Email server (for dexter9)

local configFile = ".emailServerConfig"
local config = {
    modem = "top",
    server = 0,
    users = {}
}

local function waitForSpace()
    local key = false
    while not key do
        local event, _key = os.pullEvent("key")
        key = (_key == 57)
    end
end

local function saveConfig()
    local handle = fs.open(configFile, "w")
    handle.write(textutils.serialize(config))
    handle.close()
end

local function loadConfig()
    local handle = fs.open(configFile, "r")
    if not handle then
        saveConfig() -- saves default config
        handle = fs.open(configFile, "r")
    end
    config = textutils.unserialize(handle.readAll())
    handle.close()
end

local function isValidSide(_side)
    local rsSides = rs.getSides()
    local valid = false
    for key, value in pairs(rsSides) do valid = (value == _side) or valid end
    return valid
end

local function configureModem()
    term.clear()
    term.setCursorPos(1, 1)
    print("Error: No modem on side '" .. config.modem .. "'")
    local side = false
    while not side do
        print("Enter modem side:")
        local _side = read()
        if isValidSide(_side) then
            if peripheral.getType(_side) == "modem" then
                print("Configuration updated")
                side = _side
                print("Press Space to continue...")
                waitForSpace()
            else
                print("There is no modem on side '" .. _side .. "'")
            end
        else
            print("Invalid side")
        end
    end
    config.modem = side
    saveConfig()
end

loadConfig()
if isValidSide(config.modem) then
    if peripheral.getType(config.modem) == "modem" then
        rednet.open(config.modem)
    else
        configureModem()
        rednet.open(config.modem)
    end
else
    configureModem()
    rednet.open(config.modem)
end

term.clear()
term.setCursorPos(1, 1)
print("Email server is running")

while true do
    local event, sender, message = os.pullEvent("rednet_message")
    print("\n#" .. sender .. ": " .. message)
    local fullMessage = textutils.unserialize(message)
    if fullMessage.ecommand == "send" then
        local recs = 0
        for key, value in pairs(config.users) do
            if value == fullMessage.user then
                rednet.send(key, textutils.serialize({from = sender, message = fullMessage.message}))
                recs = recs + 1
            end
        end
        if recs == 0 then
            rednet.send(sender, textutils.serialize({status = "invalid_username"}))
        else
            rednet.send(sender, textutils.serialize({status = "ok"}))
        end
    end
    if fullMessage.ecommand == "create" then
        local inuse = false
        for key, value in pairs(config.users) do
            if value == fullMessage.username then inuse = true end
        end
        if inuse then
            rednet.send(sender, textutils.serialize({status = "username_in_use"}))
        else
            config.users[sender] = fullMessage.username
            saveConfig()
            rednet.send(sender, textutils.serialize({status = "ok"}))
        end
    end
end