I fixed a memory leak in the minimax algorithm. There were copies of arrays that were not freed properly. I also changed move_t typedef to include the direction instead of an array. Now it's much faster to get the direction without comparing arrays.
Time: 3h
I added a silent mode to the AI modes so they won't print the output while running. This is used in the comparasion mode. It runs much faster now because the game board is not printed to the screen every time.
Time: 1-2h
I changed the function copy_array() to just copy the contents of the old array to the new array. Previously this function allocated and returned a pointer to a new array with the contents copied. I used this to reduce the mallocs needed in the minimax algorithm. Now the same allocation is used when going through all possible moves for max or min.
I changed the whole structure of the game array. It used to be an array of int-pointers that each pointed to int arrays. This was a bad option in terms of cache efficiency. I changed the program so that the entire game board is stored in a single 16-element array. The game board is stored there row by row. The performance of the minimax AI increased by 50-80% (benchmarks will be released later).
The disadvantage is that [][] syntax cannot be used for indexing anymore. I made an inline helper function that calculates the correct index in the table with the formula y*4+x.
Now the table can be allocated from the stack instead of needing millions of malloc calls as in the previous version.
I changed the type of the game array from int to unsigned int, because there cannot be negative numbers in the game table.
I refactored the code a lot and improved the comments. I simplified it by removing unnecessary functions.
Time: 8h