# Sudoku Construction

**Construction**

It is commonly believed that Dell Number Place puzzles are computer-generated; they typically have over 30 givens placed in an apparently random scatter, some of which can possibly be deduced from other givens. They also have no authoring credits. Wei-Hwa Huang claims that he was commissioned by Dell to write a Number Place puzzle generator in the winter of 2000; prior to that, he was told, the puzzles were hand-made. The puzzle generator was written in Visual C++, and although had options to generate a more Japanese-style puzzle, with symmetry constraints and fewer numbers, Dell opted not to use those features. Presumably the puzzles since then still use that program, although it is hard to tell.

Nikoli Sudoku are hand-constructed, with the author being credited beside each puzzle; the givens are always found in a symmetrical pattern. (Building a Sudoku with symmetrical givens can be achieved by determining in advance where the givens will be located, and only assigning an actual number to them as needed.) Dell Number Place Challenger puzzles also list author credits. The Sudoku puzzles printed in most UK newspapers are apparently computer-generated but employ symmetrical givens, implying a more humanistic algorithm; The Guardian states that its puzzles are hand-constructed "in Japan", though it does not include authoring credits.

Note that it is possible to set starting grids with more than one solution and to set grids with no solution, but such are not considered proper Sudoku puzzles; like most other pure-logic puzzles, a unique solution is expected. Great caution is required in constructing a Sudoku puzzle, as failing to recognize where a number can be logically deduced at any point in construction - regardless of how tortuous that logic may be - can result in an unsolvable puzzle when defining a future given contradicts what has already been built.

If an efficient solver is available, there is a very simple method of automatic construction: randomly add a digit to the grid, and then look for a solution. If no solution is found, remove the digit and try another. Otherwise, look for a different solution. If there is no other solution, accept the current digit; otherwise, repeat this process.

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