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Re: AW: function hash for prolog terms?

From: Daniel Diaz
Subject: Re: AW: function hash for prolog terms?
Date: Wed, 11 Feb 2004 23:43:12 +0100
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.6) Gecko/20040113


You'll find as an attachment another Prolog solution to compute a hash code from a term.


I am working using the interface of GNU-Prolog for C.

I need a function that returns me an unique number for each
prolog term. Up
to now I am using the following one

int hash(PlTerm termino_prolog) {

 return abs (Rd_Code (termino_prolog));


But there are cases in those that it stops oneself prolog term it
returns me
different values.

Thank you for your help.


while I don't know about the semantics of "Rd_Code", I suspect, that the
returned information might be not unique to a specific term.

For a true hash you need a guaranteed unique function however.

So I would suggest, that you implement it as follows:

term_hash(Term,Hash) :-

Of course, this would involve an implementation of "my_md5_digest".
You can easily wrap a "stock" MD5 function, written in C, to perform this


term_hash(T, H) :-
        term_hash1(T, H0),
        H is H0 /\ 268435455 .

term_hash1(X, _) :-
        var(X), !,
        throw(error(instantiation_error, term_hash / 2)).

term_hash1(X, H) :-
        atom(X), !,
        atom_hash(X, H).

term_hash1(X, H) :-
        integer(X), !,
        H = X.

term_hash1(X, H) :-
        float(X), !,
        H is round(float_fractional_part(X) * 100000) \/ 

term_hash1(T, H) :-
        functor(T, F, N),
        atom_hash(F, H0),
        term_hash2(0, N, T, 0, H1),
        H is H1 * 129 + H0.

term_hash2(I, N, T, H0, H) :-
        (   I = N ->
            H = H0
        ;   I1 is I + 1,
            arg(I1, T, A),
            term_hash1(A, H1),
            H2 is H0 * 129 + H1,
            term_hash2(I1, N, T, H2, H)

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