Board.groovy

void fillCandidates() {
    for (int i = 0; i < board.length; i++) {
        for (int j = 0; j < board[i].length; j++) {
            Cell cell = board[i][j]
            if (cell.value == 0) {
                cell.candidates = (1..9)
                removeKnown(i, j)
                assert cell.candidates.size() > 0
            } else {
                cell.candidates = []
            }
        }
    }
}

void removeKnown(int rowNum, int colNum) {
    Cell cell = board[rowNum][colNum]
    cell.candidates -= getValuesForColumn(colNum)
    cell.candidates -= getValuesForRecord(rowNum)
    cell.candidates -= getValuesForSquare(rowNum, colNum)
}