matrix

local chars = {}

local function new(t)
    t.y = 0
    table.insert(chars, t)
end

setmetatable(chars, {__call = function(t)
    local i = 0
    return function()
        i = i + 1
        return t[i], i
    end
end})

local function timer(period)
    return {
        id = nil;
        run = function(self)
            self.id = os.startTimer(period)
        end;
    }
end

local updateTimer = timer(0.1)
local creator = timer(0.1)

updateTimer:run()
creator:run()

term.setBackgroundColor(colors.black)
term.clear()
term.setCursorPos(1,1)

while true do
    local _, t = os.pullEvent('timer')
    local w,h = term.getSize()

    if t == updateTimer.id then
        for char, i in chars() do
            char.y = char.y + 1

            if char.y > h then
                table.remove(chars, i)
            else
                term.setCursorPos(char.x, char.y)
                if char.color == colors.black then
                    term.setBackgroundColor(colors.black)
                    term.write(' ')
                else
                    term.setTextColor(char.color)
                    term.write(string.char(math.random(38, 126)))
                end
            end
        end

        updateTimer:run()
    elseif t == creator.id then

        local time = math.random()*0.8 + 0.5
        local tail = timer(time)
        local eraser = timer(time + math.random()*0.25 + 0.5)

        new {
            x = math.random(w);
            color = colors.lime;
            tail = tail;
            eraser = eraser;
            ishead = true;
        }

        creator:run()
        tail:run()
        eraser:run()
    else
        for char in chars() do
            if char.ishead then
                if t == char.tail.id then
                    new {
                        x = char.x;
                        color = colors.green;
                    }
                    break
                elseif t == char.eraser.id then
                    new {
                        x = char.x;
                        color = colors.black;
                    }
                    break
                end
            end
        end
    end
end