-module(base32_rfc4648).

-export([encode/1, decode/1]).

-import(base32_utils, [rev_bits_list_to_binary/1]).


-spec encode(binary()) -> binary().
encode(Data) ->
    list_to_binary(encode0(Data)).


encode0(Data) ->
    Size = bit_size(Data),
    RevEncoded = case 40 - (Size rem 40) of
                     40 ->
                         encode1(Data, []);
                     32 ->
                         [$=, $=, $=, $=, $=, $= | encode1(Data, [])];
                     24 ->
                         [$=, $=, $=, $= | encode1(Data, [])];
                     16 ->
                         [$=, $=, $= | encode1(Data, [])];
                     8 ->
                         [$= | encode1(Data, [])]
                 end,
    lists:reverse(RevEncoded).


encode1(<<>>, Accu) ->
    Accu;

encode1(<<0:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$A | Accu]);
encode1(<<1:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$B | Accu]);
encode1(<<2:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$C | Accu]);
encode1(<<3:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$D | Accu]);
encode1(<<4:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$E | Accu]);
encode1(<<5:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$F | Accu]);
encode1(<<6:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$G | Accu]);
encode1(<<7:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$H | Accu]);
encode1(<<8:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$I | Accu]);
encode1(<<9:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$J | Accu]);
encode1(<<10:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$K | Accu]);
encode1(<<11:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$L | Accu]);
encode1(<<12:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$M | Accu]);
encode1(<<13:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$N | Accu]);
encode1(<<14:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$O | Accu]);
encode1(<<15:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$P | Accu]);
encode1(<<16:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$Q | Accu]);
encode1(<<17:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$R | Accu]);
encode1(<<18:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$S | Accu]);
encode1(<<19:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$T | Accu]);
encode1(<<20:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$U | Accu]);
encode1(<<21:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$V | Accu]);
encode1(<<22:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$W | Accu]);
encode1(<<23:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$X | Accu]);
encode1(<<24:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$Y | Accu]);
encode1(<<25:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$Z | Accu]);
encode1(<<26:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$2 | Accu]);
encode1(<<27:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$3 | Accu]);
encode1(<<28:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$4 | Accu]);
encode1(<<29:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$5 | Accu]);
encode1(<<30:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$6 | Accu]);
encode1(<<31:5, Next/bitstring>>, Accu) ->
    encode1(Next, [$7 | Accu]);

encode1(<<0:4>>, Accu) ->
    [$A | Accu];
encode1(<<1:4>>, Accu) ->
    [$C | Accu];
encode1(<<2:4>>, Accu) ->
    [$E | Accu];
encode1(<<3:4>>, Accu) ->
    [$G | Accu];
encode1(<<4:4>>, Accu) ->
    [$I | Accu];
encode1(<<5:4>>, Accu) ->
    [$K | Accu];
encode1(<<6:4>>, Accu) ->
    [$M | Accu];
encode1(<<7:4>>, Accu) ->
    [$O | Accu];
encode1(<<8:4>>, Accu) ->
    [$Q | Accu];
encode1(<<9:4>>, Accu) ->
    [$S | Accu];
encode1(<<10:4>>, Accu) ->
    [$U | Accu];
encode1(<<11:4>>, Accu) ->
    [$W | Accu];
encode1(<<12:4>>, Accu) ->
    [$Y | Accu];
encode1(<<13:4>>, Accu) ->
    [$2 | Accu];
encode1(<<14:4>>, Accu) ->
    [$4 | Accu];
encode1(<<15:4>>, Accu) ->
    [$6 | Accu];

encode1(<<0:3>>, Accu) ->
    [$A | Accu];
encode1(<<1:3>>, Accu) ->
    [$E | Accu];
encode1(<<2:3>>, Accu) ->
    [$I | Accu];
encode1(<<3:3>>, Accu) ->
    [$M | Accu];
encode1(<<4:3>>, Accu) ->
    [$Q | Accu];
encode1(<<5:3>>, Accu) ->
    [$U | Accu];
encode1(<<6:3>>, Accu) ->
    [$Y | Accu];
encode1(<<7:3>>, Accu) ->
    [$4 | Accu];

encode1(<<0:2>>, Accu) ->
    [$A | Accu];
encode1(<<1:2>>, Accu) ->
    [$I | Accu];
encode1(<<2:2>>, Accu) ->
    [$Q | Accu];
encode1(<<3:2>>, Accu) ->
    [$Y | Accu];

encode1(<<0:1>>, Accu) ->
    [$A | Accu];
encode1(<<1:1>>, Accu) ->
    [$Q | Accu].


-spec decode(binary()) -> {ok, binary()} | {error, atom()}.
decode(Data) ->
    decode0(Data, []).


decode0(<<>>, Accu) ->
    {ok, rev_bits_list_to_binary(Accu)};
decode0(<<"A", Next/bitstring>>, Accu) ->
    decode0(Next, [<<0:5>> | Accu]);
decode0(<<"B", Next/bitstring>>, Accu) ->
    decode0(Next, [<<1:5>> | Accu]);
decode0(<<"C", Next/bitstring>>, Accu) ->
    decode0(Next, [<<2:5>> | Accu]);
decode0(<<"D", Next/bitstring>>, Accu) ->
    decode0(Next, [<<3:5>> | Accu]);
decode0(<<"E", Next/bitstring>>, Accu) ->
    decode0(Next, [<<4:5>> | Accu]);
decode0(<<"F", Next/bitstring>>, Accu) ->
    decode0(Next, [<<5:5>> | Accu]);
decode0(<<"G", Next/bitstring>>, Accu) ->
    decode0(Next, [<<6:5>> | Accu]);
decode0(<<"H", Next/bitstring>>, Accu) ->
    decode0(Next, [<<7:5>> | Accu]);
decode0(<<"I", Next/bitstring>>, Accu) ->
    decode0(Next, [<<8:5>> | Accu]);
decode0(<<"J", Next/bitstring>>, Accu) ->
    decode0(Next, [<<9:5>> | Accu]);
decode0(<<"K", Next/bitstring>>, Accu) ->
    decode0(Next, [<<10:5>> | Accu]);
decode0(<<"L", Next/bitstring>>, Accu) ->
    decode0(Next, [<<11:5>> | Accu]);
decode0(<<"M", Next/bitstring>>, Accu) ->
    decode0(Next, [<<12:5>> | Accu]);
decode0(<<"N", Next/bitstring>>, Accu) ->
    decode0(Next, [<<13:5>> | Accu]);
decode0(<<"O", Next/bitstring>>, Accu) ->
    decode0(Next, [<<14:5>> | Accu]);
decode0(<<"P", Next/bitstring>>, Accu) ->
    decode0(Next, [<<15:5>> | Accu]);
decode0(<<"Q", Next/bitstring>>, Accu) ->
    decode0(Next, [<<16:5>> | Accu]);
decode0(<<"R", Next/bitstring>>, Accu) ->
    decode0(Next, [<<17:5>> | Accu]);
decode0(<<"S", Next/bitstring>>, Accu) ->
    decode0(Next, [<<18:5>> | Accu]);
decode0(<<"T", Next/bitstring>>, Accu) ->
    decode0(Next, [<<19:5>> | Accu]);
decode0(<<"U", Next/bitstring>>, Accu) ->
    decode0(Next, [<<20:5>> | Accu]);
decode0(<<"V", Next/bitstring>>, Accu) ->
    decode0(Next, [<<21:5>> | Accu]);
decode0(<<"W", Next/bitstring>>, Accu) ->
    decode0(Next, [<<22:5>> | Accu]);
decode0(<<"X", Next/bitstring>>, Accu) ->
    decode0(Next, [<<23:5>> | Accu]);
decode0(<<"Y", Next/bitstring>>, Accu) ->
    decode0(Next, [<<24:5>> | Accu]);
decode0(<<"Z", Next/bitstring>>, Accu) ->
    decode0(Next, [<<25:5>> | Accu]);
decode0(<<"2", Next/bitstring>>, Accu) ->
    decode0(Next, [<<26:5>> | Accu]);
decode0(<<"3", Next/bitstring>>, Accu) ->
    decode0(Next, [<<27:5>> | Accu]);
decode0(<<"4", Next/bitstring>>, Accu) ->
    decode0(Next, [<<28:5>> | Accu]);
decode0(<<"5", Next/bitstring>>, Accu) ->
    decode0(Next, [<<29:5>> | Accu]);
decode0(<<"6", Next/bitstring>>, Accu) ->
    decode0(Next, [<<30:5>> | Accu]);
decode0(<<"7", Next/bitstring>>, Accu) ->
    decode0(Next, [<<31:5>> | Accu]);

decode0(<<"======">>, [<<0:5>> | Accu]) ->
    decode0(<<>>, [<<0:3>> | Accu]);
decode0(<<"======">>, [<<4:5>> | Accu]) ->
    decode0(<<>>, [<<1:3>> | Accu]);
decode0(<<"======">>, [<<8:5>> | Accu]) ->
    decode0(<<>>, [<<2:3>> | Accu]);
decode0(<<"======">>, [<<12:5>> | Accu]) ->
    decode0(<<>>, [<<3:3>> | Accu]);
decode0(<<"======">>, [<<16:5>> | Accu]) ->
    decode0(<<>>, [<<4:3>> | Accu]);
decode0(<<"======">>, [<<20:5>> | Accu]) ->
    decode0(<<>>, [<<5:3>> | Accu]);
decode0(<<"======">>, [<<24:5>> | Accu]) ->
    decode0(<<>>, [<<6:3>> | Accu]);
decode0(<<"======">>, [<<28:5>> | Accu]) ->
    decode0(<<>>, [<<7:3>> | Accu]);

decode0(<<"====">>, [<<0:5>> | Accu]) ->
    decode0(<<>>, [<<0:1>> | Accu]);
decode0(<<"====">>, [<<16:5>> | Accu]) ->
    decode0(<<>>, [<<1:1>> | Accu]);

decode0(<<"===">>, [<<0:5>> | Accu]) ->
    decode0(<<>>, [<<0:4>> | Accu]);
decode0(<<"===">>, [<<2:5>> | Accu]) ->
    decode0(<<>>, [<<1:4>> | Accu]);
decode0(<<"===">>, [<<4:5>> | Accu]) ->
    decode0(<<>>, [<<2:4>> | Accu]);
decode0(<<"===">>, [<<6:5>> | Accu]) ->
    decode0(<<>>, [<<3:4>> | Accu]);
decode0(<<"===">>, [<<8:5>> | Accu]) ->
    decode0(<<>>, [<<4:4>> | Accu]);
decode0(<<"===">>, [<<10:5>> | Accu]) ->
    decode0(<<>>, [<<5:4>> | Accu]);
decode0(<<"===">>, [<<12:5>> | Accu]) ->
    decode0(<<>>, [<<6:4>> | Accu]);
decode0(<<"===">>, [<<14:5>> | Accu]) ->
    decode0(<<>>, [<<7:4>> | Accu]);
decode0(<<"===">>, [<<16:5>> | Accu]) ->
    decode0(<<>>, [<<8:4>> | Accu]);
decode0(<<"===">>, [<<18:5>> | Accu]) ->
    decode0(<<>>, [<<9:4>> | Accu]);
decode0(<<"===">>, [<<20:5>> | Accu]) ->
    decode0(<<>>, [<<10:4>> | Accu]);
decode0(<<"===">>, [<<22:5>> | Accu]) ->
    decode0(<<>>, [<<11:4>> | Accu]);
decode0(<<"===">>, [<<24:5>> | Accu]) ->
    decode0(<<>>, [<<12:4>> | Accu]);
decode0(<<"===">>, [<<26:5>> | Accu]) ->
    decode0(<<>>, [<<13:4>> | Accu]);
decode0(<<"===">>, [<<28:5>> | Accu]) ->
    decode0(<<>>, [<<14:4>> | Accu]);
decode0(<<"===">>, [<<30:5>> | Accu]) ->
    decode0(<<>>, [<<15:4>> | Accu]);

decode0(<<"=">>, [<<0:5>> | Accu]) ->
    decode0(<<>>, [<<0:2>> | Accu]);
decode0(<<"=">>, [<<8:5>> | Accu]) ->
    decode0(<<>>, [<<1:2>> | Accu]);
decode0(<<"=">>, [<<16:5>> | Accu]) ->
    decode0(<<>>, [<<2:2>> | Accu]);
decode0(<<"=">>, [<<24:5>> | Accu]) ->
    decode0(<<>>, [<<3:2>> | Accu]);

decode0(_, _) ->
    {error, invalid_format}.