--- a/candolint/handlers.py	Sat Sep 16 17:37:38 2017 +0800
+++ b/candolint/handlers.py	Sat Sep 16 17:41:43 2017 +0800
@@ -1,6 +1,7 @@
 from __future__ import absolute_import, division
 
 import traceback
+from difflib import unified_diff
 from math import ceil
 
 from peewee import DoesNotExist
@@ -152,6 +153,32 @@
         self.render('check.html', project=project, check=check, lines=lines, adapter=adapter)
 
 
+class CompareHandler(BaseHandler):
+    def get(self, domain, user, name, check_num, reference_num):
+        project = get_project_or_404(domain, user, name)
+        adapter = project.get_adapter()
+        check = get_check_or_404(project, check_num)
+        reference = get_check_or_404(project, reference_num)
+        lines = json_decode(check.lines)
+        reflines = json_decode(reference.lines)
+
+        rcl = [l['text'] for l in reflines if l.get('task') == 'checks']
+        cl = [l['text'] for l in lines if l.get('task') == 'checks']
+        diff = unified_diff(rcl, cl, n=max(len(rcl), len(cl)))
+        difflines = []
+        for text in list(diff)[4:]:
+            line = {'text': text}
+            if text.startswith('+'):
+                line['cls'] = 'added'
+            elif text.startswith('-'):
+                line['cls'] = 'removed'
+            difflines.append(line)
+        if not difflines:
+            difflines.append({'text': 'Diff is empty', 'cls': 'meta'})
+
+        self.render('compare.html', project=project, check=check, reference=reference, lines=difflines, adapter=adapter)
+
+
 class StatusHandler(BaseHandler):
     def get(self, domain, user, name):
         project = get_project_or_404(domain, user, name)

--- a/static/candolint.css	Sat Sep 16 17:37:38 2017 +0800
+++ b/static/candolint.css	Sat Sep 16 17:41:43 2017 +0800
@@ -157,3 +157,11 @@
 .check-log .failure {
     color: #f1bf8e;
 }
+
+.check-log .added {
+    color: #b0d89c;
+}
+
+.check-log .removed {
+    color: #f277a1;
+}

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/templates/compare.html	Sat Sep 16 17:41:43 2017 +0800
@@ -0,0 +1,28 @@
+{% extends "base.html" %}
+
+{% block title %}#{{ reference.ordinal }} and #{{ check.ordinal }}{% end %}
+
+{% block content %}
+  <div class="uk-container uk-container-center uk-margin-large-top">
+    <h2>
+      {% module Template('ui/project-link.html', project=project) %}
+      <span class="uk-text-nowrap">
+        · comparing <a href="{{ project.get_url() }}/{{ reference.ordinal }}">#{{ reference.ordinal }}</a>
+        and <a href="{{ project.get_url() }}/{{ check.ordinal }}">#{{ check.ordinal }}</a>
+      </span>
+    </h2>
+    {% if check.change.branch != reference.change.branch %}
+      <div class="uk-alert">
+        Checks are for different branches: {{ check.change.branch }} and {{ reference.change.branch }}.
+      </div>
+    {% end %}
+    {% if check.change_id == reference.change_id %}
+      <div class="uk-alert">
+        Both checks are for the same commit:
+        {% set change = check.change %}
+        <a href="{{ adapter.get_commit_url(change) }}">{% if change.rev is not None %}{{ change.rev }}:{% end %}{{ change.node[:12] }}</a>.
+      </div>
+    {% end %}
+    {% include ui/check-log.html %}
+  </div>
+{% end %}

--- a/tests/test_viewer.py	Sat Sep 16 17:37:38 2017 +0800
+++ b/tests/test_viewer.py	Sat Sep 16 17:41:43 2017 +0800
@@ -26,6 +26,16 @@
             author='alice',
             message='component: do a thing',
             project=project)
+        Check.create(
+            ordinal=Check.get_next_ordinal(project),
+            errors=0,
+            warnings=0,
+            lines='[]',
+            success=True,
+            started=datetime.fromtimestamp(0),
+            finished=datetime.fromtimestamp(0),
+            change=change,
+            project=project)
         lines = [{
             'text': '# C&O job started: 2016-08-20T02:38:06+00:00',
             'cls': 'meta'
@@ -120,9 +130,10 @@
         ns = {'atom': 'http://www.w3.org/2005/Atom'}
         title = root.find('./atom:entry/atom:title', ns)
         assert title is not None
-        assert title.text == 'Check #1: 1 error, 1 warning'
+        assert title.text.startswith('Check #')
+        assert title.text.endswith(' 1 error, 1 warning')
         updates = [el.text for el in root.findall('.//atom:updated', ns)]
-        assert len(updates) == 2
+        assert len(updates) == min(Check.select().count(), 20) + 1
         assert updates[0] == updates[1] == '2016-08-20T02:38:53Z'
 
     def test_check(self):
@@ -148,6 +159,18 @@
         assert response.headers['Content-Type'] == 'text/plain; charset=utf-8'
         assert 'test.py' in response.body
 
+    def test_compare(self):
+        base_url = '/example.com/alice/test-viewer/latest'
+        response = self.fetch(base_url + '/compare/latest')
+        assert response.code == 200
+        assert 'same commit' in response.body
+        assert 'Diff is empty' in response.body
+
+        response = self.fetch(base_url + '/compare/1')
+        assert response.code == 200
+        assert 'same commit' in response.body
+        assert 'E501' in response.body
+
     def test_status(self):
         response = self.fetch('/butt.cloud/cyber/wizard-attack/status.svg')
         assert response.code == 404

--- a/viewer.py	Sat Sep 16 17:37:38 2017 +0800
+++ b/viewer.py	Sat Sep 16 17:41:43 2017 +0800
@@ -20,6 +20,7 @@
             (project_re, h.ProjectHandler),
             (project_re + r'/atom', h.AtomHandler),
             (project_re + r'/(\d+|latest)(?:/(raw))?', h.CheckHandler),
+            (project_re + r'/(\d+|latest)/compare/(\d+|latest)', h.CompareHandler),
             (project_re + r'/status\.svg', h.StatusHandler),
             (r'.*', h.ErrorHandler, {'status_code': 404})
         ]