; c != nil && c.Type == TextNode && strings.HasPrefix(c.Data, "\n") {
		switch n.Data {
		case "pre", "listing", "textarea":
			if err := w.WriteByte('\n'); err != nil {
				return err
			}
		}
	}

Render any child nodes.

	switch n.Data {
	case "iframe", "noembed", "noframes", "noscript", "plaintext", "script", "style", "xmp":
		for c := n.FirstChild; c != nil; c = c.NextSibling {
			if c.Type == TextNode {
				if _, err := w.WriteString(c.Data); err != nil {
					return err
				}
			} else {
				if err := render1(w, c); err != nil {
					return err
				}
			}
		}

Don't render anything else. <plaintext> must be the last element in the file, with no closing tag.

			return plaintextAbort
		}
	default:
		for c := n.FirstChild; c != nil; c = c.NextSibling {
			if err := render1(w, c); err != nil {
				return err
			}
		}
	}

Render the </xxx> closing tag.

	if _, err := w.WriteString("</"); err != nil {
		return err
	}
	if _, err := w.WriteString(n.Data); err != nil {
		return err
	}
	return w.WriteByte('>')
}

writeQuoted writes s to w surrounded by quotes. Normally it will use double quotes, but if s contains a double quote, it will use single quotes. It is used for writing the identifiers in a doctype declaration. In valid HTML, they can't contain both types of quotes.

func writeQuoted(w writer, s string) error {
	var q byte = '"'
	if strings.Contains(s, `"`) {
		q = '\''
	}
	if err := w.WriteByte(q); err != nil {
		return err
	}
	if _, err := w.WriteString(s); err != nil {
		return err
	}
	if err := w.WriteByte(q); err != nil {
		return err
	}
	return nil
}

Section 12.1.2, "Elements", gives this list of void elements. Void elements are those that can't have any contents.

var voidElements = map[string]bool{
	"area":   true,
	"base":   true,
	"br":     true,
	"col":    true,
	"embed":  true,
	"hr":     true,
	"img":    true,
	"input":  true,
	"keygen": true, // "keygen" has been removed from the spec, but are kept here for backwards compatibility.
	"link":   true,
	"meta":   true,
	"param":  true,
	"source": true,
	"track":  true,
	"wbr":    true,