Creeper warning

local requiredMods = {
    "plethora:sensor",
    "plethora:glasses",
}
local hostileEntities = {
    "creeper",
    "zombie",
    "skeleton",
    "spider",
}
local RADIUS = 5
local mods = peripheral.wrap("back")

-- check mods
for _, mod in pairs(requiredMods) do
    if not mods.hasModule(mod) then
        error(("Module: %s not found"):format(mod), 0)
    end
end

local canvas = mods.canvas()

function getDistance(entity)
    local x = entity.x
    local y = entity.y
    local z = entity.z
    return math.sqrt(x*x+y*y+z*z)
end

function isHostile(entity)
    local isHostile = false
    for _, name in pairs(hostileEntities) do
        if entity.name:lower() == name then
            isHostile = true
            break
        end
    end
    return isHostile
end

function getEntities(filter)
    local found = {}
    for _, entity in pairs(mods.sense()) do
        if filter(entity) then table.insert(found, entity) end
    end
    return found
end

function drawWarning()
    canvas.clear()
    local text = canvas.addText({x = 5, y = 5}, "")
    text.setScale(2)
    text.setText("WARNING: HOSTILES")
end

function drawNoWarning()
    canvas.clear()
    local text = canvas.addText({x = 5, y = 5}, "")
    text.setScale(2)
    text.setText("All set")
end

function checkHostiles()
    local nearHostileFilter =
    function(entity)
            return getDistance(entity) <= RADIUS and isHostile(entity)
    end

    local nearHostiles = getEntities(nearHostileFilter)
    if table.getn(nearHostiles) > 0 then
        drawWarning()
    else
        drawNoWarning()
    end
end

canvas.clear()

while true do
    checkHostiles()
    os.sleep(0.5)
end