-module(utools).

-export([b32_decode/1]).
-export([b32_encode/1]).
-export([hotp/2]).
-export([rand_bytes/0, rand_bytes/1]).
-export([rand_chars/1]).
-export([rand_hash/0]).
-export([totp/1, totp/2]).
-export([totp_check/2]).
-export([totp_generate/0]).

b32_decode({<<V, "======">>, Bits}) ->
    <<Bits/bits, (b32_std_dec(V) bsr 2):3>>;
b32_decode({<<V, "====">>, Bits}) ->
    <<Bits/bits, (b32_std_dec(V) bsr 4):1>>;
b32_decode({<<V, "===">>, Bits}) ->
    <<Bits/bits, (b32_std_dec(V) bsr 1):4>>;
b32_decode({<<V, "=">>, Bits}) ->
    <<Bits/bits, (b32_std_dec(V) bsr 3):2>>;
b32_decode({<<V, R/binary>>, Bits}) ->
    b32_decode({R, <<Bits/bits, (b32_std_dec(V)):5>>});
b32_decode({<<>>, Bits}) ->
    Bits;
b32_decode(V) when is_list(V) ->
    b32_decode(list_to_binary(V));
b32_decode(V) when is_binary(V) ->
    b32_decode({V, <<>>}).

b32_encode({body, V}) ->
    O = 5 * (byte_size(V) div 5),
    <<Body:O/binary, R/binary>> = V,
    {<< <<(b32_std_enc(X))>> || <<X:5>> <= Body>>, R};
b32_encode({rest, V}) ->
    O = 5 * (bit_size(V) div 5),
    <<Body:O/bits, Rest/bits>> = V,
    B0 = << <<(b32_std_enc(X))>> || <<X:5>> <= Body>>,
    {B1, Pad} = case Rest of
		    <<I:3>> -> {<<(b32_std_enc(I bsl 2))>>, 6};
		    <<I:1>> -> {<<(b32_std_enc(I bsl 4))>>, 4};
		    <<I:4>> -> {<<(b32_std_enc(I bsl 1))>>, 3};
		    <<I:2>> -> {<<(b32_std_enc(I bsl 3))>>, 1};
		    <<>> -> {<<>>, 0}
		end,
    {<<B0/binary, B1/binary>>, Pad};
b32_encode(V) when is_list(V) ->
    b32_encode(list_to_binary(V));
b32_encode(V) when is_binary(V) ->
    {EB, _R} = b32_encode({body, V}),
    {ER, P} = b32_encode({rest, V}),
    Padding = list_to_binary(lists:duplicate(P, $=)),
    <<EB/binary, ER/binary, Padding/binary>>.

b32_std_dec(I) when I >= $2 andalso I =< $7 -> 
    I - 24;
b32_std_dec(I) when I >= $a andalso I =< $z -> 
    I - $a;
b32_std_dec(I) when I >= $A andalso I =< $Z -> 
    I - $A.

b32_std_enc(I) when is_integer(I) andalso I >= 26 andalso I =< 31 ->
    I + 24;
b32_std_enc(I) when is_integer(I) andalso I >= 0 andalso I =< 25 ->
    I + $a.

hotp(Token, Time) ->
  K = b32_decode(Token),
  M = <<Time:8/big-unsigned-integer-unit:8>>,
  D = crypto:mac(hmac, sha, K, M),
  <<_:19/binary, _O:8>> = D,
  O = _O band 15,
  <<_TB:4/integer-unit:8>> = binary:part(D, O, 4),
  TB = _TB band 16#7fffffff,
  _T = TB rem trunc(math:pow(10, 6)),
  T = integer_to_binary(_T),
  P = << <<48:8>> || _ <- lists:seq(1, 6 - byte_size(T)) >>,
  {ok, <<P/binary, T/binary>>}.

rand_bytes() ->
  rand_bytes(8).

rand_bytes(N) when is_integer(N) ->
  {ok, crypto:strong_rand_bytes(N)}.

rand_chars({0, Swap}) ->
  {ok, Swap};
rand_chars({Size, <<Swap/binary>>}) when Size > 0 ->
  Char = rand:uniform(26) + 96,
  rand_chars({Size - 1, <<Swap/binary, Char>>});
rand_chars(Size) when Size > 0 ->
  rand_chars({Size, <<"">>}).

rand_hash() ->
  {ok, R} = rand_chars(64),
  {ok, binary:encode_hex(crypto:hash(sha256, <<"utools:rand_hash(", R/binary, ")">>))}.

totp(<<Secret/binary>>) ->
  totp(Secret, erlang:timestamp()).

totp(<<Secret/binary>>, {M, S, _}) ->
  T = (M * 1000000 + S) / 30,
  Time = trunc(T),
  hotp(Secret, Time).

totp_check(<<Token/binary>>, <<Secret/binary>>) ->
  {M, S, _} = erlang:timestamp(),
  TL = [totp(Secret, {M, S - 30, 0}), totp(Secret, {M, S, 0}), totp(Secret, {M, S + 30, 0})],
  {ok, lists:member({ok, Token}, TL)}.

totp_generate() ->
  rand_chars(16).