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.

app/main/views/templates.py

@@ -1,9 +1,17 @@
 from functools import partial
 
-from flask import abort, flash, redirect, render_template, request, url_for
+from flask import (
+    abort,
+    flash,
+    jsonify,
+    redirect,
+    render_template,
+    request,
+    url_for,
+)
 from flask_login import current_user
 from notifications_python_client.errors import HTTPError
-from notifications_utils import LETTER_MAX_PAGE_COUNT
+from notifications_utils import LETTER_MAX_PAGE_COUNT, SMS_CHAR_COUNT_LIMIT
 from notifications_utils.pdf import is_letter_too_long
 
 from app import (
@@ -14,6 +22,7 @@ from app import (
     template_folder_api_client,
     template_statistics_client,
 )
+from app.formatters import character_count, message_count
 from app.main import main, no_cookie
 from app.main.forms import (
     BroadcastTemplateForm,
@@ -644,6 +653,67 @@ def edit_service_template(service_id, template_id):
         )
 
 
+@main.route(
+    "/services/<uuid:service_id>/templates/count-<template_type:template_type>-length",
+    methods=['POST'],
+)
+@user_has_permissions()
+def count_content_length(service_id, template_type):
+    if template_type not in {'sms', 'broadcast'}:
+        abort(404)
+
+    error, message = _get_content_count_error_and_message_for_template(
+        get_template({
+            'template_type': template_type,
+            'content': request.form.get('template_content', ''),
+        }, current_service)
+    )
+
+    return jsonify({
+        'html': render_template(
+            'partials/templates/content-count-message.html',
+            error=error,
+            message=message,
+        )
+    })
+
+
+def _get_content_count_error_and_message_for_template(template):
+
+    if template.is_message_empty():
+        return False, ''
+
+    if template.template_type == 'sms':
+        if template.is_message_too_long():
+            return True, (
+                f'You have '
+                f'{character_count(template.content_count_without_prefix - SMS_CHAR_COUNT_LIMIT)} '
+                f'too many'
+            )
+        if template.placeholders:
+            return False, (
+                f'Will be charged as {message_count(template.fragment_count, template.template_type)} '
+                f'(not including personalisation)'
+            )
+        return False, (
+            f'Will be charged as {message_count(template.fragment_count, template.template_type)} '
+        )
+
+    if template.template_type == 'broadcast':
+        if template.content_too_long:
+            return True, (
+                f'You have '
+                f'{character_count(template.encoded_content_count - template.max_content_count)} '
+                f'too many'
+            )
+        else:
+            return False, (
+                f'You have '
+                f'{character_count(template.max_content_count - template.encoded_content_count)} '
+                f'remaining'
+            )
+
+
 @main.route("/services/<uuid:service_id>/templates/<uuid:template_id>/delete", methods=['GET', 'POST'])
 @user_has_permissions('manage_templates')
 def delete_service_template(service_id, template_id):