ba.erl

-module(ba).
-export([tests/0, test/2, amida/1]).

tests() ->
  test("d6-7b-e1-9e", "740631825"), % #0
  test("6f-dd-ff-ff", "230685147"). % #41

test(Data, Expected) ->
  io:fwrite("~s -> ~s, ~w~n", [Data, Expected, amida(Data) =:= Expected]).

amida(Data) ->
  Hexes = string:tokens(Data, "-"),
  Init_cols = array:from_list("012345678"),
  array:to_list(lists:foldl(fun reorder_one_row/2, Init_cols, Hexes)).

% change column order of one row.
reorder_one_row(Hex, Cols) -> reorder_cols(Cols, cross_p(bin(Hex))).

% array(0123456789), [{0, 2}, {3, 4}] -> array(2104356789).
reorder_cols(A, [])         -> A;
reorder_cols(A, [{X, Y}|T]) -> reorder_cols(swap(A, X, Y), T).

% array(abcd), 1, 3 -> array(adcb). Indexes are zero origin.
swap(A, X, Y) ->
  Xvalue = array:get(X, A),
  Yvalue = array:get(Y, A),
  array:set(Y, Xvalue, array:set(X, Yvalue, A)).

% "d6" -> "11010110".
bin(Hex) -> integer_to_list(list_to_integer(Hex, 16), 2).

% "11010110" -> [{5, 7}, {3, 4}, {0, 2}].
cross_p(Bin)              -> cross_p(Bin ++ "0", $0, 0, []).
cross_p([], _, _, L)      -> L;
cross_p([$0|T], $0, S, L) -> cross_p(T, $0, S, L);
cross_p([$1|T], $0, _, L) -> cross_p(T, $1, 8 - length(T), L);
cross_p([$0|T], $1, S, L) -> cross_p(T, $0, 0, [{S, 8 - length(T)} | L]);
cross_p([$1|T], $1, S, L) -> cross_p(T, $1, S, L).