{ lib, ... }:
rec {
# PRIMARY API: generate a OPML file from a list of feeds
feedsToOpml = feeds: opmlTopLevel (opmlGroups (partitionByCat feeds));
# only keep feeds whose category is one of the provided
filterByFormat = fmts: builtins.filter (feed: builtins.elem feed.format fmts);
## INTERNAL APIS
# transform a list of feeds into an attrs mapping cat => [ feed0 feed1 ... ]
partitionByCat = feeds: builtins.groupBy (f: f.cat) feeds;
xmlTag = tag: close: attrs:
let
fmt-attrs = builtins.concatStringsSep
" "
(lib.mapAttrsToList (name: value: ''${lib.escapeXML name}="${lib.escapeXML value}"'') attrs);
fmt-close = if close then "/" else "";
in
''<${tag} ${fmt-attrs} ${fmt-close}>'';
# represents a single RSS feed.
opmlTerminal = feed: xmlTag "outline" true (
{
xmlUrl = feed.url;
type = "rss";
} // lib.optionalAttrs (feed.title != null) {
title = feed.title;
}
);
# a list of RSS feeds.
opmlTerminals = feeds: lib.concatStringsSep "\n" (builtins.map opmlTerminal feeds);
# one node which packages some flat grouping of terminals.
opmlGroup = title: feeds: ''
${opmlTerminals feeds}
'';
# a list of groups (`groupMap` is an attrs mapping groupName => [ feed0 feed1 ... ]).
opmlGroups = groupMap: lib.concatStringsSep "\n" (
builtins.attrValues (builtins.mapAttrs opmlGroup groupMap)
);
# top-level OPML file which could be consumed by something else.
opmlTopLevel = body: ''
${body}
'';
}