Add XDG applications directory scanner

Resolves the ordered list of "applications" directories to search per
the XDG Base Directory Specification: XDG_DATA_HOME (default
~/.local/share) first, then XDG_DATA_DIRS (default /usr/local/share,
/usr/share). Each variable falls back to its default independently
when unset or empty, so a Flatpak-style XDG_DATA_DIRS override is
honored in place of the default rather than merged with it.
find_desktop_files/0,1 recursively collects .desktop files from those
dirs, skipping ones that don't exist.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

1 files changed, 62 insertions, 0 deletions

diff --git a/src/er_xdg_pipe_menu_scanner.erl b/src/er_xdg_pipe_menu_scanner.erl
new file mode 100644
index 0000000..131b4f6
--- /dev/null
+++ b/src/er_xdg_pipe_menu_scanner.erl
@@ -0,0 +1,62 @@
+%%%-------------------------------------------------------------------
+%% @doc Locates .desktop files under the XDG application directories.
+%%
+%% Directory precedence follows the XDG Base Directory Specification:
+%% the user's data home (most specific, so its entries should win over
+%% system ones downstream) is searched first, then each directory in
+%% `XDG_DATA_DIRS'. Each of `XDG_DATA_HOME' and `XDG_DATA_DIRS' falls
+%% back to its spec-defined default independently when unset or empty
+%% -- so a Flatpak-aware `XDG_DATA_DIRS' (which typically adds
+%% `~/.local/share/flatpak/exports/share') is honored in place of the
+%% default rather than merged with it.
+%% @end
+%%%-------------------------------------------------------------------
+
+-module(er_xdg_pipe_menu_scanner).
+
+-export([data_dirs/0, find_desktop_files/0, find_desktop_files/1]).
+
+-define(DEFAULT_DATA_DIRS, ["/usr/local/share", "/usr/share"]).
+
+-spec data_dirs() -> [file:filename_all()].
+data_dirs() ->
+    [filename:join(Dir, "applications") || Dir <- data_home() ++ data_dirs_list()].
+
+-spec find_desktop_files() -> [file:filename_all()].
+find_desktop_files() ->
+    find_desktop_files(data_dirs()).
+
+-spec find_desktop_files([file:filename_all()]) -> [file:filename_all()].
+find_desktop_files(Dirs) ->
+    lists:flatmap(fun find_in_dir/1, Dirs).
+
+find_in_dir(Dir) ->
+    case filelib:is_dir(Dir) of
+        true -> filelib:wildcard(filename:join(Dir, "**/*.desktop"));
+        false -> []
+    end.
+
+data_home() ->
+    case env("XDG_DATA_HOME") of
+        undefined ->
+            case env("HOME") of
+                undefined -> [];
+                Home -> [filename:join([Home, ".local", "share"])]
+            end;
+        Dir ->
+            [Dir]
+    end.
+
+data_dirs_list() ->
+    case env("XDG_DATA_DIRS") of
+        undefined -> ?DEFAULT_DATA_DIRS;
+        Value -> [D || D <- string:split(Value, ":", all), D =/= ""]
+    end.
+
+%% The spec treats an unset and an empty variable the same way.
+env(Name) ->
+    case os:getenv(Name) of
+        false -> undefined;
+        "" -> undefined;
+        Value -> Value
+    end.