add GET /service/<id>/notifications/weekly

moved format_statistics to a new service/statistics.py file, and
refactored to share code. moved tests as well, to try and enforce
separation between the restful endpoints of rest.py and the logic/
data manipulation of statistics.py

app/service/statistics.py

@@ -0,0 +1,51 @@
+import itertools
+from datetime import datetime, timedelta
+
+from app.models import EMAIL_TYPE, SMS_TYPE
+
+
+def format_statistics(statistics):
+    # statistics come in a named tuple with uniqueness from 'notification_type', 'status' - however missing
+    # statuses/notification types won't be represented and the status types need to be simplified/summed up
+    # so we can return emails/sms * created, sent, and failed
+    counts = _create_zeroed_stats_dicts()
+    for row in statistics:
+        _update_statuses_from_row(counts[row.notification_type], row)
+
+    return counts
+
+
+def format_weekly_notification_stats(statistics, service_created_at):
+    preceeding_monday = service_created_at - timedelta(days=service_created_at.weekday())
+    week_dict = {
+        week: _create_zeroed_stats_dicts()
+        for week in _weeks_for_range(preceeding_monday, datetime.utcnow())
+    }
+    for row in statistics:
+        _update_statuses_from_row(week_dict[row.week_start][row.notification_type], row)
+
+    return week_dict
+
+
+def _create_zeroed_stats_dicts():
+    return {
+        template_type: {
+            status: 0 for status in ('requested', 'delivered', 'failed')
+        } for template_type in (EMAIL_TYPE, SMS_TYPE)
+    }
+
+
+def _update_statuses_from_row(update_dict, row):
+    update_dict['requested'] += row.count
+    if row.status == 'delivered':
+        update_dict['delivered'] += row.count
+    elif row.status in ('failed', 'technical-failure', 'temporary-failure', 'permanent-failure'):
+        update_dict['failed'] += row.count
+
+
+def _weeks_for_range(start, end):
+    """
+    Generator that yields dates from `start` to `end`, in 7 day intervals. End is inclusive.
+    """
+    infinite_date_generator = (start + timedelta(days=i) for i in itertools.count(step=7))
+    return itertools.takewhile(lambda x: x <= end, infinite_date_generator)