69 lines
2.4 KiB
Java
69 lines
2.4 KiB
Java
import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.List;
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public class SuccessivePrimeDifferences {
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private static Integer[] sieve(int limit) {
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List<Integer> primes = new ArrayList<>();
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primes.add(2);
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boolean[] c = new boolean[limit + 1];// composite = true
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// no need to process even numbers > 2
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int p = 3;
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while (true) {
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int p2 = p * p;
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if (p2 > limit) {
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break;
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}
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for (int i = p2; i <= limit; i += 2 * p) {
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c[i] = true;
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}
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do {
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p += 2;
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} while (c[p]);
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}
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for (int i = 3; i <= limit; i += 2) {
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if (!c[i]) {
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primes.add(i);
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}
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}
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return primes.toArray(new Integer[0]);
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}
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private static List<List<Integer>> successivePrimes(Integer[] primes, Integer[] diffs) {
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List<List<Integer>> results = new ArrayList<>();
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int dl = diffs.length;
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outer:
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for (int i = 0; i < primes.length - dl; i++) {
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Integer[] group = new Integer[dl + 1];
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group[0] = primes[i];
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for (int j = i; j < i + dl; ++j) {
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if (primes[j + 1] - primes[j] != diffs[j - i]) {
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continue outer;
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}
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group[j - i + 1] = primes[j + 1];
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}
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results.add(Arrays.asList(group));
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}
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return results;
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}
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public static void main(String[] args) {
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Integer[] primes = sieve(999999);
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Integer[][] diffsList = {{2}, {1}, {2, 2}, {2, 4}, {4, 2}, {6, 4, 2}};
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System.out.println("For primes less than 1,000,000:-\n");
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for (Integer[] diffs : diffsList) {
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System.out.printf(" For differences of %s ->\n", Arrays.toString(diffs));
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List<List<Integer>> sp = successivePrimes(primes, diffs);
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if (sp.isEmpty()) {
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System.out.println(" No groups found");
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continue;
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}
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System.out.printf(" First group = %s\n", Arrays.toString(sp.get(0).toArray(new Integer[0])));
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System.out.printf(" Last group = %s\n", Arrays.toString(sp.get(sp.size() - 1).toArray(new Integer[0])));
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System.out.printf(" Number found = %d\n", sp.size());
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System.out.println();
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}
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}
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}
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