ex1_bits.erl

-module(ex1_bits).
-export([bits_direct/1,test_bits_direct/0,bits_tail/2,test_bits_tail/0,run_tests/0]).

bits_direct(0) ->
    0;
bits_direct(N) ->
    N rem 2 + bits_direct(N div 2).

bits_tail(0, B) ->
    B;
bits_tail(N, B) ->
    bits_tail(N div 2, N rem 2 + B).

% Tests
test_bits_direct() ->
    D = [0,1,2,3,4,5,6,7,8,9,10,255,2047,14323],
    A = lists:map(fun(X) -> bits_direct(X) end, D),
    A = [0,1,1,2,1,2,2,3,1,2,2,8,11,11],
    'Tests passed'.

test_bits_tail() ->
    D = [0,1,2,3,4,5,6,7,8,9,10,255,2047,14323],
    A = lists:map(fun(X) -> bits_tail(X, 0) end, D),
    A = [0,1,1,2,1,2,2,3,1,2,2,8,11,11],
'Tests passed'.

run_tests() ->
    test_bits_direct(),
    test_bits_tail(),
    'All tests passed'.

You need Erlang installed on your machine.

1. Save this gist in some folder

2. Open erl in command line in that folder

3. Compile the file: c(ex1_bits).

4. Run tests: ex1_bits:run_tests().