If you found the software useful donation is something you might consider :-)

Janet-Sudoku - Java Android .NET Sudoku Generator & Sudoku Solver Library

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Sudoku Solver API

simple solving
path / sequence leading to the solution
evaluating solution existence
finding all solutions
puzzle / solution loading / saving
solving randomization options
error handling (error codes, messages)

Sudoku Generator API

random generator
generator based on puzzle
generating randomization options
error handling (error codes, messages)

Store API enabling variety of operations

160 built-in puzzle examples
A bunch of built-in operations enabling puzzle modification (i.e. rotation, reflection, swapping, permutation, ...)
Puzzle checking
Puzzle loading and saving
Rating puzzle difficulty

Janet Sudoku Demo App - command line tool based on the library

Puzzle loading
Puzzle generating
Puzzle evaluating
Puzzle solving
Puzzle rating
Puzzle saving

Regression tests

Large number of regression / unit tests

If you found the software useful donation is something you might consider :-)