A script to use the first knob on my novation keyboard to control my mac's volume

volume.lua

-- This is a hammerspoon (http://www.hammerspoon.org/) script to hook the 
-- volume knob on my keyboard up to my system's volume control

-- https://easings.net/ is great for easing functions
-- I tried easeInExpo, but it was too sharp (return 2 ^ (10 * x - 10))
-- experimentation suggested cubic was the most pleasant. Linear was expectedly
-- awful
function easeIn(x)
    return x * x * x
end

function onMidiMessage(object, deviceName, commandType, description, metadata)
    -- hs.alert.show(deviceName .. commandType)
    if commandType == "controlChange" then
        -- print(hs.inspect(metadata))
        if metadata["controllerNumber"] == 21 then
            -- 16256 is the max value for reasons that are unclear to me
            -- it's the first seven bytes of a fourteen-bit number: 11111110000000
            -- I suppose I should try to understand midi or something
            local vol = easeIn(metadata["fourteenBitValue"] / 16256) * 100
            if vol < 0.00001 then
                print("muting")
                hs.audiodevice.defaultOutputDevice():setMuted(true)
            else
                print("volume "..vol)
                hs.audiodevice.defaultOutputDevice():setMuted(false)
                hs.audiodevice.defaultOutputDevice():setVolume(vol)
            end
        end
    elseif commandType == "noteOn" then
        -- This lets you mash any of the drum pad buttons to play/pause 
        -- spotify, could be more specific but why I guess
        if metadata["channel"] == 9 then
            -- print("play/pause")
            -- send a play/pause keypress. I got 205 from
            -- karabiner-eventviewer, but it seems like this isn't a legit
            -- keypress so mac just ignores it
            --
            -- hs.eventtap.event.newKeyEvent(205, true):post()
            -- hs.eventtap.event.newKeyEvent(205, false):post()
            -- 
            -- so, since computers are awful, just play/pause spotify. There is
            -- hs.itunes, I don't use that, and screw Apple.
            hs.spotify.playpause()
        end
    end
end

local devices = hs.midi.devices()
device = nil
for key, deviceName in pairs(devices) do
    if deviceName.find(deviceName, "Launchkey") then
        hs.alert.show(deviceName)
        device = hs.midi.new(deviceName)
        device:callback(onMidiMessage)
    end
end