state move (int direction, state s, int length) {
   //returns a state that has moved in the direction specified if that move is valid,
   //or a state identical to that passed if not. returns NULL on a board with no blank.
   board b = newBoard(s, length);
   int x;
   int y;
   state ret = NULL;
   for (x = 0; x < b->sideLength; x++) {
      for (y = 0; y < b->sideLength; y++) {
         if (b->matrix[x][y] == BLANK_SQUARE) {
            if (direction == UP) {
               if (y > 0) { //not at top of board
                  b->matrix[x][y] = b->matrix[x][y-1];
                  b->matrix[x][y-1] = BLANK_SQUARE;
               }
            } else if (direction == RIGHT) {
               if (x < b->sideLength - 1) {
                  b->matrix[x][y] = b->matrix[x+1][y];
                  b->matrix[x+1][y] = BLANK_SQUARE;
               }
            } else if (direction == DOWN) {
               if (y < b->sideLength - 1) { //not at bottom of board
                  b->matrix[x][y] = b->matrix[x][y+1];
                  b->matrix[x][y+1] = BLANK_SQUARE;
               }
            } else if (direction == LEFT) {
               if (x > 0) {
                  b->matrix[x][y] = b->matrix[x-1][y];
                  b->matrix[x-1][y] = BLANK_SQUARE;
               }
            }
            ret = boardToState(b);
            fuckUpBoard(b);
            return ret; //so much easier than scanning the rest of the board
         }
      }
   }
   return ret;
}