Probject Eularの Problem3のプログラム②(Erlang版)

eularProblem3_2.erl

-module(eularProblem3).
-export([answer/0]).

%% Answer of Eular Problem 3 "Largest prime factor"

answer() -> largePrimeFactorOf(600851475143).

largePrimeFactorOf(X) -> largePrimeFactorOf(X, 2).

largePrimeFactorOf(X, P) ->
  if X == 1       -> P;
     X rem P == 0 -> largePrimeFactorOf(X div P, P);
     true         -> largePrimeFactorOf(X, nextPrime(P))
  end.

nextPrime(X) -> case isPrime(X+1) of
                  true -> X + 1;
                  false -> nextPrime(X+1)
                end.

isPrime(X) -> isPrime(X, numRange(2, math:sqrt(X))).
isPrime(_, []) -> true;
isPrime(X, [N|NS]) -> case X rem N of
                        0 -> false;
                        _ -> isPrime(X, NS)
                      end.

numRange(Min, Max) when Min > Max -> [];
numRange(Min, Max) -> [Min|numRange(Min+1, Max)].