Link to Wayback Diff

This commit adds support for linking to the Wayback diff view, rather
than linking to the before and after versions and asking users to be
able to determine the difference (which can be quite small).

Fixes #60

diffengine/__init__.py

@@ -3,7 +3,7 @@
 
 # maybe this module should be broken up into multiple files, or maybe not ...
 
-UA = "diffengine/0.2.6 (+https://github.com/docnow/diffengine)"
+UA = "diffengine/0.2.7 (+https://github.com/docnow/diffengine)"
 
 import os
 import re
@@ -285,6 +285,19 @@ def screenshot_path(self):
     def thumbnail_path(self):
         return self.screenshot_path.replace('.png', '-thumb.png')
 
+    @property
+    def url(self):
+
+        def snap(url):
+            m = re.match(r'^https://web.archive.org/web/(\d+?)/.*$', url)
+            return m.group(1) if m else None
+
+        return 'https://web.archive.org/web/diff/{}/{}/{}/'.format(
+            snap(self.old.archive_url),
+            snap(self.new.archive_url),
+            self.old.url
+        )
+
     def generate(self):
         if self._generate_diff_html():
             self._generate_diff_images()
@@ -445,7 +458,7 @@ def tweet_diff(diff, token):
     if len(status) >= 225:
         status = status[0:225] + "…"
 
-    status += " " + diff.old.archive_url +  " ➜ " + diff.new.archive_url
+    status += " " + diff.url
 
     try:
         twitter.update_with_media(diff.thumbnail_path, status)

setup.py

@@ -1,4 +1,4 @@
-version = "0.2.6"
+version = "0.2.7"
 
 import sys
 if sys.version_info < (3,0):

test_diffengine.py

@@ -53,6 +53,10 @@ def test_diff():
     assert os.path.isfile(diff.screenshot_path)
     assert os.path.isfile(diff.thumbnail_path)
 
+    # check that the url for the internet archive diff is working
+    assert re.match("^https://web.archive.org/web/diff/\d+/\d+/https.+$",
+                    diff.url)
+
 def test_html_diff():
     f = Feed.get(Feed.url=="https://inkdroid.org/feed.xml")
     e = f.entries[0]