Initial data commit

Task/Wieferich-primes/Java/wieferich-primes.java

@@ -0,0 +1,55 @@
+import java.util.*;
+
+public class WieferichPrimes {
+    public static void main(String[] args) {
+        final int limit = 5000;
+        System.out.printf("Wieferich primes less than %d:\n", limit);
+        for (Integer p : wieferichPrimes(limit))
+            System.out.println(p);
+    }
+
+    private static boolean[] primeSieve(int limit) {
+        boolean[] sieve = new boolean[limit];
+        Arrays.fill(sieve, true);
+        if (limit > 0)
+            sieve[0] = false;
+        if (limit > 1)
+            sieve[1] = false;
+        for (int i = 4; i < limit; i += 2)
+            sieve[i] = false;
+        for (int p = 3; ; p += 2) {
+            int q = p * p;
+            if (q >= limit)
+                break;
+            if (sieve[p]) {
+                int inc = 2 * p;
+                for (; q < limit; q += inc)
+                    sieve[q] = false;
+            }
+        }
+        return sieve;
+    }
+
+    private static long modpow(long base, long exp, long mod) {
+        if (mod == 1)
+            return 0;
+        long result = 1;
+        base %= mod;
+        for (; exp > 0; exp >>= 1) {
+            if ((exp & 1) == 1)
+                result = (result * base) % mod;
+            base = (base * base) % mod;
+        }
+        return result;
+    }
+
+    private static List<Integer> wieferichPrimes(int limit) {
+        boolean[] sieve = primeSieve(limit);
+        List<Integer> result = new ArrayList<>();
+        for (int p = 2; p < limit; ++p) {
+            if (sieve[p] && modpow(2, p - 1, p * p) == 1)
+                result.add(p);
+        }
+        return result;
+    }
+}