factorization into prime factors

Description

Demo Code


//package com.java2s;
import java.util.ArrayList;

public class Main {
    private static int[] PRIME_NUMBERS = { 2, 3, 5, 7, 11, 13, 17, 19, 23,
            29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97,
            101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157,
            163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227,
            229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283,
            293, 307 };/*  w w  w. j av  a2  s .c o  m*/

    /**
     * factorization into prime factors
     */
    public static int[] getPrimeFactors(int number) {
        int[] primeNumbers = getPrimeNumbers(number);
        ArrayList factorList = new ArrayList();
        int currentPNIndex = 0;
        int currentNumber = number;
        while (true) {
            if (currentNumber <= 1) {
                break;
            }
            int pn = primeNumbers[currentPNIndex];
            if ((currentNumber % pn) == 0) {
                currentNumber /= pn;
                factorList.add(new Integer(pn));
                continue;
            }
            currentPNIndex++;
            if (currentPNIndex >= primeNumbers.length) {
                throw new InternalError("failed factorization. : " + number);
            }
        }
        int[] ret = new int[factorList.size()];
        for (int i = 0; i < ret.length; i++) {
            ret[i] = ((Integer) factorList.get(i)).intValue();
        }
        return ret;
    }

    /**
     * make prime number array.
     */
    public static int[] getPrimeNumbers(int upperLimit) {
        if (upperLimit < 300) {
            return getPrimeNumbers_small(upperLimit);
        }
        return getPrimeNumbers_index(upperLimit);
    }

    /**
     * fastest cache method
     */
    private static int[] getPrimeNumbers_small(int upperLimit) {
        for (int i = 0; i < PRIME_NUMBERS.length; i++) {
            if (PRIME_NUMBERS[i] > upperLimit) {
                int[] ret = new int[i];
                for (int j = 0; j < ret.length; j++) {
                    ret[j] = PRIME_NUMBERS[j];
                }
                return ret;
            }
        }
        throw new InternalError("Large? : " + upperLimit);
    }

    /**
     * general array index method
     */
    private static int[] getPrimeNumbers_index(int upperLimit) {
        if (upperLimit < 2)
            return new int[0];
        boolean[] index = new boolean[upperLimit];
        index[0] = true;// not prime number
        for (int i = 2; i <= upperLimit; i++) {
            if (index[i - 1] == true) {
                continue;
            }
            for (int j = i * 2; j <= upperLimit; j += i) {
                index[j - 1] = true;
            }
        }
        int length = 0;
        for (int i = 1; i < upperLimit; i++) {
            if (index[i] == false)
                length++;
        }
        int[] ret = new int[length];
        int count = 0;
        for (int i = 1; i < upperLimit; i++) {
            if (index[i] == false) {
                ret[count++] = i + 1;
            }
        }
        return ret;
    }
}
factorization into prime factors - Java java.lang

Description

Demo Code

Related Tutorials
