folder.erl

-module(folder).
-author("Takahiro Kondo <heartery@gmail.com>").

-export([fold/1, fold/2]).
-export([example/0]).

-define(DEFAULT_SEPARATOR, ".").

fold(Dict) ->
    fold(Dict, ?DEFAULT_SEPARATOR).

fold(Dict, Separator) ->
    Keys = lists:filter(fun(Key) ->
                                is_list(Key) and (string:str(Key, Separator) > 0)
                        end, Dict:fetch_keys()),
    fold(Dict, Separator, Keys).

fold(Dict, _, []) ->
    Dict;

fold(Dict, Separator, [Key|Keys]) ->
    KeyParts = string:tokens(Key, Separator),
    Value = Dict:fetch(Key),
    fold(assign(Dict:erase(Key), KeyParts, Value), Separator, Keys).

assign(Dict, [KeyPart], Value) ->
    Dict:store(KeyPart, Value);

assign(Dict, [KeyPart|KeyParts], Value) ->
    InnerDict = case Dict:find(KeyPart) of
                    {ok, D} when is_tuple(D) and (element(1, D) =:= dict) -> D;
                    _                                                     -> dict:new()
                end,
    Dict:store(KeyPart, assign(InnerDict, KeyParts, Value)).

example() ->
    D1 = dict:new(),
    D2 = D1:store("a.b.c", 1),
    D3 = D2:store("d", 2),
    D4 = fold(D3),
    io:format("a.b.c => ~p~n", [dict:fetch("c", dict:fetch("b", dict:fetch("a", D4)))]),
    io:format("d     => ~p~n", [dict:fetch("d", D4)]).