phase2: markdown converter with goldmark, chroma, and ASCII-translit anchors

internal/converter/anchor.go

@@ -0,0 +1,123 @@
+package converter
+
+import (
+	"strings"
+
+	"github.com/yuin/goldmark"
+	emojiast "github.com/yuin/goldmark-emoji/ast"
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/text"
+	"github.com/yuin/goldmark/util"
+)
+
+type anchorExtension struct{}
+
+func (e *anchorExtension) Extend(m goldmark.Markdown) {
+	m.Parser().AddOptions(parser.WithASTTransformers(
+		util.Prioritized(&anchorTransformer{}, 900),
+	))
+}
+
+type anchorTransformer struct{}
+
+func (t *anchorTransformer) Transform(doc *ast.Document, reader text.Reader, pc parser.Context) {
+	src := reader.Source()
+	used := map[string]int{}
+
+	_ = pc
+
+	_ = ast.Walk(doc, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
+		if !entering {
+			return ast.WalkContinue, nil
+		}
+
+		h, ok := n.(*ast.Heading)
+		if !ok {
+			return ast.WalkContinue, nil
+		}
+
+		slug := translitSlug(extractHeadingText(h, src), used)
+		h.SetAttributeString("id", []byte(slug))
+
+		link := ast.NewLink()
+		link.Destination = []byte("#" + slug)
+		link.SetAttributeString("class", []byte("heading-anchor"))
+		link.SetAttributeString("aria-hidden", []byte("true"))
+		link.AppendChild(link, ast.NewString([]byte("#")))
+
+		if first := h.FirstChild(); first != nil {
+			h.InsertBefore(h, first, link)
+		} else {
+			h.AppendChild(h, link)
+		}
+
+		return ast.WalkSkipChildren, nil
+	})
+}
+
+func extractHeadingText(h *ast.Heading, src []byte) string {
+	var b strings.Builder
+
+	_ = ast.Walk(h, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
+		if !entering {
+			return ast.WalkContinue, nil
+		}
+
+		switch v := n.(type) {
+		case *ast.Link:
+			if isHeadingAnchor(v) {
+				return ast.WalkSkipChildren, nil
+			}
+		case *ast.Text:
+			b.Write(v.Segment.Value(src))
+			if v.HardLineBreak() || v.SoftLineBreak() {
+				b.WriteByte(' ')
+			}
+		case *ast.String:
+			b.Write(v.Value)
+		case *ast.CodeSpan:
+			for child := v.FirstChild(); child != nil; child = child.NextSibling() {
+				switch c := child.(type) {
+				case *ast.Text:
+					b.Write(c.Segment.Value(src))
+				case *ast.String:
+					b.Write(c.Value)
+				}
+			}
+			return ast.WalkSkipChildren, nil
+		case *ast.AutoLink:
+			b.Write(v.Label(src))
+			return ast.WalkSkipChildren, nil
+		case *emojiast.Emoji:
+			if v.Value != nil && len(v.Value.Unicode) > 0 {
+				b.WriteString(string(v.Value.Unicode))
+			} else if len(v.ShortName) > 0 {
+				b.WriteByte(':')
+				b.Write(v.ShortName)
+				b.WriteByte(':')
+			}
+			return ast.WalkSkipChildren, nil
+		}
+
+		return ast.WalkContinue, nil
+	})
+
+	return strings.TrimSpace(b.String())
+}
+
+func isHeadingAnchor(link *ast.Link) bool {
+	attr, ok := link.AttributeString("class")
+	if !ok {
+		return false
+	}
+
+	switch value := attr.(type) {
+	case []byte:
+		return string(value) == "heading-anchor"
+	case string:
+		return value == "heading-anchor"
+	default:
+		return false
+	}
+}