Is it possible to imprison an alleged criminal before committing a crime?

First, I would consider whether or not those algorithms are free and open for anybody to examine. If those algorithms are not available for anybody who wants to examine them then that alone would make me suspicious of any jurisdiction using them for "precrime" arrests.

I don't think that it can conceivably be possible to imprison anybody before a crime is committed. Such a thing seems to violate the Universal Declaration of Human Rights on several counts and would therefore be opposed by the United Nations. Article 9 of the UDHR states, "No one shall be subjected to arbitrary arrest, detention or exile." "Precrime" law enforcement would be doing just that. Articles 10 and 11 states that everyone has a right to an "impartial tribunal" and that "Everyone charged with a penal offence has the right to be presumed innocent until proved guilty...". Imprisoning someone before a crime is committed demonstrates neither impartiality or presumption of innocence.