package Maths;

import java.util.ArrayList;

import java.util.Collections;

/**

* n number theory, a vampire number (or true vampire number) is a composite natural number with an

* even number of digits, that can be factored into two natural numbers each with half as many

* digits as the original number and not both with trailing zeroes, where the two factors contain

* precisely all the digits of the original number, in any order, counting multiplicity. The first

* vampire number is 1260 = 21 × 60. *

*

* <p>* link: https://en.wikipedia.org/wiki/Vampire_number *

*

* <p>

*/

public class VampireNumber {

public static void main(String[] args) {

test(10, 1000);

}

static void test(int startValue, int stopValue) {

int countofRes = 1;

StringBuilder res = new StringBuilder();

for (int i = startValue; i <= stopValue; i++) {

for (int j = i; j <= stopValue; j++) {

// System.out.println(i+ " "+ j);

if (isVampireNumber(i, j, true)) {

countofRes++;

res.append("" + countofRes + ": = ( " + i + "," + j + " = " + i * j + ")" + "\n");

}

}

}

System.out.println(res);

}

static boolean isVampireNumber(int a, int b, boolean noPseudoVamireNumbers) {

// this is for pseudoVampireNumbers pseudovampire number need not be of length n/2 digits for

// example

// 126 = 6 x 21

if (noPseudoVamireNumbers) {

if (a * 10 <= b || b * 10 <= a) {

return false;

}

}

String mulDigits = splitIntoDigits(a * b, 0);

String faktorDigits = splitIntoDigits(a, b);

return mulDigits.equals(faktorDigits);

}

// methode to Split the numbers to Digits

static String splitIntoDigits(int num, int num2) {

StringBuilder res = new StringBuilder();

ArrayList<Integer> digits = new ArrayList<>();

while (num > 0) {

digits.add(num % 10);

num /= 10;

}

while (num2 > 0) {

digits.add(num2 % 10);

num2 /= 10;

}

Collections.sort(digits);

for (int i : digits) {

res.append(i);

}

return res.toString();

}

}