Game2048.java

/*
 * Author: Ted Yin (Maofan Yin) <ted.sybil@gmail.com>
 * Description: Implementation for performing a move in direction left in 2048 game
 */

class Game2048 {
    public int moveLeft(int[][] b, int i) {
        int bonus = 0;
        int h = 0; /* b[i][0..h-1] has its final value and cannot be changed
                      again (exactly the same definition in your hint) */
        boolean moveMade = false;
        /* loop invariant:
         * +-------------row: b[i]----------------------+
         * |###########<x>00000000??????????????????????|
         * +0-----------h---------j---------------------+
         * "#": has their final values and cannot be changed
         * "<x>": the next value to be finalized (still mergable)
         * "0": blanks
         * "?": not discovered
         */
        for (int j = 1; j < b[i].length; j++)
        {
            if (b[i][j] == 0) continue; /* skip blanks */
            if (b[i][j] == b[i][h])
            {
                bonus += (b[i][h++] += b[i][j]); /* merge with the previous same value:
                                                    <x>, z, ..., x --> (x + x), <z>, ... */
                moveMade = true;
                b[i][j] = 0;
            }
            else
            {
                /* different value, so that the value x at h already has its final value */
                if (b[i][h] == 0)
                {
                    b[i][h] = b[i][j]; /* <0>, ..., y --> <y>, ... */
                    b[i][j] = 0;
                }
                else
                {
                    b[i][++h] = b[i][j]; /* <x>, 0, ..., y --> x, <y>, ... */
                    if (h != j)
                    {
                        moveMade = true;
                        b[i][j] = 0;
                    }
                    /* if h == j, nothing is moved */
                }
            }
        }
        return moveMade ? bonus: -1;
    }

    public static void main(String [] args) {
        /* A simple test of written routine */
        Game2048 game = new Game2048();
        int[][] grid = new int[][]{new int[] {2, 4, 4, 8, 0, 8, 8}};
        System.out.printf("%d:", game.moveLeft(grid, 0));
        for (int j = 0; j < grid[0].length; j++)
            System.out.printf(" %d", grid[0][j]);
        System.out.printf("\n");
    }
}