Compute a SHA1-hash from Term. The hash is represented as a
40-byte hexadecimal atom. Unlike term_hash/2
and friends, this predicate produces a hash key for non-ground terms.
The hash is invariant over variable-renaming (see =@=/2)
and constants over different invocations of Prolog.bugThe
hash depends on word order (big/little-endian) and the wordsize (32/64
bits).
This predicate raises an exception when trying to compute the hash on
an attributed term. Attributed terms are not handled because subsumes_chk/2
is not considered well defined for attributed terms.
Cyclic terms are supported. The hash is taken over a canonical form
of the term graph, so that terms denoting the same infinite tree hash
alike however many cells they were written with. That is, given A=[a|A]
and B=[a,a|B], A and B produce the same hash. Note
that the hash of a cyclic term is not the hash of any acyclic term, as a
cyclic term is never =@=/2
to one.
This hash was developed for lookup of solutions to a goal stored in a
table. By using a cryptographic hash, heuristic algorithms can often
ignore the possibility of hash collisions and thus avoid storing the
goal term itself as well as testing using =@=/2.
