get All Divisors - Java java.lang

Java examples for java.lang:int prime

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Description

get All Divisors

Demo Code


import java.util.List;

public class Main{
    /**//from  ww  w  .j  a  v  a2  s .  c o m
     * @param number
     * @return
     */
    public static int[] getAllDivisors(int number) {
        int[] divisors = new int[number];
        divisors[0] = 1;
        int divisorIndex = 1;

        int[] primeList = PrimeUtil.getAllPrimeBelowN(number);
        int[] primeDivisorExp = new int[primeList.length];

        int i = 1;
        int temp = number;
        for (; i < primeList.length && primeList[i] <= temp; i++) {
            if (temp % primeList[i] == 0) {
                int exp = 0;
                while (temp % primeList[i] == 0) {
                    exp++;
                    temp = temp / primeList[i];
                }
                primeDivisorExp[i] = exp;
            }
        }

        //2^2 5 12
        int count = generateDivisorsFromPrimeFactors(number, primeList, 1,
                i, primeDivisorExp, divisors, 1, divisorIndex);

        int[] result = new int[count];
        System.arraycopy(divisors, 0, result, 0, count);

        return result;
    }
    /**
     * return prime list equal or below n
     * Please notice that element 0 is kept for 1 although 1 is not prime, this is preserved for usage in special occasions
     * 
     * @param n
     * @return
     */
    public static int[] getAllPrimeBelowN(int n) {
        if (n == 1) {
            return new int[] { 1 };
        }

        int[] temp = new int[n];
        boolean[] flags = new boolean[n + 1];

        temp[0] = 1;
        temp[1] = 2;
        int index = 2;

        int lastPrime = 2;
        while (lastPrime < n) {
            for (int i = 2; i * lastPrime <= n; i++) {
                flags[i * lastPrime] = true;
            }

            boolean found = false;
            for (int i = lastPrime + 1; i <= n; i++) {
                if (!flags[i]) {
                    found = true;
                    lastPrime = i;
                    temp[index++] = lastPrime;
                    break;
                }
            }

            if (!found) {
                break;
            }
        }

        int[] result = new int[index];
        System.arraycopy(temp, 0, result, 0, index);
        return result;

    }
    private static int generateDivisorsFromPrimeFactors(int number,
            int[] primeList, int primeIndex, int primeLength,
            int[] primeDivisorExp, int[] divisors, int divisor,
            int divisorIndex) {
        if (primeIndex >= primeLength) {
            return divisorIndex;
        }

        for (int k = 1; k <= primeDivisorExp[primeIndex] + 1; k++) {
            if (divisor > 1 && divisor < number
                    && divisors[divisorIndex - 1] != divisor) {
                divisors[divisorIndex++] = divisor;
            }

            divisorIndex = generateDivisorsFromPrimeFactors(number,
                    primeList, primeIndex + 1, primeLength,
                    primeDivisorExp, divisors, divisor, divisorIndex);

            divisor *= primeList[primeIndex];
        }

        return divisorIndex;
    }
}

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