add parser for blog articles

Signed-off-by: Tobias Erbshäußer <tobias@tesoft.dev>

backend/md/urls.go

@@ -0,0 +1,110 @@
+package md
+
+import (
+	"bytes"
+	"strings"
+
+	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/renderer"
+	"github.com/yuin/goldmark/renderer/html"
+	"github.com/yuin/goldmark/util"
+)
+
+type UrlTransformer = func(string) (string, error)
+
+type urlTransformerExtension struct {
+	transformer UrlTransformer
+}
+
+func (e *urlTransformerExtension) Extend(m goldmark.Markdown) {
+	m.Renderer().AddOptions(renderer.WithNodeRenderers(
+		util.Prioritized(&urlRenderer{e.transformer}, 500),
+	))
+}
+
+type urlRenderer struct {
+	transformer UrlTransformer
+}
+
+func (r *urlRenderer) RegisterFuncs(reg renderer.NodeRendererFuncRegisterer) {
+	reg.Register(ast.KindImage, r.renderImage)
+	reg.Register(ast.KindLink, r.renderLink)
+}
+
+func (r *urlRenderer) renderImage(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	if !entering {
+		return ast.WalkContinue, nil
+	}
+
+	n := node.(*ast.Image)
+
+	url, err := r.resolveUrl(string(n.Destination))
+	if err != nil {
+		return ast.WalkStop, err
+	}
+
+	_, _ = w.WriteString(`<img src="`)
+	_, _ = w.WriteString(url)
+	_, _ = w.WriteString(`" alt="`)
+	_, _ = w.Write(nodeToHTMLText(n, source))
+	_ = w.WriteByte('"')
+	if n.Title != nil {
+		_, _ = w.WriteString(` title="`)
+		_, _ = w.Write(n.Title)
+		_ = w.WriteByte('"')
+	}
+	if n.Attributes() != nil {
+		html.RenderAttributes(w, n, html.ImageAttributeFilter)
+	}
+	_, _ = w.WriteString(">")
+	return ast.WalkSkipChildren, nil
+}
+
+func (r *urlRenderer) renderLink(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	if !entering {
+		return ast.WalkContinue, nil
+	}
+
+	n := node.(*ast.Link)
+
+	url, err := r.resolveUrl(string(n.Destination))
+	if err != nil {
+		return ast.WalkStop, err
+	}
+
+	_, _ = w.WriteString(`<a href="`)
+	_, _ = w.WriteString(url)
+	_, _ = w.WriteString(`"`)
+	if n.Attributes() != nil {
+		html.RenderAttributes(w, n, html.ImageAttributeFilter)
+	}
+	_, _ = w.WriteString(`>`)
+	if n.Title != nil {
+		_, _ = w.Write(n.Title)
+	}
+	_, _ = w.WriteString("</a>")
+	return ast.WalkSkipChildren, nil
+}
+
+func (r *urlRenderer) resolveUrl(url string) (string, error) {
+	if !strings.HasPrefix(url, "http://") && !strings.HasPrefix(url, "https://") {
+		return r.transformer(url)
+	}
+
+	return url, nil
+}
+
+func nodeToHTMLText(n ast.Node, source []byte) []byte {
+	var buf bytes.Buffer
+	for c := n.FirstChild(); c != nil; c = c.NextSibling() {
+		if s, ok := c.(*ast.String); ok && s.IsCode() {
+			buf.Write(s.Text(source))
+		} else if !c.HasChildren() {
+			buf.Write(util.EscapeHTML(c.Text(source)))
+		} else {
+			buf.Write(nodeToHTMLText(c, source))
+		}
+	}
+	return buf.Bytes()
+}