gistfile1.erl

-module(p3).
-export([prime_sieve/1, solution/0, solution/1]).

% generate a list of all primes smaller than N
prime_sieve(N) when is_integer(N) ->
	prime_sieve([ 2 ] ++ [ X || X <- lists:seq(3, N) ]);
prime_sieve([]) -> [];
prime_sieve([H|C]) ->
	[ H ] ++ prime_sieve(lists:filter(fun(X)-> (X rem H) /= 0 end, C)).

solution(_, [], F) -> F;
solution(N, [H|P], F) ->
	case N rem H of
		0 ->
			io:format("prime factor ~p found~n", [ H ]),
			solution(N div H, P, [ H ] ++ F);
		_ -> solution(N, P, F)
	end.
solution(N) ->
	io:format("Generating primes... "),
	P = prime_sieve(round(math:sqrt(N))),
	io:format("done, testing~n"),
	solution(N, P, []).
% what is the largest prime factor of 600851475143?
solution() ->
	lists:max(solution(600851475143)).