package Others;

import java.util.Arrays;

/**
 * BFPRT algorithm.
 */
public class BFPRT {

 public static int[] getMinKNumsByBFPRT(int[] arr, int k) {
 if (k < 1 || k > arr.length) {
 return null;
 }
 int minKth = getMinKthByBFPRT(arr, k);
 int[] res = new int[k];
 int index = 0;
 for (int i = 0; i < arr.length; i++) {
 if (arr[i] < minKth) {
 res[index++] = arr[i];
 }
 }
 for (; index != res.length; index++) {
 res[index] = minKth;
 }
 return res;
 }

 public static int getMinKthByBFPRT(int[] arr, int k) {
 int[] copyArr = copyArray(arr);
 return bfprt(copyArr, 0, copyArr.length - 1, k - 1);
 }

 public static int[] copyArray(int[]arr) {
 int[] copyArr = new int[arr.length];
 for(int i = 0; i < arr.length; i++) {
 copyArr[i] = arr[i];
 }
 return copyArr;
 }

 public static int bfprt(int[] arr, int begin, int end, int i) {
 if (begin == end) {
 return arr[begin];
 }
 int pivot = medianOfMedians(arr, begin, end);
 int[] pivotRange = partition(arr, begin, end, pivot);
 if (i >= pivotRange[0] && i <= pivotRange[1]) {
 return arr[i];
 } else if (i < pivotRange[0]) {
 return bfprt(arr, begin, pivotRange[0] - 1, i);
 } else {
 return bfprt(arr, pivotRange[1] + 1, end, i);
 }
 }

 /**
 * wikipedia: https://en.wikipedia.org/wiki/Median_of_medians .
 * @param arr an array.
 * @param begin begin num.
 * @param end end num.
 * @return median of medians.
 */
 public static int medianOfMedians(int[] arr, int begin, int end) {
 int num = end - begin + 1;
 int offset = num % 5 == 0 ? 0 : 1;
 int[] mArr = new int[num / 5 + offset];
 for (int i = 0;i < mArr.length;i++) {
 mArr[i] = getMedian(arr, begin + i * 5, Math.min(end, begin + i * 5 + 4));
 }
 return bfprt(mArr, 0, mArr.length - 1, mArr.length / 2);
 }

 public static void swap(int[]arr, int i, int j) {
 int swap = arr[i];
 arr[i] = arr[j];
 arr[j] = swap;
 }

 public static int[] partition(int[] arr,int begin,int end,int num)
 {
 int small=begin-1;
 int cur=begin;
 int big=end+1;
 while(cur!=big)
 {
 if (arr[cur]<num)
 {
 swap(arr,++small,cur++);
 }else if (arr[cur]>num)
 {
 swap(arr,--big,cur);
 } else {
 cur++;
 }
 }
 int[] pivotRange=new int[2];
 pivotRange[0]=small+1;
 pivotRange[1]=big-1;
 return pivotRange;
 }

 public static int getMedian(int[] arr, int begin, int end) {
 insertionSort(arr, begin, end);
 int sum = begin + end;
 int mid = sum / 2 + (sum % 2);
 return arr[mid];
 }

 public static void insertionSort(int[] arr, int begin, int end) {
 if (arr == null || arr.length < 2) {
 return;
 }
 for (int i = begin + 1;i != end + 1;i++) {
 for (int j = i;j != begin;j--) {
 if (arr[j - 1] > arr[j]) {
 swap(arr, j - 1, j);
 } else {
 break;
 }
 }
 }
 }

 public static void main(String[] args) {
 int[] arr = { 11, 9, 1, 3, 9, 2, 2, 5, 6, 5, 3, 5, 9, 7, 2, 5, 5, 1, 9 };
 int[] minK = getMinKNumsByBFPRT(arr,5);
 System.out.println(Arrays.toString(minK));
 }
}