Rotate an Array

RotateArray.java

/**
 * http://web.stanford.edu/class/cs9/lectures/Coding%20Drill%203.pdf
 * Problem One: Array Rotation
 * Your job is to write a function
 * void rotateArray(int* array, size_t n, size_t amount)
 * that accepts as input an array and a “rotation amount.” You should then “rotate” the array
 * by cyclically shifting all of the elements in the array to the left by a number of steps
 * given by the rotation amount. As an example, suppose we have this array:
 * 103 106 107 108 109 110 140 161
 * Rotating it to the left by three steps would yield this array:
 * 108 109 110 140 161 103 106 107
 * (The input array may or may not be sorted.)
 * Then, analyze the big­O time and space complexity of your code.
 */
class RotateArray {
  public static void main(String[] args) { 
    int[] a = {103,106,107,108,109,110,140,161};
    rotate(a, 11);
    System.out.println(java.util.Arrays.toString(a));
    int[] b = {112,109,110,161};
    rotate(b, 2);
    System.out.println(java.util.Arrays.toString(b));
    int[] c = {112,109,110,161};
    rotate(b, 4);
    System.out.println(java.util.Arrays.toString(c));
  }

  static void rotate(int[] a, int ra) {
    int len = a.length;
    ra = ra % len;
    if (ra < 0)
      throw new IllegalArgumentException("negative ra not allowed: "  + ra);
    if (ra == 0)
      return;
    int nr  = 0; // number of elements to rotate
    // boundary: nr == len i.e. all elements are rotated
    int i = 0;
    int start = i; // we start with first element
    int m = a[i]; // remember it first
    while (true) {
      int di = i - ra; // the destiation index
      if (di < 0)
        di = len - (-di); 
      int tmp = a[di];
      a[di] = m;
      m = tmp;
      i = di; // destination is the new source
      nr += 1;
      if (nr == len)
        break;
      if (i == start) { // ra divides len evenly, and we are not yet done!
        System.out.println("back at: " + i +  ", but we aren't done yet");
        i = (i+1) % len;
        start = i;
        m = a[start];
      }
      //System.out.println(java.util.Arrays.toString(a));
    }
  }
}