My Hammerspoon init file that provides window resizing to thirds of the screen

init.lua

hs.hotkey.bind({"cmd", "alt", "ctrl"}, "Left", function()
    local win = hs.window.focusedWindow()
    local currentWindow = win:frame()
    local screen = win:screen()
    local screenFrame = screen:frame()

    local half = screenFrame.w / 2
    local third = screenFrame.w / 3
    local twothirds = screenFrame.w - third

    if currentWindow.x == screenFrame.x and currentWindow.w == half then
        currentWindow.w = third
    elseif currentWindow.x == screenFrame.x and currentWindow.w <= third then
        currentWindow.w = twothirds
    else
        -- default action is to go half left
        currentWindow.w = half
    end

    -- move window anchor to the origin of the screen
    currentWindow.x = screenFrame.x
    currentWindow.y = screenFrame.y

    -- match height of the screen 
    currentWindow.h = screenFrame.h

    win:setFrame(currentWindow)
end)

hs.hotkey.bind({"cmd", "alt", "ctrl"}, "Right", function()
    local win = hs.window.focusedWindow()
    local currentWindow = win:frame()
    local screen = win:screen()
    local screenFrame = screen:frame()

    local half = screenFrame.w / 2
    local third = screenFrame.w / 3
    local twothirds = screenFrame.w - third

    if currentWindow.w == half and currentWindow.x == screenFrame.x + half then
        currentWindow.w = third
        currentWindow.x = screenFrame.x + twothirds
    elseif currentWindow.w <= third and currentWindow.x >=
        (screenFrame.x + twothirds - 5) then
        -- have to give a small margin of error due to rounding
        currentWindow.w = twothirds
        currentWindow.x = screenFrame.x + third
    else
        currentWindow.x = screenFrame.x + half
        currentWindow.w = half
    end

    currentWindow.y = screenFrame.y

    -- match height of the screen 
    currentWindow.h = screenFrame.h

    win:setFrame(currentWindow)
end)

hs.hotkey.bind({'alt', 'ctrl', 'cmd'}, 'Up', function()
    local win = hs.window.focusedWindow()
    local currentWindow = win:frame()
    local screen = win:screen()
    local screenFrame = screen:frame()

    local half = screenFrame.h / 2
    local third = screenFrame.h / 3
    local twothirds = screenFrame.h - third

    if currentWindow.h > half then
        currentWindow.h = half
    elseif currentWindow.h > third then
        currentWindow.h = third
    else
        currentWindow.h = twothirds
    end

    currentWindow.y = screenFrame.y

    win:setFrame(currentWindow)
end)

hs.hotkey.bind({'alt', 'ctrl', 'cmd'}, 'Down', function()
    local win = hs.window.focusedWindow()
    local currentWindow = win:frame()
    local screen = win:screen()
    local screenFrame = screen:frame()

    local half = screenFrame.h / 2
    local third = screenFrame.h / 3
    local twothirds = screenFrame.h - third

    if currentWindow.h > half then
        currentWindow.y = screenFrame.y + half
        currentWindow.h = half
    elseif currentWindow.h > third then
        currentWindow.y = screenFrame.y + twothirds
        currentWindow.h = third
    else
        currentWindow.y = screenFrame.y + third
        currentWindow.h = twothirds
    end

    win:setFrame(currentWindow)
end)

hs.hotkey.bind({'alt', 'ctrl', 'cmd'}, 'n', function()
    -- get the focused window
    local win = hs.window.focusedWindow()
    -- get the screen where the focused window is displayed, a.k.a. current screen
    local screen = win:screen()
    -- compute the unitRect of the focused window relative to the current screen
    -- and move the window to the next screen setting the same unitRect 
    win:move(win:frame():toUnitRect(screen:frame()), screen:next(), true, 0)
end)

hs.hotkey.bind({'alt', 'ctrl', 'cmd'}, 'm', function()
    -- maximise to screen
    local win = hs.window.focusedWindow()
    local f = win:frame()
    local screen = win:screen()
    local max = screen:frame()

    f.x = max.x
    f.y = max.y
    f.w = max.w
    f.h = max.h
    win:setFrame(f)
end)

hs.hotkey.bind({'alt', 'ctrl', 'cmd'}, 'c', function()
    -- resize to 60% of screen, and position it in the center
    local win = hs.window.focusedWindow()
    local f = win:frame()
    local screen = win:screen()
    local max = screen:frame()

    f.x = max.x
    f.y = max.y
    f.w = max.w * 0.6
    f.h = max.h * 0.9
    win:setFrame(f)
    win:centerOnScreen()
end)