Convert mobile Twitter links to standard ones

This is the first url transformation that only applies to a specific
site (and domain).

tildes/tests/test_url_transform.py

@@ -18,3 +18,19 @@ def test_non_utm_params_unaffected():
     cleaned_url = apply_url_transformations(url)
 
     assert cleaned_url == url
+
+
+def test_twitter_mobile_conversion():
+    """Ensure that links to the Twitter mobile version are converted."""
+    url = "https://mobile.twitter.com/acarboni/status/976545648391553024"
+    cleaned_url = apply_url_transformations(url)
+
+    assert cleaned_url == "https://twitter.com/acarboni/status/976545648391553024"
+
+
+def test_other_mobile_subdomain_not_removed():
+    """Ensure that the Twitter mobile conversion isn't hitting other domains."""
+    url = "http://mobile.example.com/something"
+    cleaned_url = apply_url_transformations(url)
+
+    assert cleaned_url == url

tildes/tildes/lib/url_transform.py

@@ -115,3 +115,17 @@ class UtmQueryParamRemover(UrlTransformer):
         }
 
         return parsed_url._replace(query=urlencode(cleaned_params, doseq=True))
+
+
+class TwitterMobileConverter(UrlTransformer):
+    """Convert links to Twitter mobile version to the bare domain."""
+
+    @classmethod
+    def is_applicable(cls, parsed_url: ParseResult) -> bool:
+        """Return whether this transformation should be applied to the url."""
+        return parsed_url.hostname == "mobile.twitter.com"
+
+    @classmethod
+    def apply_transformation(cls, parsed_url: ParseResult) -> ParseResult:
+        """Apply the actual transformation process to the url."""
+        return parsed_url._replace(netloc="twitter.com")