need help to to implement external soring, given a big data file of integers, where the number of inputs is unknown but the first data of the input file is the largest array size (M) that can be used.

import java.io.BufferedReader; import java.io.FileNotFoundException; import java.io.FileReader; import java.io.FileWriter; import java.io.IOException; import java.io.PrintWriter; import java.util.Arrays; import java.util.Random;

public class ExternalSort { static int N = 2000000; // size of the file in disk static int M = 100000; // max items the memory buffer can hold public static void externalSort(String fileName) { String tfile = "temp-file-"; int[] buffer = new int[M < N ? M : N]; try { FileReader fr = new FileReader(fileName); BufferedReader br = new BufferedReader(fr); int slices = (int) Math.ceil((double) N/M); int i, j; i = j = 0; // Iterate through the elements in the file for (i = 0; i < slices; i++) { // Read M-element chunk at a time from the file for (j = 0; j < (M < N ? M : N); j++) { String t = br.readLine(); if (t != null) buffer[j] = Integer.parseInt(t); else break; } // Sort M elements Arrays.sort(buffer);

// Write the sorted numbers to temp file FileWriter fw = new FileWriter(tfile + Integer.toString(i) + ".txt"); PrintWriter pw = new PrintWriter(fw); for (int k = 0; k < j; k++) pw.println(buffer[k]); pw.close(); fw.close(); } br.close(); fr.close(); // Now open each file and merge them, then write back to disk int[] topNums = new int[slices]; BufferedReader[] brs = new BufferedReader[slices]; for (i = 0; i < slices; i++) { brs[i] = new BufferedReader(new FileReader(tfile + Integer.toString(i) + ".txt")); String t = brs[i].readLine(); if (t != null) topNums[i] = Integer.parseInt(t); else topNums[i] = Integer.MAX_VALUE; } FileWriter fw = new FileWriter("external-sorted.txt"); PrintWriter pw = new PrintWriter(fw); for (i = 0; i < N; i++) { int min = topNums[0]; int minFile = 0; for (j = 0; j < slices; j++) { if (min > topNums[j]) { min = topNums[j]; minFile = j; } } pw.println(min); String t = brs[minFile].readLine(); if (t != null) topNums[minFile] = Integer.parseInt(t); else topNums[minFile] = Integer.MAX_VALUE; } for (i = 0; i < slices; i++) brs[i].close(); pw.close(); fw.close(); } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } }

static String generateInput(int n) { String fileName = "external-sort.txt"; Random rand = new Random(); try { FileWriter fw = new FileWriter(fileName); PrintWriter pw = new PrintWriter(fw); for (int i = 0; i < n; i++) pw.println(rand.nextInt(101)); pw.close(); } catch (IOException e) { e.printStackTrace(); } return fileName; } public static void main(String[] args) { String fileName = generateInput(N); externalSort(fileName); } }