============================== USAGE =====================================
type 'make' to compile and './sudoku-solver' to run. You can change the
sudoku board to solve in the 'file/in.txt' (only the first 9 lines are
used).

=================== EXAMPLE OUTPUT AND HOW TO READ =======================
$ ./sudoku-solver 
reading from file: 'file/in.txt'
time taken: 0.09ms (horizontal solve, 1 thread) correct solution: true
time taken: 0.14ms (diagonal solve, 1 thread) correct solution: true
time taken: 0.94ms (horizontal solve, 9 threads) correct solution: true
time taken: 1.05ms (diagonal solve, 17 threads) correct solution: true
x x x 2 6 x 7 x 1       4 3 5 2 6 9 7 8 1 
6 8 x x 7 x x 9 x       6 8 2 5 7 1 4 9 3 
1 9 x x x 4 5 x x       1 9 7 8 3 4 5 6 2 
8 2 x 1 x x x 4 x       8 2 6 1 9 5 3 4 7 
x x 4 6 x 2 9 x x   ->  3 7 4 6 8 2 9 1 5 
x 5 x x x 3 x 2 8       9 5 1 7 4 3 6 2 8 
x x 9 3 x x x 7 4       5 1 9 3 2 6 8 7 4 
x 4 x x 5 x x 3 6       2 4 8 9 5 7 1 3 6 
7 x 3 x 1 8 x x x       7 6 3 4 1 8 2 5 9 
writing to file: 'file/out.txt'

Each line in the beginning shows the results of various methods I've 
implemented for trying to solve sudoku boards, ranging from single-
threaded to a fairly ludicrous 17-thread solution. The "correct solution"
indicates if that method gave a valid sudoku board as a solution to
the problem. The two actual algorithms I wrote were a horizontal solve 
algorithm and a diagonal solve algorithm (diagonals are like traces). 

I also include the running time for each method I've decided to use.
eecslinab3 reports time values as all 0.00ms, not sure why. It is
either much faster than my chromebook or time.h doesn't work on it for
some reason