package chap09_Medians_and_Order_Statistics;

import static org.junit.Assert.*;

import java.util.Random;

import org.junit.Test;

public class SearchAlorithms {
    /**
     * 分割（快速排序中对数组的分割）
     * 
     * @param n
     * @param start
     * @param end
     * @return
     */
    protected static int partition(int[] n, int start, int end) {
        int p = end - 1;
        int s = start;// s位于大于a[p]和小于a[p]之间的位置
        int tmp;
        for (int i = start; i < end; i++) {
            if (n[i] < n[p]) {
                tmp = n[i];
                n[i] = n[s];
                n[s] = tmp;
                s++;
            }
        }
        {
            tmp = n[s];
            n[s] = n[p];
            n[p] = tmp;
        }
        return s;
    }

    /**
     * 随机分割，将一个数组从随机选择的位置处分割，左边小，右边大
     * 
     * @param n
     * @param start
     * @param end
     * @return
     */
    static protected int randomPartition(int[] n, int start, int end) {
        Random rand = new Random(23);
        int t = rand.nextInt(end - start) + start;
        int tmp;
        {
            tmp = n[t];
            n[t] = n[end - 1];
            n[end - 1] = tmp;
        }
        return partition(n, start, end);
    }

    /**
     * 搜索数组n中从start到end之间第k小的数字
     * 
     * @param n
     * @param start
     * @param end
     * @param k
     * @return
     */
    static int randomSearch(int[] n, int start, int end, int k) {
        if (start == end - 1)
            return n[start];
        int q = randomPartition(n, start, end);
        int p = q - start + 1;
        if (k == p)
            return n[q+1];
        else if (k > p)
            return randomSearch(n, q, end, k - q);
        else
            return randomSearch(n, start, q, k);
    }

    @Test
    public void testName() throws Exception {
        int[] n = { 13, 19, 9, 5, 12, 8, 7, 4, 21, 2, 6, 11 };
        int a = randomSearch(n, 0, 12, 1);
        System.out.println(a);
    }

}