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Predicate term_hash/2
Availability:built-in
[det]term_hash(+Term, -HashKey)
If Term is a ground term (see ground/1), HashKey is unified with a positive integer value that may be used as a hash key to the value. If Term is not ground, the predicate leaves HashKey an unbound variable. Hash keys are in the range 0 ... 72,057,594,037,927,935 (2^56), the maximal integer that can be stored efficiently (see max_tagged_integer.

This predicate may be used to build hash tables as well as to exploit argument indexing to find complex terms more quickly.

The hash key does not rely on temporary information like addresses of atoms and may be assumed constant over different invocations and versions of SWI-Prolog.90Last change: version 9.3.28 The term_hash/2 predicate is cycle-safe.bugAll arguments that (indirectly) lead to a cycle have the same hash key. bugHashes differ between big and little endian machines and hashes for big integers (currently > 2^56) differ depending on the big integer backend (GMP or LibBF). See gmp_version.

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Jan Wielemaker said (2014-02-28T14:56:17):0 upvotes 0 0 downvotes
Picture of user Jan Wielemaker.

The predicates term_hash/2 and term_hash/4 are typically used to add indexable additional arguments. For example, if the second argument of a predicate info/2 is a compound term and you indexing if info is called as info(-,++), i.e., unbound first and ground second argument, you can do the following:

assert_info(For, Info) :-
      term_hash(Info, Hash),
      assert_info(For, Info, Hash).

info(For, Info) :-
      term_hash(Info, Hash),
      info(For, Info, Hash).

If info/2 is a static predicate, you can use:

term_expansion(info(For,Info), info(For,Info,Hash)) :-
      term_hash(Info, Hash).

info(penguine, flies(false)).

This pattern can also be used to create hashes from multiple arguments to speedup queries that have multiple arguments instantiated, but none of the individual arguments provides a good index.