CC: Debit card system

creditLimit = 0

function mainLoop()
    print('entering main loop')
    while (not (disk.isPresent('top') and disk.hasData('top')))
    do
        sleep(1)
    end
end

function drawMainScreen(monitor)
    monitor.clear()
    monitor.setCursorPos(1,1)
    monitor.write('Cash-o-matic 3000')
end

function createDebitFile(fsroot)
    local debitFile = fs.open(fsroot .. '/debit', 'w')
    debitFile.writeLine('1000')
    debitFile.flush()
    debitFile.close()
    return getDebitFile(fsroot)
end

function getDebitFile(fsroot)
    return fs.open(fsroot .. '/debit', 'r')
end


function drawDebitScreen(monitor)
    local fsroot = disk.getMountPath('top')
    if (not fs.exists(fsroot .. '/debit')) then
        -- create debit file
        debitFile = createDebitFile(fsroot)
    else
        debitFile = getDebitFile(fsroot)
    end

    creditLimit = debitFile.readLine()
    debitFile.close()

    -- actually draw the screen
    monitor.clear()
    monitor.setCursorPos(1,1)
    monitor.write('Current Balance: ' .. creditLimit)

    monitor.setCursorPos(1,10)
    monitor.write('Cancel')

    -- monitor.setCursorPos(1,15)
    -- monitor.write('Buy')

    waiting = true
    while (waiting and disk.isPresent("top"))
    do
        local event, button, x, y = os.pullEvent("monitor_touch")
        print("Button", button, "was clicked at", x, "/", y)

        if (x >= 1 and x <= 6 and y >= 10 and y <= 12) then
            disk.eject("top")
            waiting = false
        end
    end
end

local monitor = peripheral.wrap('left')
monitor.setTextScale(0.5)
monitor.setBackgroundColor(colors.yellow)
monitor.setTextColor(colors.black)

while (true) do
    drawMainScreen(monitor)
    mainLoop()

    drawDebitScreen(monitor)
end