Initial data commit

Task/Extensible-prime-generator/Java/extensible-prime-generator.java

@@ -0,0 +1,114 @@
+import java.util.*;
+
+public class PrimeGenerator {
+    private int limit_;
+    private int index_ = 0;
+    private int increment_;
+    private int count_ = 0;
+    private List<Integer> primes_ = new ArrayList<>();
+    private BitSet sieve_ = new BitSet();
+    private int sieveLimit_ = 0;
+
+    public PrimeGenerator(int initialLimit, int increment) {
+        limit_ = nextOddNumber(initialLimit);
+        increment_ = increment;
+        primes_.add(2);
+        findPrimes(3);
+    }
+
+    public int nextPrime() {
+        if (index_ == primes_.size()) {
+            if (Integer.MAX_VALUE - increment_ < limit_)
+                return 0;
+            int start = limit_ + 2;
+            limit_ = nextOddNumber(limit_ + increment_);
+            primes_.clear();
+            findPrimes(start);
+        }
+        ++count_;
+        return primes_.get(index_++);
+    }
+
+    public int count() {
+        return count_;
+    }
+
+    private void findPrimes(int start) {
+        index_ = 0;
+        int newLimit = sqrt(limit_);
+        for (int p = 3; p * p <= newLimit; p += 2) {
+            if (sieve_.get(p/2 - 1))
+                continue;
+            int q = p * Math.max(p, nextOddNumber((sieveLimit_ + p - 1)/p));
+            for (; q <= newLimit; q += 2*p)
+                sieve_.set(q/2 - 1, true);
+        }
+        sieveLimit_ = newLimit;
+        int count = (limit_ - start)/2 + 1;
+        BitSet composite = new BitSet(count);
+        for (int p = 3; p <= newLimit; p += 2) {
+            if (sieve_.get(p/2 - 1))
+                continue;
+            int q = p * Math.max(p, nextOddNumber((start + p - 1)/p)) - start;
+            q /= 2;
+            for (; q >= 0 && q < count; q += p)
+                composite.set(q, true);
+        }
+        for (int p = 0; p < count; ++p) {
+            if (!composite.get(p))
+                primes_.add(p * 2 + start);
+        }
+    }
+
+    private static int sqrt(int n) {
+        return nextOddNumber((int)Math.sqrt(n));
+    }
+
+    private static int nextOddNumber(int n) {
+        return 1 + 2 * (n/2);
+    }
+
+    public static void main(String[] args) {
+        PrimeGenerator pgen = new PrimeGenerator(20, 200000);
+        System.out.println("First 20 primes:");
+        for (int i = 0; i < 20; ++i) {
+            if (i > 0)
+                System.out.print(", ");
+            System.out.print(pgen.nextPrime());
+        }
+        System.out.println();
+        System.out.println("Primes between 100 and 150:");
+        for (int i = 0; ; ) {
+            int prime = pgen.nextPrime();
+            if (prime > 150)
+                break;
+            if (prime >= 100) {
+                if (i++ != 0)
+                    System.out.print(", ");
+                System.out.print(prime);
+            }
+        }
+        System.out.println();
+        int count = 0;
+        for (;;) {
+            int prime = pgen.nextPrime();
+            if (prime > 8000)
+                break;
+            if (prime >= 7700)
+                ++count;
+        }
+        System.out.println("Number of primes between 7700 and 8000: " + count);
+        int n = 10000;
+        for (;;) {
+            int prime = pgen.nextPrime();
+            if (prime == 0) {
+                System.out.println("Can't generate any more primes.");
+                break;
+            }
+            if (pgen.count() == n) {
+                System.out.println(n + "th prime: " + prime);
+                n *= 10;
+            }
+        }
+    }
+}