ExtTree = {}

function ExtTree:new (o)
    o = o or {}
    setmetatable(o, self)
    self.__index = self
    return o
end

function ExtTree:add_single (in_ext, out_ext)
    if self[in_ext] ~= nil then
        if type(out_ext) == "table" then
            for _, out in ipairs(out_ext) do
                table.insert(self[in_ext], out)
            end
        else
            table.insert(self[in_ext], out_ext)
        end
    else
        if type(out_ext) == "table" then
            self[in_ext] = out_ext
        else
            self[in_ext] = {out_ext}
        end
    end
end

function ExtTree:add_multiple (in_exts, out_exts)
    if type(in_exts) == "table" then
        for _, in_ext in ipairs(in_exts) do
            self:add_single( in_ext, out_exts)
        end
    else
        self:add_single( in_exts, out_exts )
    end

    if type(out_exts) == "table" then
        for _, out_ext in ipairs(out_exts) do
            self:add_single( out_ext, in_exts)
        end
    else
        self:add_single( out_exts, in_exts )
    end
end

function ExtTree:print ()
    base = "%s -> "
    for I in pairs(self) do
        local in_map = string.format(base, I)
        for _, O in ipairs(self[I]) do
            in_map = in_map .. O .. ","
        end
        print(in_map)
    end
end

p1 = ExtTree:new(nil)

p1:add_multiple("h", {"c", "cpp", "cxx", "cc", "CC"})
p1:add_multiple("H", {"C", "CPP", "CXX", "CC"})
p1:add_multiple("hpp", {"cpp", "c"})
p1:add_multiple("HPP", {"CPP", "C"})
p1:add_multiple("ads", "adb")

p1:print()