Add Markdown index HTML output mode

internal/transform/markdown/markdown.go

@@ -25,24 +25,26 @@ func (t *Transformer) Generate(ctx context.Context, req transform.Request) ([]tr
 	if t.renderer == nil {
 		t.renderer = goldmark.New()
 	}
+	switch markdownMode(req.Markdown.Mode) {
+	case transform.MarkdownModeSidecar:
+		return t.generateSidecars(ctx, req)
+	case transform.MarkdownModeIndex:
+		return t.generateIndex(ctx, req)
+	default:
+		return nil, fmt.Errorf("markdown mode %q is not supported", req.Markdown.Mode)
+	}
+}
+
+func (t *Transformer) generateSidecars(ctx context.Context, req transform.Request) ([]transform.Output, error) {
 	var outputs []transform.Output
 	for _, file := range req.SourceBundle.Manifest.Files {
 		if !strings.HasSuffix(file.Path, ".md") {
 			continue
 		}
-		sourcePath, err := storage.Join(req.SourceBundle.RootRelativePath, file.Path)
+		html, err := t.render(ctx, req, file.Path)
 		if err != nil {
 			return nil, err
 		}
-		data, err := req.SourceBackend.ReadFile(ctx, sourcePath)
-		if err != nil {
-			return nil, fmt.Errorf("read markdown source %q: %w", file.Path, err)
-		}
-		var rendered bytes.Buffer
-		if err := t.renderer.Convert(data, &rendered); err != nil {
-			return nil, fmt.Errorf("render markdown source %q: %w", file.Path, err)
-		}
-		html := wrapHTML(rendered.Bytes())
 		outputPath := strings.TrimSuffix(file.Path, ".md") + ".html"
 		outputs = append(outputs, transform.Output{
 			Path:       outputPath,
@@ -55,3 +57,78 @@ func (t *Transformer) Generate(ctx context.Context, req transform.Request) ([]tr
 	}
 	return outputs, nil
 }
+
+func (t *Transformer) generateIndex(ctx context.Context, req transform.Request) ([]transform.Output, error) {
+	input, err := selectIndexInput(req)
+	if err != nil {
+		return nil, err
+	}
+	html, err := t.render(ctx, req, input)
+	if err != nil {
+		return nil, err
+	}
+	return []transform.Output{{
+		Path:       "index.html",
+		SourcePath: input,
+		Transform:  transform.MarkdownToHTML,
+		Data:       html,
+		SHA256:     bundle.FileDigest(html),
+		Size:       int64(len(html)),
+	}}, nil
+}
+
+func selectIndexInput(req transform.Request) (string, error) {
+	if req.Markdown.Input != "" {
+		if err := storage.ValidatePath(req.Markdown.Input); err != nil {
+			return "", fmt.Errorf("markdown input %q: %w", req.Markdown.Input, err)
+		}
+		for _, file := range req.SourceBundle.Manifest.Files {
+			if file.Path != req.Markdown.Input {
+				continue
+			}
+			if !strings.HasSuffix(file.Path, ".md") {
+				return "", fmt.Errorf("markdown input %q must end in .md", req.Markdown.Input)
+			}
+			return file.Path, nil
+		}
+		return "", fmt.Errorf("markdown input %q is not listed in the source manifest", req.Markdown.Input)
+	}
+
+	var markdownFiles []string
+	for _, file := range req.SourceBundle.Manifest.Files {
+		if strings.HasSuffix(file.Path, ".md") {
+			markdownFiles = append(markdownFiles, file.Path)
+		}
+	}
+	switch len(markdownFiles) {
+	case 0:
+		return "", fmt.Errorf("markdown index mode requires one markdown source file or transform.markdown_to_html.input")
+	case 1:
+		return markdownFiles[0], nil
+	default:
+		return "", fmt.Errorf("markdown index mode found multiple markdown source files; set transform.markdown_to_html.input")
+	}
+}
+
+func (t *Transformer) render(ctx context.Context, req transform.Request, sourceFile string) ([]byte, error) {
+	sourcePath, err := storage.Join(req.SourceBundle.RootRelativePath, sourceFile)
+	if err != nil {
+		return nil, err
+	}
+	data, err := req.SourceBackend.ReadFile(ctx, sourcePath)
+	if err != nil {
+		return nil, fmt.Errorf("read markdown source %q: %w", sourceFile, err)
+	}
+	var rendered bytes.Buffer
+	if err := t.renderer.Convert(data, &rendered); err != nil {
+		return nil, fmt.Errorf("render markdown source %q: %w", sourceFile, err)
+	}
+	return wrapHTML(rendered.Bytes()), nil
+}
+
+func markdownMode(mode string) string {
+	if mode == "" {
+		return transform.MarkdownModeSidecar
+	}
+	return mode
+}