gistfile1.erl

-module(p23).
-export([solution/0, solution/1]).
-define(LIMIT, 28123).

divisors(N) ->
	Limit = round(math:sqrt(N)),
	if
		Limit < 2 -> [];
		true ->
			L1 = [ X || X <- lists:seq(2, Limit), N rem X =:= 0 ],
			lists:usort(lists:append([[ 1 ], L1, lists:map(fun (X) -> N div X end, L1)]))
	end.

property(N) ->
	X = lists:sum(divisors(N)),
	if
		X < N -> deficient;
		X > N -> abundant;
		true -> perfect
	end.

abundantNumbers(N) ->
	lists:filter(fun(X) -> property(X) =:= abundant end, lists:seq(12, N)).

is_abundant_sum(N, Abundant) ->
	L = lists:takewhile(fun(X) -> X < N end, Abundant),
	lists:any(fun(X) -> X end,
		lists:map(fun (X) -> lists:member(N - X, L) end, L)).

solution(N, _, N, A) ->
	S = is_abundant_sum(N, A),
	case S of
		false -> N;
		true -> 0
	end;
solution(X, P, N, A) ->
	S = is_abundant_sum(X, A),
	if
		P > N / 100 -> P2 = 0, io:format("\r~p% ~p/~p      ", [ round(X / N * 100), N - X, N ]);
		true -> P2 = P + 1
	end,
	case S of
		false ->
			X + solution(X + 1, P2, N, A);
		true -> 
			solution(X + 1, P2, N, A)
	end.

solution(N) -> 
	Abundant = abundantNumbers(N),
	X = solution(0, 0, N, Abundant),
	io:format("~n~p~n", [ X ]).

solution() -> solution(?LIMIT).