LaserSentry

--- This program finds hostile mobs and fires lasers at them, acting like a sentry tower.

--- We check that a manipulator exists and wrap it.
local modules = peripheral.find("manipulator")
if not modules then
    error("Cannot find manipulator", 0)
end

--- We require an entity sensor to find mobs and a laser to shoot at them. We error if neither exists.
if not modules.hasModule("plethora:laser") then error("Cannot find laser", 0) end
if not modules.hasModule("plethora:sensor") then error("Cannot find entity sensor", 0) end

--- We define a function which fires a laser towards an entity. This is a very naive implementation as it does not
--- account for the entity moving between firing and impact. You could use the `motionX`, `motionY` and `motionZ` fields
--- if you wish to add such functionality.
local function fire(entity)
    local x, y, z = entity.x, entity.y, entity.z
    local pitch = -math.atan2(y, math.sqrt(x * x + z * z))
    local yaw = math.atan2(-x, z)

    modules.fire(math.deg(yaw), math.deg(pitch), 5)
    sleep(0.2)
end

--- We build a lookup of mobs we wish to target, to avoid shooting non-hostile mobs.
local mobNames = { "Creeper", "Zombie", "Skeleton" }
local mobLookup = {}
for i = 1, #mobNames do
    mobLookup[mobNames[i]] = true
end

--- We now sense the vicinity and prepare to fire at them.
while true do
    local mobs = modules.sense()

    --- First we build up a list of all mobs that we care about.
    local candidates = {}
    for i = 1, #mobs do
        local mob = mobs[i]
        if mobLookup[mob.name] then
            candidates[#candidates + 1] = mob
        end
    end

    --- If we've got a mob then choose a random one and fire towards it. Otherwise, delay for a second before
    --- rescanning.
    if #candidates > 0 then
        local mob = candidates[math.random(1, #candidates)]
        fire(mob)
    else
        sleep(1)
    end
end