patch updated date to preserve determinism

flake.nix

@@ -75,6 +75,8 @@
               };
             };
 
+
+
           website = pkgs.stdenv.mkDerivation
             (
               let
@@ -83,6 +85,13 @@
                   name = "static";
                   gemdir = ./src;
                 };
+
+                patch-feed-date = pkgs.stdenv.mkDerivation {
+                  name = "patch-feed-date";
+                  propagatedBuildInputs = [ pkgs.python3 ];
+                  dontUnpack = true;
+                  installPhase = "install -Dm755 ${./patch-feed-date.py} $out/bin/patch-feed-date";
+                };
               in
               {
                 version = "0.0.1";
@@ -93,6 +102,7 @@
                 buildInputs = [
                   gems
                   gems.wrappedRuby
+                  patch-feed-date
                 ];
 
                 sourceRoot = "src";
@@ -104,6 +114,7 @@
                 installPhase = ''
                   mkdir -p $out
                   cp -r _site/* $out/
+                  patch-feed-date --file _site/feed.xml > $out/feed.xml
                 '';
               }
             );

patch-feed-date.py

@@ -0,0 +1,53 @@
+#!/usr/bin/env python3
+
+import argparse
+import re
+from datetime import datetime
+
+updated_pattern = r"<updated>(?P<datetime>.{25})</updated>"
+datetime_pattern = "%Y-%m-%dT%H:%M:%S%z"
+
+
+def get_post_datetimes(feed):
+    regex = re.compile(updated_pattern)
+
+    datetimes = list()
+    first_skipped = False
+
+    for match in regex.finditer(feed):
+        # Skip first found datetime, as this is what we want to replace.
+        if not first_skipped:
+            first_skipped = True
+            continue
+        dt = datetime.strptime(match.group("datetime"), datetime_pattern)
+        datetimes.append(dt)
+
+    return datetimes
+
+
+def main():
+    parser = argparse.ArgumentParser(
+        description="Patch the updated date field on an Atom feed."
+    )
+
+    parser.add_argument(
+        "--file", "-f", type=str, required=True, help="Path to the Atom feed file."
+    )
+
+    args = parser.parse_args()
+    with open(args.file, "r") as f:
+        feed = f.read()
+
+    datetimes = get_post_datetimes(feed)
+    newest_datetime = max(datetimes)
+
+    print(
+        re.sub(
+            updated_pattern, newest_datetime.strftime(datetime_pattern), feed, count=1
+        ),
+        end="",
+    )
+
+
+if __name__ == "__main__":
+    main()