Does binary addition of two bitstrings of equal length and throws away the carry.

gistfile1.hrl

-spec(add_bitstrings/2::(Bin1::bitstring(), Bin2::bitstring()) -> bitstring()).
add_bitstrings(Bin1, Bin2) when bit_size(Bin1) =:= bit_size(Bin2) ->
  add_bit_with_carry(bit_size(Bin1), Bin1, Bin2, false).

add_bit_with_carry(0, Acc, _Bin, _Carry) -> Acc;
add_bit_with_carry(BitNumber, AccBin1, Bin2, Carry) ->
  Bit = BitNumber - 1,
  <<Beginning:Bit/bitstring, B1:1/bitstring, Rest/bitstring>> = AccBin1,
  <<_DontYetCare:Bit/bitstring, B2:1/bitstring, _Rest/bitstring>> = Bin2,
  {NextBit, NextCarry} = case {B1, B2, Carry} of
    {<<0:1>>, <<0:1>>, true}  -> {1, false};
    {<<0:1>>, <<0:1>>, false} -> {0, false};
    {<<0:1>>, <<1:1>>, true}  -> {0, true};
    {<<0:1>>, <<1:1>>, false} -> {1, false};
    {<<1:1>>, <<0:1>>, true}  -> {0, true};
    {<<1:1>>, <<0:1>>, false} -> {1, false};
    {<<1:1>>, <<1:1>>, true}  -> {1, true};
    {<<1:1>>, <<1:1>>, false} -> {0, true}
  end,
  <<_:7/bitstring, B:1/bitstring>> = <<NextBit>>,
  add_bit_with_carry(Bit, 
      <<Beginning:Bit/bitstring, B:1/bitstring, Rest/bitstring>>, 
      Bin2, NextCarry).