-module(bristow_transform).

-export([parse_transform/2]).

parse_transform(Forms, _Options) ->
  {Alias, Forms1} = gen_alias(Forms),
  export_alias(Forms1, Alias).

gen_alias(Forms) ->
  gen_alias(Forms, {}, [], []).

gen_alias([], _, Alias, Acc) ->
  {Alias, lists:reverse(Acc)};
gen_alias([{function, _, Name, _, [{clause, _, Args, _, _}|_]} = Fun|Rest], _, Alias, Acc) ->
  gen_alias(Rest, {Name, Args}, Alias, [Fun|Acc]);
gen_alias([{attribute, N, alias, Name}|Rest], {Fun, Args}, Alias, Acc) ->
  gen_alias(Rest, {Fun, Args}, [{Name, length(Args)}|Alias], [{function, N, Name, length(Args),
                                                               [{clause, N,
                                                                 Args,
                                                                 [],
                                                                 [{call, N, {atom, 1, Fun}, Args}]}]}|Acc]);
gen_alias([{attribute, _, alias, _}|_], {}, _, _) ->
  erlang:throw("-alias must follow a function definition");
gen_alias([Def|Rest], _, Alias, Acc) ->
  gen_alias(Rest, {}, Alias, [Def|Acc]).

export_alias(Forms, Alias) ->
  {_, Forms1} = lists:foldl(fun
                              ({attribute, N, export, Exports}, {A, Acc}) ->
                                {[], [{attribute, N, export, Exports ++ A}|Acc]};
                              (Other, {A, Acc}) ->
                                {A, [Other|Acc]}
                            end, {Alias, []}, Forms),
  lists:reverse(Forms1).