autoheading.go

package turbine

import (
	"bytes"
	"io"
	"regexp"
	"strings"

	"github.com/russross/blackfriday/v2"
)

var reParseHeadingID = regexp.MustCompile(`id="(.+)"`)

type AutoHeadingLinkRenderer struct {
	Base blackfriday.Renderer

	// LinkText will be inserted for the anchor's content. This is inserted
	// verbatim so it could be a special character like # or an image tag.
	LinkText string

	// Outside will write the anchor outside the header tag. This can be useful
	// to avoid mixing formatting.
	Outside bool

	// CSSClass will be added to the anchor tag (optional)
	CSSClass string
}

func (r *AutoHeadingLinkRenderer) renderLink(id string) *bytes.Buffer {
	s := strings.Builder{}

	s.WriteString(`<a href="#`)
	s.WriteString(id)
	s.WriteString(`"`)
	if r.CSSClass != "" {
		s.WriteString(` class="`)
		s.WriteString(r.CSSClass)
		s.WriteString(`"`)
	}
	s.WriteString(">")
	s.WriteString(r.LinkText)
	s.WriteString("</a>")

	buf := &bytes.Buffer{}
	buf.WriteString(s.String())
	return buf
}

func (r *AutoHeadingLinkRenderer) RenderNode(w io.Writer, node *blackfriday.Node, entering bool) blackfriday.WalkStatus {
	switch node.Type {
	case blackfriday.Heading:
		heading := &bytes.Buffer{}
		// Render the node to a buffer so we can tamper with it
		status := r.Base.RenderNode(heading, node, entering)

		// entering is the opening tag, such as <h3 ... >
		if entering {
			id := reParseHeadingID.FindStringSubmatch(heading.String())

			if len(id) == 2 {
				if r.Outside {
					io.Copy(w, r.renderLink(id[1]))
				} else {
					heading.Write(r.renderLink(id[1]).Bytes())
				}
			}
		}
		if _, err := io.Copy(w, heading); err != nil {
			panic(err)
		}

		return status
	default:
		return r.Base.RenderNode(w, node, entering)
	}
}

func (r *AutoHeadingLinkRenderer) RenderHeader(w io.Writer, ast *blackfriday.Node) {
	r.Base.RenderHeader(w, ast)
}

func (r *AutoHeadingLinkRenderer) RenderFooter(w io.Writer, ast *blackfriday.Node) {
	r.Base.RenderFooter(w, ast)
}

usage.go

func Render(data []byte) []byte {
	extensions := blackfriday.FencedCode | blackfriday.Strikethrough | blackfriday.AutoHeadingIDs
	var renderer blackfriday.Renderer = blackfriday.NewHTMLRenderer(blackfriday.HTMLRendererParameters{})

	renderer = &AutoHeadingLinkRenderer{
		Base:     renderer,
		LinkText: "#",
		Outside:  true,
		CSSClass: "heading-link",
	}

	return blackfriday.Run(data, blackfriday.WithExtensions(extensions), blackfriday.WithRenderer(renderer))
}