Fabien COELHO wrote: > Most "permutation" functions are really cryptographic cyphers which are > quite expensive, and require powers of two, which is not what is needed. > ISTM that there are some constructs to deal with arbitrary sizes based on > cryptographic functions, but that would make it too expensive for the > purpose.
FWIW, you might have a look at the permuteseq extension: https://pgxn.org/dist/permuteseq It permutes an arbitrary range of 64-bit integers into itself, with a user-supplied key as the seed. Outputs are coerced into the desired range by using the smallest possible power of two for the Feistel cypher's block size, and then cycle-walking over the results. Best regards, -- Daniel Vérité PostgreSQL-powered mailer: http://www.manitou-mail.org Twitter: @DanielVerite