%-----------------------------------------------------------------------------%
% vim: ft=mercury ts=4 sw=4 et
%-----------------------------------------------------------------------------%
% Copyright (C) 1999-2006 The University of Melbourne.
% This file may only be copied under the terms of the GNU Library General
% Public License - see the file COPYING.LIB in the Mercury distribution.
%-----------------------------------------------------------------------------%

% File: term_rep.m.
% Author: Ian MacLarty.

% This module implements an abstract type, term_rep, values of which are the
% representation of some other value.  Constructing a representation from a
% term is cc_multi, but then doing comparisons on the representation is
% deterministic.
%
% This is useful when we only want to consider the representation of a term
% and don't care about it's actual value.

%-----------------------------------------------------------------------------%
%-----------------------------------------------------------------------------%

:- module mdb.term_rep.
:- interface.

:- import_module mdbcomp.
:- import_module mdbcomp.program_representation.

:- import_module univ.

%-----------------------------------------------------------------------------%

:- type term_rep.

:- pred univ_to_rep(univ::in, term_rep::out) is cc_multi.

:- pred rep_to_univ(term_rep::in, univ::out) is det.

    % argumnet(Term, N, Subterm):
    %
    % True iff Subterm is the Nth argument of Term.
    %
:- pred argument(term_rep::in, int::in, term_rep::out) is semidet.

:- pred deref_path(term_rep::in, term_path::in, term_rep::out) is semidet.

    % field_pos(FieldName, Term, N):
    %
    % True iff argument N of Term has the name FieldName.
    %
:- pred field_pos(string::in, term_rep::in, int::out) is semidet.

%-----------------------------------------------------------------------------%
%-----------------------------------------------------------------------------%

:- implementation.

:- import_module mdb.declarative_debugger.

:- import_module construct.
:- import_module deconstruct.
:- import_module exception.
:- import_module int.
:- import_module list.
:- import_module maybe.
:- import_module string.
:- import_module type_desc.

%-----------------------------------------------------------------------------%

:- type term_rep
    ---> term_rep(univ)
    where
        equality is term_rep_equal,
        comparison is term_rep_compare.

:- pred term_rep_equal(term_rep::in, term_rep::in) is semidet.

term_rep_equal(Rep1, Rep2) :-
    (=) = promise_only_solution(comp_rep_2(Rep1, Rep2)).

:- pred comp_rep_2(term_rep::in, term_rep::in, builtin.comparison_result::uo)
    is cc_multi.

comp_rep_2(Rep1, Rep2, Result) :-
    builtin.compare_representation(Result, Rep1, Rep2).

:- pred term_rep_compare(builtin.comparison_result::uo, term_rep::in,
    term_rep::in) is det.

term_rep_compare(Result, Rep1, Rep2) :-
    Result = promise_only_solution(comp_rep_2(Rep1, Rep2)).

univ_to_rep(Univ0, term_rep(Univ)) :- cc_multi_equal(Univ0, Univ).

rep_to_univ(Rep, Univ) :-
    Univ = promise_only_solution(
        pred(U::out) is cc_multi :- Rep = term_rep(U)
    ).

deref_path(Term, Path, SubTerm):-
    (
        Path = [],
        SubTerm = Term
    ;
        Path = [Head | Tail],
        argument(Term, Head, NextSubTerm),
        deref_path(NextSubTerm, Tail, SubTerm)
    ).

argument(Term, N, Arg) :-
    %
    % There is only one representation of a subterm, given
    % the representation of the containing term and a term path.
    %
    promise_equivalent_solutions [MaybeArg] (
        rep_to_univ(Term, Univ),
        % Argument indexes in the term path start from one, but
        % the argument function wants argument indexes to
        % start from zero.
        arg_cc(univ_value(Univ), N - 1, MaybeSubUniv),
        (
            MaybeSubUniv = arg(SubValue),
            univ_to_rep(univ(SubValue), Arg0),
            MaybeArg = yes(Arg0)
        ;
            MaybeSubUniv = no_arg,
            MaybeArg = no
        )
    ),
    MaybeArg = yes(Arg).

field_pos(FieldName, Term, Pos) :-
    %
    % There is only one or zero positions of a field
    % given a representation of a term and the field name.
    %
    promise_equivalent_solutions [MaybePos] (
        rep_to_univ(Term, Univ),
        Value = univ_value(Univ),
        deconstruct(Value, include_details_cc, Functor, Arity, _Args),
        Type = type_of(Value),
        find_functor(1, num_functors(Type), Type, Functor, Arity,
            MaybeFunctorNum),
        (
            MaybeFunctorNum = yes(FunctorNum),
            (
                get_functor_with_names(Type, FunctorNum - 1, 
                    _FunctorName, _Arity, _ArgTypes, ArgNames)
            ->
                ( nth_member_search(ArgNames, yes(FieldName), Pos0) ->
                    MaybePos = yes(Pos0)
                ;
                    MaybePos = no
                )
            ;
                throw(internal_error("field_pos",
                    "get_functor_with_names couldn't find functor"))
            )
        ;
            MaybeFunctorNum = no,
            throw(internal_error("field_pos",
                "find_functor couldn't find functor"))
        )
    ),
    MaybePos = yes(Pos).

:- pred find_functor(int::in, int::in, type_desc::in, string::in, int::in,
    maybe(int)::out) is det.

find_functor(Current, NumFunctors, Type, FunctorName, Arity,
        MaybeFunctorNum) :-
    ( Current =< NumFunctors ->
        ( get_functor(Type, Current - 1, FunctorName, Arity, _) ->
            MaybeFunctorNum = yes(Current)
        ;
            find_functor(Current + 1, NumFunctors, Type,
                FunctorName, Arity, MaybeFunctorNum)
        )
    ;
        MaybeFunctorNum = no
    ).