public static Cell[] GetValueCandidates(Board board, Cell cell)
{
var columnValues = board.GetAssignedColumnCells(cell);
var rowValues = board.GetAssignedRowCells(cell);
var gridValues = board.GetGridCells(cell)
.Where(i => i.IsAssigned())
.ToArray();
var all = Cell.All;
// BUG: "all" keeps bad positional value, corrupting board.
var missingColumnValues = all.Except(columnValues).ToArray();
var missingRowValues = all.Except(rowValues) .ToArray();
var missingGridValues = all.Except(gridValues) .ToArray();
var candidates = missingColumnValues
.Intersect(missingRowValues)
.Intersect(missingGridValues)
.ToArray();
return candidates;
}
