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package theweeklychallenge;
/*
Week 147:
https://theweeklychallenge.org/blog/perl-weekly-challenge-147
Task #1: Truncatable Prime
Write a script to generate first 20 left-truncatable prime numbers in base 10.
*/
import java.lang.Math;
import java.util.Arrays;
import java.util.ArrayList;
import junit.framework.TestCase;
import java.util.regex.Pattern;
import java.util.regex.Matcher;
import static junit.framework.Assert.*;
public class TruncatablePrime extends TestCase {
public static void main(String[] args) {
junit.textui.TestRunner.run(theweeklychallenge.TruncatablePrime.class);
}
public void testTruncatablePrime() {
Integer exp[] = {2, 3, 5, 7, 13, 17, 23, 37, 43, 47, 53, 67, 73, 83, 97, 113, 137, 167, 173, 197};
Integer got[] = leftTruncatablePrime(20);
assertTrue(Arrays.equals(exp, got));
}
public static Integer[] leftTruncatablePrime(int count) {
ArrayList<Integer> ltp = new ArrayList<Integer>();
int c = 0;
int n = 2;
while (c < count) {
String s = Integer.toString(n);
if (!Pattern.compile(".*0.*").matcher(s).matches() && isPrime(n)) {
Integer[] numbers = leftTruncatableNumbers(n);
boolean found = true;
if (numbers.length >=2) {
for(int i = 0; i<numbers.length; i++) {
if (!isPrime(numbers[i])) {
found = false;
}
}
}
if (found) {
ltp.add(n);
c++;
}
}
n++;
}
Integer[] ltpArray = new Integer[ltp.size()];
return ltp.toArray(ltpArray);
}
public static Integer[] leftTruncatableNumbers(int n) {
int i = 0;
String s = Integer.toString(n);
ArrayList<Integer> numbers = new ArrayList<Integer>();
while (i < s.length()) {
numbers.add(Integer.valueOf(s.substring(i)));
i++;
}
Integer[] numArray = new Integer[numbers.size()];
return numbers.toArray(numArray);
}
public static boolean isPrime(int n) {
if (n == 1) {
return false;
}
for(int i=2; i <= Math.sqrt(n); i++) {
if ((n % i) == 0) {
return false;
}
}
return true;
}
}
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