Send alert emails for particular rocket launches

Closes: #156

update.py

@@ -2,9 +2,11 @@
 """Combined update script for various data sources."""
 
 import asyncio
+import json
 import os
 import smtplib
 import sys
+import typing
 from datetime import date, datetime
 from email.message import EmailMessage
 from email.utils import formatdate, make_msgid
@@ -19,6 +21,7 @@ import agenda.types
 import agenda.uk_holiday
 import agenda.waste_schedule
 from agenda import gwr
+from agenda.types import StrDict
 from web_view import app
 
 
@@ -95,15 +98,57 @@ Agenda: https://edwardbetts.com/agenda/
     send_mail(config, subject, body)
 
 
+def report_space_launch_change(
+    config: flask.config.Config, prev_launch: StrDict, cur_launch: StrDict
+) -> None:
+    """Send mail to announce change to space launch data."""
+    subject = f'Change to {cur_launch["name"]}'
+
+    body = f"""
+A space launch of interest was updated.
+
+Get ready for two big piles of JSON.
+
+Old launch data
+{json.dumps(prev_launch, indent=2)}
+
+New launch data
+{json.dumps(cur_launch, indent=2)}
+    """
+
+    send_mail(config, subject, body)
+
+
+def get_launch_by_slug(data: StrDict, slug: str) -> StrDict:
+    """Find last update for space launch."""
+    return {item["slug"]: typing.cast(StrDict, item) for item in data["results"]}[slug]
+
+
 def update_thespacedevs(config: flask.config.Config) -> None:
     """Update cache of space launch API."""
     rocket_dir = os.path.join(config["DATA_DIR"], "thespacedevs")
 
+    existing_data = agenda.thespacedevs.load_cached_launches(rocket_dir)
+    prev_launches = {
+        slug: get_launch_by_slug(existing_data, slug)
+        for slug in config["FOLLOW_LAUNCHES"]
+    }
+
     t0 = time()
-    rockets = agenda.thespacedevs.next_launch_api(rocket_dir)
+    data = agenda.thespacedevs.next_launch_api_data(rocket_dir)
+    cur_launches = {
+        slug: get_launch_by_slug(data, slug) for slug in config["FOLLOW_LAUNCHES"]
+    }
+
+    for slug in config["FOLLOW_LAUNCHES"]:
+        prev, cur = prev_launches[slug], cur_launches[slug]
+        if prev["last_updated"] != cur["last_updated"]:
+            report_space_launch_change(config, prev, cur)
+
     time_taken = time() - t0
     if not sys.stdin.isatty():
         return
+    rockets = [agenda.thespacedevs.summarize_launch(item) for item in data["results"]]
     print(len(rockets), "launches")
     print(f"took {time_taken:.1f} seconds")