# cat examples/single_solution.txt
_ _ _ | _ 6 8 | _ 1 _
1 _ _ | _ _ _ | 7 _ _
_ 4 3 | 2 _ _ | 5 _ _
------+-------+------
3 _ _ | _ _ _ | 4 _ _
8 _ _ | _ _ _ | _ _ 9
_ _ 1 | _ _ _ | _ _ 6
------+-------+------
_ _ 6 | _ _ 4 | 8 5 _
_ _ 2 | _ _ _ | _ _ 7
_ 1 _ | 5 9 _ | _ _ _

# ./constraint-sudoku-solver examples/single_solution.txt
Trying to solve...

Solution:
  2 7 5 | 9 6 8 | 3 1 4
  1 8 9 | 4 3 5 | 7 6 2
  6 4 3 | 2 7 1 | 5 9 8
  ------+-------+------
  3 2 7 | 6 1 9 | 4 8 5
  8 6 4 | 7 5 2 | 1 3 9
  9 5 1 | 8 4 3 | 2 7 6
  ------+-------+------
  7 9 6 | 3 2 4 | 8 5 1
  5 3 2 | 1 8 6 | 9 4 7
  4 1 8 | 5 9 7 | 6 2 3

Took 0.032 seconds to find.
Given problem IS a true Sokudo.

# ./constraint-sudoku-solver --help
usage: constraint-sudoku-solver [-h] [--echo] [--duration SECONDS] FILE

positional arguments:
  FILE                file with puzzle to solve (see examples for puzzle
                      syntax)

optional arguments:
  -h, --help          show this help message and exit
  --echo              dump puzzle about to solve
  --duration SECONDS  seconds to stop searching for additional solutions after
                      (default: 0.1)