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
        -- is in left half, shrink to left third 
        currentWindow.w = third
    elseif currentWindow.x == screenFrame.x and currentWindow.w < half then
        -- is in left third, expand to left twothirds
        currentWindow.w = twothirds
    else
        -- default is to be half screen
        currentWindow.w = half
    end

    currentWindow.x = screenFrame.x -- push to the left fo the screen
    currentWindow.y = screenFrame.y
    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.x - screenFrame.x == half and currentWindow.w == half then
        -- is in right half mode, shrink to right third
        currentWindow.x = screenFrame.x + twothirds -- screenFrame could be another monitor. 
        currentWindow.w = third
    elseif currentWindow.x - screenFrame.x > half and currentWindow.w < half then
        -- is to the right of the halfway line, and is smaller than a half
        currentWindow.x = screenFrame.x + third
        currentWindow.w = twothirds
    else
        currentWindow.x = screenFrame.x + half
        currentWindow.w = half
    end

    currentWindow.y = screenFrame.y
    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 thirdHeight = screenFrame.h / 3
    local twothirdsHeight = screenFrame.h - thirdHeight

    currentWindow.x = screenFrame.x -- push to the left fo the screen
    currentWindow.w = screenFrame.w / 2
    currentWindow.y = screenFrame.y -- top edge
    currentWindow.h = twothirdsHeight

    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 thirdHeight = screenFrame.h / 3
    local twothirdsHeight = screenFrame.h - thirdHeight

    currentWindow.x = screenFrame.x
    currentWindow.w = screenFrame.w / 2

    currentWindow.y = screenFrame.y + twothirdsHeight
    currentWindow.h = thirdHeight

    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 * 0.6
    win:setFrame(f)
    win:centerOnScreen()
end)