Pure functions, no state and no hardware, because this is the part that has to
be right and the cheapest to check.
Analog and blended: how fast the arm moves depends on how far the stick is
pushed, and several axes move at once. That is the whole reason a pad beats the
web console's four-sector dial pad, which can only ask for one axis at a time at
one fixed speed.
The deadzone rescales what is left of the travel rather than just clipping it,
so the arm starts from zero just outside the band instead of jumping to a
fraction of full speed the instant the stick leaves it. Expo bends the response
because fine positioning happens near centre, where a linear stick gives almost
no resolution.
Up on the stick sends the arm along +X, matching the console's dial, so the two
agree about which way is which.
The reader owns the pad on a thread of its own, and nothing outside that file
may touch pygame: everything else sees a plain immutable Sample. That is what
lets the rest of the node be tested with no hardware, and what keeps SDL's
quirks out of the arm logic.
Sample.present is the field that matters. A pad that has been unplugged reads as
absent rather than as centred, so "the operator let go" and "the pad fell off
the desk" stay distinguishable — they call for the same stop, but only one of
them is worth logging.
Only the display and joystick subsystems are initialised. pygame.init() also
opens audio, which on a headless robot fills stderr with ALSA errors for a
device this node never uses; the display is needed despite there being no
window, because SDL refreshes joystick state during the event pump.
Settings are parsed once at startup so a deadzone that would silently make the
pad inert stops the node before it joins the dataflow.
Ignore rules, markdown lint configuration and the pointer to AGENTS.md, so the
node itself arrives as a reviewable diff rather than as an initial import.
cristhian