Use downloaded domains list to back available API

src/api/views.py

@@ -6,6 +6,41 @@ from django.http import JsonResponse
 
 from django.contrib.auth.decorators import login_required
 
+import requests
+
+from cachetools.func import ttl_cache
+
+DOMAIN_FILE_URL = "https://raw.githubusercontent.com/cisagov/dotgov-data/main/current-full.csv"
+
+
+# this file doesn't change that often, nor is it that big, so cache the result
+# in memory for ten minutes
+@ttl_cache(ttl=600)
+def _domains():
+    """Return a list of the current .gov domains.
+
+    Fetch a file from DOMAIN_FILE_URL, parse the CSV for the domain,
+    lowercase everything and return the list.
+    """
+    file_contents = requests.get(DOMAIN_FILE_URL).text
+    domains = set()
+    # skip the first line
+    for line in file_contents.splitlines()[1:]:
+        # get the domain before the first comma
+        domain = line.split(",", 1)[0]
+        # lowercase everything
+        domains.add(domain.lower())
+    return domains
+
+
+def in_domains(domain):
+    """Return true if the given domain is in the domains list.
+
+    The given domain is lowercased to match against the domains list.
+    """
+    return domain.lower() in _domains()
+
+
 @require_http_methods(["GET"])
 @login_required
 def available(request, domain=""):
@@ -15,5 +50,6 @@ def available(request, domain=""):
     Response is a JSON dictionary with the key "available" and value true or
     false.
     """
-    return JsonResponse({"available": False})
+    # a domain is available if it is NOT in the list of current domains
+    return JsonResponse({"available": not in_domains(domain)})