Below is the syntax highlighted version of MergeX.java
from §2.2 Mergesort.
/****************************************************************************** * Compilation: javac MergeX.java * Execution: java MergeX < input.txt * Dependencies: StdOut.java StdIn.java * Data files: https://algs4.cs.princeton.edu/22mergesort/tiny.txt * https://algs4.cs.princeton.edu/22mergesort/words3.txt * * Sorts a sequence of strings from standard input using an * optimized version of mergesort. * * % more tiny.txt * S O R T E X A M P L E * * % java MergeX < tiny.txt * A E E L M O P R S T X [ one string per line ] * * % more words3.txt * bed bug dad yes zoo ... all bad yet * * % java MergeX < words3.txt * all bad bed bug dad ... yes yet zoo [ one string per line ] * ******************************************************************************/ import java.util.Comparator; /** * The {@code MergeX} class provides static methods for sorting an * array using an optimized version of mergesort. * <p> * In the worst case, this implementation takes * Θ(<em>n</em> log <em>n</em>) time to sort an array of * length <em>n</em> (assuming comparisons take constant time). * <p> * This sorting algorithm is stable. * It uses Θ(<em>n</em>) extra memory (not including the input array). * <p> * For additional documentation, see * <a href="https://algs4.cs.princeton.edu/22mergesort">Section 2.2</a> of * <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne. * * @author Robert Sedgewick * @author Kevin Wayne */ public class MergeX { private static final int CUTOFF = 7; // cutoff to insertion sort // This class should not be instantiated. private MergeX() { } private static void merge(Comparable[] src, Comparable[] dst, int lo, int mid, int hi) { // precondition: src[lo .. mid] and src[mid+1 .. hi] are sorted subarrays assert isSorted(src, lo, mid); assert isSorted(src, mid+1, hi); int i = lo, j = mid+1; for (int k = lo; k <= hi; k++) { if (i > mid) dst[k] = src[j++]; else if (j > hi) dst[k] = src[i++]; else if (less(src[j], src[i])) dst[k] = src[j++]; // to ensure stability else dst[k] = src[i++]; } // postcondition: dst[lo .. hi] is sorted subarray assert isSorted(dst, lo, hi); } private static void sort(Comparable[] src, Comparable[] dst, int lo, int hi) { // if (hi <= lo) return; if (hi <= lo + CUTOFF) { insertionSort(dst, lo, hi); return; } int mid = lo + (hi - lo) / 2; sort(dst, src, lo, mid); sort(dst, src, mid+1, hi); // if (!less(src[mid+1], src[mid])) { // for (int i = lo; i <= hi; i++) dst[i] = src[i]; // return; // } // using System.arraycopy() is a bit faster than the above loop if (!less(src[mid+1], src[mid])) { System.arraycopy(src, lo, dst, lo, hi - lo + 1); return; } merge(src, dst, lo, mid, hi); } /** * Rearranges the array in ascending order, using the natural order. * @param a the array to be sorted */ public static void sort(Comparable[] a) { Comparable[] aux = a.clone(); sort(aux, a, 0, a.length-1); assert isSorted(a); } // sort from a[lo] to a[hi] using insertion sort private static void insertionSort(Comparable[] a, int lo, int hi) { for (int i = lo; i <= hi; i++) for (int j = i; j > lo && less(a[j], a[j-1]); j--) exch(a, j, j-1); } /******************************************************************* * Utility methods. *******************************************************************/ // exchange a[i] and a[j] private static void exch(Object[] a, int i, int j) { Object swap = a[i]; a[i] = a[j]; a[j] = swap; } // is a[i] < a[j]? private static boolean less(Comparable a, Comparable b) { return a.compareTo(b) < 0; } // is a[i] < a[j]? private static boolean less(Object a, Object b, Comparator comparator) { return comparator.compare(a, b) < 0; } /******************************************************************* * Version that takes Comparator as argument. *******************************************************************/ /** * Rearranges the array in ascending order, using the provided order. * * @param a the array to be sorted * @param comparator the comparator that defines the total order */ public static void sort(Object[] a, Comparator comparator) { Object[] aux = a.clone(); sort(aux, a, 0, a.length-1, comparator); assert isSorted(a, comparator); } private static void merge(Object[] src, Object[] dst, int lo, int mid, int hi, Comparator comparator) { // precondition: src[lo .. mid] and src[mid+1 .. hi] are sorted subarrays assert isSorted(src, lo, mid, comparator); assert isSorted(src, mid+1, hi, comparator); int i = lo, j = mid+1; for (int k = lo; k <= hi; k++) { if (i > mid) dst[k] = src[j++]; else if (j > hi) dst[k] = src[i++]; else if (less(src[j], src[i], comparator)) dst[k] = src[j++]; else dst[k] = src[i++]; } // postcondition: dst[lo .. hi] is sorted subarray assert isSorted(dst, lo, hi, comparator); } private static void sort(Object[] src, Object[] dst, int lo, int hi, Comparator comparator) { // if (hi <= lo) return; if (hi <= lo + CUTOFF) { insertionSort(dst, lo, hi, comparator); return; } int mid = lo + (hi - lo) / 2; sort(dst, src, lo, mid, comparator); sort(dst, src, mid+1, hi, comparator); // using System.arraycopy() is a bit faster than the above loop if (!less(src[mid+1], src[mid], comparator)) { System.arraycopy(src, lo, dst, lo, hi - lo + 1); return; } merge(src, dst, lo, mid, hi, comparator); } // sort from a[lo] to a[hi] using insertion sort private static void insertionSort(Object[] a, int lo, int hi, Comparator comparator) { for (int i = lo; i <= hi; i++) for (int j = i; j > lo && less(a[j], a[j-1], comparator); j--) exch(a, j, j-1); } /*************************************************************************** * Check if array is sorted - useful for debugging. ***************************************************************************/ private static boolean isSorted(Comparable[] a) { return isSorted(a, 0, a.length - 1); } private static boolean isSorted(Comparable[] a, int lo, int hi) { for (int i = lo + 1; i <= hi; i++) if (less(a[i], a[i-1])) return false; return true; } private static boolean isSorted(Object[] a, Comparator comparator) { return isSorted(a, 0, a.length - 1, comparator); } private static boolean isSorted(Object[] a, int lo, int hi, Comparator comparator) { for (int i = lo + 1; i <= hi; i++) if (less(a[i], a[i-1], comparator)) return false; return true; } // print array to standard output private static void show(Object[] a) { for (int i = 0; i < a.length; i++) { StdOut.println(a[i]); } } /** * Reads in a sequence of strings from standard input; mergesorts them * (using an optimized version of mergesort); * and prints them to standard output in ascending order. * * @param args the command-line arguments */ public static void main(String[] args) { String[] a = StdIn.readAllStrings(); MergeX.sort(a); show(a); } }