Implement a C program to solve the 15-puzzle problem using the A* search algorithm.

In the assignment, solving a 15-puzzle problem needs to move the tiles to their goal locations, which are as shown below. The numbers 1~15 are indexes of the tiles, and 0 means blank tile. This state is the goal state.

Your program can solve the problem using a single thread or using 4 threads, depending on the first argument (argv[1]). The program uses a single thread if argv[1] is "-s", or 4 threads if argv[1] is "-m". The initial layout of the tiles is also provided in the command line as arguments by listing tile indexes in a row-major order.

For example, the command ./your_program -s 2 3 0 4 1 6 7 8 5 9 10 12 13 14 11 15 is to use one thread to solve the 15-puzzle problem, in which the tiles are initially placed as follows, and are to be moved to their goal locations

The command ./your_program -m 2 3 0 4 1 6 7 8 5 9 10 12 13 14 11 15 is to solve the above problem using 4 threads.

Program needs to print out a sequence of states showing the movement the tiles, or text "no solution" if a solution cannot be found