appview: pages/markup: render markdown with transformations

appview/pages/markup/markdown.go

···
 
 import (
 	"bytes"
+	"path"
 
 	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/ast"
 	"github.com/yuin/goldmark/extension"
 	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/text"
+	"github.com/yuin/goldmark/util"
 )
 
-func RenderMarkdown(source string) string {
+// RendererType defines the type of renderer to use based on context
+type RendererType int
+
+const (
+	// RendererTypeRepoMarkdown is for repository documentation markdown files
+	RendererTypeRepoMarkdown RendererType = iota
+	// RendererTypeIssueComment is for issue comments
+	RendererTypeIssueComment
+	// RendererTypePullComment is for pull request comments
+	RendererTypePullComment
+	// RendererTypeDefault is the default renderer with minimal transformations
+	RendererTypeDefault
+)
+
+// RenderContext holds the contextual data for rendering markdown.
+// It can be initialized empty, and that'll skip any transformations
+// and use the default renderer (RendererTypeDefault).
+type RenderContext struct {
+	Ref          string
+	FullRepoName string
+	RendererType RendererType
+}
+
+func (rctx *RenderContext) RenderMarkdown(source string) string {
 	md := goldmark.New(
 		goldmark.WithExtensions(extension.GFM),
 		goldmark.WithParserOptions(
 			parser.WithAutoHeadingID(),
 		),
 	)
+
+	if rctx != nil {
+		var transformers []util.PrioritizedValue
+
+		transformers = append(transformers, util.Prioritized(&MarkdownTransformer{rctx: rctx}, 10000))
+
+		md.Parser().AddOptions(
+			parser.WithASTTransformers(transformers...),
+		)
+	}
+
 	var buf bytes.Buffer
 	if err := md.Convert([]byte(source), &buf); err != nil {
 		return source
 	}
 	return buf.String()
 }
+
+type MarkdownTransformer struct {
+	rctx *RenderContext
+}
+
+func (a *MarkdownTransformer) Transform(node *ast.Document, reader text.Reader, pc parser.Context) {
+	_ = ast.Walk(node, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
+		if !entering {
+			return ast.WalkContinue, nil
+		}
+
+		switch a.rctx.RendererType {
+		case RendererTypeRepoMarkdown:
+			a.rctx.relativeLinkTransformer(n.(*ast.Link))
+		case RendererTypeDefault:
+			a.rctx.relativeLinkTransformer(n.(*ast.Link))
+			// more types here like RendererTypeIssue/Pull etc.
+		}
+
+		return ast.WalkContinue, nil
+	})
+}
+
+func (rctx *RenderContext) relativeLinkTransformer(link *ast.Link) {
+	dst := string(link.Destination)
+
+	if len(dst) == 0 || dst[0] == '#' ||
+		bytes.Contains(link.Destination, []byte("://")) ||
+		bytes.HasPrefix(link.Destination, []byte("mailto:")) {
+		return
+	}
+
+	newPath := path.Join("/", rctx.FullRepoName, "tree", rctx.Ref, dst)
+	link.Destination = []byte(newPath)
+}