// This program has a method called sort that sorts an array of int values
// using the quicksort algorithm.  Method main tests it by constructing two
// arrays containing random values.  It compares the time used by this sort
// versus Arrays.sort.
//
// The program includes a "trial run" of the sorting calls prior to the timed
// executions.  Java performs some optimizations that distort the times without
// these trial runs.

import java.util.*;

public class Quick {
    public static final int SIZE = 10000000;

    public static void main(String[] args) {
        // make 2 copies for the comparision (list1, list2) and 2 copies
        // for the trial runs (list3, list4)
        int[] list1 = new int[SIZE];
        int[] list2 = new int[SIZE];
        int[] list3 = new int[SIZE];
        int[] list4 = new int[SIZE];
        fill(list1, list2, list3, list4);

        // perform trial runs to warm up the JVM
        sort(list3);
        Arrays.sort(list4);

        runTests(list1, list2);
        checkMatch(list1, list2);
    }

    // make a random list of ints and put in each of the arrays
    public static void fill(int[] list1, int[] list2, int[] list3,
                            int[] list4) {
        Random r = new Random();
        for (int i = 0; i < SIZE; i++) {
            int number = r.nextInt(2 * SIZE);
            list1[i] = number;
            list2[i] = number;
            list3[i] = number;
            list4[i] = number;
        }
    }        

    // Runs and times this sort and built-in sort using given lists
    public static void runTests(int[] list1, int[] list2) {
        // run and time our sort
        long start = System.currentTimeMillis();
        sort(list1);
        double elapsed1 = (System.currentTimeMillis() - start)/1000.0;
        System.out.println("quicksort took " + elapsed1 + " seconds");
        System.out.println();

        // run and time built-in sort
        start = System.currentTimeMillis();
        Arrays.sort(list2);
        double elapsed2 = (System.currentTimeMillis() - start)/1000.0;
        System.out.println("Arrays.sort took " + elapsed2 + " seconds");
        System.out.println();

        System.out.println("Ratio = " + elapsed1 / elapsed2);
    }

    // compares the lists and reports whether they match
    public static void checkMatch(int[] list1, int[] list2) {
        boolean match = true;
        for (int i = 0; i < SIZE; i++) {
            if (list1[i] != list2[i])
                match = false;
        }
        if (match)
            System.out.println("lists match");
        else
            System.out.println("lists don't match");
    }

    // pre : 0 <= index1, index2 < list.length
    // post: Values of list[x] and list[y] are exchanged.
    private static void swap(int[] list, int index1, int index2) {
        int temp = list[index1];
        list[index1] = list[index2];
        list[index2] = temp;
    }

    // post: uses the middle value of the list as a "pivot" to
    //       partition the list into two sublists: all those values less
    //       than or equal to the pivot appearing first followed by the
    //       pivot followed by all those values greater than the pivot;
    //       places the pivot value between the two sublists and returns
    //       the index of the pivot value
    private static int partition(int[] list, int low, int high) {
        // swap middle value into first position
        swap(list, low, (low + high) / 2);
        // remember pivot
        int pivot = list[low];

        // loop invariant: list divided into 3 parts: values <= pivot followed
        //                 by unknown values followed by values > pivot
        int index1 = low + 1; // index of first unknown value
        int index2 = high;    // index of last unknown value
        while (index1 <= index2) // while some values still unknown
            if (list[index1] <= pivot)
                index1++;
            else if (list[index2] > pivot)
                index2--;
            else {
                swap(list, index1, index2);
                index1++;
                index2--;
            }

        // put the pivot value between the two sublists and return its index
        swap(list, low, index2);
        return index2;
    }

    // post: elements low through high of given list are in sorted
    //       (nondecreasing) order
    public static void sort(int[] list, int low, int high) {
        // uses recursive quicksort to sort elements low through high; base
        // case is a list of 0 or 1 elements, which requires no sorting; in
        // recursive case, the list is partitioned using a pivot value and the
        // two resulting lists are recursively sorted
        if (low < high) {
            int pivotIndex = partition(list, low, high);
            sort(list, low, pivotIndex - 1);
            sort(list, pivotIndex + 1, high);
        }
    }

    // post: given list is in sorted (nondecreasing) order
    public static void sort(int[] list) {
        sort(list, 0, list.length - 1);
    }
}