import java.io.*;

public class SortLargeFile {
  public static final int MAX_ARRAY_SIZE = 43;
  public static final int BUFFER_SIZE = 100000;

  public static void main(String[] args) throws Exception {
    // Sort largedata.dat to sortedfile.dat
    sort("largedata.dat", "sortedfile.dat");
    
    // Display the first 100 numbers in the sorted file
    displayFile("sortedfile.dat");
  }
  
  /** Sort data in source file and into target file */
  public static void sort(String sourcefile, String targetfile) 
      throws Exception {
    // Implement Phase 1: Create initial segments
    int numberOfSegments =
      initializeSegments(MAX_ARRAY_SIZE, sourcefile, "f1.dat");

    // Implement Phase 2: Merge segments recursively
    merge(numberOfSegments, MAX_ARRAY_SIZE,
      "f1.dat", "f2.dat", "f3.dat", targetfile);
  }

  /** Sort original file into sorted segments */
  private static int initializeSegments
      (int segmentSize, String originalFile, String f1)
      throws Exception {
    int[] list = new int[segmentSize];
    DataInputStream input = new DataInputStream(
      new BufferedInputStream(new FileInputStream(originalFile)));
    DataOutputStream output = new DataOutputStream(
      new BufferedOutputStream(new FileOutputStream(f1)));

    int numberOfSegments = 0;
    while (input.available() > 0) {
      numberOfSegments++;
      int i = 0;
      for ( ; input.available() > 0 && i < segmentSize; i++) {
        list[i] = input.readInt();
      }

      // Sort an array list[0..i-1]
      java.util.Arrays.sort(list, 0, i);

      // Write the array to f1.dat
      for (int j = 0; j < i; j++) {
        output.writeInt(list[j]);
      }
    }

    input.close();
    output.close();
    return numberOfSegments;
  }

  private static void merge(int numberOfSegments, int segmentSize, 
      String f1, String f2, String f3, String targetfile) 
      throws Exception {
    if (numberOfSegments > 1) {
      mergeOneStep(numberOfSegments, segmentSize, f1, f2, f3);
      merge((numberOfSegments + 1) / 2, segmentSize * 2, 
        f3, f1, f2, targetfile);
    }
    else { // Rename f1 as the final sorted file
      File sortedFile = new File(targetfile);
      if (sortedFile.exists()) sortedFile.delete();
      new File(f1).renameTo(sortedFile);
    }
  }

  private static void mergeOneStep(int numberOfSegments,
      int segmentSize, String f1, String f2, String f3)
      throws Exception {
    DataInputStream f1Input = new DataInputStream(
      new BufferedInputStream(new FileInputStream(f1), BUFFER_SIZE));
    DataOutputStream f2Output = new DataOutputStream(
      new BufferedOutputStream(new FileOutputStream(f2), BUFFER_SIZE));

    // Copy half number of segments from f1.dat to f2.dat
    copyHalfToF2(numberOfSegments, segmentSize, f1Input, f2Output);
    f2Output.close();

    // Merge remaining segments in f1 with segments in f2 into f3
    DataInputStream f2Input = new DataInputStream(
      new BufferedInputStream(new FileInputStream(f2), BUFFER_SIZE));
    DataOutputStream f3Output = new DataOutputStream(
      new BufferedOutputStream(new FileOutputStream(f3), BUFFER_SIZE));

    mergeSegments(numberOfSegments / 2,
      segmentSize, f1Input, f2Input, f3Output);

    f1Input.close();
    f2Input.close();
    f3Output.close();
  }

  /** Copy first half number of segments from f1.dat to f2.dat */
  private static void copyHalfToF2(int numberOfSegments,
      int segmentSize, DataInputStream f1, DataOutputStream f2)
      throws Exception {
    for (int i = 0; i < (numberOfSegments / 2) * segmentSize; i++) {
      f2.writeInt(f1.readInt());
    }
  }

  /** Merge all segments */
  private static void mergeSegments(int numberOfSegments,
      int segmentSize, DataInputStream f1, DataInputStream f2,
      DataOutputStream f3) throws Exception {
    for (int i = 0; i < numberOfSegments; i++) {
      mergeTwoSegments(segmentSize, f1, f2, f3);
    }

    // If f1 has one extra segment, copy it to f3
    while (f1.available() > 0) {
      f3.writeInt(f1.readInt());
    }
  }

  /** Merges two segments */
  private static void mergeTwoSegments(int segmentSize,
    DataInputStream f1, DataInputStream f2,
    DataOutputStream f3) throws Exception {
    int intFromF1 = f1.readInt();
    int intFromF2 = f2.readInt();
    int f1Count = 1;
    int f2Count = 1;
  
    while (true) {
      if (intFromF1 < intFromF2) {
        f3.writeInt(intFromF1);
        if (f1.available() == 0 || f1Count++ >= segmentSize) {
          f3.writeInt(intFromF2);
          break;
        }
        else {
          intFromF1 = f1.readInt();
        }
      }
      else {
        f3.writeInt(intFromF2);
        if (f2.available() == 0 || f2Count++ >= segmentSize) {
          f3.writeInt(intFromF1);
          break;
        }
        else {
          intFromF2 = f2.readInt();
        }
      }
    }
  
    while (f1.available() > 0 && f1Count++ < segmentSize) {
      f3.writeInt(f1.readInt());
    }
  
    while (f2.available() > 0 && f2Count++ < segmentSize) {
      f3.writeInt(f2.readInt());
    }
  }
  
  /** Display the first 100 numbers in the specified file */
  public static void displayFile(String filename) {
    try {
      DataInputStream input = 
        new DataInputStream(new FileInputStream(filename));
      for (int i = 0; i < 100; i++)
        System.out.print(input.readInt() + " ");
      input.close();
    }
    catch (IOException ex) {
      ex.printStackTrace();
    }
  }
}