MergeBars.java

Below is the syntax highlighted version of MergeBars.java from §2.2 Mergesort.

/******************************************************************************
 *  Compilation:  javac MergeBars.java
 *  Execution:    java MergeBars m n
 *  Dependencies: StdDraw.java
 *
 *  Sort n random real numbers between 0 and 1 (with m disintct values)
 *  using mergesort with cutoff to insertion sort.
 *
 *  Visualize the results by ploting bars with heights proportional
 *  to the values.
 *
 *  % java MergeBars 1000 96
 *
 *  Comments
 *  --------
 *   - suggest removing the 10% default StdDraw border
 *   - if image is too large, it may not display properly but you can
 *     still save it to a file
 *
 ******************************************************************************/


public class MergeBars {
    private static final int VERTICAL = 70;
    private static final int CUTOFF = 12;

    private static int numberOfRows;
    private static int row = 0;


    // stably merge a[lo .. mid] with a[mid+1 .. hi] using aux[lo .. hi]
    public static void merge(double[] a, double[] aux, int lo, int mid, int hi) {

        // copy to aux[]
        for (int k = lo; k <= hi; k++) {
            aux[k] = a[k];
        }

        // merge back to a[]
        int i = lo, j = mid+1;
        for (int k = lo; k <= hi; k++) {
            if      (i > mid)              a[k] = aux[j++];
            else if (j > hi)               a[k] = aux[i++];
            else if (less(aux[j], aux[i])) a[k] = aux[j++];
            else                           a[k] = aux[i++];
        }
    }

    // mergesort a[lo..hi] using auxiliary array aux[lo..hi]
    private static void sort(double[] a, double[] aux, int lo, int hi) {
        int n = hi - lo + 1;
        if (n <= CUTOFF) {
            insertionSort(a, lo, hi);
            show(a, lo, hi);
            return;
        }
        if (hi <= lo) return;
        int mid = lo + (hi - lo) / 2;
        sort(a, aux, lo, mid);
        sort(a, aux, mid + 1, hi);
        merge(a, aux, lo, mid, hi);
        show(a, lo, hi);
    }

    public static void sort(double[] a) {
        double[] aux = new double[a.length];
        sort(a, aux, 0, a.length-1);
    }

    // sort from a[lo] to a[hi] using insertion sort
    private static void insertionSort(double[] 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);
    }

    private static boolean less(double v, double w) {
        return v < w;
    }

    private static void exch(double[] a, int i, int j) {
        double t = a[i];
        a[i] = a[j];
        a[j] = t;
    }

    // draw one row of trace
    private static void show(double[] a, int lo, int hi) {
        double y = numberOfRows - row - 1;
        for (int k = 0; k < a.length; k++) {
            if      (k < lo)             StdDraw.setPenColor(StdDraw.LIGHT_GRAY);
            else if (k > hi)             StdDraw.setPenColor(StdDraw.LIGHT_GRAY);
            else                         StdDraw.setPenColor(StdDraw.BLACK);
            StdDraw.filledRectangle(k, y + a[k] * 0.25, 0.25, a[k] * 0.25);
        }
        row++;
    }

    public static void main(String[] args) {
        int m = Integer.parseInt(args[0]);
        int n = Integer.parseInt(args[1]);
        if (args.length == 3) {
            long seed = Long.parseLong(args[2]);
            StdRandom.setSeed(seed);
        }
        double[] a = new double[n];
        double[] b = new double[n];
        for (int i = 0; i < n; i++) {
            a[i] = (1 + StdRandom.uniformInt(m)) / (double) m;
            b[i] = a[i];
        }

        StdDraw.enableDoubleBuffering();

        // precompute the number of rows
        numberOfRows = 0;
        sort(b);
        numberOfRows = row;
        row = 0;
        StdDraw.clear();

        StdDraw.setCanvasSize(800, numberOfRows*VERTICAL);
        StdDraw.show();
        StdDraw.square(0.5, 0.5, 0.5);
        StdDraw.setXscale(-1, n);
        StdDraw.setYscale(-0.5, numberOfRows);
        StdDraw.show();
        sort(a);
        StdDraw.show();
    }
}