Delayed Callback

--[[
    Lua delayed callback library.

    Copyright (c) 2016 Mitchell Nelson <me@mitchfizz05.net>

    Permission is hereby granted, free of charge, to any person obtaining a copy
    of this software and associated documentation files (the "Software"), to deal
    in the Software without restriction, including without limitation the rights
    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
    copies of the Software, and to permit persons to whom the Software is
    furnished to do so, subject to the following conditions:

    The above copyright notice and this permission notice shall be included in
    all copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
    THE SOFTWARE.
]]

local callbacks = {}

local function typeValidate(value, expectedType, argumentNumber, argumentName)
    if type(value) ~= expectedType then
        local a = tostring(argumentNumber)
        if argumentName ~= nil then a = a .. " (" .. argumentName .. ")" end
        error("Argument " .. a .. " expected " .. expectedType .. ", got " .. type(value))
    end
end

function new(func, delay)
    typeValidate(func, "function", 1, "func")
    typeValidate(delay, "number", 2, "delay")

    if delay > 0 then
        local timer = os.startTimer(delay)
        callbacks[timer] = func
        sleep(0)
        return timer
    else
        func()
    end
end

--[[
    Should be called every os.pullEvent.
    Allows library to check if any callbacks need to be executed.
]]
function pullEvent(e)
    if e[1] == "timer" then
        local toRemove = {}
        -- Check if any of the callback timers match.
        for timer, func in pairs(callbacks) do
            if timer == e[2] then
                -- Run callback, then queue for removal from callbacks.
                func()
                toRemove[#toRemove + 1] = timer
            end
        end

        -- Remove any callbacks queued for removal.
        for _, timer in ipairs(toRemove) do
            callbacks[timer] = nil
        end

        --[[
            If there are no callbacks left, raise an event.
            This is so if the program wants to, it can exit once all callbacks are finished.
            This probably won't be used often.
        ]]
        os.queueEvent("no_callbacks_left")
    end
end