// ... implements parser.ASTTransformer.
type PreserveTaskListSourcePosition struct{}

func (a *PreserveTaskListSourcePosition) Transform(node *ast.Document, reader text.Reader, pc parser.Context) {
	ast.Walk(node, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
		if entering && n.Kind() == extast.KindTaskCheckBox {
			c := n.(*extast.TaskCheckBox)
			c.SetAttributeString(`data-source-position`, fmt.Sprintf(`[%d,%d]`, c.Segment.Start, c.Segment.Stop))
		}
		return ast.WalkContinue, nil
	})
}

var source = []byte(`
- [ ] Task1
- [ ] Task2
`)

func main() {
	md := goldmark.New(
		goldmark.WithExtensions(extension.TaskList),
		goldmark.WithParserOptions(
			parser.WithASTTransformers(util.Prioritized(&PreserveTaskListSourcePosition{}, 1000)),
		),
	)

	doc := md.Parser().Parse(text.NewReader(source))
	md.Renderer().Render(os.Stdout, source, doc)
	// <ul>
	// <li><input disabled="" type="checkbox" data-source-position="[4,5]"> Task1</li>
	// <li><input disabled="" type="checkbox" data-source-position="[16,17]"> Task2</li>
	// </ul>

	// This piece of code should be implemented by the Server
	// to respond to a Click event on the Web page.
	// We assume that we got a request to finish
	// the task at `data-source-position="[16,17]"`.
	{
		var taskPos = `[16,17]`
		var start, stop int
		_, _ = fmt.Sscanf(taskPos, `[%d,%d]`, &start, &stop)

		_ = stop

		// mark it accomplished
		source[start] = 'x'

		doc = md.Parser().Parse(text.NewReader(source))
		md.Renderer().Render(os.Stdout, source, doc)
		// <ul>
		// <li><input disabled="" type="checkbox" data-source-position="[4,5]"> Task1</li>
		// <li><input checked="" disabled="" type="checkbox" data-source-position="[16,17]"> Task2</li>
		// </ul>

		// TODO: Save the new source and rendered content.
	}
}