Improve sections

sections.go

@@ -3,6 +3,9 @@ package main
 import (
 	"bytes"
 	"regexp"
+
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/text"
 )
 
 var sectionHeadingRe = regexp.MustCompile(`(?m)^#{1,6} `)
@@ -25,6 +28,29 @@ func splitSections(raw []byte) [][]byte {
 	return sections
 }
 
+// headingIDs returns the auto-generated id of every heading in raw markdown,
+// in document order. The kth heading (1-indexed) corresponds to section k from
+// splitSections. Uses the package-level goldmark parser so duplicate-id
+// numbering matches what the renderer emits.
+func headingIDs(raw []byte) []string {
+	doc := md.Parser().Parse(text.NewReader(raw))
+	var ids []string
+	ast.Walk(doc, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
+		if !entering {
+			return ast.WalkContinue, nil
+		}
+		if _, ok := n.(*ast.Heading); ok {
+			if v, ok := n.AttributeString("id"); ok {
+				if b, ok := v.([]byte); ok {
+					ids = append(ids, string(b))
+				}
+			}
+		}
+		return ast.WalkContinue, nil
+	})
+	return ids
+}
+
 // joinSections reassembles sections produced by splitSections.
 // Inserts a newline between sections when a non-empty section lacks a
 // trailing newline, so an edited section cannot inline the next heading.