feat(nix): flake + home manager module

flake.nix

@@ -0,0 +1,29 @@
+{
+  inputs = {
+    nixpkgs.url = "nixpkgs/nixos-unstable";
+    systems.url = "github:nix-systems/default-linux";
+  };
+
+  outputs = inputs @ {
+    self,
+    nixpkgs,
+    systems,
+    ...
+  }: let
+    inherit (nixpkgs) lib;
+    eachSystem = lib.genAttrs (import systems);
+    pkgsFor = eachSystem (system:
+      import nixpkgs {
+        localSystem = system;
+      });
+  in {
+    packages = eachSystem (system: let
+      pkgs = pkgsFor.${system};
+    in {
+      default = self.packages.${system}.battery-notifier;
+      battery-notifier = pkgs.callPackage ./default.nix {};
+    });
+
+    homeManagerModule.default = import ./nix/hm-module.nix self;
+  };
+}

nix/hm-module.nix

@@ -0,0 +1,90 @@
+self: {
+  config,
+  pkgs,
+  lib,
+  ...
+}:
+with lib; let
+  inherit (pkgs.stdenv.hostPlatform) system;
+  tomlFormat = pkgs.formats.toml {};
+  flake-pkgs = self.packages.${system};
+in {
+  options.programs.battery-notifier = let
+    settingsModule = types.submodule {
+      options = {
+        interval_ms = mkOption {
+          type = types.int;
+          default = 700;
+        };
+
+        reminder_threshold = mkOption {
+          type = types.int;
+          default = 30;
+        };
+
+        warn_threshold = mkOption {
+          type = types.int;
+          default = 15;
+        };
+
+        threat_threshold = mkOption {
+          type = types.int;
+          default = 5;
+        };
+      };
+    };
+  in {
+    enable = mkEnableOption "battery-notifier";
+
+    settings = mkOption {
+      default = null;
+      type = types.nullOr settingsModule;
+    };
+  };
+
+  config = let
+    cfg = config.programs.battery-notifier;
+  in
+    mkIf cfg.enable {
+      assertions = mkIf (cfg.settings != null) [
+        {
+          assertion = builtins.length (lib.attrsets.attrValues (lib.attrsets.filterAttrs (k: v: lib.strings.hasSuffix k "threshold" && v >= 0 && v <= 100) cfg.settings)) == 0;
+          message = "threshold values must be greater equal than 0 and less equal than 100";
+        }
+        {
+          assertion = cfg.settings.reminder_threshold > cfg.settings.warn_threshold;
+          message = "'reminder' threshold must be greater than 'warn' threshold";
+        }
+        {
+          assertion = cfg.settings.warn_threshold > cfg.settings.threat_threshold;
+          message = "'warn' threshold must be greater than 'threat' threshold";
+        }
+        {
+          assertion = cfg.settings.sleep_ms > 0;
+          message = "sleep time must be greater than zero";
+        }
+      ];
+
+      xdg.configFile = mkIf (cfg.settings != null) {
+        "battery-notifier/config.toml".source = tomlFormat.generate "battery-notifier-config" cfg.settings;
+      };
+
+      systemd.user.services = {
+        battery-notifier = {
+          Unit = {
+            Description = "A very useful battery notifier for window managers";
+          };
+
+          Service = {
+            Type = "simple";
+            ExecStart = "${flake-pkgs.battery-notifier}/bin/battery-notifier";
+            Restart = "on-failure";
+          };
+
+          Install = {
+            WantedBy = ["default.target"];
+          };
+        };
+      };
+    };
+}