School, 12.09.2017, Ex8, merge sorting

public class Ex08 {
    public static int[] copy(int[] arr, int start, int end) {
        int[] res = new int[end - start + 1];

        for (int i = 0; i < res.length; i++) {
            res[i] = arr[start + i];
        }

        return res;
    }

    public static int[] merge(int[] arr1, int[] arr2) {
        int[] res = new int[arr1.length + arr2.length];
        int p1 = 0;
        int p2 = 0;
        int p3 = 0;

        while (p1 < arr1.length && p2 < arr2.length) {
            if (arr1[p1] <= arr2[p2]) {
                res[p3] = arr1[p1];
                p1 += 1;
                p3 += 1;
            } else {
                res[p3] = arr2[p2];
                p2 += 1;
                p3 += 1;
            }
        }

        while (p1 < arr1.length) {
            res[p3] = arr1[p1];
            p1 += 1;
            p3 += 1;
        }

        while (p2 < arr2.length) {
            res[p3] = arr2[p2];
            p2 += 1;
            p3 += 1;
        }

        System.out.println(asString(arr1) + " and " + asString(arr2) + " was merged to " + asString(res));
        return res;
    }

    public static int[] mergeSort(int[] arr) {
        System.out.println("start sorting " + asString(arr));
        if (arr.length == 1) {
            System.out.println(asString(arr) + " was sorted " + asString(arr));
            return arr;
        }

        int mid = arr.length / 2;
        int[] left = copy(arr, 0, mid - 1);
        int[] right = copy(arr, mid, arr.length - 1);
        int[] res;
        // sort . . . .
        int[] sortedLeft = mergeSort(left);
        int[] sortedRight = mergeSort(right);

        res = merge(sortedLeft, sortedRight);
        System.out.println(asString(arr) + " was sorted " + asString(res));
        return res;
    }

    public static String asString(int[] arr) {
        String res = "[";
        for (int i = 0; i < arr.length; i++) {
            res += arr[i] + " ";
        }

        res += "]";

        return res;
    }

    public static void main(String[] args) {
        int[] arr = { 1, 90, 2, 21, 9, 13, 1, 2, 4 };
        int[] res; // = {1, 1, 2, 4,5,6,7,8,9,13,21,90};

        System.out.println("source data : " + asString(arr));
        res = mergeSort(arr);
        System.out.println("the sorted data : " + asString(res));

    }

}