S.D. Codified Laws § 22-7-7
If a defendant has been convicted of one or two prior felonies under the laws of this state or any other state or the United States, in addition to the principal felony, the sentence for the principal felony shall be enhanced by changing the class of the principal felony to the next class which is more severe, but in no circumstance may the enhancement exceed the sentence for a Class C felony. The determination of whether a prior offense is a felony for purposes of this chapter shall be determined by whether the prior offense was a felony under the laws of this state or under the laws of the United States at the time of conviction of such prior offense. For the purpose of this section, if the principal felony is not classified it shall be enhanced to the class which has an equal maximum imprisonment. For the purposes of this section, if the maximum imprisonment for the principal felony falls between two classifications, the principal felony shall be enhanced to the class which has the less severe maximum authorized imprisonment.
SDCL 22-7-7