import static java.lang.Math.ceil;
import static java.lang.Math.sqrt;
import static java.nio.file.StandardOpenOption.*;
import java.nio.file.*;
import java.nio.channels.*;
import java.nio.*;
import java.util.*;
import java.io.IOException;

public class PrimesToFile3 {
  public static void main(String[] args) {
    int primesRequired = 100;                                          // Default count
    if (args.length > 0) {
      try {
        primesRequired = Integer.valueOf(args[0]).intValue();
      } catch (NumberFormatException e) {
        System.out.println("Prime count value invalid. Using default of "
                           + primesRequired);
      }
    }

      long[] primes = new long[primesRequired];                        // Array to store primes
      primes[0] = 2L;                                                  // Seed the first prime
      primes[1] = 3L;                                                  // and the second
      // Count of primes found – up to now, which is also the array index
      int count = 2;
      // Next integer to be tested
      long number = 5L;

      outer:
      for (; count < primesRequired; number += 2) {

        // The maximum divisor we need to try is square root of number
        long limit = (long)ceil(sqrt((double)number));

        // Divide by all the primes we have up to limit
        for (int i = 1; i < count && primes[i] <= limit; ++i)
          if (number % primes[i] == 0L)                                // Is it an exact divisor?
            continue outer;                                            // yes, try the next number

        primes[count++] = number;                                      // We got one!
      }

    Path file = Paths.get("D:/Junk/primes.txt");
    try {
      Files.createDirectories(file.getParent());                       // Make sure we have the directory
    } catch (IOException e) {
      e.printStackTrace();
    }
    final int BUFFERSIZE = 1024;                                       // Buffer size in bytes – bigger!

    try (WritableByteChannel channel = Files.newByteChannel( file, EnumSet.of(WRITE, CREATE))){
      ByteBuffer buf = ByteBuffer.allocate(BUFFERSIZE);
      String primeStr = null;
      int primesWritten = 0;
      while (primesWritten < primes.length) {
        while (primesWritten < primes.length) {
          primeStr = "prime = " + primes[primesWritten];
          if ((buf.position() + 2*primeStr.length() + 16) > buf.limit()) {
            break;
          }
          buf.asDoubleBuffer().put(0, (double)primeStr.length());
          buf.position(buf.position() + 8);
          buf.position(buf.position() + 2*buf.asCharBuffer().put(primeStr).position());
          buf.asLongBuffer().put(primes[primesWritten++]);
          buf.position(buf.position() + 8);
        }
        buf.flip();
        channel.write(buf);
        buf.clear();
      }
      System.out.println("File written is " + ((FileChannel)channel).size() + " bytes.");
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
}