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Ingy döt Net 2023-07-01 11:58:00 -04:00
parent 7387c8f97b
commit cb5bb5e222
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66
Task/Safe-primes-and-unsafe-primes/Java/safe-primes-and-unsafe-primes.java Normal file
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public class SafePrimes {
public static void main(String... args) {
// Use Sieve of Eratosthenes to find primes
int SIEVE_SIZE = 10_000_000;
boolean[] isComposite = new boolean[SIEVE_SIZE];
// It's really a flag indicating non-prime, but composite usually applies
isComposite[0] = true;
isComposite[1] = true;
for (int n = 2; n < SIEVE_SIZE; n++) {
if (isComposite[n]) {
continue;
}
for (int i = n * 2; i < SIEVE_SIZE; i += n) {
isComposite[i] = true;
}
}
int oldSafePrimeCount = 0;
int oldUnsafePrimeCount = 0;
int safePrimeCount = 0;
int unsafePrimeCount = 0;
StringBuilder safePrimes = new StringBuilder();
StringBuilder unsafePrimes = new StringBuilder();
int safePrimesStrCount = 0;
int unsafePrimesStrCount = 0;
for (int n = 2; n < SIEVE_SIZE; n++) {
if (n == 1_000_000) {
oldSafePrimeCount = safePrimeCount;
oldUnsafePrimeCount = unsafePrimeCount;
}
if (isComposite[n]) {
continue;
}
boolean isUnsafe = isComposite[(n - 1) >>> 1];
if (isUnsafe) {
if (unsafePrimeCount < 40) {
if (unsafePrimeCount > 0) {
unsafePrimes.append(", ");
}
unsafePrimes.append(n);
unsafePrimesStrCount++;
}
unsafePrimeCount++;
}
else {
if (safePrimeCount < 35) {
if (safePrimeCount > 0) {
safePrimes.append(", ");
}
safePrimes.append(n);
safePrimesStrCount++;
}
safePrimeCount++;
}
}
System.out.println("First " + safePrimesStrCount + " safe primes: " + safePrimes.toString());
System.out.println("Number of safe primes below 1,000,000: " + oldSafePrimeCount);
System.out.println("Number of safe primes below 10,000,000: " + safePrimeCount);
System.out.println("First " + unsafePrimesStrCount + " unsafe primes: " + unsafePrimes.toString());
System.out.println("Number of unsafe primes below 1,000,000: " + oldUnsafePrimeCount);
System.out.println("Number of unsafe primes below 10,000,000: " + unsafePrimeCount);
return;
}
}