Show international letters on the usage page

The api returns letter details split by postage, so international
letters are returned with a postage of `europe` or `rest-of-world` not
`international` and these rows need to be added together when the rate
is the same before they are displayed on the usage page.

To do this, we need to replace the postage of `europe` and
`rest-of-world` with `international`. The data then needs to be sorted
by postage and rate before the letter units for rows which are
international and have the same rate are added together.

app/main/views/dashboard.py

@@ -3,6 +3,7 @@ from collections import namedtuple
 from datetime import datetime
 from functools import partial
 from itertools import groupby
+from operator import itemgetter
 
 from flask import (
     Response,
@@ -418,16 +419,8 @@ def get_free_paid_breakdown_for_billable_units(year, free_sms_fragment_limit, bi
             [billing_month for billing_month in sms_units if billing_month['month'] == month]
         )
 
-        LetterDetails = namedtuple('LetterDetails', ['billing_units', 'rate', 'cost', 'postage'])
-
-        letter_billing = [LetterDetails(billing_units=x['billing_units'],
-                                        rate=x['rate'],
-                                        cost=(x['billing_units'] * x['rate']),
-                                        postage=x['postage'])
-                          for x in letter_units if x['month'] == month]
-
-        if letter_billing:
-            letter_billing.sort(key=lambda x: (x.postage, x.rate))
+        letter_units_for_month = [x for x in letter_units if x['month'] == month]
+        letter_billing = format_letter_details_for_month(letter_units_for_month)
 
         letter_total = 0
         for x in letter_billing:
@@ -443,6 +436,49 @@ def get_free_paid_breakdown_for_billable_units(year, free_sms_fragment_limit, bi
         }
 
 
+def format_letter_details_for_month(letter_units_for_month):
+    # Format postage descriptions in letter units e.g. to 'international' not 'europe'
+    for month in letter_units_for_month:
+        for k, v in month.items():
+            if k == 'postage':
+                month[k] = get_postage_description(v)
+
+    # letter_units_for_month must be sorted before international postage values can be aggregated
+    postage_order = {'first class': 0, 'second class': 1, 'international': 2}
+    letter_units_for_month.sort(key=lambda x: (postage_order[x['postage']], x['rate']))
+
+    LetterDetails = namedtuple('LetterDetails', ['billing_units', 'rate', 'cost', 'postage_description'])
+
+    # Aggregate the rows for international letters which have the same rate
+    result = []
+    for _key, rate_group in groupby(letter_units_for_month, key=itemgetter('postage', 'rate')):
+        rate_group = list(rate_group)
+
+        letter_details = LetterDetails(
+            billing_units=sum(x['billing_units'] for x in rate_group),
+            rate=format_letter_rate(rate_group[0]['rate']),
+            cost=(sum(x['billing_units'] for x in rate_group) * rate_group[0]['rate']),
+            postage_description=rate_group[0]['postage']
+        )
+
+        result.append(letter_details)
+
+    return result
+
+
+def get_postage_description(postage):
+    if postage in ('first', 'second'):
+        return f'{postage} class'
+    return 'international'
+
+
+def format_letter_rate(number):
+    if number >= 1:
+        return f"£{number:,.2f}"
+
+    return f"{number * 100:.0f}p"
+
+
 def get_free_paid_breakdown_for_month(
     free_sms_fragment_limit,
     cumulative,

app/templates/views/usage.html

@@ -109,8 +109,8 @@
               {% endif %}
               {% for letter in month.letters%}
                 {% if letter.billing_units %}
-                  <li class="tabular-numbers">{{ "{:,} {}".format(letter.billing_units, letter.postage)}} class {{ message_count_label(letter.billing_units, 'letter', '') }}at
-                  {{ '{:.0f}p'.format(letter.rate * 100) }}</li>
+                  <li class="tabular-numbers">{{ "{:,} {}".format(letter.billing_units, letter.postage_description) }} {{ message_count_label(letter.billing_units, 'letter', '') }}at
+                  {{ letter.rate }}</li>
                 {% endif %}
               {% endfor %}
               {% if not (month.free or month.paid or month.letters) %}