import java.util.Random;

/**
 * This class illustrates various algorithms for shuffling an array of integers
 * 
 * @author alchambers
 * @version sp17
 */
public class Shuffler{

    // This variable was promoted to have class scope since
    // it is used in more than one method. Note, however, that
    // this is *not* an instance variable but a static variable
    private static Random rng;

    /**
     * Shuffles by swapping random elements in the array
     */
    private static void shuffle1(int[] values){
        final int NUM_TIMES = values.length;
        for(int i = 0; i < NUM_TIMES; i++){
            int pos1 = rng.nextInt(values.length);
            int pos2 = rng.nextInt(values.length);            
            int temp = values[pos2];
            values[pos2] = values[pos1];
            values[pos1] = temp;
        }        
    }

    /**
     * Repeatedly removes and then re-inserts the first
     * element in the array
     */
    private static void shuffle2(int[] values){
        for(int i = 0; i < values.length; i++){
            int firstElement = values[0];
            int newPos = rng.nextInt(values.length);

            // Shift all elements in the range [0...newPos] left by one spot
            for(int j = 0; j < newPos; j++){
                values[j] = values[j+1];
            }

            // Insert the previously first element into the
            // hole we made in the array
            values[newPos] = firstElement;            
        }
    }


    /**
     * Splits array into two halves. Then interleaves the values
     * from these two halves back into the array.
     */
    private static void shuffle3(int[] values){
        int length1 = values.length / 2;
        int length2 = values.length - length1;        
        int[] firstHalf = new int[length1];
        int[] secondHalf = new int[length2];

        final int NUM_TIMES = values.length;
        for(int i = 0; i < NUM_TIMES; i++){

            // Split the array into two halves
            for(int j = 0; j < values.length; j++){
                if(j < length1){
                    firstHalf[j] = values[j];
                }
                else{
                    secondHalf[j-length1] = values[j];
                }
            }

            // Interleave the two halves. Note that firstHalf.length is always
            // less than or equal to secondHalf.length
            int j, curr = 0;
            for(j = 0; j < firstHalf.length; j++){
                values[curr] = firstHalf[j];
                curr++;
                values[curr] = secondHalf[j];                
                curr++;
            }

            // Check if there is a remaining value in secondHalf that needs to be copied over
            if(curr == values.length-1){
                values[curr] = secondHalf[j];
            }
        }        
    }

    /**
     * Assigns each element to a brand new randomly chosen spot in the array
     */
    private static void shuffle4(int[] values){
        int[] newValues = new int[values.length];
        boolean[] taken = new boolean[values.length]; // starts out as all false

        for(int i = 0; i < values.length; i++){
            int element = values[i];

            // Find a new random spot for this element in the array
            // May need to generate more than one spot until we find
            // one that hasn't been taken yet
            int newPos = -1;
            do{
                newPos = rng.nextInt(newValues.length);                
            }while(taken[newPos]);

            // Store the element in its new spot and mark this
            // spot as taken
            taken[newPos] = true;
            newValues[newPos] = element;
        }

        // Need to copy over the new array to the old array!
        for(int i = 0; i < values.length; i++){
            values[i] = newValues[i];
        }

    }

    /**
     * Prints an array to the screen
     */
    private static void printArray(int[] values){
        for(int i = 0; i < values.length; i++){
            System.out.print(values[i] + " ");
        }
        System.out.println();
    }

    public static void main(String[] args){
        rng = new Random();
        int[] array = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

        // Call various shuffling algorithms below
        printArray(array);
        shuffle4(array);
        printArray(array);
    }
}