local function new(file)
    local set = {}
    local fh = fs.open(file, "r")
    if fh == nil then
        -- no such file, return empty set
        return {inner=set, file=file}
    end
    local line = fh.readLine()
    while line ~= nil do
        if line then
            set[line] = true
        end
        line = fh.readLine()
    end
    fh.close()
    return {inner=set, file=file}
end

local function writeSet(set)
    local fh = fs.open(set.file, "w")
    for it, allowed in pairs(set.inner) do
        if allowed then
            fh.writeLine(it)
        end
    end
    fh.flush()
    fh.close()
end

local function add(set, item)
    set.inner[item] = true
    writeSet(set)
end

local function remove(set, item)
    set.inner[item] = false
    writeSet(set)
end

local function has(set, item)
    return set.inner[item] == true
end

return {new=new, writeSet=writeSet, add=add, remove=remove, has=has}