package cabana

import (
	"bytes"
	"fmt"
	"regexp"

	"github.com/yuin/goldmark"
)

var (
	markdownEngine = goldmark.New()

	markdownUnsafeTag    = regexp.MustCompile(`(?i)<(?:script|iframe|object|embed)\b`)
	markdownEventHandler = regexp.MustCompile(`(?i)\son[a-z]+\s*=`)
	markdownDangerousURL = regexp.MustCompile(`(?i)(?:javascript|vbscript|data):`)
)

// RenderMarkdown converts source to HTML using the pinned goldmark engine
// without html.WithUnsafe. Output that still contains script/iframe tags,
// event handlers, or javascript/vbscript/data URLs is rejected, matching
// postcard's mail HTML gate.
func RenderMarkdown(src string) (string, error) {
	var buf bytes.Buffer
	if err := markdownEngine.Convert([]byte(src), &buf); err != nil {
		return "", fmt.Errorf("cabana: markdown: %w", err)
	}
	html := buf.String()
	if err := rejectUnsafeMarkdownHTML(html); err != nil {
		return "", err
	}
	return html, nil
}

func rejectUnsafeMarkdownHTML(html string) error {
	if markdownUnsafeTag.MatchString(html) {
		return fmt.Errorf("cabana: rendered HTML contains raw unsafe tags")
	}
	if markdownEventHandler.MatchString(html) {
		return fmt.Errorf("cabana: rendered HTML contains event handlers")
	}
	if markdownDangerousURL.MatchString(html) {
		return fmt.Errorf("cabana: rendered HTML contains a dangerous URL scheme")
	}
	return nil
}