Add an endpoint to count the length of templates

Users sending text messages are sometimes unaware that long messages
will cost more.

Users sending broadcast messages need to be aware that there’s a
character limit, so they can take this into account when planning their
messages.

This commit adds an endpoint which counts the number of characters in
some template content, and returns a snippet of useful info about how
long the message is.

In subsequent commits we’ll be able to use AJAX to fetch this snippet as
the user types.

There’s a surprising amount of complexity in counting the length of
messages. So we’ll need to do this in Python because it would be too
convoluted to re-implement the length counting in client side code, let
alone ensuring it had parity with its Python equivalent.

tests/app/main/views/test_templates.py

@@ -1,7 +1,9 @@
+import json
 from functools import partial
 from unittest.mock import ANY, Mock
 
 import pytest
+from bs4 import BeautifulSoup
 from flask import url_for
 from freezegun import freeze_time
 from notifications_python_client.errors import HTTPError
@@ -2543,3 +2545,181 @@ def test_should_not_create_broadcast_template_with_placeholders(
         'You can’t use ((double brackets)) to personalise this message'
     )
     assert mock_create_service_template.called is False
+
+
+@pytest.mark.parametrize(
+    'template_type, prefix_sms, content, expected_message, expected_class', (
+        (
+            'sms', False, '',
+            '',
+            None,
+        ),
+        (
+            'sms', False, 'a' * 160,
+            'Will be charged as 1 text message',
+            None,
+        ),
+        (
+            'sms', False, 'a' * 161,
+            'Will be charged as 2 text messages',
+            None,
+        ),
+        (
+            # service name takes 13 characters, 147 + 13 = 160
+            'sms', True, 'a' * 147,
+            'Will be charged as 1 text message',
+            None,
+        ),
+        (
+            # service name takes 13 characters, 148 + 13 = 161
+            'sms', True, 'a' * 148,
+            'Will be charged as 2 text messages',
+            None,
+        ),
+        (
+            'sms', False, 'a' * 918,
+            'Will be charged as 6 text messages',
+            None,
+        ),
+        (
+            # Service name increases fragment count but doesn’t count
+            # against total character limit
+            'sms', True, 'a' * 918,
+            'Will be charged as 7 text messages',
+            None,
+        ),
+        (
+            # Can’t make a 7 fragment text template from content alone
+            'sms', False, 'a' * 919,
+            'You have 1 character too many',
+            'govuk-error-message',
+        ),
+        (
+            # Service name increases content count but character count
+            # is based on content alone
+            'sms', True, 'a' * 919,
+            'You have 1 character too many',
+            'govuk-error-message',
+        ),
+        (
+            # Service name increases content count but character count
+            # is based on content alone
+            'sms', True, 'a' * 920,
+            'You have 2 characters too many',
+            'govuk-error-message',
+        ),
+        (
+            'sms', False, 'Ẅ' * 70,
+            'Will be charged as 1 text message',
+            None,
+        ),
+        (
+            'sms', False, 'Ẅ' * 71,
+            'Will be charged as 2 text messages',
+            None,
+        ),
+        (
+            'sms', False, 'Ẅ' * 918,
+            'Will be charged as 14 text messages',
+            None,
+        ),
+        (
+            'sms', False, 'Ẅ' * 919,
+            'You have 1 character too many',
+            'govuk-error-message',
+        ),
+        (
+            'sms', False, 'Hello ((name))',
+            'Will be charged as 1 text message (not including personalisation)',
+            None,
+        ),
+        (
+            # Length of placeholder body doesn’t count towards fragment count
+            'sms', False, f'Hello (( {"a" * 999} ))',
+            'Will be charged as 1 text message (not including personalisation)',
+            None,
+        ),
+        (
+            'broadcast', False, '',
+            '',
+            None,
+        ),
+        (
+            'broadcast', False, 'a',
+            'You have 1,394 characters remaining',
+            None,
+        ),
+        (
+            'broadcast', False, 'a' * 1395,
+            'You have 0 characters remaining',
+            None,
+        ),
+        (
+            'broadcast', False, 'a' * 1396,
+            'You have 1 character too many',
+            'govuk-error-message',
+        ),
+        (
+            'broadcast', False, 'a' * 1397,
+            'You have 2 characters too many',
+            'govuk-error-message',
+        ),
+        (
+            'broadcast', False, 'Ẅ' * 615,
+            'You have 0 characters remaining',
+            None,
+        ),
+        (
+            'broadcast', False, 'Ẅ' * 616,
+            'You have 1 character too many',
+            'govuk-error-message',
+        ),
+    ),
+)
+def test_content_count_json_endpoint(
+    logged_in_client,
+    service_one,
+    template_type,
+    prefix_sms,
+    content,
+    expected_message,
+    expected_class,
+):
+    service_one['prefix_sms'] = prefix_sms
+    response = logged_in_client.post(
+        url_for(
+            'main.count_content_length',
+            service_id=SERVICE_ONE_ID,
+            template_type=template_type,
+        ),
+        data={
+            'template_content': content,
+        },
+    )
+    assert response.status_code == 200
+
+    html = json.loads(response.get_data(as_text=True))['html']
+    snippet = BeautifulSoup(html, 'html.parser').select_one('span')
+
+    assert normalize_spaces(snippet.text) == expected_message
+
+    if snippet.has_attr('class'):
+        assert snippet['class'] == [expected_class]
+    else:
+        assert expected_class is None
+
+
+@pytest.mark.parametrize('template_type', (
+    'email', 'letter', 'banana',
+))
+def test_content_count_json_endpoint_for_unsupported_template_types(
+    client_request,
+    template_type,
+):
+    client_request.post(
+        'main.count_content_length',
+        service_id=SERVICE_ONE_ID,
+        template_type=template_type,
+        content='foo',
+        _expected_status=404,
+    )