add middleware to add signposting links into html head (#3)

* add middleware to add signposting links into html head

django_signposting/middleware.py

@@ -42,8 +42,49 @@ class SignpostingMiddleware:
         response["Link"] = " , ".join(link_snippets)
 
 
-class JsonLdSignpostingParserMiddleware(MiddlewareMixin):
+class HtmlSignpostingMiddleware(SignpostingMiddleware):
+    def __init__(self, get_response: Callable[[HttpRequest], HttpResponse]):
+        self.get_response = get_response
 
+    def __call__(self, request: HttpRequest) -> HttpResponse:
+        response = self.get_response(request)
+
+        if not hasattr(response, "_signposts"):
+            return response
+
+        # Adding Signposts via HTML is only supported for HTML responses
+        if not response.headers["Content-Type"].startswith("text/html"):
+            return response
+
+        content = response.content.decode("utf-8")
+        soup = BeautifulSoup(content, "html.parser")
+
+        # Html should have a root element
+        if not soup.html:
+            raise Exception("Could not find HTML root element")
+
+        # Add head element if not present
+        if not soup.head:
+            head_tag = soup.new_tag("head")
+            soup.html.insert(0, head_tag)
+
+        # BUild links for each signpost and add them to the html
+        for signpost in response._signposts:
+            link_tag = soup.new_tag("link")
+            link_tag["rel"] = signpost.rel
+            link_tag["href"] = signpost.target
+            if signpost.type:
+                link_tag["type"] = signpost.type
+            soup.head.append(link_tag)
+
+        # Override the original content with the new HTML
+        response.content = soup.prettify().encode("utf-8")
+        response["Content-Length"] = len(response.content)
+
+        return response
+
+
+class JsonLdSignpostingParserMiddleware(MiddlewareMixin):
     def process_response(
         self, request: HttpRequest, response: HttpResponse
     ) -> HttpResponse: