chore(web/atom-feed): Move feed generator out of //users/tazjin

I intend to use this for updates on TVL projects, which will end up on
the homepage, which is outside of //users.

Change-Id: I03542d1bcef3d9fc4599294655caab5ed22ba5d9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3728
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

web/atom-feed/default.nix

@@ -0,0 +1,145 @@
+# This file defines functions for generating an Atom feed.
+
+{ depot, lib, pkgs, ... }:
+
+with depot.nix.yants;
+
+let
+  inherit (builtins) map readFile replaceStrings;
+  inherit (lib) concatStrings concatStringsSep removeSuffix;
+  inherit (pkgs) runCommandNoCC;
+
+  # 'link' describes a related link to a feed, or feed element.
+  #
+  # https://validator.w3.org/feed/docs/atom.html#link
+  link = struct "link" {
+    rel = string;
+    href = string;
+  };
+
+  # 'entry' describes a feed entry, for example a single post on a
+  # blog. Some optional fields have been omitted.
+  #
+  # https://validator.w3.org/feed/docs/atom.html#requiredEntryElements
+  entry = struct "entry" {
+    # Identifies the entry using a universally unique and permanent URI.
+    id = string;
+
+    # Contains a human readable title for the entry. This value should
+    # not be blank.
+    title = string;
+
+    # Content of the entry.
+    content = option string;
+
+    # Indicates the last time the entry was modified in a significant
+    # way (in seconds since epoch).
+    updated = int;
+
+    # Names authors of the entry. Recommended element.
+    authors = option (list string);
+
+    # Related web pages, such as the web location of a blog post.
+    links = option (list link);
+
+    # Conveys a short summary, abstract, or excerpt of the entry.
+    summary = option string;
+
+    # Contains the time of the initial creation or first availability
+    # of the entry.
+    published = option int;
+
+    # Conveys information about rights, e.g. copyrights, held in and
+    # over the entry.
+    rights = option string;
+  };
+
+  # 'feed' describes the metadata of the Atom feed itself.
+  #
+  # Some optional fields have been omitted.
+  #
+  # https://validator.w3.org/feed/docs/atom.html#requiredFeedElements
+  feed = struct "feed" {
+    # Identifies the feed using a universally unique and permanent URI.
+    id = string;
+
+    # Contains a human readable title for the feed.
+    title = string;
+
+    # Indicates the last time the feed was modified in a significant
+    # way (in seconds since epoch). Recommended element.
+    updated = int;
+
+    # Entries contained within the feed.
+    entries = list entry;
+
+    # Names authors of the feed. Recommended element.
+    authors = option (list string);
+
+    # Related web locations. Recommended element.
+    links = option (list link);
+
+    # Conveys information about rights, e.g. copyrights, held in and
+    # over the feed.
+    rights = option string;
+
+    # Contains a human-readable description or subtitle for the feed.
+    subtitle = option string;
+  };
+
+  # Feed generation functions:
+
+  renderEpoch = epoch: removeSuffix "\n" (readFile (runCommandNoCC "date-${toString epoch}" {} ''
+    date --date='@${toString epoch}' --utc --iso-8601='seconds' > $out
+  ''));
+
+  escape = replaceStrings [ "<" ">" "&" "'" ] [ "&lt;" "&gt;" "&amp;" "&#39;" ];
+
+  elem = name: content: ''<${name}>${escape content}</${name}>'';
+
+  renderLink = defun [ link string ] (l: ''
+    <link href="${escape l.href}" rel="${escape l.rel}" />
+  '');
+
+  # Technically the author element can also contain 'uri' and 'email'
+  # fields, but they are not used for the purpose of this feed and are
+  # omitted.
+  renderAuthor = author: ''<author><name>${escape author}</name></author>'';
+
+  renderEntry = defun [ entry string ] (e: ''
+    <entry>
+      ${elem "title" e.title}
+      ${elem "id" e.id}
+      ${elem "updated" (renderEpoch e.updated)}
+      ${if e ? published
+        then elem "published" (renderEpoch e.published)
+        else ""
+      }
+      ${if e ? content
+        then ''<content type="html">${escape e.content}</content>''
+        else ""
+      }
+      ${if e ? summary then elem "summary" e.summary else ""}
+      ${concatStrings (map renderAuthor (e.authors or []))}
+      ${if e ? subtitle then elem "subtitle" e.subtitle else ""}
+      ${if e ? rights then elem "rights" e.rights else ""}
+      ${concatStrings (map renderLink (e.links or []))}
+    </entry>
+  '');
+
+  renderFeed = defun [ feed string ] (f: ''
+    <?xml version="1.0" encoding="utf-8"?>
+    <feed xmlns="http://www.w3.org/2005/Atom">
+      ${elem "id" f.id}
+      ${elem "title" f.title}
+      ${elem "updated" (renderEpoch f.updated)}
+      ${concatStringsSep "\n" (map renderAuthor (f.authors or []))}
+      ${if f ? subtitle then elem "subtitle" f.subtitle else ""}
+      ${if f ? rights then elem "rights" f.rights else ""}
+      ${concatStrings (map renderLink (f.links or []))}
+      ${concatStrings (map renderEntry f.entries)}
+    </feed>
+  '');
+in {
+  inherit entry feed renderFeed renderEpoch;
+}