If you think anyone might care about your software in a hundred years, explain it in writing in a technology-independent way that would make it easy for anyone to re-implement it by reading your explanation, along with a reference implementation in pure lisp, lambda calculus, or the MIX architecture from Knuth's TAOCP. Self-publish your explanation and reference implementation in a book printed on acid-free paper, get an ISBN for it, and send the book to the Library of Congress. Spread the risk by also sending it to other national depository libraries.