Hamming weight of 64bit integers in Erlang using the popcount_3() method found http://en.wikipedia.org/wiki/Hamming_weight counts the number of bits set to 1.

bitpop.erl

%% -*- coding: utf-8 -*-
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%% Copyright (C) 2013 Gregory Burd. All Rights Reserved.
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-module(bitpop).

-ifdef(TEST).
-include_lib("eunit/include/eunit.hrl").
-endif.

-export([count/1]).

count(0) -> 0;
count(X)
  when is_integer(X), X > 0, X < 16#FFFFFFFF ->
    ((c4(X) bsr 16) + c4(X)) band 16#0000FFFF.
c1(V) -> V - ((V bsr 1) band 16#55555555).
c2(V) -> ((c1(V) bsr 2) band 16#33333333) + (c1(V) band 16#33333333).
c3(V) -> ((c2(V) bsr 4) + c2(V)) band 16#0F0F0F0F.
c4(V) -> ((c3(V) bsr 8) + c3(V)) band 16#00FF00FF.

-ifdef(TEST).

bitpop_test_() ->
    [?_assertEqual(0, count(0)),
     ?_assertEqual(1, count(1)),
     ?_assertEqual(2, count(3)),
     ?_assertEqual(3, count(7)),
     ?_assertEqual(4, count(15)),
     ?_assertEqual(5, count(31)),
     ?_assertEqual(6, count(63)),
     ?_assertEqual(7, count(127)),
     ?_assertEqual(8, count(255)),
     ?_assertEqual(1, count(4)),
     ?_assertEqual(1, count(8)),
     ?_assertEqual(1, count(16)),
     ?_assertEqual(1, count(32)),
     ?_assertEqual(1, count(64)),
     ?_assertEqual(1, count(128)),
     ?_assertEqual(1, count(256)),
     ?_assertEqual(1, count(512)),
     ?_assertEqual(1, count(1024)),
     ?_assertEqual(1, count(2048)),
     ?_assertEqual(1, count(16#FFFF + 1)),
     ?_assertEqual(19, count(16#FFFFE)),
     ?_assertEqual(1, count(16#FFFFF + 1)),
     ?_assertEqual(23, count(16#FFFFFE)),
     ?_assertEqual(1, count(16#FFFFF + 1)),
     ?_assertEqual(27, count(16#FFFFFFE)),
     ?_assertEqual(1, count(16#FFFFF + 1)),
     ?_assertEqual(31, count(16#FFFFFFFE)),
     ?_assertException(error, function_clause, count(-1))].

-endif.