feat: TUIs through ex_ratatui

.gitignore

@@ -23,4 +23,4 @@ erl_crash.dump
 .mise.local.toml
 
 # ignore macOS files
-.DS_Store
+.DS_Store

config/runtime.exs

@@ -0,0 +1,34 @@
+import Config
+
+# Register the live system monitor TUI as a `nerves_ssh` subsystem so
+# you can drop into a full-color dashboard from your laptop without
+# disturbing the badge's e-ink screen:
+#
+#     ssh -t nerves@wisteria.local -s Elixir.NameBadge.ExRatatui.SystemMonitorTui
+#
+# Plain `ssh nerves@wisteria.local` still gives you the regular IEx
+# prompt — the subsystem only kicks in when you pass `-s`.
+#
+# This config lives in `runtime.exs` on purpose: `ExRatatui.SSH.subsystem/1`
+# is a normal function call, and on a fresh `MIX_TARGET=trellis mix
+# compile` the compile-time config files (`config.exs`, `target.exs`)
+# run before Mix has compiled deps for the target — `ExRatatui.SSH`
+# isn't on the code path yet, so calling it there crashes with
+# `module ExRatatui.SSH is not available`.
+#
+# `runtime.exs` is evaluated on device boot, after every beam file in
+# the release is loaded but before the OTP application controller
+# starts `:nerves_ssh`, so the config it writes is in place by the
+# time the daemon reads it. Standard Elixir release pattern for any
+# config that can't be a pure data literal.
+#
+# On host builds (`MIX_TARGET=host mix run`, tests, etc.) this file
+# still runs but it's harmless: `:nerves_ssh` isn't a host dep, so no
+# one ever reads the env key we're writing.
+if Application.spec(:nerves_ssh) do
+  config :nerves_ssh,
+    subsystems: [
+      :ssh_sftpd.subsystem_spec(cwd: ~c"/"),
+      ExRatatui.SSH.subsystem(NameBadge.ExRatatui.SystemMonitorTui)
+    ]
+end

config/target.exs

@@ -22,7 +22,17 @@ config :nerves, :erlinit, update_clock: true
 # set tzdata dir for Nerves device
 config :tzdata, :data_dir, "/data/tzdata"
 
-# Configure the device for SSH IEx prompt access and firmware updates
+# Configure the device for SSH IEx prompt access and firmware updates.
+#
+#   ssh nerves@wisteria.local                        # IEx shell
+#   cat name_badge.fw | ssh -s nerves@... fwup        # via fwup subsystem
+#   ssh -t nerves@wisteria.local \                   # live system monitor TUI
+#     -s Elixir.NameBadge.ExRatatui.SystemMonitorTui
+#
+# The TUI subsystem is registered in `config/runtime.exs` because
+# `ExRatatui.SSH.subsystem/1` is a function call and target deps
+# aren't compiled when this file is evaluated. Authorized keys, by
+# contrast, are baked in at build time so you never need a password.
 #
 # * See https://hexdocs.pm/nerves_ssh/readme.html for general SSH configuration
 # * See https://hexdocs.pm/ssh_subsystem_fwup/readme.html for firmware updates

lib/name_badge/battery.ex

@@ -18,10 +18,11 @@ defmodule NameBadge.Battery do
     min_voltage = 3.0
     max_voltage = 4.2
 
-    percentage = ((v - min_voltage) / (max_voltage - min_voltage) * 100)
-    |> max(0)
-    |> min(100)
-    |> round()
+    percentage =
+      ((v - min_voltage) / (max_voltage - min_voltage) * 100)
+      |> max(0)
+      |> min(100)
+      |> round()
 
     percentage
   end